United States
Environmental Protection
Agency
EPA-452/R-96-001d
June 1996
Air
Mercury Study
Report to Congress
Volume IV:
Health Effects of Mercury
and Mercury Compounds
SAB REVIEW DRAFT
&EPA
Office of Air Quality Planning & Standards
and
Office of Research and Development
1-2-4
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MERCURY STUDY REPORT TO CONGRESS
VOLUME IV:
HEALTH EFFECTS OF MERCURY AND MERCURY COMPOUNDS
SAB REVIEW DRAFT
June 1996
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
Office of Air Quality Planning and Standards
and
Office of Research and Development
U.S. Environmental Protection Agency
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iry
TABLE OF CONTENTS
Page
U.S. EPA AUTHORS [[[ v
SCIENTIFIC PEER REVIEWERS .............................................. vi
LIST OF TABLES [[[ viii
LIST OF FIGURES [[[ xi
LIST OF SYMBOLS, UNITS AND ACRONYMS ........................... '. ...... xii
EXECUTIVE SUMMARY ................................................. ES-1
1. INTRODUCTION ........................ . ......................... 1-1
2. TOXICOKINETICS ........................... . ..................... 2-1
2.1 Absorption ........ . ......................................... 2-1
2.1.1 Elemental Mercury ............. -. ......................... 2-1
2.1.2 Inorganic Mercury ....................................... 2-2
2.1.3 Methylmercury ......................................... 2-3
2.2 Distribution ............................................ . ..... 2-4
2.2.1 Elemental Mercury ....................................... 2-4
2.2.2 Inorganic Mercury ....................................... 2-4
2.2.3 Methylmercury ......................................... 2-5
2.3 Metabolism ................................................. 2-6
2.3.1 Elemental Mercury . . ..................................... 2-6
2.3.2 Inorganic Mercury ....................................... 2-6
2.3.3 Methylmercury ......................................... 2-6
2.4 Excretion [[[ 2-7
2.4.1 Elemental Mercury ....................................... 2-7
2.4.2 Inorganic Mercury ^ ....................................... 2-8
2.4.3 Methylmercury . ' ....................................... 2-8
2.5 Biological Monitoring .......................................... 2-9
2.5.1 Elemental Mercury ....................................... 2-9
2.5.2 Inorganic Mercury ...................................... 2-11
2.5.3 Methylmercury ...... .... .............................. 2-11
2.5.4 Methods of Analysis for Measuring Mercury in Biological Samples ... 2-12
2.6 Studies on Pharmacokinetic Models ............................... 2-12
2.6.1 Introduction .......................................... 2-12
2.6.2 Inorganic mercury ...................................... 2-13
2.6.3 Methylmercury ........................................ 2-13
2.6.4 Discussion ........................................... 2-15
3. BIOLOGICAL EFFECTS ............................................. 3-1
3.1 Elemental Mercury ................................ . ....... .... 3-1
3.1.1 Critical Noncancer Data ................................... 3-1
3.1.2 Cancer Data ..................................... . ..... 3.5
3.1.3 Other Data ............................................ 3-9
3.2 Inorganic Mercury ............................................ 3-35
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TABLE OF CONTENTS (continued)
3.2.3 . Other Data 3-43
3.3 Methylmercury 3-59
3.3.1 Critical Noncancer Data 3-59
3.3.2 Cancer Data '. 3-64
3.3.3 Other Data 3-71
4. POPULATIONS UNUSUALLY SUSCEPTIBLE TO MERCURY 4-1
5. INTERACTIONS 5-1
6. HAZARD IDENTIFICATION AND DOSE-RESPONSE ASSESSMENT 6-1
6.1 Background . ..:........ 6-1
6.1.1 Hazard Identification 6-1
6.1.2 Dose-response Assessment 6-4
6.2 Hazard Identification for Mercury 6-7
6.2.1 Developmental Effects . 6-8
6.2.2 Germ Cell Mutagenicity 6-11
6.2.3 Carcinogenic Effects 6-13
6.3 Dose-Response Assessment For Mercury 6-15
6.3.1 Systemic Noncancer Effects 6-15
6.3.2 Developmental Effects 6-35
6.3.3 Germ Cell Mutagenicity 6-36
6.3.4 Carcinogenic Effects 6-36
6.4 Risk Assessments Done By Other Groups 6-37
6.4.1 Food and Drug Administration 6-37
6.4.2 ATSDR 6-38
6.4.3 Department of Energy 6-39
6.4.4 National Institute of Environmental Health Sciences (NIEHS) 6-39
6.4.5 Department of Labor 6-39
6.4.6 Various States 6-39
6.4.7 World Health Organization 6-39
6.4.8 ACGIH 6-40
7. ONGOING RESEARCH AND RESEARCH NEEDS 7-1
7.1 Ongoing Research 7-1
7.2 Research Needs 7-3
June 1996 ii SAB REVIEW DRAFT
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TABLE OF CONTENTS (continued)
Page
APPENDIX A DOSE CONVERSIONS A-1
APPENDIX B INTEGRATED RISK INFORMATION SYSTEM (IRIS)
SUMMARIES B-l
APPENDIX C ATTENDEES OF U.S. EPA PEER REVIEW WORKSHOP
ON MERCURY ISSUES C-l
APPENDIX D TECHNICAL DETAILS OF UNCERTAINTY MODEL IN
SUPPORT OF THE METHYLMERCURY RfD D-l
June 1996 iii SAB REVIEW DRAFT
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LIST OF TABLES
Paee
ES-1 Summary of U.S. EPA Hazard Identification/Dose-response
Assessment for Mercury and Mercury Compounds ES-6
2-1 Reference Values for Total Mercury in Biological Media in the General
Population 2-10
2-2 Analytical Methods for the Detection of Mercury in Biological Samples 2-12
3-1 Carcinogenic Effects of Elemental Mercury in Humans: Epidemiological Studies . . . 3-7
3-2 Lethality of Elemental Mercury hi Humans: Case Studies 3-9
3-3 Lethality of Elemental Mercury in Animals: Inhalation Exposure 3-10
3-4 Neurotoxicity of Elemental Mercury in Humans: Case Studies 3-11
3-5 Neurotoxicity of Elemental Mercury in Humans: Epidemiological Studies 3-12
3-6 Neurotoxicity of Elemental Mercury in Animals: Inhalation Exposure 3-15
3-7 Renal Toxicity of Elemental Mercury in Humans: Case Studies 3-16
3-8 Renal Toxicity of Elemental Mercury in Humans: Epidemiological Studies 3-17
3-9 Renal Toxicity of Elemental Mercury hi Animals: Inhalation Exposure 3-18
3-10 Respiratory Toxicity of Elemental Mercury in Humans: Case Studies 3-18
3-11 Respiratory Toxicity of Elemental Mercury in Animals: Inhalation Exposure 3-19
3-12 Cardiovascular Toxicity of Elemental Mercury hi Humans: Case Studies 3-20
3-13 Cardiovascular Toxicity of Elemental Mercury hi Humans: Epidemiological
Studies 3-21
3-14 Cardiovascular Toxicity of Elemental Mercury hi Animals: Inhalation
Exposure 3-21
3-15 Gastrointestinal Toxicity of Elemental Mercury hi Humans: Case Studies 3-22
3-16 Gastrointestinal Toxicity of Elemental Mercury hi Animals: Inhalation
Exposure 3-23
3-17 Hepatic Toxicity of Elemental Mercury hi Humans: Case Study 3-24
3-18 Hepatic Toxicity of Elemental Mercury hi Animals: Inhalation Exposure 3-24
3-19 Hematological Toxicity of Elemental Mercury in Humans: Case Studies 3-25
3-20 Hematological Toxicity of Elemental Mercury in Humans: Epidemiological
Studies . 3-25
3-21 Immunotoxicity of Elemental Mercury hi Humans: Case Study 3-26
3-22 Immunotoxicity of Elemental Mercury hi Humans: Epidemiological Studies 3-26
3-23 Immunotoxicity of Elemental Mercury hi Animals: Inhalation Exposure 3-27
3-24 Dermal Toxicity of Elemental Mercury hi Humans: Case Studies 3-27
3-25 Developmental Toxicity of Elemental Mercury hi Humans: Case
Studies 3-28
3-26 Developmental Toxicity of Elemental Mercury hi Humans: Epidemiological
Studies . 3-29
3-27 Developmental Toxicity of Elemental Mercury hi Animals 3-30
3-28 Reproductive Toxicity of Elemental Mercury in Humans: Epidemiological
Studies 3-31
3-29 Reproductive Toxicity of Elemental Mercury in Animals 3-33
3-30 Genotoxicity of Elemental Mercury hi Humans 3-34
3-31 Incidence of Selected Lesions hi Rats in the NTP (1993) 2-Year Gavage
Study 3-40
June 1996 vii SAB REVIEW DRAFT
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LIST OF TABLES (continued)
Page
3-32 Incidence of Renal Tubule Tumors in Male Mice in the NTP (1993)
2-Year Gavage Study 3-41
3-33 Carcinogenic Effects of Inorganic Mercury in Animals: Oral Exposure 3-42
3-34 Lethality of Inorganic Mercury in Humans: Case Study 3-43
3-35 Lethality of Inorganic Mercury in Animals: Oral Exposure 3-44
3-36 Neurotoxicity of Inorganic Mercury in Humans: Case Studies 3-44
3-37 Neurotoxicity of Inorganic Mercury in Animals: Oral Exposure 3-45
3-38 Renal Toxicity of Inorganic Mercury in Humans: Case Studies 3-45
3-39 Renal Toxicity of Inorganic Mercury in Anjmals: Oral Exposure . 3-46
3-40 Renal Toxicity of Inorganic Mercury in Animals: Inhalation Exposure 3-48
3-41 Cardiovascular Toxicity of Inorganic Mercury in Animals 3-48
3-42 Gastrointestinal Toxicity of Inorganic Mercury in Humans: Case Studies 3-49
3-43 Gastrointestinal Toxicity of Inorganic Mercury in Animals 3-50
3-44 Hepatic Toxicity of Inorganic Mercury in Animals 3-50
3-45 Immunotoxicity of Inorganic Mercury in Animals 3-51
3-46 Developmental Toxicity of Inorganic Mercury in Animals: Inhalation
Exposure 3-52
3-47 Developmental Toxicity of Inorganic Mercury in Animals: Oral Exposure 3-53
3^8 Reproductive Toxicity of Inorganic Mercury in Humans: Case Study 3-56
3-49 Reproductive Toxicity of Inorganic Mercury in Animals 3-56
3-50 Genotoxicity of Inorganic Mercury hi Humans 3-57
3-51 Genotoxicity of Inorganic Mercury in Animals 3-59
3-52 Carcinogenic Effects of Methylmercury in Humans: Epidemiological Studies .... 3-66
3-53 Carcinogenic Effects of Methylmercury hi Animals: Oral Exposure 3-68
3-54 Lethality of Methylmercury hi Humans: Case Study of Oral Exposure 3-71
3-55 Lethality of Methylmercury hi Humans: Case Studies of Inhalation Exposure .... 3-72
3-56 Lethality of Methylmercury hi Animals 3-73
3-57 Neurotoxicity of Methylmercury hi Humans: Case Studies of Oral Exposure 3-75
3-58 Neurotoxicity of Methylmercury hi Humans:. Case Studies of Inhalation
Exposure 3-76
3-59 Neurotoxicity of Methylmercury hi Animals 3-78
3-60 Renal Toxicity of Methylmercury hi Animals 3-80
3-61 Cardiovascular Toxicity of Methylmercury hi Humans: Case Study ' 3-81
3-62 Cardiovascular Toxicity of Methylmercury hi Animals 3-81
3-63 Gastrointestinal Toxicity of Methylmercury hi Animals 3-82
3-64 Immunotoxicity of Methylmercury hi Animals 3-82
3-65 Dermal Toxicity of Methylmercury hi Humans: Epidemiological Study 3-83
3-66 Developmental Toxicity of Methylmercury hi Humans: Case Studies 3-85
3-67 Developmental Toxicity of Methylmercury hi Humans: Epidemiologic
Studies 3-85
3-68 Developmental Toxicity of Methylmercury in Animals 3-87
3-69 Reproductive Toxicity of Methylmercury hi Animals 3-92
3-70 Genotoxicity of Methylmercury in Humans: Case Study 3-94
3-71 Genotoxicity of Methylmercury in Humans: Epidemiology Study 3-94
3-72 Genotoxicity of Methylmercury in Cats 3.95
June 1996 viii SAB REVIEW DRAFT
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LIST OF TABLES (continued)
Page
5-1 Interactions of Mercury with Other Compounds 5-2
6-1 t Consensus Decisions of-Peer Review Panel 6-17
6-2 ' Available Data on HairBlood Ration (Total Hg) 6-22
6-3 Incidence of Effects in Iraqi Children by Exposure Group 6-24
6-4 Incidence of Effects in Iraqi Adults by Exposure Group 6-24
6-5 Methylmercury Benchmark Dose Estimates (ppm hair) 6-26
6-6 Methylmercury Benchmark Dose Estimates (ppb blood) 6-26
6-7 Density-Based Dose Groupings 6-27
6-8 Uniform Dose Groupings 6-27
7-1 Ongoing Research * 7-1
June 1996 ix SAB REVIEW DRAFT
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LIST OF FIGURES
6-1 Density of Data Points Relative to Hg Concentration in Hair
for Iraqi Cohort Data 6-27
June 1996 x SAB REVIEW DRAFT
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U.S. EPA AUTHORS
Principal Authors:
Beth Hassett-Sipple
Office of Air Quality
Planning and Standards
Research Triangle Park, NC
Contributing Authors:
Harlal Choudhury, Ph.D., D.A.B.T.
National Center for Environmental
Assessment-Cincinnati
Office of Research and Development
Cincinnati, OH
John Cicmanec, D.V.M.
National Center for Environmental
Assessment-Cincinnati
Office of Research and Development
Cincinnati, OH
Kathryn R. Mahaffey, Ph.D.
National Center for Environmental
Assessment-Cincinnati
Office of Research and Development
Cincinnati, OH
Glenn E. Rice
National Center for Environmental
Assessment-Cincinnati
Office of Research and Development
Cincinnati, OH
Jeff Swartout
National Center for Environmental
Assessment-Cincinnati
Office of Research and Development
Cincinnati, OH
Rita Schoeny, Ph.D.
National Center for Environmental
Assessment-Cincinnati
Office of Research and Development
Cincinnati, OH
June 1996
IV
SAB REVIEW DRAFT
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SCIENTIFIC PEER REVIEWERS
Brian J. Alice, Ph.D.
Harza Northwest, Incorporated
Thomas D. Atkeson, Ph.D.
Florida Department of Environmental
Protection
Steven M. Bartell, Ph.D.
SENES Oak Ridge, Inc.
Mike Bolger, Ph.D.
U.S. Food and Drug Administration
James P. Butler, Ph.D,
University of Chicago
Argonne National Laboratory
Rick Canady, Ph.D.
Agency for Toxic Substances and Disease
Registry
Rufus Chaney, Ph.D.
U.S. Department of Agriculture
Tim Eder
Great Lakes Natural Resource Center
National Wildlife Federation for the
States of Michigan and Ohio
William F. Fitzgerald, Ph.D.
University of Connecticut
Avery Point
Robert Goyer, Ph.D.
National Institute of Environmental Health
Sciences
George Gray, Ph.D.
Harvard School of Public Health
Terry Haines, Ph.D.
National Biological Service
Joann L. Held
New Jersey Department of Environmental
Protection & Energy
Gerald J. Keeler, Ph.D.
University of Michigan
Ann Arbor
Leonard Levin, Ph.D.
Electric Power Research Institute
Malcom Meaburn, Ph.D.
National Oceanic and Atmospheric
Administration
U.S. Department of Commerce
Paul Mushak, Ph.D.
PB Associates
Jozef M. Pacyna, Ph.D.
Norwegian Institute for Air Research
Ruth Patterson, Ph.D.
Cancer Prevention Research Program
Fred Gutchinson Cancer Research Center
Donald Porcella, Ph.D.
Electric Power Research Institute
Charles Schmidt
U.S. Department of Energy
Pamela Shubat, Ph.D.
Minnesota Department of Health
Alan H. Stern, Dr.P.H.
New Jersey Department of Environmental
Protection & Energy
Edward B. Swain, Ph.D.
Minnesota Pollution Control Agency
M. Anthony Verity, M.D.
University of California
Los Angeles
June 1996
SAB REVIEW DRAFT
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WORK GROUP AND U.S. EPA/ORD REVIEWERS
Core Work Group Reviewers:
Dan Axelrad, U.S. EPA
Office of Policy, Planning and Evaluation
Angela Bandemehr, U.S. EPA
Region 5
Jim Darr, U.S. EPA
Office of Pollution Prevention and Toxic
-Substances
Thomas Gentile, State of New York
Department of Environmental Conservation
Arnie Kuzmack, U.S. EPA
Office of Water
David Layland, U.S. EPA
Office of Solid Waste and Emergency
Response
Karen Levy, U.S. EPA
Office of Policy Analysis and Review
Steve Levy, U.S. EPA
Office of Solid Waste and Emergency
Response
Lorraine Randecker, U.S. EPA
Office of Pollution Prevention and Toxic
Substances
Joy Taylor, State of Michigan
Department of Natural Resources
U.S. EPA/ORD Reviewers:
Robert Bellies, Ph.D., D.A.B.T.
National Center for Environmental Assessment
Washington, DC
Eletha Brady-Roberts
National Center for Environmental Assessment
Cincinnati, OH
Annie M. Jarabek
National Center for Environmental Assessment
Research triangle Park, NC
Matthew Lorber
National Center for Environmental Assessment
Washington, DC
Susan Braen Norton
National Center for Environmental Assessment
Washington, DC
Terry Harvey, D.V.M.
National Center for Environmental Assessment
Cincinnati, OH
June 1996
VI
SAB REVIEW DRAFT
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LIST OF SYMBOLS, UNITS AND ACRONYMS
ATSDR
BML
bw
CAA
CHO
C.I.
CNS
CRAVE
DDST
DHHS
DNA
DWEL
ECG
EEG
EPA
FDA
GABA
Gd
HEC
Hg
Hg-U
IgG
IRIS
LC
50
LOAEL
MF
MMAD
MMC
MMH
MRL
MTD
NAG
NADH
NADPH
NOAEL
NS
NTP
PMA
ppd
RfD
RfDr
RfC
SCE
SGPT
SH
SMR
•'DT
Agency for Toxic Substances and Disease Registry
Biological monitoring level
Body weight
Clean Air Act as amended in 1990
Chinese hamster ovary
Confidence interval
Central nervous system
Carcinogen Risk Assessment Verification Endeavor
Denver Developmental Screen Test
Department of Health and Human Services
Deoxyribonucleic acid
Drinking water equivalent level *
Electrocardiogram
Electroencephalogram
Environmental Protection Agency
Food and Drug Administration
Gamma aminobutyric acid
Gestation day
Human equivalent concentration
Mercury
Urinary mercury
Immunoglobulin G
Integrated Risk Information System
Lethal concentration killing 50 percent of the animals tested.(inhalation)
Lethal dose killing 50 percent of the animals tested
Lowest-observed-adverse-effect level
Modifying factor
Mass median aerodynamic diameter
Methylmercuric chloride
Methylmercuric hydroxide
Minimal risk level
Maximum tolerated dose
N-acetyl-b-glycosaminidase
Reduced nicotinamide adenine dinucleotide
Reduced nicotinamide adenine dinucleotide phosphate
No-observed-adverse-effect level
Not specified
National Toxicology Program
Phenyl mercuric acetate
Postpartum day
Reference dose (oral)
Reference dose for developmental toxicity
Reference concentration (inhalation)
Sister chromatid exchange
Serum glutamic-pyruvic transaminase
Sulfhydryl groups
Standard mortality ratio
June 1996
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LIST OF SYMBOLS, UNITS AND ACRONYMS (continued)
TOLD Test of Language Development
TWA Time-weighted average.
UF Uncertainty factor
UFA Uncertainty factor for interspecies extrapolation
UFH Uncertainty factor for intraspecies extrapolation (animal to human)
UFL Uncertainty factor for use of a LOAEL
UFS Uncertainty factor for use of a subchronic-duration study
WHO World Health Organization
June 1996 xii SAB REVIEW DRAFT
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EXECUTIVE SUMMARY
Section 112(n)(l)(B) of the Clean Air Act (CAA), as amended in 1990, directs the U.S.
Environmental Protection Agency (U.S. EPA) to submit to Congress a comprehensive study on
atmospheric emissions of mercury. This document, which covers the human health effects of mercury
and mercury compounds, is one volume of U.S. EPA's seven-volume report in response to this
directive.
Mercury is a naturally occurring element that is found in air, water and soil. It exists in any
of three valence states: Hg° (elemental mercury), Hg22+ (mercurous mercury), or Kg2* (mercuric
mercury). Most of the population of the earth have some exposure to mercury as a result of normal
daily activities. The general population may be exposed to mercury through inhalation of ambient air;
consumption of contaminated food, water, or soil; and/or dermal exposure to substances containing
mercury. In addition, some quantity of mercury is released from dental amalgam.
The health effects literature contains many investigations of populations with potentially high
exposure to mercury, including industrial workers, people living near point sources of mercury
emissions, people who consume large amounts of fish, and dental professionals. There also are
numerous studies of populations unintentionally exposed to high levels of mercury, such as the
Minamata poisoning episode in Japan. Volume III (An Assessment of Exposure from Anthropogenic
Mercury Emissions in the United States) presents measured and predicted mercury exposure for
various U.S. populations.
The purpose of this volume, Volume IV, is to summarize the available health effects
information for mercury and mercury compounds and'to present U.S. EPA's analysis for two critical
pieces of the risk assessment paradigm described by the National Academy of Sciences in 1983.
Specifically, this volume contains the hazard identification and dose-response assessments for three
forms of mercury, elemental mercury, mercuric chloride (inorganic mercury),and methylmercury
(organic mercury). In order to characterize risk for any populations, the evaluations presented in this
volume must be combined with the assessment of exposure presented in Volume III.
Volume IV is not intended to be an exhaustive survey of the voluminous health effects
literature available for mercury. Rather, the purpose is to present a brief survey of the studies relevant
for assessing potential human health effects and to present more detailed information of those studies
which form the basis for U.S. EPA's hazard identification and dose-response assessments. The three
forms of mercury which are emphasized in this volume were selected based on data indicating that
these are the predominant forms of mercury to which humans are exposed. In addition, examination
of the published literature indicates that most health data are on these forms. It is acknowledged that
certain populations can be exposed to many types of organic mercurials, such as antiseptics and
pesticides. Volume IV, however, deals with methylmercury except in cases where information on
another organic is presented for illustrative purposes.
Toxicokinetics
The toxicokinetics (i.e., absorption, distribution, metabolism, and excretion) of mercury is
highly dependent on the form of mercury to which a receptor has been exposed. Below is a brief
summary of the toxicokinetics information for elemental mercury, mercuric chloride, and
methylmercury. Chapter 2 contains a more complete summary of the toxicokinetics information
available for mercury.
June 1996 ES-1 SAB REVIEW DRAFT
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Elemental Mercury
The absorption of elemental mercury vapor occurs rapidly through the lungs, but it is poorly
absorbed from the gastrointestinal tract Once absorbed, elemental mercury is readily distributed
throughout the body; it crosses both placental and blood-brain barriers. Elemental mercury is oxidized
to inorganic divalent mercury by the hydrogen peroxidase-catalase pathway, which is present in most
tissues. The distribution of absorbed elemental mercury is limited primarily by the oxidation of
elemental mercury to the mercuric ion as the mercuric ion has a limited ability to cross the placental
and blood-brain barriers. Once elemental mercury crosses these barriers and is oxidized to the
mercuric ion, return to the general circulation is impeded, and mercury can be retained in brain tissue.
The elimination of elemental mercury occurs via urine, feces, exhaled air, sweat, and saliva. The
pattern of excretion changes depending upon the extent the elemental-mercury has been oxidized to
mercuric mercury.
Inorganic Mercury
Absorption of inorganic mercury through the gastrointestinal tract varies with the particular
mercuric salt involved. Absorption decreases with decreasing solubility. Estimates of the percentage
of inorganic mercury that is absorbed vary; as much as 20% may be absorbed. Available data indicate
that absorption of mercuric chloride from the gastrointestinal tract results from an electrostatic
interaction with the brush border membrane and limited passive diffusion. Increases in intestinal pH,
high doses of mercuric chloride causing a corrosive action, a milk diet (e.g., neonates) and increases in
pinocytotic activity in the gastrointestinal tract (e.g., neonates) have all been associated with increased
absorption of inorganic mercury. Inorganic mercury has a reduced capacity for penetrating the blood-
brain or placental barriers. There is some evidence indicating that mercuric mercury in the body
following oral exposures can be reduced to elemental mercury and excreted via exhaled air. Because
of the relatively poor absorption of orally administered inorganic mercury, the majority of the ingested
dose in humans is excreted through the feces.
Methvlmercury
Methylmercury is rapidly and extensively absorbed through the gastrointestinal tract.
Absorption information following inhalation exposures is limited. This form of mercury is distributed
throughout the body and easily penetrates the blood-brain and placental barriers in humans and
animals. Methylmercury transport into tissues appears to be mediated by the formation of a
methylmercury-cysteine complex. This complex is structurally similar to methionine and is transported
into cells via a widely distributed neutral amino acid carrier protein. Methylmercury in the body is
considered to be relatively stable and is only slowly demethylated to form mercuric mercury in rats. It
is hypothesized that methylmercury metabolism may be related to a latent or silent period observed in
epidemiological studies observed as a delay in the onset of specific adverse effects. Methylmercury
has a relatively long biological half-life in humans; estimates range from 44 to 80 days. Excretion
occurs via the feces, breast milk, and urine.
Biological Monitoring/Pharmacokinetic Models
Chapter 2 provides information on biological monitoring of mercury as well as a summary of
the development of pharmacokinetic models for mercury. The most common biological samples
analyzed for mercury are blood, urine, and scalp hair. The methods most frequently used to determine
the mercury levels in these sample types include atomic absorption spectrometry, neutron activation
analysis, X-ray fluorescence, and gas chromatography.
June 1996 ES-2 SAB REVIEW DRAFT
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Both simple and complex multi-compartmental models have been described in the literature.
A recent report (Gearhart et al. 1995) presents an approach based upon data from human, rat, and
monkey data that could be used for characterizing dose-response data both adults and neonates.
Biological Effects
Chapter 3 presents summary information on the toxicity of elemental mercury, mercuric
mercury and methylmercury to various organ systems. The primary targets for toxicity of mercury and
mercury compounds are the nervous system, the kidney, and the developing fetus. Other systems that
may be affected include the respiratory, cardiovascular, gastrointestinal, hematologic, immune, and
reproductive systems. For each form of mercury and each of the endpoints addressed, information
from epidemiological studies, human case studies, and animal toxicity studies is summarized in tabular
form. Critical studies are discussed in the accompanying text
Elemental Mercury
A number of epidemiological studies have been conducted that examined cancer mortality
and/or morbidity among workers occupationally exposed to elemental mercury. All of these studies,
however, have limitations which compromise the interpretation of their results; these limitations
include small sample sizes, probable exposure to other known lung carcinogens, failure to consider
confounding factors such as smoking, and/or failure to observe correlations between estimated
exposure and the cancer incidence. Only one animal study was identified that examined cancer
incidence in animals exposed (by injection) to elemental mercury. While tumors were found at contact
sites, the study was incompletely reported as to controls and statistics and, thus, considered inadequate
for the purpose of risk assessment. Findings from genotoxicity assays are limited and do not provide
supporting evidence for a carcinogenic effect of elemental mercury.
Effects on the nervous system appear to be the most sensitive lexicological endpoint observed
following exposure to elemental mercury. Symptoms associated with elemental mercury-induced
neurotoxicity include the following: tremors, initially affecting the hands and sometimes spreading to
other parts of the body; emotional lability, often referred to as "erethism" and characterized by
irritability, excessive shyness, confidence loss, and nervousness; insomnia; neuromuscular changes
(e.g., weakness, muscle atrophy, muscle twitching); headaches; polyneuropathy (e.g., paresthesia,
stocking-glove sensory loss, hyperactive tendon reflexes, slowed sensory and motor nerve conduction
velocities); and memory loss and performance deficits in test of cognitive function. At higher
concentrations, adverse renal effects and pulmonary dysfunction may also be observed.
• A few studies have provided suggestive evidence for potential reproductive toxicity associated
with exposure to elemental mercury. Data from two studies in rats demonstrate developmental effects
of elemental mercury exposure. These were behavioral changes associated with both in utero and
perinatal exposure. Data for other toxicological effects are limited.
Inorganic Mercury
There is no evidence in humans linking exposure to mercuric chloride with carcinogenic
effects. Data in animals are limited. Focal hyperplasia and squamous cell papillomas of the
forestomach as well as thyroid follicular adenomas and carcinomas were observed in male rats
gavaged with mercuric chloride. In the same study, evidence for an increased incidence of squamous
cell forestomach papillomas in female rats and renal adenomas and carcinomas in male mice were
considered equivocal. All increased tumor incidences were observed in excess of the maximum
June 1996 ES-3 SAB REVIEW DRAFT
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tolerated dose (MTD). In this context, the relevance of the tumors to human health evaluation has
been questioned. Results from in vitro and in vivo tests for genotoxicity have been mixed and do not
provide strong supporting data for carcinogenicity.
There are some data indicating that mercuric chloride may be a germ cell mutagen. Positive
results have been obtained for chromosomal aberrations in multiple systems, and evidence suggests
that mercuric chloride can reach female gonadal tissue.
The most sensitive general systemic adverse effect observed following exposure to inorganic
mercury is the formation of mercuric mercury-induced autoimmune glomerulonephritis. The
production and deposition of IgG antibodies to the glomerular basement membrane can be considered
the first step in the formation of this mercuric-mercury-induced autoimmune glomerulonephritis.
Several studies in animals have evaluated the potential for developmental toxicity to occur
following exposure to various inorganic salts. While the evidence suggests that developmental effects
may occur, all of the studies have significant limitations.
Methylmercury
Three human studies that examined the relationship between methylmercury and cancer
incidence were considered extremely limited because of study design inappropriate for risk assessment
or incomplete data reporting. Evidence from animal studies provides limited evidence of '
carcinogenicity. Male ICR and B6C3F1 mice exposed orally to methylmercuric chloride were
observed to have an increased incidence of renal adenomas, adenocarcinomas, and carcinomas. Renal
epithelial cell hyperplasia and tumors, however, were observed only in the presence of profound
nephrotoxicity suggesting that the tumors may be a consequence of reparative changes to the damaged
kidneys. Tumors were observed at a single site, in a single species and sex.
Methylmercury appears to be clastogenic but not a potent mutagen. Studies have also shown
evidence that methylmercury may induce mammalian germ cell chromosome aberrations. There are a
number of studies in both humans and experimental animals that show methylmercury to be a
developmental toxicant. Neurotoxicity in offspring is the most commonly observed effect and the
effect seen at lowest exposures.
A significant body of human studies exists for evaluating the potential systemic toxicity of
methylmercury. This data base is the result of studying two large scale poisoning episodes in Japan
and Iraq as well as several epidemiological studies assessing populations that consume significant
quantities of fish. In addition, much research on the toxicity of methylmercury has been conducted in
animals including non-human primates.
The critical target for methylmercury toxicity is the nervous system. The developing fetus
may be at particular risk from methylmercury exposure. Offspring born of women exposed to
methylmercury during pregnancy have exhibited a variety of developmental neurological abnormalities,
including the following: delayed onset of walking, delayed onset of talking, cerebral palsy, altered
muscle tone and deep tendon reflexes, and reduced neurological test scores. Maternal toxicity may or
may not have been present during pregnancy for those offspring exhibiting adverse effects. For the
general population, the critical effects observed following methylmercury exposure are multiple central
nervous system effects including ataxia and paresthesia.
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Table ES-1
Summary of U.S. EPA Hazard Identification/Dose-response Assessment
for Mercury and Mercury Compounds
Form
of
Mercury
Elemental
Inorganic
Organic
OralRfD
(mg/kg-day)
n/aa
0.0003°
(mercuric
chloride)
0.0001*
(methyl-
mercury)
Inhalation
RfC
(mg/m3)
0.0003"
Notd
verifiable
n/a
Cancer
Weight-of-
evidence
Rating
D, not classifiable
as to human
carcinogemcity
C, possible
human carcinogen
C, possible
human carcinogen
Cancer
Slope
Factor
n/a
n/a
n/a
Germ Cell
Mutagenicity
Low weight of
evidence
Moderate weight
of evidence
High weight of
evidence
Developmental
Toxicity
Data Base
Characterization
Insufficient human
evidence; sufficient
animal evidence
Insufficient
evidence
Sufficient human
and animal data
a Not available; data do not support development of a value at this time.
b Critical effect is neurological toxicity (hand tremor; increases in memory disturbances; slight subjective and objective
evidence of autoimmune dysfunction) in adults.
0 Critical effect is renal toxicity resulting from an autoimmune disease caused by the accumulation of a hapten-mercury
complex in the glomerular region of the kidneys.
d Data were judged insufficient for calculation of RfC.
e Critical effect is neurological toxicity in progeny of exposed women, RfD calculated using a benchmark dose (10%).
Ongoing Research
While much data has been collected on the potential toxicity of mercury and mercury
compounds, much is still unknown. Two ongoing epidemiological studies should provide critical
information on the developmental toxicity of methylmercury. One study, being conducted in the
Seychelles Islands, is evaluating dose-response relationships in a human population with dietary
exposures (fish) at levels believed to be in the range of the threshold for developmental toxicity. The
second study, being conducted in the Faroe Islands, is assessing mercury exposure in a population that
consumes a relatively large quantity of marine fish and marine mammals. Children exposed to
methylmercury in utero and followed through 6 years of age will be assessed for mercury exposure
and neurological developmental. Potential for interaction of mercury with selenium and possible
confounding coexposure to polychlorinated biphenyls (PCBs) is being examined.
Research Needs
Specifically, information is needed to reduce the uncertainties associated with the current oral
RfDs and inhalation RfCs. More work with respect to both dose and duration of exposure would also
allow for potentially assessing effects above the RfD/RfC. Limited evidence suggests that
methylmercury and mercuric chloride are possible human carcinogens. Data are not sufficient to
classify the potential carcinogenicity of elemental mercury. Research on mode of action in induction
of tumors at high mercury dose will be of particular use in defining the nature of the dose response
relationship for carcinogenicity. At this time data have been judged insufficient for calculation of
quantitative developmental toxicity estimates for elemental and inorganic mercury; research toward this
end should be encouraged. While some pharmacokinetic models have been developed additional work
June 1996
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to ensure the applicability of these to risk assessment should be pursued. In particular work aimed at
validation of a fetal pharmacokinetic model and research in support of toxicokinetics will be useful.
Conclusions
The following conclusions are presented in approximate order of degree of certainty in the
conclusion, based on the quality of the underlying database. The conclusions progress from
those with greater certainty to those with lesser certainty.
• The three forms of mercury discussed in this report can present a human health hazard.
• Neurotoxicity is the most sensitive indicator of adverse effects in humans exposed to
elemental mercury and methylmercury.
• Immune-mediated kidney toxicity is the most sensitive indicator of toxic effects of
exposure to inorganic mercury. This judgement is largely based on results in
experimental animals.
• Methylmercury is a developmental toxicant in humans.
• Methylmercury is likely to be a human germ cell mutagen. This judgement is based
on data from human studies, genetic toxicology studies in animals and a consideration
of the pharmacokinetics of methylmercury.
• Methylmercury and inorganic mercury produce tumors in experimental animals at toxic
doses. If the mechanisms of action which induced tumors in the animal models could
occur in humans, it is possible that tumors could be induced in exposed humans by
these forms of mercury. It is likely that cancer induction by mercury is a high dose
effect.
• An RfD fqr ingested methylmercury based on neurotoxic effects observed in Iraqi
children exposed in utero is 1 x ICf4 mg/kg-day. The threshold estimate derived using
a benchmark dose approach is not model dependent (polynomial vs. Weibull). The
estimate is not much affected by data grouping, but is dependent on response
classification and on parameters used in determination of ingestion relative to
measured mercury in hair.
• An RfD for ingested inorganic mercury based on immune-mediated kidney effects in
Brown-Norway rats is 3 x 10"4 mg/kg-day.
• An RfC for inhaled elemental mercury based on neurotoxic effects in exposed workers
is 3 x ID"4 mg/m3.
• Elemental mercury is a developmental toxicant in experimental animals. If the
mechanisms of action producing developmental toxicity in animals occur in humans,
elemental mercury is very likely to produce developmental effects in exposed human
populations. U.S. EPA has made no estimate of dose response for developmental
effects of elemental mercury.
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There are many uncertainties associated with this analysis, due to an incomplete understanding
of the toxicitv of mercury and mercury compounds. The sources of uncertainty include the
following:
• The data serving as the basis for the methyhnercury RfD were from a population
ingesting contaminated seed grain. The nutritional status of this group may not be
similar to that of U.S. populations. The exposure was for a short albeit critical period
of time; extrapolation to a chronic RfD is a source of uncertainty. It is likely that
there is a range of response among individuals to methylmercury exposure. The
selenium status of the exposed Iraqi population is not certain, nor is it established the
extent to which selenium has an inhibitory effect on mercury toxicity.
• There was no NOAEL (no-observable-adverse-effect level) for estimation of a
threshold for all developmental endpoints. A benchmark was estimated using a
Weibull model on grouped data. Use of an estimate other than the 95% lower limit on
10% response provides alternate estimates. Other modeling approaches using data
which have not been grouped provide alternate estimates.
• Ingestion levels of methyhnercury associated with measured mercury in hair were
estimated based on pharmacokinetic parameters derived from evaluation of the extant
literature. Use of other plausible values for these parameters results in (relatively
small) changes hi the exposure estimate.
• It has not been established that the developing fetus, is ha fact more sensitive to the
effects of methyhnercury than are adult humans.
• The RfD for inorganic mercury is based on data in experimental animals; there is
uncertainty in extrapolation to humans. It is thought that these animals constitute a
good surrogate for a sensitive population. The data were from less than lifetime
exposures; there is uncertainty in extrapolation to a lifetime RfD. There was no
NOAEL in the studies; there is uncertainty in extrapolation to a NOAEL or in
estimation of a threshold for effects hi animals.
• The RfC for elemental mercury was based on studies in exposed workers for which
there is no reported NOAEL; there is uncertainty hi estimating the no effect level in
these populations. There is uncertainty as to whether reproductive effects could be
occurring at lower exposure levels than those which produced the observed
neurotoxicity.
• There are insufficient data to determine whether elemental mercury induces
carcinogenic effects hi experimental animals.
• Data are not sufficient to judge if elemental and inorganic mercury are germ cell
mutagens.
• U.S. EPA did not formally evaluate data on mercury for reproductive effects.
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To improve the risk assessment for mercury and mercury compounds. U.S. EPA would need
the following:
• Results from ongoing studies in human populations with presumptive high exposure to
methylmercury.
• Results for immune-mediated kidney effects from lifetime studies of sensitive animals
exposed to inorganic mercury. Definitive data from human studies on effects of
exposure to inorganic mercury.
• Data on inhalation effects of inorganic mercury exposure.
• Dose response data for developmental effects of elemental and inorganic mercury.
• Reproductive studies and analysis for all forms of mercury.
• Data on mode of action of inorganic and methylmercury tumor induction.
• Validated physiologically-based pharmacokinetic models for mercury which include a
fetal component.
Based on the extant data and knowledge of developing studies, the U.S. EPA predicts the
following:
• Human populations exposed to sufficiently high levels of elemental mercury will have
increased incidence of neurotoxic effects.
• Human populations exposed to sufficiently high levels of methylmercury either in
utero or post partum will have increased incidence of neurotoxic effects.
•
• Human populations exposed to sufficiently high levels of inorganic mercury will have
increased incidence of systemic effects including immune-mediated kidney effects.
• The RfDs and RfC calculated by U.S. EPA for systemic toxic effects of mercury are
protective of human health including sensitive subpopulations.
• The RfDs are protective against carcinogenic effects; tumor induction in animals was
observed only at doses likely to produce systemic toxic effects.
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1. INTRODUCTION
Section 112(n)(l)(B) of the Clean Air Act (CAA), as amended in 1990, requires the U.S.
Environmental Protection Agency (U.S. EPA) to submit a study on atmospheric mercury emissions to
Congress. The sources of emissions that must be studied include electric utility steam generating
units, municipal waste combustion units and other sources, including area sources. Congress directed
that the Mercury Study evaluate many aspects of mercury emissions, including the rate and mass of
emissions, their health and environmental effects, technologies to control such emissions and the costs
of such controls.
In response to this mandate, U.S. EPA has prepared a seven-volume Mercury Study Report to
Congress. The seven volumes are as follows:
I. Executive Summary
II. An Inventory of Anthropogenic Mercury Emissions in the United States
in. An Assessment of Exposure from Anthropogenic Mercury Emissions in the United
States
IV. Health Effects of Mercury and Mercury Compounds
V. An Ecological Assessment for Anthropogenic Mercury Emissions in the United States
VI. Characterization of Human Health and Wildlife Risks from Anthropogenic Mercury
Emissions in the United States
VII. An Evaluation of Mercury Control Technologies and Costs
This volume (Volume IV) addresses the potential human health effects associated with
exposure to mercury. It summarizes the available human and animal studies and other supporting
information relevant to the toxicity of mercury and mercury compounds in humans. It also
summarizes U.S. EPA's current overall assessments of hazard and quantitative dose-response for
various categories of toxic effects. This volume presents data relevant to assessment of potential
effects on human health for elemental mercury, inorganic mercury and methylmercury. Organic
mercury compounds other than methylmercury are generally not considered in this volume. Chapter 2
discusses the toxicokinetics of mercury, including information on absorption, distribution, metabolism
and excretion. Chapter 3 is a summary of the toxicity literature for mercury. It is organized into three
main subsections, corresponding to elemental mercury, inorganic mercury and methylmercury. Within
each of these subsections, the study data are presented according to the effect type (e.g., death, renal
toxicity, developmental toxicity, cancer). For each effect type, separate summary tables in similar
formats are used to present the available data from human epidemiological studies, human case studies,
and animal studies.
Chapter 6, Hazard Identification and Dose-Response Assessment, presents U.S. EPA's
assessments of the hazard presented by various forms of mercury and, where possible, the quantitative
dose-response information that is used in risk assessments of mercury. Chapters 4 and 5 briefly
discuss populations with increased susceptibility to mercury and interactions between exposure to
mercury and other substances. Ongoing research and research needs are described in Chapter 7, and
Chapter 8 lists the references cited. Appendix A documents the dose conversion equations and factors
used. Appendix B consists of RfD, RfC and cancer risk summaries for U.S. EPA's Integrated Risk
Information System (IRIS). Appendix C lists the participants of a U.S. EPA-sponsored workshop on
mercury issues held in 1987. Appendix D presents an analysis of uncertainty and variability in the
methylmercury human effects threshold estimate.
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2. TOXICOKINETICS
This chapter describes the toxicokinetics (i.e., absorption, distribution, metabolism and
excretion) of mercury and mercury compounds in the body. Biomarkers of exposure and methods of
analysis for measuring mercury levels in biological samples are discussed. The biotransformation of
mercury in the environment is discussed in Volume III.
The absorption of elemental mercury vapor occurs rapidly through the lungs, but it is poorly
absorbed from the gastrointestinal tract Oral absorption of inorganic mercury involves absorption
through the gastrointestinal tract; absorption information for the inhalation route is limited.
Methylmercury is rapidly and extensively absorbed through the gastrointestinal tract.
Once absorbed, elemental mercury is readily distributed throughout the body; it crosses both
placental and blood-brain barriers. Elemental mercury is oxidized to inorganic divalent mercury by the
hydrogen peroxidase-catalase pathway, which is present in most tissues. The oxidation of elemental
mercury to the inorganic mercuric cation in the brain can result in retention in the brain. Inorganic
mercury has poor lipophilicity and a reduced capacity for penetrating the blood-brain or placental
barriers. Once elemental mercury crosses the placental or blood-brain barriers and is oxidized to the
mercuric ion, return to the general circulation is impeded, and mercury can retained in brain tissue.
Recent studies indicate that transport and distribution of methylmercury is carrier-mediated.
Methylmercury penetrates the blood-brain and placental barriers, can be converted to mercuric ion, and
may accumulate in the brain and fetus.
The elimination of elemental mercury occurs via the urine, feces and expired air. Exposure to
mercuric mercury results in the elimination of mercury in the urine and feces. Methylmercury is
excreted primarily in the feces (mostly in the inorganic form) by humans.
2.1 Absorption
2.1.1 Elemental Mercury
2.1.1.1 Inhalation
Elemental mercury vapors are readily absorbed through the lungs. Studies in human
volunteers have shown that approximately 75-85% of an inhaled dose of elemental mercury vapor was
absorbed by the body (Nielsen-Kudsk 1965; Oikawa et al. 1982; Teisinger and Fiserova-Bergerova
1965; Hursh 1985; Hursh et al. 1985). The high lipid solubility of elemental mercury vapor relative to
its vapor pressure favors its rapid diffusion across alveolar membranes and dissolution in blood lipids
(Berlin et al. 1969b).
2.1.1.2 Oral
Liquid metallic mercury is very poorly absorbed from the gastrointestinal tract In rats, less
than 0.01% of an ingested dose of metallic mercury was absorbed (Bornmann et al. 1970). The
release of mercury vapor from liquid elemental mercury in the gastrointestinal tract and the subsequent
absorption of the released vapor is limited by reaction of the mercury with sulfur to form mercuric
sulfide. The mercuric sulfide coats ingested metallic mercury, preventing release of elemental vapor
(Berlin 1986).
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2.1.1.3 Dermal
Elemental mercury vapor is absorbed through the skin of humans at an average rate of 0.024
ng Hg/cm2 (skin) for every one mg/m3 in the air (Hursh et al. 1989). This rate of dermal absorption
is sufficient to account for less than 3% of the total amount absorbed during exposure to mercury
vapor (greater than 97% of the absorption occurs through the lungs). Dermal absorption of liquid
metallic mercury has been demonstrated in experimental animals (Schamberg et al. 1918); however,
the extent of absorption was not quantified. Koizumi et al. (1994) measured mercury absorption
through the skin of F344 rats exposed to solutions of industrially generated dust containing mercury.
After 3 days mean blood concentration in dust-exposed rats was 15.5 ug/L compared to 3 ug/L for
saline controls.
2.1.2 Inorganic Mercury
2.1.2.1 Inhalation
There is limited information suggesting that absorption occurs after inhalation of aerosols of
mercuric chloride. Clarkson (1989) reported absorption to be 40% in dogs via inhalation. Inhalation
exposure of rats to an aerosol of a 1% mercuric chloride solution for 1 hour/day, 4 days/week for 2
months resulted in retention of 5-6 ug HgCl/HR aerosol/100 g body weight or approximately 37-44
ug Hg/kg-day (Bernaudin et al. 1981). The authors prepared the aerosol "with reference to the
maximum allowable air concentrations (0.10 mg Hg/m3) for a man". Retention was defined at the end
of exposure as total mercury in the rat carcass minus skin and hair. It is unknown to w,hat extent the
amount retained represented absorption through the lungs or absorption of material cleared from the
respiratory tract by mucociliary activity and ultimately swallowed
2.1.2.2 Oral
The absorption of mercuric mercury from the gastrointestinal tract has been estimated at
approximately 7-15% in human volunteers following oral administration of radiolabeled inorganic
mercury (Miettinen 1973; Rahola et al. 1973). Recent data from studies in mice, however, suggest
that "true" absorption may be closer to 20% but appears lower due to intestinal pH, compound
dissociation, age, diet, rapid biliary secretion and excretion hi the feces (Kostial et al. 1978; Nielsen
1992). Because the excretion of absorbed mercury is rapid, mercury levels detected in the
gastrointestinal tract most likely represent both unabsorbed and excreted mercury in the studies by
Miettinen (1973) and Rahola et al. (1973). The absorption of mercuric chloride from the
gastrointestinal tract is not believed to depend on any specific transport mechanism, reactive sulfhydryl
groups, or oxidative metabolism (Foulkes and Bergman 1993). Rather, uptake appears to result from
an electrostatic interaction with the brush border membrane and limited passive diffusion. Several
factors have been identified that modulate absorption of mercuric mercury from the gastrointestinal
tract. At high doses, the corrosive action of mercuric chloride may increase its uptake by breaking
down membrane barriers between the ions and the blood. Increases hi intestinal pH also increase
absorption (Endo et al. 1990). Increased uptake also occurs in neonates (Kostial et al. 1978). The
increased absorption in neonates is believed to be due in part to the milk diet of neonates (increased
absorption was observed hi adults given a milk diet) and in part to the increased pinocytotic activity in
the gastrointestinal tract that occurs in the very young (Kostial et al. 1978). Diffusion through
aqueous channels present in the immature brush border of neonates has also been suggested to account
for the greater absorption in the very young (Foulkes and Bergman 1993).
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Absorption of mercuric salts from the gastrointestinal tract varies with the particular salt
involved. Absorption decreases with decreasing solubility (Endo et al. 1990). For example, the poorly
soluble salt mercuric sulfide is not absorbed from the gastrointestinal tract as well as the more soluble
mercuric chloride salt (Sin et al. 1983).
Mercurous salts in the form of calomel (long in use as a therapeutic agent) are insoluble in
water and are poorly absorbed from the gastrointestinal tract (Clarkson 1993a). Long term use of
calomel, however, has resulted in toxicity in humans (Davis et al. 1974).
2.1.2.3 Dermal
Dermal absorption of mercuric chloride has been observed in treated guinea pigs (Skog and
Wahlberg 1964). Approximately 2-3% of an applied dose was absorbed during a 5-hour period.
Absorption was measured both by disappearance of the applied compound and by appearance in
kidney,-liver, urine and blood.
2.1.3 Methvlmercurv
2.1.3.1 Inhalation
Inhaled methylmercury vapors are absorbed through the lungs. Fang (1980) did not measure
percent absorbed but showed a correlation between tissue mercury levels and both exposure level and
duration in rats exposed to radioactively labelled methylmercury vapor. •>
2.1.3.2 Oral
Methylmercury is efficiently absorbed from the gastrointestinal tract Approximately 95% of
methylmercury in fish ingested by volunteers was absorbed from the gastrointestinal tract (Aberg et al.
1969; Miettinen 1973). Similarly, when radiolabeled methyl mercuric nitrate was administered in
water to volunteers, uptake was greater than 95% (Aberg et al. 1969).
Reports of the percentage of absorbed methylmercury distributed to the blood range from 1%
to 10%. Following the ingestion of a single meal of memylmercuiy-contaminated fish, Kershaw et al.
(1980) found that blood accounted for 5.9% of absorbed methylmercury, while Miettinen et al. (1971)
found an initial value of 10%, decreasing to about 5% over the first 100 days. In a population that
chronically ingested fish with high methylmercury levels, approximately 1% of the absorbed dose was
distributed to the blood (Sherlock et al. 1982).
2.1.3.3 Dermal
Dermal absorption of the methylmercuric cation (CH3Hg)+ (as the dicyandiamide salt) has also
been observed hi treated guinea pigs (Skog and Wahlberg 1964). Approximately 3-5% of the applied
dose was absorbed during a 5-hour period. Absorption was measured both by disappearance of the
applied compound and by appearance in kidney, liver, urine and blood.
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2.2 Distribution
2.2.1 Elemental Mercury
Because of its lipophilicity, absorbed elemental mercury vapor readily distributes throughout
the body, crossing the blood-brain barrier in humans (Hursh et al., 1976; Nordberg and Serenius,
1969) and the placenta in rats and mice (Clarkson et al., 1972). The distribution of absorbed
elemental mercury is limited primarily by the oxidation of elemental mercury to the mercuric ion and
reduced ability of the mercuric ion to cross membrane barriers. The oxidation is sufficiently slow,
however, to allow distribution to all tissues and organs. Once it is oxidized to the mercuric ion, it is
indistinguishable from Kg2* from inorganic sources (i.e., the highest levels of mercury accumulate in
the kidneys) (Hursh et al. 1980; Rothstein and Hayes 1964). Based on an in vitro study by Hursh et
al. (1988), oxidation of mercury in the blood is slow and, therefore, inhaled mercury reaches the brain
primarily unoxidized (i.e., as dissolved*vapor) and is available for rapid penetration into brain cells.
Once in the brain, oxidation of elemental mercury to mercuric mercury in the brain enhances for the
accumulation of mercury in these tissues (Hursh et al. 1988; Takahata et al. 1970). For example, ten
years after termination of exposure, miners exposed to elemental mercury vapor had high
concentrations of mercury (>120 ppm) in the brain (Takahata et al. 1970). A similar effect occurs
when elemental mercury reaches the fetus and (after oxidation) accumulates in the tissues as inorganic
mercury (Dencker et al. 1983).
In the blood, elemental mercury initially distributes predominantly to-the red blood cells; at
20 minutes, 98% of the mercury in the blood is found in the red blood cells. Several hours following
parenteral, oral or inhalation exposure, however, a stable ratio of red blood cell mercury to plasma
mercury of approximately 1:1 is established (Gerstner and Huff, 1977; Clarkson, 1972; Cherian et al.,
1978). The rise in plasma mercury levels was suggested to be due to binding to protein sulfhydryl
groups by mercuric mercury formed when the elemental mercury was oxidized.
2.2.2 Inorganic Mercury
In contrast to elemental mercury vapor and methylmercury, mercuric mercury does not
penetrate the blood-brain or placenta! barriers easily. Levels of mercury observed in the rat brain after
injection of mercuric nitrate were 10-fold lower than after inhalation of an equivalent dose of
elemental mercury vapor (Berlin et al. 1969a). Similarly, mercuric mercury shows only limited ability
to penetrate to the fetus (Garrett et al. 1972). Mercuric mercury does, however, accumulate in the
mouse placenta (Berg and Smith 1983; Mitani et al. 1978; Suzuki et al. 1984). In the blood, the
mercuric ion is bound to sulfhydryl groups present in the plasma and erythrocytes. The ratio of
human red blood cell mercuric mercury to plasma mercuric mercury is approximately 1:1 (0.53:1.20)
(Hall et al. 1994). The half-life in blood for humans was reported to range from 19.7 to 65.6 days in
a study of five subjects treated with i.v. mercuric nitrate (Hall et al. 1994). From the blood, mercuric
mercury initially distributes to liver, but the highest levels are generally observed in the kidneys
(Newton and Fry 1978). With time after exposure, accumulation in the kidneys may account for up to
90% of the total body burden (Rothstein and Hayes 1960). The mercury levels in the kidney are dose
dependent, with increasing amounts occurring with higher administered dose levels (Cember, 1962).
The highest concentration of mercuric mercury in the kidneys is found in the proximal tubules.
Mercuric mercury induces metallothionein production in the kidneys (Piotrowski et al. 1974). The
high metallothionein levels in the kidneys may contribute to the kidney's accumulation of mercuric
mercury (Piotrowski et al. 1973).
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In neonates, lower proportions of mercuric mercury distribute to the kidneys than in adult
animals (Jugo 1976). This results in higher distribution to other tissues. The protective blood-brain
barrier is incomplete in fetal and neonatal animals, which may also contribute to the increased mercury
levels in immature brain. For example, the higher levels in the neonatal brain of rats and guinea pigs
are believed to be associated with the decrease in renal sequestration of the mercuric ion (Jugo 1976;
Yoshida et al. 1989). The higher levels observed in the livers of rat neonates may be attributable to
increased distribution to organs other than the kidney as well as to higher levels of neonatal hepatic
metallothionein (Daston et al. 1986).
2.2.3 Methylmercurv
Methylmercury is distributed throughout.the body, easily penetrating the blood-brain and
placenta! barriers in humans and animals (Clarkson 1972; Hansen 1988; Hansen et al. 1989; Nielsen
and Andersen 1992; Sofia et al. 1992; Suzuki et al. 1984). By contrast with elemental mercury,
studies in rats indicate that methylmercury transport into tissues is mediated by the formation of a
methylmercury-cysteine complex (Aschner and Aschner 1990; Tanaka et al. 1991, 1992; Kerper et al.
1992). The complex is structurally similar to methionine and is transported into cells via a widely
distributed neutral amino acid carrier protein. Methylmercury associates with water-soluble molecules
(e.g., proteins) or thiol-containing amino acids because of the high affinity of the methyl mercuric
cation (CH3Hg)+ for the sulfhydryl groups (SH)~. Complexes of methylmercury with cysteine have
been identified in blood, liver and bile of rats (Aschner and Aschner 1990).
Al-Shahristani and Shihab (1974) calculated a "biological half-life" of methylmercury in a
study of 48 male and female subjects who had ingested seed grain contaminated by organic mercurials.
The half-life ranged from 35 to 189 days with a mean of 72 days; it was determined from distribution
of mercury along head hah-.
The blood half-life is 49-164 days in humans (Aberg et al. 1969; Miettinen et al. 1971) and
10-15 days in monkeys (Rice et al. 1989). Smith et al. (1994) determined a blood half-life of 32-60
days in a study of seven adult males given i.v. methylmercury. In the blood, methylmercury is found
predominantly in the red blood cells (Kershaw et al. 1980; Thomas et al. 1986). In humans, the ratio
of red blood cell methylmercury to plasma methylmercury is approximately 20:1. This ratio varies in
animal species; the ratio is approximately 20:1 in primates and guinea pigs, 7:1 in mice, greater than
100:1 in rats and 42:1 in cats (Hollins et al. 1975; Magos 1987).
The clinical significance of the differences in the distribution of various forms of mercury in
the blood is that it permits diagnosis of the type of mercury to which an individual has been exposed.
Short-chain alkyl mercury compounds such as methylmercury or ethyl mercury are very stable in the
body, whereas long-chain compounds may be metabolized over time to the mercuric ion. The mercury
distribution in the blood, therefore, may shift from a distribution characteristic of methylmercury to
one more suggestive of inorganic mercury (Berlin 1986; Gerstner and Huff 1977).
Mercury has been found in the umbilical cord of human newborns at levels comparable to
maternal blood levels (Grandjean et al. 1992a). For lactating mothers, the clearance of mercury from
the blood appears to be faster than for non-lactating women. Lactating individuals have a blood half-
life of 42 days compared to 75 days for non-lactating females among a group of people who had
consumed contaminated seed grain (Greenwood et al. 1978). This finding may be due to excretion of
mercury via the milk, increased food intake by mothers (which enhances biliary excretion) and/or
altered hormonal patterns in lactating mothers (which affect the excretion pattern).
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Methylmercury transport across the blood-brain barrier in rats may involve an amino acid
carrier (Kerper et al. 1992). Following acute exposure to methylmercury, most of the mercury in the
brain is in the organic form; however, with chronic exposures, a greater amount of the mercury in the
brain is in the inorganic form, suggesting that the rate of demethylation increases with long-term
exposure (Aschner and Aschner 1990). Rice (1989a, 1989b) demonstrated that tissue half-life in the
brain may be significantly longer than the blood half-life for methylmercury.
The bioaccumulation of methylmercury can be affected by age and sex (Thomas et al. 1982,
1986, 1988). After administration of methylmercury to rats, the females had higher peak levels of
mercury in the kidneys, primarily as methylmercury, compared to the males; inorganic mercury levels
did not differ significantly between the sexes (Thomas et al. 1986). Accumulation of mercury in the
body is also found to be higher in neonatal rats (Thomas et al. 1988) than in adult rats (Thomas et al.
1982). Ten days after administration of methylmercury," 94% of the dose was still detected in neonates
while =60% was retained in adults (Thomas et al. 1988). The longer retention of mercury in the
neonates may be attributed to various factors including the high amount of mercury accumulated in the
pelt of the neonates due to lack of clearance (Thomas et al. 1988) and the lack of a fully developed
biliary transport system in the neonates (Ballatori and Clarkson 1982).
2.3 Metabolism
2.3.1 Elemental Mercury
Elemental mercury dissolved in the blood is rapidly oxidized in red blood cells to mercuric
mercury by catalase in the presence of hydrogen peroxide (Halbach and Clarkson 1978). Catalase is
found in many tissues, and oxidation by this pathway probably occurs throughout the body (Nielsen-
Kudsk 1973). The pathway is saturable, however, and hydrogen peroxide production is the rate-
limiting step (Magos et al. 1989). Blood and tissue levels of mercuric mercury following exposure to
high concentrations of elemental mercury are, therefore, lower than would be expected based on levels
observed following exposure to low levels.
2.3.2 Inorganic Mercury
Several investigators have observed exhalation of elemental mercury vapor after oral
administration of mercuric mercury to rats and mice, indicating that mercuric mercury in the body can
be reduced to elemental mercury (Clarkson and Rothstein 1964; Dunn et al. 1981a, 1981b; Sugata and
Clarkson 1979). The reduction of mercuric ion to elemental mercury may occur via cytochrome c,
NADPH and NADH, or a superoxide anion produced by the xanthine-xanthine oxidase system (Ogata
et al. 1987). There is no evidence that mercuric mercury is methylated to form methylmercury in
mammalian cells. The studies of Rowland et al. on the intestinal flora of the Wistar rat show that
microbes are responsible for at least a portion of mercuric chloride methylation in the gut.
Mercurous mercury is unstable in biological fluids and rapidly disassociates to one molecule of
elemental mercury and one ion of mercuric mercury (Clarkson 1972).
2.3.3 Methvlmercurv
Methylmercury in the body is relatively stable and is only slowly demethylated to form
mercuric mercury in rats (Norseth and Clarkson 1970). The demethylation appears to occur in tissue
macrophages (Suda and Takahashi 1986), intestinal microflora (Nakamura et al. 1977; Rowland et al.
1980) and fetal liver (Suzuki et al. 1984). In vitro demethylation has been reported to involve
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hydroxyl radicals produced by cytochrome P-450 reductase (Suda and Hirayama 1992) or
hypochlorous acid scavengers (Suda and Takahashi 1992). Organic mercury compounds with longer
alkyl chains are more readily metabolized over time to the mercuric ion (Berlin, 1986).
Methylmercury metabolism may be related to the latent or silent period observed in
epidemiological studies from two methylmercury poisonings. During the latent period, both during
and after the cessation of exposure, the patient feels no untoward effects. It is possible that a number
of biochemical changes may take place in parallel during this period, and some may not be causatively
related to the clinical outcome. Ganther (1978) has hypothesized that the carbon-mercury bond in
methylmercury undergoes homolytic cleavage to release methyl free radicals. The free radicals are
expected to initiate a chain of events involving peroxidation of lipid constituents of the neuronal cells.
The onset of symptoms is delayed for the period of time that cellular systems are able to prevent or
repair effects of lipid peroxidation. When the cellular defense mechanisms are overwhelmed, rapid
and progressive degeneration of the tissue results. In the Iraqi poisoning incident, the latent period
before toxic signs were noted varied from a matter of weeks to months. By contrast, in the Japanese
poisoning incident, the latency was as long as a year or more. The difference in duration of the latent
period may in part be due to the presence of selenium in the fish ingested by the Japanese population.
The role of selenium in mercury toxicity is discussed further in Chapter 5.
2.4 Excretion
2.4.1 Elemental Mercury
Excretion of mercury after exposure to elemental mercury vapor may occur via exhaled air,
urine, feces, sweat and saliva. The pattern of excretion of elemental mercury changes as elemental
mercury is oxidized to mercuric mercury. During and immediately after an acute exposure, when
dissolved elemental mercury is still present in the blood, glomerular filtration of dissolved mercury
vapor occurs, and small amounts of mercury vapor can be found in the urine (Stopford et al. 1978).
Mercury vapor present in the blood may also be exhaled; human volunteers exhaled approximately 7%
of the retained dose within the first few days after exposure (Hursh et al. 1976). The half-life for
excretion via the lungs is approximately 18 hours. Approximately 80% of the mercury accumulated in
the body is eventually excreted as mercuric mercury. As the body burden of mercury is oxidized from
elemental mercury to mercuric mercury, the pattern of excretion becomes more similar to mercuric
mercury excretion. The majority of the excretion of mercuric mercury occurs in the feces and urine
(Cherian et al. 1978). During the first few days after exposure of humans to mercury vapor,
approximately four times more mercury was excreted in the feces than in the urine (Cherian et al.
1978). With time, as the relative mercury content of the kidneys increases, excretion by the urinary
route also increases (Rothstein and Hayes 1964). Tissue levels of mercury decrease at different rates,
but the half-life for excretion of whole-body mercury in humans (58 days) is estimated to be
approximately equal to the half-life of elimination from the kidneys (64 days), where most of the body
burden is located (Hursh et al. 1976). Excretion via the urine may be increased if mercury-induced
damage of the renal tubular epithelium has happened and exfoliation of damaged mercury-containing
cells occurs (Magos 1973).
Excretion via sweat and saliva are thought to contribute only minimally to total excretion
under normal circumstances. In workers who have perspired profusely, however, the total amount of
mercury excreted in the sweat during 90 minutes ranged from 50% to 200% of that found in a 16-hour
composite sample of urine (Lovejoy et al. 1974).
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2.4.2 Inorganic Mercury
Because of the poor absorption of orally administered mercuric mercury, the majority (=85%)
of an ingested dose in humans is excreted in the feces within a few days after administration
(Miettinen 1973). Hall et al. (1994) showed that for five adult male volunteers given i.v. mercuric
nitrate and evaluated for 70 days, 6.3-35% of the dose was excreted in urine and 17.9-38.1% in feces.
For absorbed inorganic mercury, the half-life for excretion has been estimated to be =40 days (Rahola
et al. 1973) and 67 days with a range of 49-96 days (Hall et al. 1994). Information on the routes of
exaction for absorbed inorganic mercury are limited, but excretion would be expected to be similar to
that of inorganic mercury formed in rats by the oxidation of elemental mercury (Rothstein and Hayes
1964). The majority of absorbed inorganic mercury is excreted in the urine (Berlin 1986).
Glomerular filtration is not thought to contribute substantially to urinary excretion of mercuric
mercury (Cherian et al. 1978). Rather, mercuric mercury is excreted in the urine primarily as
sulfhydryl conjugates (with cysteine or N-acetylcysteine) actively transported into the tubular lumen.
Urinary levels correlate with renal mercury concentrations rather-than blood mercury levels.
Fecal excretion of mercury occurs as the result of excretion in the saliva, secretion through the
epithelium of the small intestines and colon and secretion in the bile (Berlin 1986). Secretion of
mercuric mercury in the bile is believed to result from active transport of a mercury-glutathiohe
complex across the canalicular membrane via the glutathione carrier (Ballatori and Clarkson 1982).
Mercuric mercury may also be excreted in breast milk during lactation (Yoshida et al. 1992).
The levels in breast milk are proportional to the plasma content. In maternal guinea pigs, milk levels
were approximately half of that found in plasma; After termination of exposure, however, mercury
levels in milk decreased at a slower rate than plasma mercury levels.
2.4.3 Methvlmercurv
Like inorganic mercury, methylmercury has a relatively long half-life of approximately 70-80
days in the human body (Aberg et al. 1969; Bernard and Purdue 1984; Miettinen 1973). Recently a
shorter half-life of 44 days was estimated by Smith et al. (1994) in their study of seven adult males
treated i.v. with methylmercury. In this study methylmercury and inorganic mercury concentrations in
blood and excreta were determined separately based on differential extractability into benzene. The
predominant species in the blood was methylmercury; there was no detectable methylmercury in the
urine.
The long half-life of methylmercury in the body is due, in part, to reabsorption of
methylmercury secreted into the bile (hepato-biliary cycling) (Norseth and Clarkson, 1971). In this
cycle, methylmercury forms a complex with glutathione in the hepatocyte, and the complex is secreted
into the bile via a glutathione carrier protein (Clarkson, 1993b). The methylmercury-glutathione
complex in the bile may be reabsorbed from the gallbladder and intestines into the blood. When
microorganisms found in the intestines demethylate methylmercury to form mercuric mercury, this
cycle is broken, and fecal excretion of mercury from methylmercury occurs (Rowland et al. 1980).
Mercuric mercury is poorly absorbed from the intestines, and that which is not reabsorbed is excreted
in the feces. In humans, approximately 90% of the absorbed dose of methylmercury is excreted in the
feces as mercuric mercury. Excretion via the urine is minor but slowly increases with time; at 100
days after dosing, urinary excretion of mercury accounted for 20% of the daily amount excreted. The
urinary excretion of mercury may reflect the deposition of demethylated mercury in the kidneys and its
subsequent excretion.
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In animals, the predominant route of methylmercury elimination also is the feces (Farris et .al.
1993; Rollins et al. 1975; Thomas et al. 1987). As in humans, biliary excretion of methylmercury and
its demethylation in gastrointestinal flora have been reported in rats (Farris et al., 1993). After a
single oral dose of methylmercury, the major elimination route was the feces (65% of the administered
dose as inorganic mercury and 15% of the administered dose as methylmercury) and the minor route
was urine (1% of the administered dose as inorganic mercury and 4% of the administered dose as
methylmercury) (Farris et al. 1993).
In rat and monkey neonates, excretion of methylmercury is severely limited (Lok 1983;
Thomas et al. 1982). In rats dosed prior to 17 days of age, essentially no mercury was excreted
(Thomas et al. 1982). By the time of weaning, the rate of excretion had increased to adult levels. The
failure of neonates to excrete methylmercury may be associated with the inability of suckling infants to
secrete bile (Ballatori and Clarkson 1982) and the decreased ability of intestinal microflora to
demethylate methylmercury during suckling (Rowland et al. 1977).
Methylmercury is also excreted in breast milk (Bakir et al. 1973; Sundberg arid Oskarsson
1992). The ratio of mercury in breast milk to mercury in whole blood was approximately 1:20 in
women exposed to methylmercury via contaminated grain in Iraq between 1971 and 1972 (Bakir et al.
1973). Evidence from the Iraqi poisoning incident also showed that lactation decreased blood mercury
clearance half-times from 75 days in males and nonlactating females to 42 days in lactating females;
the faster clearance due to lactation was confirmed in mice (Greenwood et al. 1978). In mice, of the
total mercury in the breast milk, approximately 60% was estimated to be methylmercury. Skerfving
(1988) has found that 16% of mercury in human breast milk is methylmercury. Studies in animals
indicate that the mercury content of breast milk is proportional to the mercury content of plasma
(Sundberg and Oskarsson, 1992; Skerfving; 1988).
2.5 Biological Monitoring
This section describes the various biological media most frequently used when assessing
mercury exposure. In addition, this section describes the available analytical methods for measuring
mercury in biological samples. Reference values for mercury in standard biological media from the
general population are shown in Table 2-1. These values represent total mercury; individual mercury
species have not been separated.
2.5.1 Elemental Mercury
Blood and urinary mercury are common indicators used to assess occupational mercury
exposure.
2.5.1.1 Blood
In workers chronically exposed to mercury vapor, a good correlation was observed between
intensity of mercury vapor exposure and levels of mercury in the blood at the end of a workshift
(Roels et al. 1987). The usefulness of blood as a biomarker for exposure to elemental mercury
depends on the time elapsed since exposure and the level of exposure. For recent, high-level
exposures, whole blood analysis may be used to assess exposure (Clarkson et al. 1988). Mercury in
the blood peaks rapidly, however, and decreases with an initial half-life of approximately two to four
days (Cherian et al. 1978). Thus, evaluation of blood mercury is of limited value if a substantial
amount of time has elapsed since exposure. Also, dietary methylmercury contributes to the amount of
mercury measured in blood. At low levels of elemental mercury exposure, the contribution of dietary
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Table 2-1
Reference Values for Total Mercury in Biological Media
in the General Population
Medium
Whole blood
Fish consumption:
No fish meals
2 meals/week
2-4 meals/week
more than 4 meals/week
Urine
Scalp hair
Fish consumption:
once/mo
once/2 wk
once/wk
once/day
Mercury Level
l-8|ig/L
2ug/L
2.0ng/L
4.8 ng/L
8.4ug/L
44.4 Mg/L
4-5(ig/L
2ng/g
1.4 (ig/g
1.9 U£/g
2~5 ug/s
n.6ug/g
Reference
WHO (1990)
Nordberg et al. (1992)
Brune (1991)
WHO (1990)
WHO (1990)
Airey (1983)
methylmercury to the total blood mercury may be high relative to that of the inhaled mercury, limiting
the sensitivity of this biomarker. Several studies have separated whole blood into its plasma and
erythrocyte fractions in order to evaluate potential confounding factors due to the presence of
methylmercury (95% of methylmercury is found in the red blood cell).
2.5.1.2 Urine
Urinary mercury is thought to indicate most closely the mercury levels present in the kidneys
(Clarkson et al. 1988). For most occupational exposures, urinary mercury has been used to estimate
exposure. In contrast to blood mercury levels, urinary mercury peaks approximately 2-3 weeks after
exposure and decreases at a much slower rate with a half-life of 40-60 days for short-term exposures
and 90 days for long-term exposures (Barregard et al. 1992; Roels et al. 1991). The urine remains,
therefore, a more appropriate indicator for longer exposures than blood samples. As little dietary
methylmercury is excreted in the urine, the contribution of ingested methylmercury to the measured
levels is not expected to be high. Good correlations have been observed between urinary mercury
levels and air levels of mercury vapor; however, these correlations were obtained after correcting
urinary mercury content for variations in the urinary excretion rate (using urinary creatinine content or
specific gravity) and after standardizing the amount of time elapsed after exposure (Roels et al. 1987).
Such steps are necessary because considerable intra- and interindividual variability has been observed
in the urinary excretion rate (Barber and Wallis 1986; Piotrowski et al. 1975). Even when such
precautions are taken, intraindividual variability remains at =18% (Barregard et al. 1992; Roels et al.
1987).
2.5.1.3 Exhaled Air
Exhaled air has been suggested as a possible biomarker of exposure to elemental mercury
vapor because a portion of absorbed mercury vapor is excreted via the lungs. Excretion by this route
has a half-life of approximately 18 hours (Hursh et al. 1976). At low levels of exposure, however,
mercury vapor released from dental amalgam may contribute substantially to the measured amount of
mercury.
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2.5.2 Inorganic Mercury
No information was identified in the literature that specifically assessed biological indicators
for inorganic mercury exposure. The information presented above for detection of mercury in blood
and urine after occupational exposure to elemental mercury vapor should also apply to inorganic
mercury exposures because elemental mercury vapor is rapidly converted to mercuric mercury after
absorption.
2.5.3 Methylmercurv
Blood and scalp hair are the primary indicators used to'assess methylmercury exposure.
2.5.3.1 Blood
Because methylmercury freely distributes throughout the body, blood is a good indicator
medium for estimating methylmercury exposure. Because an individual's intake may fluctuate, blood
levels may not necessarily reflect mercury intake over time (Sherlock et al. 1982; Sherlock and Quinn,
1988). At steady state, blood levels have been related to dose by the following equation (Kershaw et
al. 1980):
. C xbxV
a -
A xf
Where:
C = concentration hi blood (expressed in ug/L)
V = volume of blood (expressed as L)
b = the kinetic rate constant (day"1)
A = absorption rate (unitless)
F = fraction of dose that is present in blood
d = intake (ug/day)
It is useful to measure blood hematocrit and mercury concentrations hi both whole blood and
plasma. From these data, the red blood cell to plasma mercury ratio may be determined, and
interference from exposure to high levels of elemental or inorganic mercury may be estimated
(Clarkson et al. 1988).
2.5.3.2 Scalp Hair
Scalp hair can also be a good indicator medium for estimating methylmercury exposure
(Phelps et al. 1980). Methylmercury is incorporated into scalp hair at the hair follicle in proportion to
its content in blood. The hair-to-blood ratio in humans has been estimated as approximately 250:1
expressed as ug Hg/g hair to mg Hg/L blood, but some difficulties in measurements, interindividual
variation in body burden, differences in hair growth rates, and variations in fresh and saltwater ,fish
intake have led to varying estimates (Birke et al. 1972; Skerfving 1974). Once incorporated into the
hair, the methylmercury is stable, and, therefore, gives a longitudinal history of blood methylmercury
levels (Phelps et al. 1980; WHO, 1990). Analysis of hair mercury levels may be confounded by
adsorption of mercury vapor onto the hair strands (Francis et al. 1982).
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2.5.4 Methods of Analysis for Measuring Mercury in Biological Samples
The most common methods used to determine mercury levels in blood, urine and hair of
humans and animals include atomic absorption spectrometry (AAS), neutron activation analysis
(NAA), X-ray fluorescence (XRF) and gas chromatography (GC). Table 2-2 identifies the major
characteristics of these methods.
Table 2-2
Analytical Methods for the Detection of Mercury in Biological Samples
Method
NAA
AAS
GC — Electron capture
XRF
Able to Distinguish
Methylmercury?
No
No
No"
Yes
No
Detection limit
(ppm)
0.1
2
0.5
1.0
"low ppm"
References
Byrne and'Kosta (1974)
WHO (1976)
Hatch and Ott (1968)
Magos and Clarkson (1972)
Von Burg et al. (1974)
Cappon and Smith (1978)
Marsh et al. (1987)
*The Magos and Clarkson method estimates methylmercury by subtracting the inorganic mercury content from
the total mercury content.
2.6 Studies on Pharmacokinetic Models
2.6.1 Introduction
Pharmacokinetic modeling is a process by which administered dose, such as the amount of a
compound instilled into the body via inhalation, ingestion or parenteral route is used to estimate
measures of tissue dose which may not always be accessible to measurement by direct
experimentation. A pharmacokinetic model is employed to predict relevant measures of tissue dose
under a wide range of exposure conditions. In practice, the pharmacokinetic models used may
incorporate features such as compartmental analysis and physiologically-based models.
Reports available on the in vivo distribution of several types of mercury compounds provide
different physiokinetic relationships between the structure of mercury compounds and their behavior in
living organisms because the studies reported have been carried out under different experimental
conditions. Takeda et al. (1968) reported that in the rat, alkyl mercury compounds such as
ethylmercuric chloride and butylmercuric chloride were excreted more slowly and were retained in
higher concentration for a longer time in the body than mercuric chloride and phenylmercuric chloride.
The distribution of mercury in the brain was found to depend on the structure of the mercury
compounds; relatively high accumulation was observed for ethyl and n-butyl mercury compounds.
Sebe and Itsuno (1962) reported that after oral administration methyl-, ethyl-, and n-propylmercury
compounds were neurotoxic to rats; n-butylmercury was not neurotoxic and thus presumably did not
cross the blood-brain barrier. By contrast, Suzuki et al. (1963, 1964) reported that ethylmercuric
acetate and n-butylmercuric acetate had similar patterns of distribution when subcutaneously
administered to mice.
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2.6.2 Inorganic mercury
Few controlled laboratory studies of pharmacokinetics of mercury in humans have been
published (Hursh et al. 1976, Rahola et al. 1973). Rahola et al. (1973) examined mercury absorption
and elimination after oral administration of mercuric nitrate to five male and five female volunteers,
and reported very low and variable rate of gastrointestinal absorption (8 to 25% dose). They reported
a half-time for inorganic mercury in human red blood cells of 16 days and whole body of 46 (32-60)
days in males and somewhat lower values in females. Hursh et al. (1976) found half-times for
mercury cle'arance from the body of 58 (35-90) days after exposure to mercury vapor. Whole body
clearance from the Rahola et al. (1973) study appeared biphasic with half-times of 2.3 days for the fast
compartment and 42 (39-45) days for the slow compartment
Low and variable rates of absorption of orally administered inorganic mercury in the Rahola et
al. (1973) study prompted Hall et al. (1994) to examine distribution of intravenously administered
inorganic mercury in human volunteers. In order to describe retention of mercury after transient
distributional effects, a one-compartment model was fit to the blood and body burden data after day
10, assuming first order kinetics. The half-lives observed in the single compartment model for blood
and body burden were 30 (19.7-65.6 days) and 67 (48.6-95.5 days) days, respectively. The authors
attempted closer agreement between observed and predicted values by structuring a multicompartment
model. Measured mercury concentrations in blood, urine, feces, and whole body radioactive levels of
mercury were used hi an a posteriori fashion to develop a model comprising six blood compartments,
one compartment each for feces and urine and a delayed compartment for feces. Inter-subject
variability (temporal pattern of blood mercury) and the existence of a kinetically distinct plasma pool
(three distinct compartments) for mercury resulted hi equivocal predictions for blood, urine and feces;
whether these findings point to uncertainties of measurement of body burden or incomplete collection
of excreta or suggest other pathways of excretion, such as exhalation or sweating, is unknown. The
authors concluded that this type of complex pattern of blood kinetics, although unusual, is not without
precedent Four kinetically distinct plasma pools of selenium has been reported after oral dosing with
a stable isotopic tracer (Patterson and Zech 1992). Hall et al. (1994) noted that the apparently linear
kinetics observed for the small tracer doses of i.v. inorganic mercury would likely change with toxicity
associated with larger or more frequent doses.
2.6.3 Methylmercury
Methylmercury is structurally the simplest of the organic mercurials; it bioaccumulates in
certain species of fish, some of which are important human and wildlife foods. In order to elucidate
the mechanisms that influence the pharmacokinetics of both methylmercury and mercuric mercury and
to extrapolate further both intra- and inter-species extrapolation of experimental data for these toxins,
Farris and associates (1993) developed a physiological pharmacokinetic model for methylmercury and
its metabolite, mercuric mercury. This was done in growing rats dosed orally with labeled
methylmercury over a period of 98 days. Mercuric mercury accounted for less than 0.5% of total
activity. Extensive sets of metabolism and distribution data were collected to understand the processes
that influence the pharmacokinetics of both methylmercury and mercuric mercury. The model
consisted of nine lumped compartments, each of which represented a major site of mercury
accumulation, distribution or elimination. The carcass served as a residual compartment, which
included all tissues and organs not separately incorporated into the model. Model simulations in this
study were made with experimentally determined concentrations of both inorganic and methylmercury
in blood, brain, kidney and liver. The data showed bidirectional and symmetric transport of both
chemical species between blood and tissues with relatively slow movement into and out of the brain.
Some key parameters remained uncertain; for example, the rate constant for demethylation is one of
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the most critical in adopting the model to other species. This model, however, established a
foundation for more complete understanding of methylmercury pharmacokinetics. With further
refinements, it could be applied to other species including humans. To characterize health hazard from
dietary methylmercury better, one needs to understand the distribution of methylmercury in the body,
the extent to which it accumulates and the rate at which it is eliminated. Farris et al. (1993) noted that
following methylmercury dosing there was a buildup of inorganic mercury in tissues and that excreted
mercury was predominantly mercuric; methylmercury behaved as a single body pool, while mercuric
mercury was handled differently in different tissues.
Smith and associates (1994) made further refinements to the Farris et al. (1993) model. They
reported a multicompartment pharmacokinetic model for methylmercury and mercuric mercury in
seven human volunteers. This model simulated the long-term disposition of methylmercury and
inorganic mercury in humans following a single i.v. dose of radio-labeled methylmercury. This was a
tracer amount to avoid toxic or saturation effects. The behavior of both methylmercury and inorganic
mercury in the body was modeled with the simplest compartmental model which fitted the data; blood,
urine and feces data were used to fit the model. In this model the tracer dose was delivered to the
first blood compartment and subsequently distributed to two extra-vascular methylmercury.
compartments; two distinct compartments (urine and feces) for inorganic mercury were added features.
This five-compartment model showed that inorganic mercury accumulated in the body and at longer
times was the predominant form of mercury present. The biological half-life of methylmercury in the
body was calculated to be 44 days, and 1.6% of the body burden was lost each day by both
metabolism and excretioa
To characterize neurological impairments of prenatal methylmercury exposure in children,
Gearhart and associates (1995) applied a more sophisticated multispecies pharmacokinetic model and
statistical dose-response analysis to an epidemiological study of a large population in New Zealand
(Kjellstrom et al. 1989) which featured relatively constant chronic exposure to methylmercury in fish.
The model for methylmercury in this study consisted of an adult with 11 compartments representing
both organ-specific and lumped tissues; eight compartments represented transport of methylmercury as
flow^limited, and three other compartments represented transport as diffusion-limited. The flow-
limited compartments were plasma, kidney, richly perfused, slowly perfused, brain-blood, placenta,
liver and gut compartments; RBC, brain and fetus were the diffusion-limited compartments. There
were also four other compartments in the model which were involved in methylmercury uptake and
elimination: methylmercury in the urine; and methylmercury and inorganic mercury in the hair, feces
and the intestinal lumen. The fetal sub-model for methylmercury consisted of four compartments:
fetal plasma, RBCs, brain and the remaining fetal body. This modeling effort was designed to create
a multispecies model that would be amenable to simulation of the kinetics of methylmercury by
simply changing the species-specific parameters. Unlike Farris et al. (1993), separate red blood cell
and plasma compartments were used to predict changes in kinetics of methylmercury across species
due to differences in the red blood cell/plasma ratio. Different pharmacokinetic parameters, such as
tissue/blood partition coefficients and volume distributions for humans, rats and monkeys, were taken
from different studies published in the current literature. The authors provided a benchmark dose on
results of a battery of neurobehavioral tests in 6-year-old children prenatally exposed to methylmercury
in seafood. Their calculations suggested a NOAEL of 17 ppm Hg in maternal hair for the most
sensitive neurological event in children. The analysis of the pharmacokinetic model indicated that the
fetal brain concentrations of methylmercury at this NOAEL were on the order of 50 ppb and were
associated with maternal dietary intakes of methylmercury ranging from 0.8 to 2.5 ug/kg-day. These
analyses provided support to the Iraqi data used in the development of the RfD for methylmercury,
presented in the risk assessment chapter (Chapter 6) of this volume.
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2.6.4 Discussion
Both simple and complex multi-compartment models have been reported by Hall et al. (1994),
Farris et al. (1993), Smith et al. (1994) and Gearhart et al. (1995). The Hall et al. (1994) paper
discussed a model which employed inorganic mercury data obtained from human studies; hdwever,
temporal patterns of blood mercury and the existence of kinetically distinct plasma pools for mercury
present uncertainties which limit the use of this model in risk assessment. Farris et al. (1993) reported
a multicompartment model using data obtained from rats exposed to methylmercury in diets over a
period of 98 days. They observed a buildup of inorganic mercury in tissues and the conversion of
methylmercury to inorganic mercury could not be accurately predicted by whole-body counting, which
was also subjected to errors from low sensitivity and the inability to compensate for geometric changes
due to redistribution of methylmercury or translocation of inorganic mercury to its target tissues.
Smith and associates (1994) refined this model and presented a multicompartment model using data
obtained from humans given a single i.v. dose of methylmercury. Uncertainties, however, persist in
prediction of methylmercury exposures hi food. Since methylmercury causes subtle neurotoxicity-in
children, this model may not be predictive of exposure in children. This potential neurotoxicity
observed in prenatally exposed children prompted Gearhart et al. (1995) to develop multicompartment
adult and fetal model using data from rat, monkey and humans. This model was applied to an
epidemiology study on which benchmark dose analysis was used to better characterize the dose-
response information rather than the traditional NOAEL approach. In the risk assessment chapter of
this volume, U.S. EPA utilizes a benchmark dose approach for setting the RfD for methylmercury. A
multispecies compartment model discussed in the Gearhart et al. (1995) report may provide a viable
approach because it can use data from both adults and neonates. This approach can use adult and
neonatal effects data from several animal and human studies to account for evidences of non-linearities
hi dose-responses. Research is needed to reduce uncertainties in racial, ethnic, and cultural differences
which exist in epidemiological studies.
June 1996 . 2-15 SAB REVIEW DRAFT
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3. BIOLOGICAL EFFECTS
This chapter summarizes the available toxicity data on mercury compounds; the information is
tabulated specifically for each form of mercury (i.e., elemental, inorganic and methylmercury) and
each toxicity endpoint. Case studies in humans are distinguished from epidemiological studies and
animal studies. In addition, critical studies for a given endpoint are briefly summarized in the
narrative preceding the corresponding table.
The tables provide information on study design, observed effects, study limitations and any
reported biological monitoring levels (BMLs) of mercury. To the extent possible, BML values have
been reported in consistent units throughout this chapter (ug/L in body fluids, ug/g in tissue, ug/g
creatinine in urine). It was not possible, however, to use completely consistent units because
investigators measured mercury in different media (e.g., blood, urine, or tissue) or used different time
frames (e.g., ug/L urine, ug/24 hour urine). In addition, some investigators normalized urine,,
concentrations to the amount of creatinine; while most did not. An explanation is provided in
Appendix A for any dose conversions required during review and evaluation of the toxicity and
carcinogenicity studies reported in the discussions presented below.
3.1 Elemental Mercury
3.1.1 Critical Noncancer Data
This section describes studies evaluated by U.S. EPA for use hi assessing general systemic
health risks, primarily toxicity in exposed workers. Chapter 6 describes the derivation of an inhalation
Reference Concentration (RfC) for elemental mercury based on neurotoxicity observed in several
human occupational studies. For completeness, some of these studies are also presented hi tabular
form in succeeding sections.
Fawer et al. (1983) used a sensitive measure of intention tremor (tremors that occur at the
initiation of voluntary movements) hi workers occupationally exposed for an average of 26 years to
metallic mercury vapor. A statistically significant difference was seen in the frequency of these
tremors in mercury-exposed workers compared with unexposed workers. The concentration of metallic
mercury hi the air was measured, and a time-weighted-average (TWA) of 0.026 mg/m3 over an
average of 15.3 years was derived. This was based on the assumption that the workers were exposed
to the same concentration of mercury for the duration of their employment. It should be noted that
very little detail was presented with regard to the measurement of the exposure levels, and that it is
likely that there were variations in the mercury air levels during the period of exposure. Furthermore,
the tremors may have resulted from intermittent exposure to concentrations higher than the TWA.
Piikivi and Tolonen (1989) studied the effects of long-term exposure to mercury vapor on the
electroencephalograms (EEG) of 41 chloralkali workers exposed for a mean of 15.6 ± 8.9 years by
comparison to matched referent controls. They found that the exposed workers, who had mean blood
mercury levels of 12 ug/L and mean urine mercury levels of 20 ug/L, tended to have an increased
number of EEG abnormalities when analyzed by visual inspection only. When the EEGs were
analyzed by computer, the exposed workers had significantly slower and attenuated EEGs as compared
to the referents. These changes were observed in 15% of the exposed workers. The frequency of
these changes correlated with cortical mercury content (measured in other studies); the changes were
most prominent in the occipital cortex, less prominent in the parietal cortex and almost absent in the
frontal cortex. The authors extrapolated an exposure level associated with these EEG changes of 0.025
mg/m3 from blood levels based on a conversion factor calculated by Roels et al. (1987).
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Piikivi and Hanninen (1989) studied the subjective symptoms and psychological performances
on a computer-administered test battery in 60 chloralkali workers exposed to mercury vapor for a
mean of 13.7 ±5.5 years as compared to matched referent controls. The exposed workers had mean
blood mercury levels of 10 ug/L and mean urine mercury levels of 17 ug/L. A statistically significant
increase in subjective measures of memory disturbance and sleep disorders was found in the exposed
workers. The exposed workers also reported more anger, fatigue and confusion. No objective
disturbances in perceptual motor, memory or learning abilities were found in the exposed workers.
The authors extrapolated an exposure level associated with these subjective measures of memory
disturbance of 0.025 mg/m3 from blood levels based on a conversion factor calculated by Roels et al.
(1987).
Both subjective and objective symptoms of autonomic dysfunction were investigated in «
41 chloralkali workers exposed to mercury vapor for a mean of 15.6 ± 8.9 years as compared to
matched referent* controls (Piikivi 1989). The quantitative non-invasive test battery consisted of
measurements of pulse rate variation in normal and deep breathing in the Valsalva maneuver and in
vertical tilt, as well as blood pressure responses during standing and isometric work. The exposed
workers had mean blood levels of 11.6 ug/L and mean urinary levels of 19.3 ug/L. The exposed
workers complained of more subjective symptoms of autonomic dysfunction than the controls, but the
only statistically significant difference was an increased reporting of palpitations in the exposed
workers. The quantitative tests revealed a slight decrease in pulse rate variations, indicative of
autonomic reflex dysfunction, in the exposed workers. The authors extrapolated an exposure level
associated with these subjective and objective measures of autonomic dysfunction of 0.03 mg/m3 from
blood levels based on the conversion factor calculated by Roels et al. (1987).
Sensory and motor nerve conduction velocities were studied in 18 workers from a mercury cell
chlorine plant (Levine 1982). Time-integrated urine mercury levels were used as an indicator of
mercury exposure. Using linearized regression analysis, the authors found that motor and sensory
nerve conduction velocity changes, (i.e., prolonged distal latencies) were correlated with the time-
integrated urinary mercury levels in asymptomatic exposed workers and occurred when urinary
mercury levels exceeded 25 ug/L. This study demonstrated that elemental mercury exposure can be
associated with preclinical evidence of peripheral neurotoxicity.
Miller et al. (1975) investigated several subclinical parameters of neurological dysfunction in
142 workers exposed to inorganic mercury in either the chloralkali industry or a factory for the
manufacture of magnetic materials. They found that there was a significant increase in average
forearm tremor frequency in workers whose urinary mercury concentration exceeded 50 ug/L as
compared to unexposed controls. Also observed were eyelid fasciculation, hyperactive deep tendon
reflexes and dermatographia, but there was no correlation between the incidence of these findings and
urinary mercury levels.
Roels et al. (1985) examined 131 male and 54 female workers occupationally exposed to
mercury vapor for an average duration of 4.8 years. Urinary mercury (52 and 37 ug/g creatinine for
males and females, respectively) and blood mercury levels (14 and 9 ug/L for males and females,
respectively) were recorded, but atmospheric mercury concentration was not provided. Symptoms
indicative of central nervous system (CNS) disorders were reported but were not related to mercury
exposure. Minor renal tubular effects were detected in mercury-exposed males and females and were
attributed to current exposure intensity rather than exposure duration. Male subjects with urinary
mercury levels of >50 ug/g creatinine exhibited preclinical signs of hand tremor. It was noted that
females did not exhibit this effect, and that their urinary mercury never reached the level of 50 ug/g
creatinine. A companion study (Roels et al. 1987) related air mercury levels to blood mercury
June 1996 3-2 SAB REVIEW DRAFT
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(Hg-blood) and urinary mercury (Hg'U) values in 10 workers in a chloralkali battery plant. Duration
of exposure was not specified. A high correlation was reported for Hg-air and Hg-U for pre-shift
exposure (r=0.70, p<0.001) and post-shift (r=0.81, p<0.001) measurements. Based on these data and
the results of their earlier (1985) study, the investigators suggested that some mercury-induced effects
may occur when Hg-U levels exceed 50 ug/g creatinine and that this value corresponds to a mercury
TWA of =0.04 mg Hg/m3.
A survey of 567 workers at 21 chloralkali plants was conducted to ascertain the effects of
mercury vapor inhalation (Smith et al. 1970). Mercury levels ranged from <0.01 to 0.27 mg/m3, and
chlorine concentrations ranged from 0.1 to 0.3 ppm in most of the working stations of these plants.
Worker exposure to mercury levels (TWA) varied with 10.2% of the workers exposed to <0.01 mg
Hg/m3, 48.7% exposed to 0.01-0.05 mg Hg/m3, 25.6% exposed to 0.06-0.10 mg Hg/m3, and 4.8%
exposed to 0.24-0.27 mg Hg/m3 (approximately 85% were exposed to mercury levels <0.1 mg/m3).
The duration of employment for the examined workers ranged from one year (13.3%) to >10 years
(31%) with 55.7% of the workers employed for 2 or 9 years. A group of 600 workers not exposed to
chlorine served as a control group. A strong positive correlation (p<0.001) was found between the
mercury TWAs and the reporting of subjective neuropsychiatric symptoms (nervousness, insomnia),
occurrence of objective tremors and weight and appetite loss. A positive correlation (rxO.OOl) was
also found between mercury exposure levels and urinary and blood mercury levels of test subjects. No
adverse alterations in cardiorespiratory, gastrointestinal, renal, or hepatic functions were attributed to
the mercury vapor exposure. Additionally, biochemical (hematologic data, enzyme activities) and
clinical measurements (electrocardiogram, chest X-rays) were not different between the mercury-
exposed and non-exposed workers. No significant signs or symptoms were noted for individuals
exposed to mercury vapor concentrations <0.1 mg Hg/m3. This study provides data indicative of a no-
observed-adverse-effect level (NOAEL) of 0.1 mg Hg/m3 and a lowest-observed-adverse-effect level
(LOAEL) of 0.18 mg Hg/m3. In a follow-up study conducted by Bunn et al. (1986), however, no
significant differences hi the frequency of objective or subjective findings such as weight loss and
appetite loss were seen in workers exposed to mercury at levels that ranged between 50 and
100 mg/m3. The study by Bunn et al. (1986) was limited, in that little information was provided
regarding several methodological questions such as quality assurance measures and control of possible
confounding variables.
Neurological signs and symptoms (i.e., tremors) were observed in 79 workers exposed to
metallic mercury vapor; urinary mercury levels in affected subjects exceeded 500 ug/L. Short-term
memory deficits were seen in workers whose urine levels were less than 500 ug/L (Langolf et al.
1978). Impaired performance hi mechanical and visual memory tasks and psychomotor ability tests
was reported by Forzi et al. (1978) hi exposed workers whose urinary mercury levels exceeded
100 ug/L.
Decreased strength, decreased coordination, increased tremor, decreased sensation and
increased prevalence of Babinski and snout reflexes were exhibited by 247 exposed workers whose
urinary mercury levels exceeded 600 ug/L. Evidence of clinical neuropathy was observed at urinary
mercury levels that exceeded 850 ug/L (Albers et al. 1988). Preclinical psychomotor dysfunction was
reported to occur at a higher incidence in 43 exposed workers (mean exposure duration of 5 years)
whose mean urinary excretion of mercury was 50 ug/L. In the same study, workers whose mean
urinary mercury excretion was 71 ug/L had a higher incidence of total proteinuria and albuminuria
(Roels et al. 1982). Postural and intention tremor was observed in 54 exposed workers (mean
exposure duration of 7.7 years) whose mean urinary excretion of mercury was 63 ug/L (Roels et al.
1989). Verbeck et al. (1986) observed an increase hi tremor parameters with increasing urinary
June 1996 3-3 SAB REVIEW DRAFT
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excretion of mercury in 21 workers exposed to mercury vapor for 0.5-19 years. The LOAEL for this
effect was a mean urinary excretion of 35 ug/g creatinine.
The elemental mercury levels reported to be associated with preclinical and symptomatic
neurological dysfunction are generally lower than those found to affect kidney function, as discussed
below.
Piikivi and Ruokonen (1989) found no evidence of glomerular or tubular damage in
60 chloralkali workers exposed to mercury vapor for an average of 13.7 ± 5.5 years as compared to
their matched referent controls. Renal function was assessed by measuring urinary albumin and N-
acetyl-p-glucosaminidase (NAG) activity. The mean blood mercury level in the exposed workers was
14 ug/L, and the mean urinary mercury level was 17 ug/L. The authors extrapolated a NOAEL for
kidney effects based on these results of 0.025 mg/m3 from blood levels based on the conversion factor
calculated by Roels et al. (1987).
Stewart et al. (1977) studied urinary protein excretion in 21 laboratory workers exposed to
0.01-0.05 mg/m of mercury. Their urinary level of mercury was -35 ug/L. Increased proteinuria
was found in the exposed workers by comparison to unexposed controls. When preventive measures
were instituted to limit exposure to mercury, proteinuria was no longer observed in the exposed
technicians.
Lauwerys et al. (1983) found no change in several indices of renal function (e.g., proteinuria,
albuminuria, urinary excretion of retinol-binding protein, aminoaciduria, creatinine in serum,
P2-microglobulin in serum) hi 62 workers exposed to mercury vapor for an average of 5.5 years. The
mean urinary mercury excretion in the exposed workers was 56 ug/g creatinine, which corresponds to
an exposure level of -0.046 mg/m3 according to a conversion factor of 1:1.22 (airurine [ug/g
creatinine]) (Roels et al. 1987). Despite the lack of renal effects observed, 8 workers were found to
have an increase in serum anti-laminin antibodies, which can be indicative of immunological effects.
In a follow-up study conducted by Bernard et al. (1987), however, there was no evidence of increased
serum anti-laminin antibodies in 58 workers exposed to mercury vapor for an average of 7.9 years.
These workers had a mean urinary mercury excretion of 72 ug/g creatinine, which corresponds to an
exposure level of -0.059 mg/m3.
Renal function in 100 chloralkali workers exposed to inorganic mercury vapor for an average
of 8 years was studied (Stonard et al. 1983). No changes in the following urinary parameters of renal
function were observed at mean urinary mercury excretion rates of 67 ug/g creatinine: total protein,
albumin, c^-acid glycoprotein, p^-microglobulin, NAG and y-glutamyl transferase. When urinary
mercury excreiion exceeded 100 ug/g creatinine, a small increase in the prevalence of higher activities
of NAG and y-glutamyl transferase were observed.
Rosenman et al. (1986) evaluated routine clinical parameters (physical exams, blood chemistry,
urinalysis), neuropsychological disorders, urinary NAG, motor nerve conduction velocities and
occurrence of lenticular opacities in 42 workers of a chemical plant producing mercury compounds. A
positive correlation (p<0.05 to p<0.001) was noted between urinary mercury (levels ranged from
100-250 ug/L) and the number of neuropsychological symptoms, NAG excretions and decrease in
motor nerve conduction velocities. Evidence of renal dysfunction was seen in 63 chloralkali workers.
This included increased plasma and urinary concentrations of p-galactosidase, increased urinary
excretion of high-molecular weight proteins and a slightly increased plasma p^-microglobulin
concentration. The incidence of these effects increased in workers whose urinary mercury excretion
exceeded 50 ug/g creatinine (Buchet et al. 1980).
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Increased urinary NAG levels were found in workers whose urinary mercury levels exceeded
50 ug/L (Langworth et al. 1987). An increase in the concentration of urinary brush border proteins
(BB-50) was observed in 20 workers whose mean urinary mercury excretion exceeded 50 ug/g
creatinine (Mutti et al. 1985). Foa et al. (1976) found that 15 out of 81 chloralkali workers exposed to
0.06-0.30 mg/m3 mercury exhibited proteinuria. An increased excretion of p-glutamyl transpeptidase,
indicative of renal dysfunction, was found in 509 infants dermally exposed to phenylmercury via
contaminated diapers (Gotelli et al. 1985).
The elemental mercury levels reported to be associated with preclinical and symptomatic
neurological dysfunction and kidney effects are lower than those found to affect pulmonary function,
as discussed below.
McFarland and Reigel (1978) described the cases of six workers who were acutely exposed
(4-8 hours) to calculated metallic mercury vapor levels'of 1.1—44 mg/m3. These men exhibited a
combination of chest pains, dyspnea, cough, hemoptysis, impairment of pulmonary function (reduced
vital capacity), diffuse pulmonary infiltrates and evidence of Interstitial pneumonitis. Although the
respiratory symptoms resolved, all six cases exhibited chronic neurological dysfunction, presumably as
a result of the acute, high-level exposure to mercury vapor.
Lilis et al. (1985) described the case of a 31-year-old male who was acutely exposed to high
levels of mercury vapor in a gold extracting facility. Upon admission to the hospital, the patient
exhibited dyspnea, chest pain with deep inspiration, irregular infiltrates in the lungs and reduced
pulmonary function (forced vital capacity). The level of mercury to which he was exposed is not
known, but a 24-hour urine collection contained 1,900 ug Hg/L. Although the patient improved*
gradually over the next several days, he still showed signs of pulmonary function abnormalities (e.g.,
restriction and diffusion impairment) 11 months after exposure.
Levin et al. (1988) described four cases of acute high-level mercury exposure during gold ore
purification. The respiratory symptoms observed in these four cases ranged from minimal shortness of
breath and cough to severe hypoxemia. The most severely affected patient exhibited mild interstitial
lung disease both radiographically and on pulmonary function testing. One patient had a urinary
mercury level of 245 ug/L upon hospital admission. The occurrence of long-term respiratory effects
in these patients could not be evaluated since all but one refused follow-up treatment.
3.1.2 Cancer Data
3.1.2.1 Human Data
A number of epidemiological studies were conducted that examined mortality among elemental
mercury vapor-exposed workers. Conflicting data regarding a correlation between mercury exposure
and an increased incidence of cancer mortalities have been obtained. All of the studies have
limitations which compromise interpretation of their results. These studies are summarized in Table 3-
1.
A retrospective cohort study examined mortality among 5,663 white males who worked
between 1953 and 1958 at a plant in Oak Ridge, Tennessee, where elemental mercury was used for
lithium isotope separation (Cragle et al. 1984). The workers were divided into three cohorts: exposed
workers who had been monitored on a quarterly basis for mercury levels in urine (n=2,133); workers
exposed in the mercury process section for whom urinalysis monitoring data were not collected
(n=270); and unexposed workers from other sections of the nuclear weapons production facility
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(n=3,260). The study subjects worked at least 4 months during 1953-1958 (a period when mercury
exposures were likely to be high); mortality data from death certificates were followed through the end
of 1978. The mean age of the men at first employment at the facility was 33 years, and average
length of their employment was greater than 16 years with a mean 3.73 years of estimated mercury
exposure. Air mercury levels were monitored beginning in 1955, and during 1955 through the third
quarter of 1956, air mercury levels were above 100 |ig/m3 in 30-80% of the samples. Thereafter, air
mercury levels decreased to concentrations below 100 ug/m3. The mortality experience [standard
mortality ratio (SMR)] of each group was compared with the age-adjusted mortality experience of the
U.S. white male population. Among exposed and monitored workers, there were no significant
increases in mortality from cancer at any site, even after the level or length of exposure was
considered. A significantly lower mortality from all causes was observed. There was an excess of
deaths due to lung cancer in the exposed, monitored workers (42 observed, 31.36 expected) but also in
the unexposed workers (71 observed and 52.93 expected). The SMR for each group was 1.34; the
elevated incidence of lung ctocer deaths was, therefore, attributed to some other factor at the plant
and/or to lifestyle factors (e.g., smoking) opmmon to both the exposed and unexposed groups. Study
limitations include small cell sizes for cancer mortality, which limited the statistical stability of many
comparisons.
Barregard et al. (1990) studied mortality and cancer morbidity between 1958 and 1984 in
1,190 workers from eight Swedish chloralkali plants that used the mercury cell process in the
production of chlorine. The men included in the study had been monitored for urinary or blood
mercury for more than one year between 1946 and 1984. Vital status and cause of death were
ascertained from the National Population Register and the National Bureau of Statistics. The cancer
incidence of the cohort was obtained from the Swedish Cancer Register. The observed total mortality
and cancer incidences were compared with those of the general Swedish male population.
Comparisons were not made between exposed and unexposed workers. Mean urinary mercury levels
indicated a decrease in exposure between the 1950s and 1970s; the mean urinary mercury level was
200 ug/L during the 1950s, 150 ug/L during the 1960s and 50 ug/L in the 1970s. Mortality from all
causes was not significantly increased in exposed workers. A significant increase in deaths from lung
tumors with greater than 10 years of latency was seen in exposed workers (rate ratio, 2.0; 95% C.I.
1.0-3.8), but 9 of the 10 observed cases of lung cancer occurred among workers (457 of the 1,190)
possibly exposed to asbestos as well as to mercury. No dose response was observed with respect to
mercury exposure and lung tumors. This study is limited because no quantitation was provided on
smoking status, and results were confounded by exposure to asbestos.
Ahlbom et al. (1986) examined the cancer mortality during 1961 to 1979 of cohorts of
Swedish dentists and dental nurses aged 20-64 and employed in 1960 (3,454 male dentists, 1,125
female dentists, 4,662 female dental nurses). Observed incidences were compared with those expected
based on cancer incidence during 1961-1979 among all Swedes employed during 1960 and the
proportion of all Swedes employed as dentists and dental nurses. Data were stratified by sex, age (5-
year age groups), and county. The incidence of glioblastomas among the dentists and dental nurses
combined was significantly increased (SMR, 2.1; 95% C.I. 1.3-3.4); the individual groups had
elevated SMRs (2.0-2.5), but the 95% confidence intervals of these groups included unity. By
contrast, physicians and nurses had SMRs of only 1.3 and 1.2, respectively. Exposure to mercury
could not be established as the causative factor because exposure to other chemicals and X-rays was
not ruled out.
Amandus and Costello (1991) examined the association between silicosis and lung cancer
mortality between 1959 and 1975 in white male metal miners (n=9,912) employed in the United States
between 1959 and 1961. Mercury exposures were not monitored. Exposures to specific metals among
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the silicotic and nonsilicotic groups were analyzed separately. Lung cancer mortality in both silicotic
and nonsilicotic groups was compared with rates in white males in the U.S. population. Both silicotic
(n=ll) and nonsilicotic mercury miners (n=263) had significantly increased lung cancer mortality
(SMR, 14.03, 95% C.I., 2.89-40.99 for silicotics; SMR, 2.66, 95% C.I. 1.15-5.24 for nonsilicotics).
The analysis did not focus on mercury miners, and confounders such as smoking and radon exposure
were not analyzed with respect to mercury exposure. This study is also limited by the small sample
size for mercury miners.
A case-control study of persons admitted to a hospital in Florence, Italy with lung cancer
between 1981-1983 was performed to. evaluate occupational risk factors (Buiatti et al. 1985). Cases
were matched with one or two controls (persons admitted to the hospital with diagnoses other than
lung cancer or suicide) with respect to sex, age, date of admission, and smoking status. Women who
had "ever worked" as hat makers had a significantly increased risk of lung cancer (p=0.01; determined
using the Mafctel-Haenszel Chi-square test). The duration of employment as a hat maker averaged
22.2 years, and latency averaged 47.8 years. Workers in the Italian hat industry were known to be
occupationally exposed to mercury; however, the design of this study did not allow evaluation of the
relationship between cumulative exposure and cancer incidence. In addition, interpretation of the
results of this study is limited by the small sample size (only 6/376 cases reported this occupation) and
by exposure of hat makers to other pollutants including arsenic, a known lung carcinogen.
Ellingsen et al. (1992) examined the total mortality and cancer incidence among 799 workers
employed for more than 1 year in two Norwegian chloralkali plants. Mortality incidence between
1953 and 1988 and cancer incidence between 1953 and 1989 were examined. Mortality and cancer
incidence were compared with that of the age-adjusted general male Norwegian population. No
increase in total cancer incidence was reported, but lung cancer was significantly elevated in the
workers (ratio, 1.66; 95% C.I. 1.0-2.6). No causal relationship can be drawn between mercury
exposure and lung cancer because no correlation existed between cumulative mercury dose, years of
employment, or latency time. Also, the prevalence of smoking was 10-20% higher in the exposed
workers and many workers were also exposed to asbestos.
Table 3-1
Carcinogenic Effects of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/
2,133 M
Human/376
cases of lung
cancer (6 were
hatmakers), 892
controls
Exposure
Duration
>4 months
(occup)
NS
(occup)
Dose
(mg/m3)
NS, but up
to 80% of
air samples
in early
years were
>0.10; this
declined to
1-10% in
later years
NS
Effects/Limitations/BML
No biologically significant increase in cancer mortality in
workers at an isotope enrichment plant, compared with
unexposed workers at the same plant, or with age-adjusted
mortality of U.S. males. Lung cancer mortality was increased in
exposed workers, but the increase was not statistically significant
and a significant increase in lung cancer mortality was observed
in unexposed workers.
Limitations: small cell size for cancer mortality, limiting
statistical power of comparisons
BML not reported
Increased lung cancer incidence among female hat makers
(p = 0.01). Controls were matched by age, sex and smoking
history.
Limitations: Hat makers were also exposed to arsenic
BML not reported
Reference
Cragle et al. 1984
Buiatti et al. 1985
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Table 3-1 (continued)
Carcinogenic Effects of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/
cohort/3454 M,
5787 F
Human/9,912 M
(369 silicotics,
9,543
nonsilicotics)
Human/
cohoit/799 M
Human/1 190 M
*
Exposure
Duration
NS
(occup)
.
NS
(occup)
SI year
(occup)
at least one
year
•
Dose
(mg/m3)
NS
NS
NS
NS
grouped by
years x U-
Hg;
>1000
ug/L,
1000-2000
ug/L,
>2000 ug/L
Effects/Limitations/BML
Increased incidence of glioblastomas among dental professionals.
95% C.I. = 1.3-3.4. Expected incidence was based on all
employed people (Sweden), stratified by age, sex and county.
Limitations: No information was provided on the duration or
level of exposure; subjects were also exposed to chloroform and
X-rays.
BML not reported
Increased lung cancer mortality among metal miners (95%
CI. = 0.94-2.90 for silicotics, 0.98-1.42 for nonsilicotics).
Limitations: Miners were exposed to a variety of metals. Only
274 worked in mercury mines, and data were not reported
separately for this group. Workers were also exposed to radon,
increase may have been related to silicosis
BML not reported
Increased incidence of lung cancer among chloralkali workers,
but there was no association with cumulative mercury dose,
years of employment or latency (95% C.I. = 1.0-2.6). The
increase could be partly explained by an assumed higher
smoking incidence and asbestos exposure. Cancer mortality and
incidence compared with age-adjusted Norwegian population.
Limitations: Subjects were also exposed to chlorine and low
levels of asbestos dust. Limited data available, since reported as
an abstract.
BML not reported
Cancer and mortality rates compared with general population; no
increase in mortality; excess of lung cancers (rate ratio = 2.0;
95% CI 1.0-3.8)
Limitations: co-exposure to asbestos
Reference
Ahlbom et al. 1986
Amandus and
Costello 1991
Ellingsen et al. 1992
Barregard et al. 1990
3.1.2.2 Animal Data
Druckrey et al. (1957) injected 0.1 mL of metallic mercury intraperitoneally into 39 rats (males
and females; numbers of each not specified) of the BD III and BD IV strains. Among the rats
surviving longer than 22 months, 5 out of 12 developed peritoneal sarcomas (three females and two
males). All sarcomas were observed to have droplets of mercury present. Although severe kidney
damage was reported in all treated animals, there were no renal tumors or tumors at any site other than
the peritoneal cavity.
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3.1.3 Other Data
3.1.3.1 Death
*
Accidental exposure to high concentrations of elemental mercury vapor for short amounts of
time has led to deaths in humans (Table 3-2). The cause of death in all available reports was
respiratory failure. The onset of death occurred six hours to 23 days after exposure to mercury vapors
(Campbell 1948; Kanluen and Gottlieb 1991; Rowens et al. 1991; Soni et al. 1992; Taueg et al. 1992).
Urinary mercury concentrations indicated that body levels were up to 10 times higher than controls.
Only acute-duration studies were found that directly linked elemental mercury vapor exposure to death.
Table 3-2
Lethality of Elemental Mercury in Humans: Case Studies
Species/
No. per Sex
Human/ 1 F
(4-month old)
Human/2 M,
2 F (adult,
2 elderly)
Human/1 F
(1-yr old)
Human/2 M,
2 F (adults)
Human/2
F (children)
Exposure
Duration
5hr
-24 hr
<6hr
NS
(Acute)
Several
months
Dose
(mg/m3)
NS
NS
NS
<0.91 at 11-
lS days post-
exposure
0.01-0.04
several
months after
initial spill
Effects/Limitations/BML
Increased creatimne excretion; necrotic stomach mucosa;
degeneration of convoluted tubules; death due to
pulmonary edema
Limitation: Limited exposure data
BML not reported
Respiratory distress; CNS alterations; nausea; tubular
necrosis of proximal tubules in kidneys
Limitation: Limited exposure data
BML Range: 4.6-219 ug/L in urine
Breathing difficulty; distended abdomen
Limitation: Limited exposure data
BML not reported
Dyspnea; respiratory failure; death at 11-24 days
postexposure
BML Range: 94-423 ug/L in urine
Numbness in fingers and toes; absence of deep tendon
reflexes; visual field defects; weakness
BML not reported
Reference
Campbell 1948
Kanluen and
Gottlieb 1991;
Rowens et al. 1991
Soni et al. 1992
Taueg et al. 1992
Taueg et al. 1992
Animal studies reveal that pulmonary edema and asphyxiation result from acute high-
concentration exposure to elemental mercury vapors (Table 3-3). Exposure to elemental mercury
vapors for two hours at a concentration of 27 mg Hg/m3 resulted in death of 20 of 32 rats
(Livardjani et al. 1991). Rabbits exposed for 1 to 30 hours to 28.8 mg Hg/m3 of elemental mercury
vapors appeared to be less affected. Death occurred in only one of two rabbits exposed for 30 hours
(Ashe et al. 1953). Exposure to the same concentration for a shorter duration resulted in no deaths.
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Table 3-3
Lethality of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per Sex
per Group
Rabbit/strain
NS/14 (sex
NS)
Rat/Wistar/64
M/duration
Exposure
Duration
1-30 hr
I or2hr
Dose
(mg/m3)
28.8
0,30
Effects/Limitations/BML
Death in 1/2 rabbits exposed for 30 hours; all rabbits exposed
for shorter periods survived.
Limitations: There was no control group, and details on
effects were lacking.
BML: 5,320 ug/L in blood
Death was due to asphyxiation; pulmonary edema and fibrosis
were observed. No animals exposed for 1 hour died by 15
days, and all animals exposed for 2 hours died within 5 days.
Limitations: No control group; limited reporting of histology
BML Range: 391-4,558 ng/L in blood at 1-15 days
postexposure
Reference
Ashe et al. 1953
Livardjani et al.
1991
3.1.3.2 Neurological
Case reports from accidental exposures to high concentrations of mercury vapors (Adams et al.
1983; Aronow et al. 1990; Barber 1978; Bluhm et al. 1992a; Fagala and Wigg 1992; Foulds et al.
1987; Friberg et al. 1953; Hallee 1969; Jaffe et al. 1983; Karpathios et al. 1991; Lilis et al. 1985;
McFarland and Reigel 1978; Sexton et al. 1976; Snodgrass et al. 1981; Taueg et al. 1992) as well as
studies of populations chronically exposed to potentially high concentrations (Ehrenberg et al. 1991;
Friberg et al. 1953; Roels et al. 1982; Sexton et al. 1978) have provided considerable information
about the neurotoxicity of elemental mercury vapor. These studies have shown effects on a wide
variety of cognitive, sensory, personality and motor functions.
Occasionally, hearing loss, visual disturbances (visual field constriction), and/or hallucinations
have also occurred. In general, symptoms have been observed to subside after removal from exposure.
However, persistent effects (tremor, cognitive deficits) have been observed in occupationally exposed
subjects 10 to 20 years after cessation of exposure (Albers et al. 1988; Ellingsen et al. 1993; Kishi et
al. 1993).
Studies of workers exposed to elemental mercury vapor have reported frank neurotoxicity at
exposure levels greater than 0.1 mg/m3 (Smith et al. 1970) or at levels resulting in urinary mercury of
greater than 300 ug in a 24-hour urine sample (Bidstrup et al. 1951). Several studies, however, have
shown evidence of neurotoxicity at approximately 2- to 4-fold lower concentrations. Self-reported
memory disturbances, sleep disorders, anger, fatigue, confusion and/or hand tremors were increased in
workers chronically exposed to an estimated 0.025 mg/m3 (blood levels of approximately 10 ug/L)
(Langworth et al. 1992a; Piikivi and Hanninen 1989). Also, objective measures of cognitive and/or
motor function in exposed populations have shown significant differences from unexposed controls
(Ehrenberg et al. 1991; Fawer et al. 1983; Liang et al. 1993; Ngim et al. 1992; Piikivi and Tolonen
1989; Piikivi et al. 1984; Roels et al. 1982, 1989).
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Symptoms of Mercury Vapor-induced Neurotoxicity
The most prominent symptoms associated with mercury vapor-induced neurotoxicity include the
following:
tremors - initially affecting the hands and sometimes spreading to other parts of the body
emotional lability - often referred to as "erethism" and characterized by irritability, excessive
shyness, confidence loss and nervousness
insomnia
neuromuscular changes - weakness, muscle atrophy, muscle twitching
headaches
polyneuropathy - paresthesias, stocking-glove sensory loss, hyperactive tendon reflexes,.slowed
sensory and motor nerve conduction velocities
memory loss and performance deficits in tests of cognitive function
Table 3-4
Neurotoxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per Sex
Human/1 M
(adult)
Human/6 M
Human/5 M, 6
F (adults and
children)/
12 controls
(sex NS)
Human/2 M, 2
F (adults)
Human/1 M
(adult)
Human/1 F (8-
month old)
Human/1 M
Exposure
Duration
8-9 mo
(occup)
<8hr
51-176 d
3 d
2d
=1 d
-2hr
Dose
(mg/m3)
0.02-0.45
44.3
(est)
0.1-1.0
NS
NS
NS
NS
Effects/Limitations/BML
Fatigue, irritability in an electrochemical industry
worker
Limitations: small sample size; concomitant exposure
to chlorine; limited data repotting
BML: 680-900 ug/L in urine
Tremor; irritability; visual and hearing abnormalities
Limitations: small sample size; limited data reporting
BML Range: 1,060-3,280 pig/24 hr urine
Nervousness, insomnia and inattentiveness were more
common than in controls; altered EEGs and personality
changes also noted
Limitations: small sample size
BML: 183-620 |ig/L in blood at first measure
Headache, slowed speech
Limitation: Small sample size; limited exposure data
BML: 82-5700 ug/24 hr urine
Delayed neurotoxicity: paresthesias; muscle
fasciculations; hyperactive deep tendon reflexes
Limitation: small sample size; exposure data limited
BML: 98.75 ug/L in urine 3.5 months after exposure
Seizures; weakness; short-term hearing deficit; cortical
atrophy
Limitations: Exposure data limited
BML Range: 16-43 ng/24 hr urine
Dizziness, weakness
Limitation: small sample size; limited exposure data
BML: 1,900 ng/L urine on first day
Reference
Friberg et al. 1953
McFarland and
Reigel 1978
Sexton et al. 1978
Snodgrass et al.
1981
Adams et al. 1983
Jaffe et al. 1983
Lilis et al. 1985
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Table 3-4 (continued)
Neurotoxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per Sex
Human/1 F
(child)
Human/1 M
(child)
Human/17-26
M
Human/1 F
(child)
Human/2 F
(children)
Exposure
Duration
2 mo
2wk
<16hr
6 mo
Several
months
Dose
(mg/m3)
NS
NS
NS
NS
0.01-0.04
several
months after
initial spill
Effects/Limitations/BML
Lethargy; irritability
Limitations: small sample size; limited reporting of
symptoms; limited exposure data
BML: 214 pg/L in 24 hr urine
Tremor; sleep disturbance; anxiety; cold hands and feet
Limitation: small sample size; limited exposure data
BML: 130 ug/24 hr urine
Fatigue, headaches, irritability, depression, anxiety,
tremor, impaired performance on visual-motor tests
(p<0.05) reported in welders following accidental
exposure.
Limitation: Chronic exposure to other metals; exposure
data limited
BML: -60 ug/L in blood at 20 d postexposure
Peripheral neuropathy; erethism; dizziness; depression;
irritability
Limitation: small sample size; exposure data limited
BML: 686 ug/24 hr urine
Numbness in fingers and toes; absence of deep tendon
reflexes; visual field defects; weakness
BML not reported
Reference
Foulds et al. 1987
Karpathios et al.
1991
Bluhm et al. 1992a
Fagala and Wigg
1992
Taueg et al.
1992
Table 3-5
Neurotoxicity of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/27
cases (sex NS)
Human/3 M, 6
F exposed/10
M, 30 F
controls
Exposure
Duration
3 mo-39 yr
(occup)
NS
(occup)
Dose
(mg/m3)
0-1.67
(est)
NS
Effects/Limitations/BML
161 electric meter repair workers were examined, and 22
were found to be symptomatic; there were 5 index cases.
Tremor, irritability; visual impairment were observed
Limitation: Concomitant exposure to other chemicals is
likely.
BML Range: 1,495-7,950 ug/24 hr urine
Neuropsychological tests showed irritability, tremor,
memory loss, poor coordination, visual impairment; altered
electrophysiology (pefl.05) in thermometer manufacturing
employees
Limitation: Exposure data limited
BML Range: 4-1,101 ug/24 hr urine
Reference
Bidstrup et al. 1951
Vroom and Greer
1972
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Table 3-5 (continued)
Neurotoxicity of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/43
exposed/47
controls (sex
NS)
Human/23 M
exposed/22 M
control
Human/12
exposed/12
controls (sex
NS)
Human/26
exposed M/25
control M
Human/36 M
exposed/36
controls
Human/60 M
exposed/60 M
" controls
Exposure
Duration
>6 months
Mean: 5.3 yr
(occup)
NS (occup)
3 mo-8 yr
(occup)
Avg: 15.3 yr
(occup)
Avg: 16.9 yr
(10-37 yr)
(occup)
Avg: 13.7 yr
(5-28 yr)
(occup)
Dose
(mg/m3)
NS
NS
NS
0.026
crwA)
personal
monitoring
0.022-
0.028 (est.)a
0.025 (est)
k
Effects/Limitaiions/BML
Objectively assessed tremor and eye-hand coordination
tended to be higher in the exposed group, with a significant
(p<0.05) difference on one test. There was a tendency
toward a dose-response, but it was not statistically
significant
Exposed group worked in amalgam or chloralkali plants;
control workers were matched from the same plants, but
unexposed.
Limitation: Exposure data limited
BML: 29.2 ug/L in blood (range: 5.3-135); 95.5 ug/g
creatmine in urine (range 9.9-286)
Decreased nerve conduction velocity (p<0.05); visual
impairment (p<0.01); higher distress levels
Of a sample of 298 dentists, the exposed group had "tissue"
mercury levels in the top 20%; the controls were age-
matched, with no detectable tissue mercury. Tissue
mercury in the.head and wrist was measured using x-ray
fluorescence.
Limitation: Exposure data limited
BML: >20 ug/g in tissue
In a battery of objective tests, .the following findings were
significant: tremor (p<0.025); decreased verbal intelligence;
short- and long-term memory impairment (p<0.01); fatigue
(fxO.Ol).
The exposed group worked with amalgam (n=4) or were
exposed to mercuric chloride, methoxyethyl mercuric
chloride, methoxyethylmercuric acetate (n=8). Controls
were matched by age, sex, education, ethnic background.
Limitation: Exposure data limited; small sample size
BML Range: <10-670 ug/L urine
Objectively assessed tremor was significantly (p = 0.001)
elevated in the exposed group and correlated with exposure
duration. Exposed group worked in fluorescent tube
factories (n=7), chloralkali plants (n=12). or in acetaldehyde
production. The control subjects worked at the same
factories but had not been exposed to mercury.
Mean BML: 8,280 ug/L in blood; 20 ug/g creatmine in
urine
By comparison to age-matched controls, chloralkali workers
had memory impairment, decreased verbal intelligence
(p<0.01).
BML: >15ug/L in blood; >56 ug/L in urine
In a psychological and psychomotor test battery, there were
statistically significant differences in subjective tests
(memory disturbance, mood; p<0.01) and an objecUve test
(hand-eye coordination, p<0.001). Subjects were chlorine-
alkali workers and controls were age-matched.
BML: 10.4 ug/L avg. in blood; 17.9 ug/g creatmine avg. in
urine
Reference
Roels et al. 1982
.•
Shapiro et al. 1982
Williamson et al.
1982
a
Fawer et al. 1983
Piikivi et al. 1984
Piikivi and
Hanninen 1989
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In animals, as in humans, adverse neurological effects are observed after exposure to elemental
mercury vapor. Effects observed in rabbits and mice after subchronic exposures included tremors,
ataxia, paralysis, failure to respond to light and decreased conditioned avoidance responding (Fukuda
1971; Ganser and Kirschner 1985; Kishi et al. 1978). Pathologic changes (unspecified) were observed
in the brains of rabbits at 0.86 mg Hg/m3 (Ashe et al. 1953).
Table 3-6
Neurotoxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per Sex
per Group
Rat/AIbino/7 M
exposed/6 M
control
Mouse/
C57BL6J/
No. and sex NS
*
Rabbit/774
strain NS/
31 (sexNS)
Rabbit/strain
NS/6M
Exposure
• Duration
12-42 wk
5d/wk
3hr/d
3.5 wk
5d/wk
20-40 min/d
1-12 wk
5d/wk
7hr/d
13 wk
4d/wk
6hr/d
Dose
'(mg/m3)
0, 3
NS
(saturated)
0.86
0,4
*
Effects/Limitations/BML
Tremor; decline in conditioned avoidance and conditioned
escape responses. First significant effect at 20 weeks
(p<0.05)
Limitation: Only one level tested
BML Range: 11.18-17.83 ug/g in cerebrum (wet weight)
Ataxia; motor dysfunction
Limitations: Poorly defined exposure conditions; limited
data reporting on effects
BML not reported
Mild to moderate pathological changes in brains
Limitations: One exposure level; limited data reporting
BML: Brain level 1.2 ug/g
Tremor
Limitation: No control
BML: 0.8-3.9 ug/g wet weight (brain)
Reference
Kishi et al. 1978
Ganser and
Kirschner 1985
Ashe et al. 1953
Fukuda 1971
3.1.3.3 Renal
The kidney is a sensitive target organ following inhalation exposure to elemental mercury.
Acute accidental exposure in private homes or as a result of industrial accidents resulted in symptoms
ranging from slight changes in urinary acid excretion to transient renal failure with proteinuria,
nephrosis and necrosis of the proximal convoluted tubules (Bluhm et al. 1992b; Jaffe et al. 1983;
Rowens et al. 1991; Tubbs et al. 1982). Proteinuria, proximal tubule damage and glomerulosclerotic
changes were also reported in a workers occupationally exposed for up to 2.5 years; in two cases the
exposure levels were measured at 0.02 to 0.45 mg/m3 (Friberg et al. 1953; Kazantzis et al. 1962).
Comparisons of exposed workers to unexposed controls found increased urinary N-acetyl-p-D-
glucosaminidase in workers exposed to 0.025 mg/m3 and increased incidence of proteinuria (Roels et
al. 1982).
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Table 3-7
Renal Toxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per
Sex
Human/2 M
(adult)
Human/3 M
Human/2 M/
41 M controls
Human/1 F
(8-month old)
Human/
2M.2F
Human/11 M
Exposure
Duration
8-9 mo
(occup)
4 mo- 2.5 yr
(occup)
'
NS (occup)
"1 day
Once
<16hr
,
Dose
(mg/m3)
0.02-0.45
NS
NS
NS
NS
NS
Effects/Limitations/BML
Protemuria and nephrosis in electrochemical industry workers
Limitations: small sample size; concomitant exposure to
chlorine
BML: 160-900 ug/L in urine
Heavy albuminuria; transient renal failure; proximal tubule
damage; glomeruloscierotic changes »
Limitations: small sample size; concomitant exposure to other
mercurials and other compounds; limited exposure'data
BML Range: 1,100-1,440 ug/L in urine
Proteinuria; glomerulonephritis in chemical plant workers
Limitation: small sample size; concomitant exposure to other
metals; limited exposure data
BML Range: 174-548 ug/24 hr urine
Acute renal failure (proteinuria, glucosuna, granular casts)
BML Range: 16 ug/24 hr urine
Necrosis of proximal tubule; increased serum urea nitrogen
and creatinine in 2 subjects
Limitations: small sample size; limited exposure data
BML Range: 94-220 ug/L Hg in urine
Hyperchloremia, low normal bicarbonate in urine in welders
following accidental exposure
Limitations: No information on pre-exposure range
BML: -60 ug/L in blood at 20 d postexposure
Reference
Friberg et al. 1953
Kazantzis et al.
1962
Tubbs et al. 1982
Jaffe et al. 1983
Rowens et al.
1991
Bluhm et al. 1992b
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Table 3-8
Renal Toxicity of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/21 NS
Human/43
exposed/47
controls (sex
NS)
Human/ 62 M
exposed, 60 M
controls
Human/100 M
Human/58 M
exposed
Human/41 M
exposed, 41 M
controls
Human/ 60 M
exposed, 60 M
controls
Exposure
Duration
NS
(occup)
>6 months
Mean: 5.3 yr
(occup)
1-25 yr
(avg 5.5 yr)
(occup)
8 yr (avg)
(occup)
7.9 yr (avg.)
1-20 yr
(occup)
13.7±5.5vr
(occup)
Dose
(mg/m3)
0.01-0.05
NS
0.046 (est)
NS
•
0.059 (est)
0, 0.025
NS
Effects/Limitations/BML
Increased proteinuria in exposed pathology laboratory
workers compared to unexposed controls. Proteinuria
cleared when mercury exposure was limited.
BML: »35 ug/L in urine
Proteinuria significantly elevated (p<0.05). Exposed group
worked in amalgam or chloralkali plants; control workers
were matched from the same plants but unexposed.
Limitations: Exposure data limited
BML: 29.2 ug/L in blood (range: 53-135); 95.5 ug/g
creatinine in urine (range 9.9-286)
Among exposed chloralkali plant or zinc-amalgam factory
workers, renal function parameters were not different from
unexposed controls. Circulating anu-laminin antibodies
found in eight exposed, 0 controls. No dose-effect
relationship between blood or urine levels and occurrence of
anti-laminin antibodies. Exposure level estimated using
Roels et al (1987) conversion factor.
BML: 16 ug/L (range 2.5-75.6) in blood; 56 ug/g creatinine
(range 3-272) in urine
Small increase in prevalence of higher activities of NAG
and gamma-glutamyl transferase in chloralkali workers
w/urinary mercury excretion >100 ng/g creatinine. No renal
function changes in workers w/mean urine 67 ug/g
creatinine
Limitation: Exposure data limited
BML: 67->100 ug/g creatinine in urine
Follow-up to Lauwerys et al. (1983). In contrast to the
earlier study, there was no evidence of anu-laminin
antibodies in exposed workers
BML: 72 ug/g creatinine in urine
Increased urinary N-acetyl-p"-D-glucosaminidase in a group
of chloralkali workers. Controls were age-matched.
BML: 15.6 ug/L in blood
No evidence of glomerular or tubular damage (effect on
urinary albumin or N-acetyl-p'-glucosaminidase activity) in
chloralkali workers compared to controls. NOAEL of 25
mg/m3 based on Roels et al. (1987) conversion factor.
Limitation: Exposure data limited
BML: 14 ug/L in blood; 17 ug/L in urine
Reference
Stewart et al. 1977
Roels et al. 1982
Lauwerys et al 1983
Stonard et al. 1983
Bernard et al. 1987
Barregard et al.
1988
Pilkivi and
Ruokonen 1989
Only one study was found of kidney effects in animals from exposure to elemental mercury
vapor (Ashe et al. 1953). The observed effects supported the human data, with kidney effects ranging
from moderate unspecified pathological changes at shorter durations to necrosis and cellular
degeneration at longer durations. Limited quantitative data were reported.
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Table 3-9
Renal Toxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
. No. per Sex
per Group
Rabbit/strain
NS/14 (sex
NS)
Exposure
Duration
1-30 hr
Dose
(mg/m3)
28.8
Effects/Limitations/BML
Kidney pathology correlated with exposure duration.
ranging from moderate changes at 1 hour to widespread
necrosis at 30 hours.
Limitations: No control group, limited data reporting
BML Range: 20-5,320 ug/L in blood
Reference
Ashe et al. 1953
3.1.3.4 Respiratory
Respiratory toxicity in humans following exposure to elemental mercury vapors has been
characterized by pulmonary edema and congestion, coughing, interstitial pneumonitis, respiratory
failure and absence of air in lungs at time of histopathological examination (Bluhm et al. 1992a;
Hallee 1969; McFarland and Reigel 1978; Milne et al. 1970; Snodgrass et al. 1981; Taueg et al. 1992).
One case of occupational exposure to elemental mercury vapor occurred due to a faulty thermostat that
heated to 450°F and vaporized the mercury it contained. Signs included cough, chest pains, reduced
vital capacity and pneumonitis, which began within hours of the onset of exposure (McFarland and
Reigel 1978). Accidental exposure to elemental mercury vapors in private homes has led to interstitial
pneumonia, dyspnea, lung disease and respiratory failure (Hallee 1969; Snodgrass et al. 1981; Taueg et
al. 1992). In each case, signs of toxicity persisted for days to months following acute exposure. No
studies were identified regarding respiratory effects in humans following intermediate or chronic
exposures to elemental mercury vapor.
Table 3-10
Respiratory Toxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per
Sex
Human/1 M,
1 F (adults)
Human/4 M
Human/6 M
Exposure
Duration
<12hr
2.5-5 hr
(occup)
<8hr
Dose
(mg/m3)
NS
1.1-1.7
(est.)
44.3
(est)
Effects/Limitations/BML
Dyspnea; interstitial pneumonia; fibrosis; moderate restrictive
lung disease
Limitation: Case study
BML Range: 191-557 ug/24 hr urine
Cough; chest tightness occurred following an accidental
exposure of electrochemical industry workers
Limitation: Case study
BML Range: 100-130 ug/L in urine 10-14 days postexposure
Pneumonitis; cough; chest pain
Limitations: Case study; limited data reporting
BML Range: 1,060-3,280 ug/24 hr urine
Reference
Hallee 1969
Milne et al. 1970
McFarland and
Reigel 1978
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Table 3-10 (continued)
Respiratory Toxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per
Sex
Human/2 M,
2 F (adults)
Human/1 M
Human/17 M
Human/2 M,
2 F (adults)
Exposure
Duration
3 d
-2hr
<16hr
-24 hr
Dose
(mg/m3)
NS
NS
NS
NS
Effects/Limitations/BML
Cough; dyspnea
Limitation: Case study
BML: 82-5700 ug/24 hr urine
Reduced vital capacity and dynamic lung volumes, shortness
of breath
Limitation: Case study
BML: 1,900 ug/L urine on first day
Congestion; dyspnea; lung infiltrates in up to 15/17 welders
interviewed following accidental exposure
Limitation: Limited data reporting of effects or exposure
BML: -60 ug/L in blood 20 d postexposure
Adult respiratory distress syndrome; respiratory failure
BML Range: 4.6-219 ug/L in urine
Reference
Snodgrass et al.
1981
Lilis et al. 1985
Bluhm et al. 1992a
Taueg et al. 1992
Rats exposed to 27 mg Hg/m3 as elemental mercury vapor for one hour exhibited dyspnea, and
exposure for two hours resulted in death by asphyxiation (Livardjani et al. 1991). Histopathological
analyses revealed necrosis of the alveolar membrane, presence of hyaline membranes and evidence of
pulmonary edema. Acute-duration studies with rabbits revealed degeneration and necrosis of the lungs
(Ashe et al. 1953). Gage (1961) reported congestion and necrosis of the lungs following intermediate-
duration exposure to elemental mercury vapor at a concentration of 1 mg Hg/m3.
Table 3-11
Respiratory Toxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Wistar/
6F
Rat/Wistar/64
M/ duration
Rabbit/strain
NS/14 (sex NS)
Exposure
Duration
7 wk
100 hr/wk
5d/wk
1 or2hr
1-30 hr
Dose
(mg/m3)
i
0,27
28.8
Effects/Limitations/BML
Congestion; necrosis of lung
Limitation: Limited data reporting
BML: 10 ug/rat in lungs
Death by asphyxiation; lung edema; hyaline membranes;
necrosis of alveolar epithelium
BML Range: 391-4,558 ug/L in blood
Pathology correlated with exposure duration and ranged
from mild changes at 1 hour to marked cellular
degeneration and necrosis at 30 hours. In another study,
Ashe reported no respiratory damage in rats exposed to 0. 1
mg/m3 for 72 weeks.
BML Range: 20-5,320 ug/L in blood
Reference
Gage 1961
Livardjani et al.
1991
Ashe et al. 1953
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3.1.3.5 Cardiovascular
Signs of cardiovascular toxicity in humans after acute exposure to elemental mercury include
tachycardia, elevated blood pressure and heart palpitations (Bluhm et al. 1992a; Snodgrass et al. 1981;
Soni et al. 1992). Intermediate-duration exposure to elemental mercury vapors produced similar
effects (i.e., tachycardia and elevated blood pressure) (Fagala and Wigg 1992; Foulds et al. 1987).
Barregard et al. (1990) performed a study on chloralkali workers and showed that they had an
increased risk of ischemic heart disease and cerebrovascular disease. These workers, however, were
exposed to other chemicals and to magnetic fields which may have affected the results. Piikivi (1989)
demonstrated a positive correlation between heart palpitations and urinary mercury concentrations in
workers from a chloralkali plant. It is unclear from the available scientific literature, however, whether
the effects on cardiovascular function (e.g., tachycardia, elevated blood pressure) are due to direct
cardiac toxicity or to indirect toxicity (e.g., due to effects on neural control of cardiac function) of
elemental mercury.
Table 3-12
Cardiovascular Toxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per Sex
Human/2 M, 2
F (adults)
Human/1 F
(child)
Human/17 M
Human/1 F
(child)
Human/1 M
(3-yr old)
Exposure
Duration
3d
2 mo
<16nr
6 mo
<6hr
Dose
(mg/m3)
NS
NS
NS
NS
NS '
Effects/Limitations/BML
Elevated blood pressure; tachycardia
Limitations: Case study; limited exposure data
BML Range: 82-5,700 ug/24 hr urine
Elevated blood pressure; tachycardia
Limitations: Case study; limited exposure data
BML Range: 214-296 ug/L in 24 hr urine
Palpitations in 5/17 welders interviewed following accidental
exposure
Limitation: Exposure data limited
BML: -60 ug/L in blood 20 d postexposure
Elevated blood pressure; tachycardia
Limitations: Case study; limited exposure data
BML: 686 ug/24 hr urine
Tachycardia
Limitations: Case study; limited exposure data
BML not reported
Reference
Snodgrass et al.
1981
Foulds et aL 1987
Bluhm et al. 1992a
Fagala and Wigg
1992
Soni et al. 1992
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Table 3-13
Cardiovascular Toxicity of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per
Sex
Human/41 M
exposed/41
M controls
Human/
26 M
«
Exposure
Duration
16 yr (avg)
5-27 yr
(occup)
10 yr (avg)
(occup)
Dose
(mg/m3)
0.03
(est)
Avg
samples:
0.025-
0.050
Effects/Limitations/BML
Palpitations in chloralkali workers (p<0,05); no significant effect
on cardiovascular reflex responses compared to matched
controls.
BML Range: 3-5-52.5 ug/L in urine; avg 19.3 ug/L in urine
Increased mortality due to ischemic heart and cerebrovascular
disease in chloralkali workers, compared to matched controls.
Limitation: Possible confounding^due to shift work
BML: Decrease from 200 ug/L in urine in 1950's to <50 ug/L
in 1990
Reference
Piitovi 1989
Barregard et al.
1990
Few animal studies were located regarding cardiovascular effects after exposure to elemental
mercury vapor. Studies in rabbits report unspecified cellular degeneration and necrosis of the
cardiovascular system following both acute and intermediate exposure (Ashe et al. 1953). Ashe et al.
(1953), however, concluded that the concentration of mercury is a better indicator of cardiovascular
toxicity than the duration of exposure, especially at lower exposure levels.
Table 3-14
Cardiovascular Toxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per
Sex per
Group
Rabbit/strain
NS/14 (sex
NS)
Rabbit/strain
NS/I6 (sex
NS)
Exposure
Duration
1-30 hr
1-11 wk
5d/wk
7hr/d
Dose
(mg/m3)
28.8
6
Effects/Limitations/BML
Pathology correlated with exposure duration and ranged from
mild changes in the heart at 1 hour to marked cellular
degeneration and necrosis at >12 hours.
Limitations: No controls; limited data reporting;
only one dose level tested
BML Range: 20-5,320 ug/L in blood
Mild to moderate pathological changes of the heart.
Pathological changes observed in subchronic studies were
correlated with the exposure concentration but not exposure
duration.
Limitations: No controls; limited data reporting;
only one dose level tested
BML Range: 70-3,000 ug/L in blood
Reference
Ashe et al. 1953
Ashe et al. 1953
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Table 3-14 (continued)
Cardiovascular Toxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per
Sex per
Group
Rabbit/strain
NS/31 (sex
NS)
Exposure
Duration
12 wk
5d/wk
7hr/d
Dose
(mg/m3)
0.86
Effects/Limitations/BML
Most animals had mild heart pathology, but 2 animals each at 6
and 7 weeks had marked cellular degeneration and necrosis.
with focal fibrosis.
Limitations: Limited data reporting; only one dose level
BML Range: 50-620 ug/L blood
Reference
Ashe et al. 1953
3.1.3.6 Gastrointestinal
Gastrointestinal effects have been reported by persons exposed to elemental mercury vapor.
The most common sign of mercury poisoning is stomatitis (inflammation of the oral mucosa), which is
usually reported following acute, high concentration exposure to elemental mercury vapors (Bluhm et
al. 1992a; Snodgrass et al. 1981). Sexton et al. (1978), however, reported signs of bleeding gingiva in
12 people exposed to mercury vapors for two months after metallic mercury was spilled in two homes,
and Schwartz et al. (1992) reported bleeding gums in a child exposed to mercury vapors for two to
four weeks. In addition, Vroom and Greet (1972) documented mercury intoxication in nine workers at
a thermometer manufacturing plant; the workers complained of sore gums and lesions on the oral
mucosa after long-term exposure. Other commonly reported gastrointestinal effects include nausea,
vomiting, diarrhea and abdominal cramps (Bluhm et al. 1992a; Campbell 1948; Lilis et al. 1985;
Sexton et al. 1978; Snodgrass et al. 1981; Vroom and Greer 1972).
Table 3-15
Gastrointestinal Toxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per
Sex
Human/1 F
(4-month old)
Human/3 M,
6F
Human/5 M,
6 F (adults
and
children)/12
controls (sex
NS)
Exposure
Duration
5hr
NS
(occup)
51-176 d
Dose
(mg/m3)
NS
NS
0.1-1.0
Effects/Limitations/BML
Difficulty swallowing; abdominal pain; necrosis of stomach
mucosa and duodenum
Limitations: Case study; exposure data limited
BML not repotted
Sore gums; diarrhea in thermometer manufacturing employees
Limitation: Exposure data limited
BML Range: 4-1,101 ug/24 hr urine
Nausea, vomiting, abdominal pain, anorexia, diarrhea.
bleeding gingiva more common than in controls
Limitations: Small sample size; no statistical analysis
BML avg: 3.7 jig/L in urine;
BML Range: 183-620 ug/L in blood
Reference
Campbell 1948
Vroom and Greer
1972
Sexton et al. 1978
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Table 3-15 (continued)
Gastrointestinal Toxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per
Sex
Human/2 M,
2 F (adults)
Human/1 M
Human/17 M
Exposure
Duration
3 days
-2hr
<16hr
Dose
(mg/m3)
NS
NS
NS
Effects/Limitations/BML
Nausea; vomiting; swelling of gums
Limitation: Case study
B ML Range: 13-5,700 ug/24hr urine
Nausea; vomiting
Limitations: Case study; limited repotting of symptoms;
exposure data limited
BML Range: 900-1,900 ng/L in urine (over 3 days)
Diarrhea; cramps in up to 11/17 welders accidentally exposed
Limitations: Case study; exposure data limited
BML not reported
Reference
Snodgrass et al.
1981
Lilis et al. 1985
Bluhm et al. 1992a
Very little information is available concerning gastrointestinal toxicity after exposure to
elemental mercury vapors. Ashe et al. (1953) exposed rabbits to mercury vapors for 1-30 hours at a
concentration of 28.8 mg Hg/m3 and found unspecified cellular degeneration and necrosis. When
rabbits were exposed to 6 mg Hg/m3 for 1-11 weeks, changes in the colon were seen during
histopathological analysis (Ashe et al. 1953).
Table 3-16
Gastrointestinal Toxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per
Sex per
Group
Rabbit/strain
NS/14 (sex
NS)
Rabbit/strain
NS/16 (sex
NS)
Exposure
Duration
1-30 hr
1-11 wk
5d/wk
7 hr/d
Dose
(mg/m3)
28.8
6.0
Effects/Limitations/BML
Earliest effect on colon (mild pathological changes) occurred at
2 hr; marked cellular degeneration and necrosis observed at 30
hr
Limitations: No controls; limited data reporting
BML Range: 20-5,320 ug/L in blood
No effects or mild histopathological changes in colon
Limitations: No controls; limited data reporting
BML Range: 70-3,600 ug/L blood
Reference
Asheetal. 1953
Ashe et al. 1953
3.1.3.7 Hepatic
Biochemical changes in hepatic enzymes were noted in a child who was exposed for
approximately one day to an unspecified concentration of elemental mercury vapors (Jaffe et al. 1983).
Serum glutamic-pyruvic transaminase (SGPT) and bilirubin levels were elevated, and synthesis of
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hepatic coagulation factors was reduced. No human studies were identified regarding the hepatic
toxicity of mercury following intermediate or chronic exposures to elemental mercury vapors.
Table 3-17
Hepatic Toxicity of Elemental Mercury in Humans: Case Study
Species/
No. per
Sex
Human/1 F
(8-month old)
Exposure
Duration
-Id
Dose
(mg/m3)
NS
Effects/Umitations/BML
Elevated serum alanine arnino-transferase and bilirubin
Limitations: Case study; exposure data limited
BML: 16 ug/24 hr urine
Reference
Jaffe et al. 1983
Ashe et al. (1953) performed histopathological analyses on rabbits after exposing them for one
to 30 hours or for one to 11 weeks to elemental mercury vapors. The analyses revealed necrosis and
cellular degeneration of the liver. No other animal studies were identified regarding the hepatic
toxicity of mercury vapors following inhalation exposure.
Table 3-18
Hepatic Toxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per
Sex per
Group
Rabbit/strain
NS/14 (sex
NS)
Rabbit/strain
NS/16 (sex
NS)
Exposure
Duration
1-30 hr
1-11 wk
5d/wk
7hr/d
Dose
(mg/m3)
28.8
6.0
Effects/Limitations/BML
•
Pathology correlated with exposure duration. Moderate changes
first occurred at 2 hr and widespread necrosis at 30 hr
Limitations: No controls; limited data reporting
BML Range: 20-5,320 ug/L in blood
Pathology was somewhat correlated with exposure duration and
ranged from mild to marked cellular degeneration with necrosis.
Limitations: No controls; limited data reporting
BML Range: 70-3,600 ug/L blood
Reference
Ashe et al. 1953
Ashe et al. 1953
3.1.3.8 Hematological
After acute-duration exposure to high concentrations of elemental mercury vapor, onset of
"metal fume fever" may occur; this syndrome is characterized by leukocytosis with fever, chills and
fatigue (Campbell 1948; Haddad and Stenberg 1963; Jaffe et al. 1983). Intermediate-duration
exposure to mercury vapors led to an elevated white blood cell count in a 12-year-old female after
exposure for six months (Fagala and Wigg 1992). Volunteers with dental amalgams had significantly
decreased hemoglobin and hematocrit compared to controls without dental amalgams (Siblerud 1990).
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Table 3-19
Hematological Toxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per
Sex
Human/1 F
(child)
Human/1 M
(3.5 yr old)
Exposure
Duration
6 mo
2-4 wk
Dose
(mg/m3)
NS
NS
Effects/Limitations/BML
Elevated white cell count
Limitations: Case study; exposure data limited; skin. lesions
could have led to elevated count
BML: 686 ug/24 hr urine
Thrombocytopema
Limitation: Exposure data limited
BML: 151 ug/L in^blood
Reference
Fagala and Wigg
1992
Schwartz et al. 1992
Table 3-20
Hematological Toxicity of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/47 (sex
NS)
Human/41 M
exposed/55
controls
Human/20 M,
30 F
exposed/21 M,
30 F control
Exposure
Duration
NS
(occup)
NS
(occup)
NS
Dose
(mg/m3)
<0.1
Range:
0.106-
0.783
NS
Effects/Limitations/BML
Decreased y-aminolevulinic acid dehydratase and
cholinesterase activity in erythrocytes, effects were
significantly (p<0.01) correlated to urinary mercury
BML Range: 2-472 ug/g of creatinine in urine.
Increased o2-macroglobulin and ceruloplasmin in mercury
plant workers compared to unexposed controls (rxO.OOl)
BML Range: 29-545 ug/L in urine
Subjects with amalgams had decreased (p<0.02) mean
hemoglobin (14.66*1.09 g/dL in subjects vs. 14.88±1.14
g/dL in controls) and mean hematocrit (43.15±3.66% in
subjects vs. 43 .91 ±3 .61% in controls). These reductions
were significantly (p<0.01) correlated with increasing urine
mercury in the subjects with amalgam.
Limitations: Subjects identified through newspaper ads may
have introduced self-selection bias; only mean data reported.
BML avg: 3.7 ug/L in urine
Reference
Wada et al. 1969
Bencko et al. 1990
Siblerud 1990
No animal studies were identified regarding the hematological toxicity of mercury vapors
following inhalation exposure.
3.1.3.9 Immunological
The available evidence suggests that the immune reaction to elemental mercury exposure is
idiosyncratic, with either increases or decreases in immune activity depending on genetic
predisposition. Although there is evidence for an overall suppression of the humoral immune response
among exposed workers (Moszczynski et al. 1990), this effect has not been consistently observed
(Bencko et al. 1990; Langworth et al. 1992b). The failure to observe consistent decreases in antibody
content of the serum may be due to small numbers of workers in each group who develop an
autoimmune reaction upon exposure to mercury. For example, small numbers of workers exposed to
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elemental mercury vapors have had elevated levels of antiglomerular basement membrane and anti-
DNA antibodies (Cardenas et al. 1993; Langworth et al. 1992b) or granular deposition of IgG and
complement C3 in the renal glomeruli (Tubbs et al. 1982).
Table 3-21
Immunotoxicity of Elemental Mercury in Humans: Case Study
Species/
No. per
Sex
Human/2 M
Exposure
Duration
NS
(occup)
Dose
(mg/m3)
<0.1
Effects/Limitations/BML
t Deposition of IgG and C3 in glomeruli of "chemical plant
workers
Limitation: Case»study
BML Range: 174-548 ug/24 hr urine
Reference
Tubbs et al. 1982
Table 3-22
Immunotoxicity of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/41 M
exposed/55
controls
Human/50
exposed/50
controls
Exposure
Duration
NS
(occup)
1.5-25 yr
Avg: 11 yr
(occup)
Dose
(mg/m3)
Range:
0.106-
0.783
NS
Effects/Limitations/BML
Decreased IgG; increased IgA and IgM in mercury plant
workers (p<0.05)
BML Range: 29-545 ug/L in urine
Abnormally high anti-DMA antibody litre (jxO.Ol)
Mean BML: 31.9 ug/L in urine
Reference
Bencko et al. 1990
Cardenas et al. 1993
An autoimmune response to mercury has been produced in a susceptible strain of rats (Brown
Norway) exposed to mercury vapor (Hua et al. 1993). In these rats, increased levels of serum IgE and
antilaminin autoantibodies, deposition of IgG deposits in the renal glomeruli and proteinuria were
observed.
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Table 3-23
Immunotoxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per
Sex per
Group
Rat/BN/3-4
M, 3-4 F
Exposure
Duration
5 wk
6 or 24 hr/d
Dose
(mg/m3)
0, 1
Effects/Limitations/BML
Increased serum IgE; anti-laminin autoantibody litre, IgG
deposits along glomerular capillary walls (rxO.OOl)
Mean BML: 90.3 ug/L in blood
Reference
Hua et al. 1993
3.1.3.10 Dermal
Exposure to elemental mercury vapors for acute or intermediate durations may elicit a response
known as acrodynia or "pink disease", which is characterized by peeling palms of hands and soles of
feet, excessive perspiration, itching, rash, joint pain and weakness, elevated blood pressure and
tachycardia (Fagala and Wigg 1992; Karpathios et al 1991; Schwartz et al 1992). Children seem to be
the most susceptible to acrodynia, although adults may be affected to a lesser degree (Warkany and
Hubbard 1953). One man experienced a rash and stomatitis after inhalation exposure to mercury when
repairing a cell in a chloralkali plant (Bluhm et al. 1992a); however, dermal exposure may have also
occurred.
Table 3-24
Dermal Toxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per Sex
Human/1 M
(child)
Human/17 M
Human/1 F
(child)
Human/1 M
(3-yr old)
Exposure
Duration
2 wk
<16hr
6 mo
2-4 wk
Dose
(mg/m3)
NS
NS
NS
NS
Effects/Limitations/BML
Red palms and soles; perspiration; rash
Limitations: Case study; concomitant dermal exposure
possible; exposure data limited
BML: 130 ug/24 hr urine
Conjunctivitis; dermatitis in 8/17 welders exposed in an
accident
Limitation: Exposure data limited
BML not reported
Peeling skin on palms and soles
Limitations: Case study; exposure data limited
BML: 686 ug/24 hr urine
Maculopapular whole body rash
Limitations: Case study; exposure data limited
BML: 151 ug/L in blood
Reference
Karpathios et al.
1991
Bluhm et al. 1992a
Fagala and Wigg
1992
Schwartz et al. 1992
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No animal studies were identified regarding the dermal toxicity of mercury vapors following
inhalation exposure.
3.1.3.11 Developmental
Although few reports have addressed the effects of maternal exposure to elemental mercury
vapor on the developing fetus, the available information suggests that maternal exposure to sufficiently
high concentrations of elemental mercury vapor may adversely affect the developing fetus. A study of
the pregnancies of Polish dental professionals showed a high frequency of malformations of a
nonspecified nature (Sikorski et al. 1987). In contrast, a study of Swedish dental professionals found
no increases in malformations, abortions, or stillbirths (Ericson and Kallen 1989). An increase in low
birth weight infants was noted hi the offspring of female dental nurses (Ericson and Kallen 1989);
however, in this same study similar effects were not observed for either dentists or dental technicians,
and socioeconomic factors may have contributed to the effects observed. It is unknown to what extent
discrepancies In the results of the above studies are attributable to differences in mercury exposure
levels (only the study by Sikorski et al. (1987) attempted to assess exposure levels) or to other
confounders.
Table 3-25
Developmental Toxicity of Elemental Mercury in Humans: Case Studies
Species/
No. per Sex
Human/1 F
Human/1 F
Human/1 F
Exposure
Duration
8 mo
(occup)
2yr
(occup)
»17 wk
Dose
(mg/m3)
NS
NS
0.02-0.06
Effects/Limitations/BML
Infant death at birth; fetal hepatomegaly; spontaneous
abortion in 2 successive pregnancies of a thermometer-
manufacturing worker. Maternal toxicity included tremors,
motor incoordination, hyperreflexivity, stomatitis.
Limitations: Case study; exposure data limited; maternal
toxicity also occurred.
BML not reported
Delivery of viable infant at term to thermometer factory
worker with mild peripheral neuropathy attributed to
mercury. Maternal toxicity included slight decrease in
sensory reflexes.
Limitations: Case study; exposure data limited; no
neurological assessment of infant; slight maternal toxicity
also reported.
BML: Mother 875 ug/L in urine; Offspring: 2.5 ug/L in
urine
Delivery of normal child who met all developmental
milestones. No maternal toxicity reported.
Limitation: Case study; no psychodevelopmental testing
BML: Mother: 230 ug/L in 24 hr urine at 17 wk, then
declined; Offspring: 3,000 ug/g in hair
Reference
Derobert et al. 1950
Melkonian and
Baker 1988
Thorp et al. 1992
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Table 3-26
Developmental Toxicity of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/ 349 F
exposed, 215 F
controls
Human/57 F
Human/8157 F
exposed
Exposure
Duration
NS
(occup)
0.5-27 yr
(occup)
NS
(occup)
Dose
(mg/m3)
NS
NS
NS
Effects/Limitations/BML
Rates of pregnancy and labor complications were high
among women exposed to elemental mercury. Insufficient
detail provided to evaluate dose-response relationship.
Limitation: Lack of exposure or effect data
BML not reported
In a study of 57 dental professionals (117 pregnancies),
reproductive failure (spontaneous abortion, stillbirth or
congenital malformation-no* described further) was higher
than among unexposed controls, and the effect correlated
with exposure level (p=0.004). No maternal toxicity
reported.
Limitations: Small study group; control group not
described; exposure data limited
BML avg: 0.527 ug/g in scalp hair
Study of infants born to dental workers, compared with the
general population. Based on medical registry, no increase
in malformations, abortions, or stillbirths. Increased
incidence of low birth weight infants among offspring of .
dental assistants (risk ratio 1.2, 95% C.I. 1.0-1.3), but the
• risk ratio was decreased for dentists, suggesting a
socioeconomic effect Case-control study of infants with
neural tube defects found none bom to dentists, but the
expected number was only 0.5. No maternal toxicity
reported.
Limitation: Exposure data limited
BML not reported.
Reference
Mishinova et al.
1980
Sikorski et al. 1987
Ericson and Kallen
1989
The few animal studies that were identified indicate that inhalation of elemental mercury vapor
may be toxic to the developing animal. In an abstract, Steffek et al. (1987) reported decreased fetal
weight in offspring of rats exposed to elemental mercury vapor during gestation. Increased fetal and
postnatal deaths were also reported by Baranski and Szymczyk (1973) among rats exposed to
elemental mercury vapor for three weeks prior to mating and then again on gestation days 1-20, and
increased resorptions in rats exposed on gestation days 10-15 or 1-20.
Pregnant Sprague-Dawley rats (12/group) were exposed on gestation days 11-14 and 17-20 to
elemental mercury vapors (1.8 mg/m3) for one or three hours/day (Danielsson et al. 1993). Litters
were culled to 4 males and 4 females. Behavioral testing was done on one male and one female adult
from each litter; the authors state that for behavioral testing 8 were tested for each group. There was
no difference between controls and treatment groups for maternal weight gain. There was no obvious
mercury toxicity in the dams. Offspring exposed in utero were no different from controls in the
following measures: body weight; clinical signs; pinna unfolding; surface righting reflex development;
tooth eruption; and results of a negative geotaxis test at days 7, 8 or 9 post partum. At 3 months of
age, exposed male but not female rats showed significant decrements in four measures of spontaneous
motor activity: locomotion, rearing, rearing time and total activity. By 14 months, the high-dose
animals showed hyperactivity in the same test. Females were not evaluated in other adult behavioral
tests. A test for habitation to novel environment at 7 months of age showed significant differences
between controls and treated males on four measures. At 4 months, mercury-treated males had
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significantly higher latency in a maze learning test; at 15 months, there was no difference between
controls and treated rats in a circular swim maze test.
Early postnatal exposure (during a period of rapid brain growth) resulted in subtle behavioral
changes when the rats were tested as young adults (Fredriksson et al. 1992). Eight litters/group, culled
to 8 individuals, were exposed to 0.05 mg/m3 for either 1 or 4 hr/day. Exposure was on days 11-17
of age. There were no signs of overt toxicity or changes in body weight. Spontaneous motor activity
was evaluated at 2 and 4 months. The high-dose group showed increased rearing at the early test, but
the repeat test indicated hypoactivity. The low-dose group was no different from controls at two
months; at four months this group showed increased total activity and decreased rearing. In the spatial
learning test administered at 6 months, low- dose rats had increased time to complete the task. High-
dose animals were observed to have increases in time to complete the task and in numbers of errors.
No information was given on the number of males and females tested or on any differences in
behavior dependent on gender.
Table 3-27
Developmental Toxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Strain
NS/23-24 F
Rat/Sprague-
Dawley/NS F
Rat/Sprague
Dawley/ 4M, 4F
Exposure
Duration
Group I: 6
hr/d, 6-8 weeks
before
fertilization:
Group H: 3 wk
before mating
and Gd 7-20
6or20d
24 hr/d
Gd 10-15 or
Gd 1-20
1 or 3 hr/day
onGd 11-14
plus 17-20
Dose
(mg/m3)
0, 2.5
0, 0.1, 0.5,
1.0
0, 1.8
mg/m
Effects/Limitatibns/BML
Group I: Decreased number of live pups (p<0.05);
decreased relative kidney (p<0.01) and liver weights
(p<0.05) and increased ovaries (p<0.05) in 2-month-old
pups. Group II: Mean number of live fetuses lower
than in controls
Limitations: Wide range in actual mercury
concentration (0.5-4.8 mg/m3); only one level tested;
maternal toxicity
BML not reported
Increased resorptions (LOAEL = 0.5 for Gd 10-15 and
1.0 for Gd 1-20); decreased maternal and fetal weights
in group exposed to 1.0 on Gd 1-20.
Limitations: Reported only as an abstract; limited study
details; maternal toxicity
BML not reported
Hypoactivity at 3 months; hyperactivity at 14 months;
decrement in habituation to novel environment at 7
months; retarded learning in radial arm maze at 4
months but no difference from controls in circular swim
maze at 15 months. BML ranges for control through
high dose group (mg Hg/kg in organs): 0.001-0.012
(brain); 0.004- 0.112 (liver); 0.002-0.068 (kidney).
Limitations: Limited testing of female offspring; no
evaluation of differences between males and females;
small numbers of rats/group.
Reference
Baranski and
Szymczyk 1973
Steffek et al. 1987
Damelsson et al.
1993
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Table 3-27 (continued)
Developmental Toxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Sprague-
Dawley/8 F
Exposure
Duration
7d
1 or 4 hr/d
on post-partum
days 11-17
Dose
(mg/m3)
0, 0.05
Effects/Limitations/BML
Impaired spatial leaning at 6 months (p<0.01);
increased locomotor activity in objective test (p<0.01)
BML Range: 1,700-63,000 ug/g in brain
Limitations: No infonnation on gender-specific
behavioral effects; small number of animals/group.
Reference
Fredriksson et al.
1992
3.1.3.12 Reproductive
Most studies that have examined the effects of occupational exposure to elemental mercury
vapor on reproductive function have failed to find evidence of adverse effects (Alcser et al. 1989;
Brodsky et al. 1985; Erfurth et al. 1990; Ericson and Kallen 1988; Heidam 1984; Lauwerys et al.
1985; McGregor and Mason 1991). A few studies have shown at least suggestive evidence that
elemental mercury exposure may adversely affect reproductive function. In females exposed
occupationally to metallic mercury vapor, a correlation was observed between scalp hair mercury and
reproductive failure or menstrual abnormalities (Sikorski et al. 1987). An increased incidence of
pregnancy complications such as toxicosis or prolonged or hemorrhagic parturition was observed in
exposed females when compared to unexposed controls (Mishonova et al. 1980). A slightly increased
incidence of menstrual disorders in exposed females was reported by DeRosis et al. (1985); however,
the statistical significance of this finding was not presented. No evidence for an effect on fertility was
observed in exposed males, but one study of wives of exposed workers found an increased rate of
spontaneous abortions (Cordier et al. 1991). It is possible that the wives were exposed to mercury as
the result of handling contaminated clothing. None of the above studies presented infonnation on
exposure levels, and few presented biomonitoring data. Thus, it is difficult to compare findings in the
various studies.
Table 3-28
Reproductive Toxicity of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/728 F
exposed/1034 F
controls
Human/29,514
M, 30,272 F
Exposure
Duration
NS
(occup)
NS
(occup)
Dose
(mg/m3)
NS
NS
Effects/Limitations/BML
No increase in rate of spontaneous abortions in gardeners,
dental assistants, painters, and factory workers.
BML not reported
No correlation between mercury exposure
-------
Table 3-28 (continued)
Reproductive Toxicity of Elemental Mercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/153 F
exposed/193 F
controls
Human/103 M
exposed/101 M
controls
Human/57 F
exposed
Human/8157 F
Human/247 M
exposed/255 M
controls
Human/20 M
exposed/21 M
controls
Human/152 F
exposed/374 F
controls
Human/
40 M exposed/
63 M controls
Exposure
Duration
<5-17yr
(occup)
•
Avg: 5.9 yr
(1-25 yr)
(occup)
0.5-27 yr
(occup)
NS
(occup)
4mo-8 yr
(occup)
2-18 yr
(occup)
NS
(occup)
2-20 yr
(occup)
Dose
(mg/m°)
<0.01
TWA at
study;
>0.05 for 4
yr
NS
NS
NS
NS
NS
NS
NS
Effects/Limitations/BML
Slightly increased prevalence of menstrual disorders in
mercury lamp manufacturers, compared with workers subject
to similar stresses but not exposed to mercury.
Limitations: Subjective measures; no statistical analysis
BML not reported
No effect of paternal exposure on fertility of chloralkali.
amalgam or electrical equipment workers, compared to
controls with similar workloads.
Avg BML: 52.4 ug/g creatinine in urine (range 5.1-272)
In a study of 57 dental professionals (117 pregnancies),
reproductive failure (spontaneous abortion, stillbirth or
congenital malformation) was higher than among unexposed
controls, and the effect was correlated with exposure level
(extrapolated from hair Hg levels) (p=0.004). Irregular,
painful, or hemorrhagic menses was correlated with exposure
duration (p=0.005).
Limitations: Small study size; control group not described;
exposure data limited
Avg BML: 0.527 ug/g in scalp hair
Based on medical registry, there was no increase in
spontaneous abortions or stillbirths in pregnancies of dental
professionals, compared to the general population.
BML not reported
No association between paternal exposure and rate of
miscarriages in Department of Energy plant workers.
Limitation: Potential recall bias
BML: reported only as value integrated over time
No correlation between blood or urinary mercury, and male
gonadotropic hormones of chloralkali workers, other
industrially-exposed workers, or dentists and matched
controls.
BML: Avg. 46 ug/g creatinine in urine (workers); 2.3 ug/g
creatinine (dentists)
Increased spontaneous abortions in women whose husbands
were exposed to mercury vapors in chloralkali plants (rate
doubled above 50 ug/L in urine; 95% C.I. = 0.99-5.23)
Limitation: Exposure data limited
Avg BML: 61.9 ug/L in urine (range 26.9-75.9 ug/L)
No correlation between blood or urinary Hg and male
gonadotropic hormones in workers from different industries
(not specified)
Avg BML: 103 ug/g creatinine in urine
Reference
DeRosis et al. 1985
Lauwerys et al.
1985
Sikorski et al. 1987
Ericson and Kallen
1988
Alcser et al. 1989
Erfuth et al. 1990
Cordier et al. 1991
McGregor and
Mason 1991
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In rats exposed to elemental mercury vapor, prolongation of estrous cycles was observed both
when compared to either unexposed controls or preexposure rates of cycling (Baranski and Szymczyk
1973).
Table 3-29
Reproductive Toxicity of Elemental Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Strain
NS/24F
Exposure
Duration
3wk
5d/wk
6 hr/d and
Gd 7-20
Dose
(mg/m3)
2.5
Effects/Limitations/BML
Longer estrous cycles, but the effect was not statistically
significant
BML not reported
Reference
Baranski and
Szymczyk 1973
3.1.3.13 Genotoxicity
Cytogenetic monitoring studies in populations exposed occupationally to elemental mercury
vapor provide conflicting evidence for a clastogenic effect of elemental mercury. Early studies
reported increased frequencies of chromosomal aberrations among exposed workers (Popescu et al.
1979; Verschaeve et al. 1976). These studies, however, were not well-controlled, and the results could
not be reproduced in later studies (Mabille et al. 1984; Verschaeve et al. 1979). Popescu et al. (1979)
compared two groups of men exposed to elemental mercury vapor (Group I, n=4; Group II, n=18)
with an unexposed group of ten individuals and found a statistically significant increase in incidence
of chromosome aberrations in the exposed groups. Verschaeve et al. (1976) found an increase in
aneuploidy in lymphocytes of 28 subjects exposed to low concentrations of mercury vapor (by
comparison to seven controls), but these results were not repeated in later studies (Verschaeve et al.
1979). Mabille et al. (1984) did not find increases in structural chromosomal aberrations of
lymphocytes of exposed workers.
More recently, Barregard et al. (1991) demonstrated a correlation between cumulative mercury
exposure and induction of micronuclei among a group of chloralkali workers, suggesting a clastogenic
effect. This study did not show significant differences in frequency or size of micronuclei between the
exposed group to unexposed controls who were matched for age and smoking habits. Neither did they
find a correlation between the induction of micronuclei and current mercury exposure as measured by
blood or urine mercury levels. A correlation, however, was observed between cumulative exposure to
mercury and micronuclei induction in T-lymphocytes in exposed workers suggesting a genotoxic
effect.
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Table 3-30
Genotoxicity of Elemental Mercury in Humans
Species/
No. per Sex
Human/
8 M, 6 F (exposed)/
3 M, 4 F (control)
Human/4 M
exposed/10 controls
(sex NS)
-
Human/
28 exposed/
20 controls (sex NS)
Human/
22 exposed/
25 controls (sex NS)
Human/26 M
„
Exposure
Duration
NS
(occup)
9.25 yr (avg)
(occup)
1-11 yr
(occup)
4 yr (avg)
(0.3-15.3 yr)
(occup)
10 yr (avg)
(rain. 1 yr)
(occup)
Dose
(mg/m3)
NS
Range:
0.15-0.44
<0.05at
time of
study
NS
Avg
samples:
0.025-
0.050
Effects/Limitations/BML
Aneuploidy was significantly (fxO.OOl) increased in
subjects exposed due to an unstated occupation or as a
result of an accident at a university. Structural
aberrations were not increased.
Limitations: Small study size; smoking status not
reported
BML Range: 1-114 ug/L in urine
Chromosome breaks (excluding gaps) were significantly
(p<0.001) increased in whole blood cultures taken from
chemical plant workers. There was no effect on
numerical chromosome aberrations.
Limitations: Small sample size; smoking status not
reported.
BML Range: 142-386 ug/L in urine at study
The incidence of structural and numerical chromosome
aberrations in exposed chloralkali plant workers did not
differ from controls. Eight of the controls were
unexposed workers at the same plant and 12 were taken
from the general population.
Limitations: Small sample size; there were 12 smokers in
the exposed group, 4/8 among the internal controls, and
an unknown number of smokers in the external controls
Avg BML: 35.4 ug/L in urine
No increase in structural chromosome aberrations in zinc
amalgam or chloralkali workers, compared to unexposed
control workers at the same plant
Limitations: Small sample size; there were 15 smokers in
the exposed group and 12 among the controls; limited
exposure data
Avg BML: 30.6 ug/L in blood; range: 7.5-105 ug/L
The frequency of micronuclei in lymphocytes was
correlated with cumulative exposure in Swedish
chloralkali workers (p = 0.0035). There was no
significant difference between the frequency in the
exposed and control populations.
Controls were matched by age; exposed and control
groups each had 14 smokers
Limitation: Small study size
Avg BML: 9.6 ug/L in blood
Reference
Verschaeve et al.
1976
Pepescu et al.
1979
Verschaeve et al.
1979
Mabille et al. 1984
Barregard et al.
1991
No studies were identified that examined the genotoxicity of elemental mercury in animals
following inhalation exposure. Likewise no studies of genotoxic effects of mercury exposure in vitro
were recovered.
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3.2 Inorganic Mercury
Inorganic mercury occurs in numerous forms/compounds; the most common include mercuric
chloride (HgCl2), mercurous chloride (Hg2Cl2), mercuric oxide (HgG>). The tables in this section
include a notation in the dose column indicating the specific form of inorganic mercury involved in
that study. Oral doses, shown in mg/kg-day, have been converted to mg Hg/kg-day using the method
shown in Appendix A.
3.2.1 Critical Noncancer Data
This section describes studies evaluated by U.S. EPA for use in assessing general systemic
health risks. Chapter 6 describes the derivation of an oral Reference Dose (RfD) for inorganic
mercury based on several studies wherein kidney diseases consequent to immunological effects were
observed. For completeness, some of these studies are also presented in tabular form in succeeding
sections.
3.2.1.1 Human Data
Singer et al. (1987) studied nerve conduction velocity of the median motor, median sensor and
sural nerves in 16 workers exposed to various inorganic mercury compounds (e.g., mercuric oxides
and mercurial chlorides) for an average of 7.3 ±7.1 years and compared to an unexposed control
group using t-tests. They found a slowing of nerve conduction velocity in motor, but not sensory,
nerves that correlated with increased blood and urine mercury levels and an increased number of
neurologic symptoms. The mean mercury levels in the exposed workers were 1.4 ng/L and 10 ug/L
for blood and urine, respectively. These urine levels are 2-fold less than those associated with
peripheral neurotoxicity in other studies (e.g., Levine et al. 1982). There was considerable variability
in the data presented by Singer et al. (1987), however, and the statistical analyses (t-test) were not as
rigorous as those employed by Levine et al. (1982) who used linearized regression analysis.
Furthermore, the subsections in the Levine et al. (1982) study were asymptomatic at higher urinary
levels than those reported to be associated with subjective neurological complaints in the workers
studied by Singer et al. (1987). These results, therefore, are not considered to be as reliable as those
reported by Levine et al. (1982).
Kazantzis et al. (1962) performed renal biopsies in 2 (out of 4) workers with nephrotic
syndrome who had been occupationally exposed to mercuric oxide, mercuric acetate and probably
mercury vapors. The authors felt that the nephrotic syndrome seen in 3 of the 4 workers may have
been an idiosyncratic reaction since many other workers in a factory survey had similarly high levels
of urine mercury without developing proteinuria. This conclusion was strengthened by work in Brown
Norway rats indicating a genetic (strain) susceptibility and that similar mercury-induced immune
system responses have been seen in affected humans and the susceptible Brown Norway rats (U.S.
EPA 1987b).
3.2.1.2 Animal Data
Bernaudin et al. (1981) reported that mercurials administered by inhalation or ingestion to
Brown Norway rats resulted in the development of a systemic autoimmune disease. The mercuric
chloride ingestion portion of the study involved the forcible feeding of either 0 or 3 mg/kg/week of
mercuric chloride to male and female Brown Norway rats for up to 60 days. No abnormalities were
reported using standard histological techniques in either experimental or control rats.
Immunofiuorescence histology revealed that 80% (4/5) of the mercuric-exposed rats were observed
June 1996 3-35 SAB REVIEW DRAFT
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with a linear IgG deposition in the glomeruli after 15 days of exposure. After 60 days of mercuric
chloride exposure, 100% (5/5) of the rats were- seen with a mixed linear and granular pattern of IgG
deposition in the glomeruli and granular IgG deposition in the arteries. Weak proteinuria was
observed in 60% (3/5) of the rats fed mercuric chloride for 60 days. The control rats were observed to
have no deposition of IgG in the glomeruli or arteries as well as normal urine protein concentrations,
Andres (1984) administered mercuric chloride (3 mg per kg of body weight in 1 mL of water)
by gavage to five Brown Norway rats and two Lewis rats twice a week for 60 days. A sixth Brown
Norway rat was given only 1 mL of water by gavage twice a week for 60 days. All rats had free
access to tap water and pellet food. After 2-3 weeks of exposure, the Brown Norway mercuric
chloride-treated rats started to lose weight and hair. Two of the mercuric chloride-treated Brown
Norway rats died 30-40 days after beginning the study. No rats were observed to develop detectable
proteinuria during the 60-day study. The kidneys appeared normal in all animals when evaluated
using standard histological techniques, but examination by immuno-fluorescence showed deposits of
IgG present in the renal glomeruli of only the mercuric-treated Brown Norway rats. The Brown
Norway treated rats were also observed with mercury-induced morphological lesions of the ileum.and
colon with abnormal deposits of IgA in the basement membranes of the intestinal glands and of IgG in
the basement membranes of the lamina propria. All observations in the Lewis rats and the control
Brown Norway rat appeared normal.
The only chronic oral study designed to evaluate the toxicity of mercury salts was reported by
Fitzhugh et al. (1950). In this study, rats of both sexes (20-24/group) were given 0.5, 2.5, 10, 40 or
160 ppm mercury as mercuric acetate in their food for up to 2 years. Assuming food consumption
was equal to 5% body weight per day, the daily intake would have been 0.025, 0.125, 0.50, 2.0 and
8.0 mg Hg/kg for the five groups, respectively. At the highest dose level, a slight depression of body
weight was detected in male rats only. The statistical significance of this body weight depression was
not stated. Kidney weights were significantly (rxO.05) increased at the 2rand 8-mg Hg/kg-day dose
levels. Pathologic changes originating in the proximal convoluted tubules of the kidneys were also
noted with more severe effects in females than males. The primary weaknesses of this study were the
lack of reporting (which adverse effects were observed with which dosing groups) and that the most
sensitive strain, the Brown Norway rat, was not used for evaluating the mercury-induced adverse
health effects.
NTP (1993) conducted subchronic and chronic gavage toxicity studies on Fischer 344 rats and
B6C3F1 mice to evaluate the effects of mercuric chloride, and the kidney appeared to be the major
organ of toxicity. These studies were also summarized by Dieter et al. (1992). In the 6-month study,
Fischer 344 rats (10/sex/group) were administered 0, 0.312, 0.625, 1.25, 2.5, or 5 mg/kg-day of
mercuric chloride (0.23, 0.46, 0.92, 1.9, and 3.7 mg Hg/kg-day), 5 days/week, by gavage. Survival
was not affected, although body weight gains were decreased in males at high dose and in females at
or above 0.46 mg Hg/kg-day. Alkaline phosphatase and gamma-glutamyl transferase levels in the
urine were significantly elevated in the females exposed to 3.7 mg Hg/kg-day at four and six months
of exposure. Absolute and relative kidney weights were significantly increased in both sexes with
exposure to at least 0.46 mg Hg/kg-day. The kidney weight changes were slightly dose-related in the
females. Histopathology revealed corresponding changes in the kidneys. In males, the incidence of
nephropathy was 80% in controls and 100% for all treated groups; however, severity was minimal in
the two low-dose groups and minimal to mild in the 0.92-mg Hg/kg-day group and higher. In
females, there was a significant increased incidence of nephropathy only at the high-dose group (4/10
with minimal severity). Nephropathy was characterized by foci of tubular regeneration, thickened
tubular basement membrane and scattered dilated tubules containing hyaline casts. No treatment-
June 1996 3-36 SAB REVIEW DRAFT
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related effects were observed in the other organs: however, histopathology on the other organs was
performed only on control and high-dose rats.
B6C3F1 mice (10/sex/group) were administered gavage doses of 0, 1.25, 2.5, 5, 10, or 20
mg/kg-day mercuric chloride (0, 0.92, 1.9, 3.7, 7.4, or 14.8 mg Hg/kg-day) 5 days/week for 6 months
(NTP 1993), There was a decrease in body weight gain in males at the highest dose tested.
Significant increases occurred in absolute kidney weights of male mice dosed with 3.7 mg Hg/kg-day
or more, and relative kidney weights were increased in male mice at the 7.4 and 14.8 mg Hg/kg-day
doses. The kidney weight changes corresponded to an increased incidence of cytoplasmic vacuolation
of renal tubule epithelium in males exposed to at least 3.7 mg Hg/kg-day. The exposed female mice
did not exhibit any histopathologic changes in the kidneys.
In the 2-year NTP study, Fischer 344 rats (60 per sex per group) were administered 0, 2.5, and
5 mg/kg-day mercuric chloride (1.9 and 3.7 mg Hg/kg-day), 5 days a week, by gavage (Dieter et al.
1992; NTP 1993). After two years, survival was significantly reduced hi the treated male rats
compared to the controls. Mean body weights were significantly decreased m both treated males and
females (9-10% and 14-15% decrease from control, respectively). At 15 months, relative kidney
weights were significantly elevated (not dose-related) in all treated groups (15-20% increase from
control), and relative brain weights were significantly elevated (slightly dose-related) in treated females
(13-18%). The increased kidney weights were accompanied by an increase in severity of nephropathy.
After two years, there was an increased incidence of nephropathy of moderate-to-marked severity and
increased incidence of tubule hyperplasia in the* kidneys of exposed males compared to the controls.
The control males exhibited nephropathy, primarily of mild-to-moderate severity.
Hyperparathyroidism, mineralization of the heart and fibrous osteodystrophy were observed and
considered secondary to the renal impairment. There were no significant differences found in renal
effects between exposed and control females. Other nonneoplastic effects included an increased
incidence of forestomach hyperplasia in the exposed males and high dose females, increased incidence
of nasal inflammation at the high-dose animals, slightly increased incidence of acute hepatic necrosis
in the high-dose males and increased incidence of inflammation of the cecum in exposed males.
Statistical analyses, however, were not performed on these histopathologic changes.
NTP (1993) also administered to B6C3F1 mice (60/sex/group) daily oral gavage doses of 0, 5,
or 10 mg/kg mercuric chloride (0, 3.7 and 7.4 mg Hg/kg-day), 5 days a week, by gavage for 2 years.
Survival and body weights of mice were slightly lower in mercuric chloride treated mice compared to
controls. Absolute kidney weights were significantly increased in the treated males while relative
kidney weights were significantly increased in high-dose males and both low- and high-dose females.
Histopathology revealed an increase in the incidence and severity of nephropathy in exposed males
(mild severity in low dose and moderate-to-marked severity in high dose) and females (minimal
severity in low dose and minimal-to-mild severity in high dose). Nephropathy was defined as foci of
proximal convoluted tubules with thickened basement membrane and basophilic cells with scant
cytoplasm. Some affected convoluted tubules contained hyaline casts. There was also an increase in
nasal cavity inflammation (primarily infiltration of granulocytes in nasal mucosa) in the exposed
animals.
In a 4-week oral study (Jonker et al. 1993), Wistar rats (5-10/sex/group) were fed a diet
containing 15 and 120 ppm mercuric chloride (0.56 or 4.4 mg Hg/kg-day). A significant increase in
relative kidney weight was reported for the low-dose females and high-dose males. There was also an
increase in the incidence of high-dose males that had occasional basophilic tubules in the outer cortex
of the kidneys. In the range-finding study by Jonker et al. (1993), rats were administered 75, 150, or
300 ppm mercuric chloride (2.8, 5.6, 11.1 mg Hg/kg-day) in the diet for four weeks. A significant
June 1996 3-37 SAB REVIEW DRAFT
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increase in the relative kidney weights was observed in both sexes for all dose groups: the effect was
dose related. Nephrosis and proteinaceous casts in the kidneys were reported in both sexes at the
lowest dose. At 5.6 mg/kg-day, the body weight was significantly decreased in males and serum
alkaline phosphatase levels were elevated in females. At 11.1 mg/kg-day, increased serum aspartate
aminotransferase (both sexes), decreased urinary density (males), increased relative adrenal weight
(males), increased serum sodium and phosphate levels (females) and decrease in body weight (females)
were reported.
A series of studies (Boscolo et al. 1989; Carmignani et al. 1989, 1992) reported renal and
cardiovascular changes in rats exposed to mercuric chloride in drinking water. These studies were
limited due to the small number of animals and dose levels tested. Boscolo et al. (1989) evaluated the
renal effects of mercuric chloride in two different rat strains. Male Sprague-Dawley rats (8/group)
were administered 0 or 0.05 mg/mL mercury (0 or 7 mg Hg/kg-day), and male Wistar rats (8/group)
received 0, 0.05, or 0.2 mg/mL mercury (0, 7, or 28 mg Hg/kg-day) in drinking water for 350 days.
Increases in blood pressure and cardiac inotropism, without changes in heart rate, occurred in exposed
rats of both strains. Hydropic degeneration and desquamation of the proximal tubular cells were
exhibited in kidneys of Sprague-Dawley rats, with alterations and lysis of lysosomes in tubular cells
and thickening of the basal membrane in the glomeruli. Wistar rats displayed tubular degeneration and
membranous glomerulonephritis in 30% of the glomeruli at 7 mg/kg-day and all glomeruli at 28
mg/kg-day. Thickening of basal membrane and hypercellularity and alteration of the mesangial matrix
hi the glomeruli and hydropic degeneration of tubules were seen in Wistar rats. Similar findings of
renal histopathology alterations and cardiovascular changes were reported by Carmignani et al. (1989)
who administered 0 or 0.05 mg/L of mercury (7 mg/kg-day) to male Sprague-Dawley rats (8/group)
for 350 days.
In Carmignani et al. (1992), male Sprague-Dawley rats (8/group) received 0 or 0.2 mg/mL of
mercury (28 mg Hg/kg-day) as mercuric chloride in drinking water for a shorter duration (180 days).
Similar renal changes were observed, as well as IgM deposition hi the glomeruli (as shown by
immunofluorescence). In addition, the treated group displayed significantly decreased urinary
kallikrein and creatinine, decreased plasma renin and increased plasma angiotensin-converting enzyme.
The cardiovascular effects were slightly different from Boscolo et al. (1989) and Carmignani et al.
(1989); there was an increase in blood pressure but a decrease hi cardiac inotropism in the exposed
rats. The increase in blood pressure was suggested to be due to a vasoconstrictor effect, likely related
to a greater release of noradrenaline from adrenergic neurons and to baroreflex hyposensitivity. The
decrease in contractility was attributed to a direct toxic effect of the mercury on the cardiac muscle
because of the high levels of mercury detected in the heart The differences in the results of
cardiovascular changes for the studies were not explained.
To evaluate the effect of mercuric chloride on the development of autoimmunity, female
SJL/N mice (7/group) received 0.625, 1.25, 2.5, or 5 ppm mercuric chloride (0.07, 0.14, 0.28, or
0.56 mg Hg/kg-day) in drinking water ad libitum for 10 weeks (Hultman and Enestrom 1992). An
increase in circulating antinucleolar antibodies was observed at 0.28 mg Hg/kg-day. The high-dose
group had elevated granular IgG deposits in the renal mesangium and in vessel walls of glomerular
capillaries, arteries and arterioles of the spleen and in intramyocardial arteries. Slight glomerular cell
hyperplasia and discrete widening of the centrolobular zone were also exhibited in the 0.56-mg/kg-day
group.
Agrawal and Chansouria (1989) administered 0, 2.6, 5.2, and 10.4 mg Hg/kg-day as mercuric
chloride in drinking water to male Charles Foster rats for 60, 120, or 180 days (5/group). The relative
adrenal gland weight was significantly increased for the dose groups at all durations compared to
June 1996 3-38 SAB REVIEW DRAFT
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controls. Significant increases in adrenal and plasma corticosterone levels occurred in all dose groups
at 60 and 120 days; however, changes were not seen after 180 days. The authors suggested that
mercuric chloride may have acted as a chemical stressor in a dose- and duration-dependent manner.
The study was limited because histopathology was not performed on the kidneys, and the adrenal
gland was the only tissue evaluated.
Both male and female Brown Norway rats 7-9 weeks of age were divided into groups of 6-20
animals each (Druet et al. 1978). The numbers of each sex were not stated. The animals were
injected subcutaneously with mercuric chloride 3 times weekly, for 8 weeks, with doses of 0, 0.07,
0.2, 0.4, 0.7, and 1.5 mg Hg/kg. An additional group was injected with a 0.04 mg/kg for 12 weeks.
Antibody formation was measured by the use of kidney cryostat sections stained with a fluoresceinated
sheep anti-rat IgG antiserum; urinary protein was assessed by the biuret method. Tubular lesions were
seen at the higher dose levels. Proteinuria was seen at doses of 0.07 mg/kg and above, but not at 0.04
mg/kg. Proteinuria was considered a highly deleterious effect in that affected animals developed
hypoalbuminemia and many died. Fixation of IgG antiserum was detected in all groups except
controls.
3.2.2 Cancer Data
3.2.2.1 Human data
No data are available on the carcinogenic effects of inorganic mercury in humans.
3.2.2.2 Animal data
The results from a dietary study in rats and mice show equivocal evidence for carcinogenic
activity in male mice and female rats and some evidence for carcinogenic activity in male rats. Two
other dietary studies show negative evidence for carcinogenicity, but these studies are limited by
inadequacies in the data and experimental design.
Mercuric chloride was administered by gavage in water at doses of 0, 2.5, or 5 mg/kg-day (0,
1.9 and 3.7 mg Hg/kg-day) to Fischer 344 rats (60/sex/group), 5 days a week, for over 104 weeks
(NTP 1993). An interim sacrifice (10/sex/dose) was conducted after 15 months of exposure.
Complete histopathological examinations were performed on all animals found dead, killed in
extremis, or killed by design. Survival after 24 months was statistically significantly (p<0.01) lower in
low- and high-dose males; survival was 43%, 17% and 8% in control, low-, and high-dose males,
respectively, and 58%, 47%, and 50% in control, low-, and high-dose females, respectively. During
the second year of the study, body weight gains of low- and high-dose males were 91% and 85% of
controls, respectively, and body weight gains of low- and high-dose females were 90% and 86% of
controls, respectively. At study termination, nephropathy was evident in almost all male and female
rats including controls, but the severity was much greater hi treated males; the incidence of "marked"
nephropathy was 6/50, 29/50, and 29/50 in control, low- and high-dose males, respectively.
Squamous cell papillomas of the forestomach showed a statistically significant (p<0.001)
positive trend with dose by life table adjusted analysis; the incidences were 0/50, 3/50 and 12/50 in
control, low-, and high-dose males, respectively. The incidence in female rats was 0/50, 0/49 and 2/50
in control, low- and high-dose groups, respectively. These neoplasms are rare neoplasms in rats and
occurred in only 1 out of 264 historical controls. The incidence of papillary hyperplasia of the
stratified squamous epithelium lining of the forestomach was statistically significantly (p<0.01)
elevated in all dosed males (3/49, 16/50 and 35/50 in control, low- and high-dose males, respectively)
June 1996 3-39 SAB REVIEW DRAFT
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and in high-dose females (5/50, 5/49 and 20/50 in control, low- and high-dose females, respectively).
The incidence of thyroid follicular cell carcinomas was marginally significantly (p=0.044 by logistic
regression analysis; tumors not considered to be fatal) increased in high-dose males (1/50, 2/50 and
6/50 in control, low- and high-dose groups, respectively). The data, adjusted for survival, also showed
a significant (p=0.017) positive trend in males. The combined incidence of thyroid follicular cell
neoplasms (adenoma and/or carcinoma), however, was not significantly increased (2/50, 6/50 and 6/50
in control, low- and high-dose males, respectively). In female rats a significant decrease in the
incidence of mammary gland fibroadenomas was observed (15/50, 5/48 and 2/50 in control, low- and
high-dose females, respectively). Table 3-31 gives the incidences of lesions which were increased in
treated animals.
Table 3-31
Incidence9 of Selected Lesions in Rats in the NTP (1993) 2-Year Gavage Study
Tumor Site and Type
Forestomach
Papillary hyperplasia
Squamous cell papilloma
Thyroid Follicular Cell"1
Adenoma
Carcinoma
Adenoma or carcinoma
Dose Group (mg Hg/kg-day)
Males
0
1.9
3/49
0/50
1/50
1/50
2/50
16/50"
3/50
4/50
2/50
6/50
3.7
Females
0
1.9
3.7
35/50"
12/50°
0/50
6/50°
6/50
5/50
0/50
-
-
-
5/49
0/49
-
-
--
20/50"
2/50
--
-
-
a Overall rate
" p S 0.01
c p <0,001; trend test also p<0.001
Data on thyroid follicular cell lesions were reported for males only.
e p = 0.044, logistic regression
The high mortality in both groups of treated males indicates that the maximally tolerated dose
(MTD) was exceeded in these groups and limits the interpretation of the study. NTP (1993)
considered the forestomach tumors to be of limited relevance to humans because the tumors did not
appear to progress to malignancy. NTP (1993) also questioned the relevance of the thyroid
carcinomas because these neoplasms are usually seen in conjunction with increased incidences of
hyperplasia and adenomas, but increases in hyperplasia (2/50, 4/50 and 2/50 in control, low- and high-
dose males, respectively) or adenomas (1/50, 4/50 and 0/50 in control, low- and high-dose males,
respectively) were not observed.
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In the same study, mercuric chloride was administered by gavage in water at doses of 0. 5. or
10 mg/kg-day (0, 3.7 and 7.4 mg Hg/kg-day), 5 days a week, for 104 weeks to B6C3F1 mice
(60/sex/group) (NTP 1993). An interim sacrifice (10/sex/dose) was conducted after 15 months of
exposure. Terminal survival of male mice was not affected by the administration of mercuric chloride;
survival of high-dose females was slightly lower (p=0.051) than controls (41/60, 35/60 and 31/60 in
control, low- and high-dose females, respectively). Body weight gain was not affected. Female mice
exhibited a significant increase in the incidence of nephropathy (21/49, 43/50 and 42/50 in control,
low- and high-dose females, respectively). Nephropathy was observed in 80-90% of the males in all
groups. The severity of nephropathy increased with increasing dose (1.08, 1.74 and 2.51 in control,
low- and high-dose males, respectively; 0.47, 1.02 and 1.24 in control, low- and high-dose females,
respectively). The incidence of renal tubule hyperplasia was 1/50, 0/50 and 2/49 in control, low- and
high-dose males.
As shown in Table 3-32", the combined incidence of renal tubule adenomas and
adenocarcinomas was 0/50, 0/50 and 3/49 in control, low- and high-dose males, respectively.
Although no tumors were seen in the low-dose group, a statistically significant (p=0.032) positive
trend for increased incidence with increased dose was observed. These observations were considered
important because renal tubule hyperplasia and tumors in mice are rare. The two-year historical
incidence of renal tubule adenomas or adenocarcinomas in male mice dosed by gavage with water was
0/205, and only four of the nearly 400 completed NTP studies have shown increased renal tubule
neoplasms in mice. NTP did not report a statistical comparison of the study data to historical control
data. Analysis of the reported data with Fisher's Exact test, however, showed that the incidence of
renal tubule adenomas or adenomas and carcinomas (combined) in the high-dose males was
significantly elevated when compared to historical controls (Rice and Knauf 1994).
Table 3-32
Incidence* of Renal Tubule Tumors in Male Mice in the
NTP (1993) 2-Year Gavage Study
•
Adenoma
Adenoraacarcinoma
Adenoma or adenomacarcinoma
Dose Group (mg/kg-day)
0
0/50
0/50
0/50
5
0/50
0/50
0/50
10
2/49
1/49
3/49b
' Overall rate
1 p = 0.107; trend test p = 0.032
A 2-year feeding study in rats (20 or 24/sex/group; strain not specified) was conducted in
which mercuric acetate was administered in the diet at doses of 0, 0.5, 2.5, 10, 40, and 160 ppm (0,
0.02, 0.1, 0.4, 1.7, and 6.9 mg Hg/kg-day) (Fitzhugh et al. 1950). Survival was not adversely affected
in the study. Increases in kidney weight and renal tubular lesions were observed at the two highest
doses. No statement was made in the study regarding carcinogenicity. This study was not intended as
a carcinogenicity assay, and the number of animals/dose was rather small. Histopathological analyses
were conducted on only 50% of the animals (complete histopathological analyses were conducted on
June 1996
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only 31% of the animals examined), and no quantitation of results or statistical analysis was
performed.
No increase in tumor incidence was observed in a carcinogenicity study using white Swiss
mice (Schroeder and Mitchener 1975). Groups of mice (54/sex/group) were exposed until death to
mercuric chloride in drinking water at 5 ppm mercury (0.95 mg Hg/kg-day). No effects on survival or
body weights were observed. After dying, mice were weighed, dissected, gross tumors were detected,
and some sections were made of the heart, lung, liver, kidney and spleen for microscopic examination.
Mercuric chloride was nontoxic in the study. No statistically significant differences were observed in
tumor incidences for treated animals and controls. This study is limited because complete histological
examinations were not performed, only a single dose was tested, and the MTD was not achieved.
The increasing trend for renal tubular cell tumors in mice observed in the NTP (1993) study is
supported by similar findings in mice after chronic dietary exposure to methylmercury (Hirano et al.
1986; Mitsumori et al. 1981, 1990). In these studies, dietary exposure to methylmercuric chloride
resulted in increases in renal tubular tumors at doses where substantial nephrotoxicity was observed.
Table 3-33
Carcinogenic Effects of Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/strain
NS/20-24 M,
20-24 F
Rat/F344/
60H60F
Mouse/Swiss/54
M, 54 F
Mouse/
B6C3F1/
60 M, 60 F
Exposure
Duration
2yr
ad lib
2yr
5 d/wk
1 x/d
(gavage)
Lifetime
ad lib
2yr
5 d/wk
1 x/d
(gavage)
Dose
(mg/kg-day)
0, 0.02, 0.1,
0.4, 1.7, 6.9*
0, IS, 3.7
(Hgcy
0.95
(Hgcy
0, 3.7, 7.4
(Hgcy
-
Effects/Limitations//BML
No carcinogenicity reported
Limitations: small number of animals/dose; complete
histopathological examinations conducted on only 31% of
animals; no statistical analyses
Thyroid follicular cell carcinomas in males at 3.7.
Limitations: MTD exceeded (high mortality in treated
males); limited relevance of lesions to humans
•
No carcinogenicity reported
Limitations: Complete histopathological examinations not
performed; only single dose level tested; MTD not
achieved
Renal tubule tumors (adenoma or adenomacarcinoma) in
3/49 males at 7.4 (positive trend test, p=0.0032)
Limitations: Severe nephropathy also observed in high-
dose males.
Reference
Fitzhugh et al. 1950
NTP 1993
Schroeder and
Mitchener 1975
NTP 1993
a Phenylmercuric acetate and mercuric acetate
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3.2.3 Other Data
3.2.3.1 Death
The estimated lethal dose of inorganic mercury for a 70 kg adult is 10-42 mg Hg/kg (Gleason
et al. 1957). Most deaths attributed to inorganic mercury occur soon after a person ingests a single
large amount of mercury. Causes of death include cardiovascular failure, gastrointestinal damage and
acute renal failure (Troen et al. 1951).
Table 3-34
Lethality of Inorganic Mercury in Humans: Case Study
Species/
No. per
- Sex
Human/25
M, 29 F
Exposure
Duration
Once
Dose
(mg/kg-day)
21-37 (est)
(HgCl2)
Effects/Limitations/BML
Case studies of mercuric chloride poisonings in victims age
2-60 yr; 9 resulted in death (all adults).
BML not reported
Reference
Troen et al. 1951
The estimated LD50 for rats following oral exposure to mercuric chloride is 25.9 mg Hg/kg;
however, LD50 levels as high as 77.7 mg Hg/kg have been observed in rats (Kostial et al. 1978).
Male rats appear to be more sensitive to the effects of mercuric chloride* This was demonstrated in a
chronic-duration oral study with rats, in which 40/50 males and 21/49 females died at the low dose,
45/50 males and 20/50 females died at the high dose, compared to 24/50 males and 15/50 females in
the controls (Dieter et al. 1992; NTP 1993). The increase in deaths in the male rats was statistically
significant and were considered to be due to renal lesions. Mortality incidence was not significantly
increased in exposed female groups.
Table 3-35
Lethality of Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Albino
(Nsye NS
Rat/F344/5 M,
5F
Exposure
Duration
Once
(gavage)
14 d
5d/wk
Ix/d
(gavage)
Dose
(mg/kg-day)
NS
(6 levels)
(Hgcy
0, 0.93, 1.9,
3.7, 7.4, 14.8
(HgClj)
Effects/Limitations/BML
LD50 = 25.8 rag/kg for 2-week old pups; older rats had
higher LDJO values.
Limitation: Incomplete data reporting (i.e., doses not
reported, toxic effects not specified)
BML not reported
2/5 males died at 14.8; no other animals died.
BML: 45.4 ug/g in kidney of males, 43.3 ng/g in kidney
of females
Reference
Kostial et al. 1978
Dieter et al. 1992
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Table 3-35 (continued)
Lethality of Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/F344/
50 M, 50 F
Mouse/
B6C3Fl/5 M, 5
F
Exposure
Duration
2yr
5d/wk
1 x/d
(gavage)
14 d
Sd/wk
1 x/d
(gavage)
Dose
(mg/kg-day)
0, 1.9, 3.7
(Hgcy
0, 3.7, 7.4,
14.8, 29, 59
(Hgcy
Effects/Limitations/BML
40/50 males died at 1.9 mg/kg, vs. 24/50 control males;
survival of dosed females was not significantly different
from controls.
BML not reported
9/10 died (LOAEL = 59).
Limitations: small number of aqJTTvto
BML not reported
Reference
Dieter et al. 1992
NTP1993
3.2.3.2 Neurological
Limited studies are available concerning neurological toxicity following oral exposure to
inorganic mercury. These studies are summarized below.
Table 3-36
Neurotoxicity of Inorganic Mercury in Humans: Case Studies
Species/
No. per Sex
Human/2 F
Human/2 M (1
child), 1 adult
F
Exposure
Duration
6-25 yr
3 mo
Dose
(mg/kg-day)
0.73
(HgjCy
NS
(HgjCy
(HgS)
Effects/Limitations/BML
•
Dementia, irritability, decreased cerebellar neurons, low
brain weight
Limitation: Case study
BML: 3.4-4.7 ug/g in frontal cortex
Drooling, dysphagia, irregular arm movements, impaired
gait, convulsions following ingestion of patent medicines
containing mercuric sulfide and mercurous chloride
Limitation: Case studies; concomitant exposure to other
metals; limited exposure data
BML: 39-2800 ug/L in 24 hr urine
Reference
Davis et al. 1974
Kang-Yum and
Oransky 1992
There are several animal studies in which inorganic mercury-induced neurotoxicity has been
reported.
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Table 3-37
Neurotoxicity of Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/
Holtzman/
8 M exposed,
8 M control
Rat/Sprague
Dawley/12 F
exposed, 10 F
controls
Mouse/C57BL
6J/NS
Exposure
Duration
11 wk
3 mo ad lib
in feed
17 mo
ad lib in
drinking
water
Dose
(mg/kg-day)
0, 0.74
(Hgcy
0,2.2
(HgClj)
-
0.74 for 1 10 d,
then 7.4-14.8
for 400 d; 2.2
for 17 mo
(HgCl2)
Effects/Limitations/BML
Weakening of hind legs, crossing reflex of limbs, ataxia;
degenerative changes in neurons of dorsal root ganglia
and Purkinje and granule cells of cerebellum
Limitation: One dose level tested
• BML not reported
Inactivity and abnormal gait
Limitation: One dose level tested
BML not reported
No clinical signs of neurotoxicity; no effect on optic or
peripheral nerve structure
Limitation: Lack of statistical analyses due to
insufficient number of animals tested; uncertainty of
dosage due to large variation in water consumption
BML not reported
Reference
Chang and
Hartmann 1972
Goldman and
Blackburn 1979
Ganser and
Kirschner 1985
3.2.3.3 Renal
The kidney appears to be the critical target organ for the effects of acute ingestion of inorganic
mercury. Case studies of poisonings by mercuric chloride report acute renal failure, including
proteinuria, oliguria and hematuria, in people ingesting estimated doses of 3.5-37 mg Hg/kg (Afonso
and deAlvarez 1960; Pesce et al. 1977; Troen et al. 1951). These effects are attributed to tubular and
glomerular pathology.
Table 3-38
Renal Toxicity of Inorganic Mercury in Humans: Case Studies
Species/
No. per Sex
Human/25 M,
29 F
Human/1 F
(adult)
Exposure
Duration
Once
Once
(tablet)
Dose
(mg/kg-day)
3.5-37 (est.)
(HgCl2)
30
(HgCIj)
Effects/Limitations/BML
Case studies of mercuric chloride poisonings in victims
age 2-60 yr; 18 cases resulted in renal effects
(albuminuria, anuria)
BML not reported
Oliguria; proteinuria; hematuria following ingestion of
mercuric chloride
Limitation: Case study
BML not reported
Reference
Troen et al. 1951
Afonso and
deAlvarez 1960
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Table 3-38 (continued)
Renal Toxicity of Inorganic Mercury in Humans: Case Studies
Species/
No. per Sex
Human/1 M
Exposure
Duration
Once
Dose
(mg/kg-day)
21.4
(Hgcy
Effects/Limitations/BML'
Proteinuria indicating glomerular and' tubular damage
Limitation: Case study
BML: Avg 370 ug/L in blood
Reference
Pesce et al. 1977
There are numerous animal studies reporting kidney damage in rats and mice ingesting
inorganic mercury. Acute exposures result in increased kidney weight with at least 0.46 mg Hg/kg-
day and tubular necrosis at higher doses; males appear to have greater sensitivity for the histological
changes than females (Fowler 1972; NTP 1993). Similarly, longer-term studies have found
histopathologic effects affecting the tubules and glomeruli, including thickening of basement
membranes and degeneration of tubular cells (Carmignani et al. 1989; Jonker et al. 1993; NTP 1993).
A study monitoring kidney function reported ketonuria and proteinaceous casts (Jonker et al. 1993).
Table 3-39
Renal Toxicity of Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Sprague
Dawley/8 M
Rat/Sprague-
Dawley/8 M
Rat/Wistar/5 M, 5
F exposed/10 M,
10 F controls
Rat/Wistar/
5M, 5F
exposed/10 M, 10
F controls
Exposure
Duration
350 d
ad lib in
drinking
water
180 d
ad lib in
drinking
water
4 wk
ad lib in feed
4 wk
ad lib in feed
Dose
(mg/kg-day)
0,7
(Hgcy
0, 28
(Hgcy
0, 0.56, 4.4
(Hgcy
2.8,5.6, 11.1
(HgCl2)
Effects/Limitations/BML
Hydropic degeneration of tubular cells
Limitation: Only one dose tested
BML: 140 ug/g in kidney
Hydropic degeneration of tubular cells, IgM deposition
in glomeruli, decreased urinary kallikrein and
creatinine, decreased plasma renin, increased plasma
angiotensra-converting enzyme
Limitation: Only one dose tested
BML: 0.94 ug/g in blood
Ketonuria in males at 4.4 mg/k-day; increased kidney
weight in males and females (LOAEL = 0.56 in
females, 4.4 in males)
BML not reported
Ketonuria in males at all levels; increased relative
kidney weight, increased nephrosis and proteinaceous
casts in males and females (LOAEL = 2.8)
BML not reported
Reference
Carmignani et al.
1989
Carmignani et al.
1992
Jonker et al. 1993
Jonker et al. 1993
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Table 3-39 (continued)
Renal Toxicity of Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/F344/
5M, 5F
Rat/F344/
10 M, 10 F
Rat/F344/
60M.60F
Mouse/NMRI/20
(sex NS)
exposed/10
controls
Mouse/NMRI/24
(sex NS)
Mouse/B6C3F!/
5M, 5F
Mouse/B6C3Fj/
10 M, 10 F
Mouse/B6C3Fj/
60 M, 60 F
Exposure
Duration
14 d
5 d/wk
Ix/d
(gavage)
6 mo
5 d/wk
1 x/d
(gavage)
2yr
5 d/wk
1 x/d
(gavage)
Once
(gavage)
Once
(gavage)
14 d
5 d/wk
1 x/d
(gavage)
6 mo
5 d/wk
1 x/d
(gavage)
2yr
5 d/wk
Ix/d
(gavage)
Dose
(mg/kg-day)
0, 0.93, 1.9,
3.7, 7.4, 14.8
(HgCl2) -
0, 0.23, 0.46,
0.93, 1.8, 3.7
(Hgd2)
0, 1.9, 3.7
(HgCLj)
0, 5, 10, 20, 40
(HgCty
0,20
(Hgd2)
0, 3.7, 7.4,
14.8, 29, 59
(Hgcy
0, 0.93, 1.9,
3.7, 7.4, 14.8
(Hgcy
0, 3.7, 7.4
(HgCl2)
Effects/limitations/BML
Increased absolute and relative kidney weight in males
and females (LOAEL = 1.9); acute renal tubule
necrosis at S3 .7 mg/kg-day in both sexes
BML: 43-46 ug/g in kidney at J4.8 mg/kg
Increased absolute and relative kidney weight in mal^s
and females (LOAEL = 0.46); increased severity of
nephropathy in males (LOAEL = 0.93)
BML: 86.2-89.6 ug/g in kidney at 0.93 mg/kg
Increased severity of nephropathy in males (thickening
of glomerular and tubular basement membranes;
degeneration and atrophy of tubule epithelium)
(LOAEL = 1.9)
BML not reported
Decreased selenium-dependent glutathione peroxidase
activity in kidney; minor renal tubular damage
(LOAEL = 10)
BML: 260 ug/L in blood at 10 mg/kg
Necrosis of proximal tubules
BML not reported
Increased absolute and relative kidney weight (LOAEL
= 3.7); acute renal tubular necrosis at 29 mg/kg-day in
males and at 59 mg/kg-day in males and females
BML: 116-171 ug/g in kidney at 29 mg/kg-day
Increased absolute and relative kidney weight of males
(LOAEL = 3.7); Cytoplasmic vacuolation of tubule
epithelium in males (LOAEL = 3.7)
BML: 36.1-40.6 ug/g in kidney at 3.7 mg/kg-day
Increased severity of nephropathy (foci of proximal
tubules with thickened basement membrane; basophilic
cells with scant cytoplasm (LOAEL = 3.7)
BML not reported
Reference
NTP 1993
NTP 1993
NTP 1993
Nielsen et al. 1991
Nielsen et al. 1991
NTP 1993
NTP 1993
NTP 1993
Bernaudin et al. (1981) exposed male and female Brown Norway rats (number not specified)
to mercuric chloride via aerosols (4 hours/week) and intratracheal instillation for 2 months. The
aerosol exposures resulted in a retention of 0.05-0.06 mg HgCl2/kg/hour (based on radiolabeled
mercury); the parameters of the aerosol were not well characterized (e.g., no mass median aerodynamic
diameter (MMAD) or geometric standard deviation provided and the particle generation system was
not adequately described). The autoimmune response was typified by a linear pattern of IgG conjugate
fixation in kidney glomeruli and a granular pattern of fixation in kidney glomeruli and arteries, lung
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and spleen; evidence of autoimmune disease was noted at all but the lowest intratracheal exposure
level (60 |ag HgCl2/kg/week). In two of three rats exposed to aerosols and examined when sacrificed,
weak proteinuria (1, 28 and 47 mg/day) was detected. No significant proteinuria was observed in the
animals administered mercuric chloride by intratracheal instillations. •
Table 3-40
Renal Toxicity of Inorganic Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Brown
Norway/ 3-8
both sexes
Rat/Brown
Norway/5 both
sexes
Exposure
Duration
2 mo, Ix/wk
(intra-
tracheal)
2 mo,
4d/wk,
1 hr/d
(aerosol)
Dose
(mg/m3)
0,6, 11,47,
79 mg/kg-day
(HgCl2)
1
(HgCl2/m3)
(estimate of
minimum air
concentration)
Effects/Limitations/BML
Autoimmune effect in spleen at 6 mg/kg-day and in
spleen, lung and kidney at higher doses
BML not reported
Weak proteinuria; autoimmune effect in kidney, lung
and spleen
BML not reported
Reference
Bernaudin et al.
1981
Bernaudin et al.
1981
3.2.3.4 Cardiovascular
No studies were located regarding the cardiovascular toxicity of inorganic mercury in humans
following oral exposure.
Limited information was located regarding the cardiovascular toxicity of inorganic mercury
following oral exposure in animals. Signs of cardiovascular toxicity in rats include increased blood
pressure and varying changes in the contractility of the heart (Carmignani et al. 1989, 1992). These
signs manifested after oral exposure to mercuric chloride in drinking water for 180 or 350 days. No
other animal studies were located.
Table 3-41
Cardiovascular Toxicity of Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Sprague
Dawley/8 M
Exposure
Duration
350 d
ad lib in
drinking
water
Dose
(mg/kg-day)
0,7
(Hgdj)
Effects/Limitations/BML
Increased blood pressure; positive inotropic response
(p<0.05)
Limitation: Only one dose tested; small number of
animals
BML: 0.9 ug/g in heart
Reference
Carmignani et al.
1989
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Table 3-41 (continued)
Cardiovascular Toxicity of. Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Wistar/8 M
Exposure
Duration
ISOd
ad lib in
drinking
water
Dose
(mg/kg-day)
0,28
(Hgcy
Effects/Limitations/BML
Increased blood pressure (jxO.05); negative inotropic
response (not significant)
Limitation: Only one dose tested; small number of
animals
BML: 940 fig/L in blood, 4.1 ug/g in hean
Reference
Carmignam et al.
1992
3.2.3.5 Gastrointestinal
Irritation of the gastrointestinal mucosa is a common outcome of mercury toxicity following
ingestion of mercuric chloride (Murphy et al. 1979). Ingestion of inorganic mercury may also cause
vomiting, nausea, severe abdominal pain and diarrhea (Afonso and deAlvarez 1960; Murphy et al.
1979). No studies were located regarding the gastrointestinal toxicity of inorganic mercury after
ingestion in humans for intermediate or chronic durations.
Table 3-42
Gastrointestinal Toxicity of Inorganic Mercury in Humans: Case Studies
Species/
No. per Sex
Human/25 M,
29 F
Human/ 1 F
(adult)
Exposure
Duration
Once
Once
(tablets)
Dose
(mg/kg-day)
3.5-37 (est.)
(Hgcy
30
(Hgcy
Effects/Limitations/BML
Case studies of mercuric chloride poisonings in victims
age 2-60 yr; effects ranged from nausea to severe
corrosive gastritis
Limitation: exposure data limited
BML not reported
Nausea; vomiting; abdominal cramps; diarrhea after
ingestion of mercuric chloride
Limitation: Case study
BML not reported
Reference
Troen et al. 1951
Afonso and
deAlvarez 1960
Similar signs of gastrointestinal irritation appear in mice after intermediate duration oral
exposure to mercuric chloride (NTP 1993). Histopathologic analyses reveal inflammation and necrosis
of the stomach tissue. Further damage occurs to the gastrointestinal tract with continued dosing (NTP
1993). The incidence of hyperplasia of the forestomach epithelium increases in high-dose rats fed
mercuric chloride for two years (NTP 1993).
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Table 3-43
Gastrointestinal Toxicity of Inorganic Mercury in Animals: Oral- Exposure
Species/
Strain/'
No. per Sex
per Group
Rat/F344/60
M.60F
Mouse/
BeOF^S M, 5
F
Exposure
Duration
2yr
5d/wk
Ix/d
(gavage)
14 d
5d/wk
1 x/d
(gavage)
Dose
(mg/kg-day)
0, 1.9, 3.7
(Hgcy
0, 3.7, 7.4,
14.8, 29, 59
-------
3.2.3.7 Immunologicai
In addition to the inorganic mercury-induced autoimmune glomerulonephritis discussed earlier
(see discussion of renal effects in Section 3.2.1), several studies identified other immunotoxicity
endpoints in animals after oral exposure to inorganic mercury.
Table 3-45
Immunotoxicity of Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Brown-
Norway/6 both
sexes
exposed/22
controls both
sexes
Mouse/
B6C3Fj/10 M
Mouse/SJL or
DBA/5 F
Mouse/SJL/7 F
Mouse/
B6C3Fj/5 M, 5
F
Exposure
Duration
2 mo
Ix/wk
(gavage)
7 wk
ad lib in
drinking
water
2wk
ad lib in
drinking
water
10 wk
ad lib in
drinking
water
14 d
5d/wk
Ix/d
(gavage)
Dose
(mg/kg-day)
2.2
(HgCI2)
0, 0.6, 2.9, 14.3
CHgcy
0,0.7
(Hgcy
0, 0.07, 0.14,
0.28, 0.56
(HgCI2)
0, 3.7, 7.4,
14.8, 29, 59
(HgClj)
•
&
Effects/Limitations/BML
IgG deposits in glomerular capillary wall of kidney and
renal arteries, suggestive of autoimmune disease; similar
deposits also observed in lungs and spleen; no deposits
observed in controls
Limitation: Only one dose tested; small number of
animals tested
BML not reported
Suppression of lymphoproliferative response to T-cell,
concavalin A and phytohemagglutinin (LOAEL = 2.9
mg/kg-day; p<0.05)
BML: 600 ng/L in blood at 2.9 mg/kg-day
Increased lymphoproliferative response to concanavalin A
and E. coli lipopolysaccharide (p<0.02)
Limitations: only one dose tested; small number of
animals tested
BML not reported
Increased antinucleolar antibodies in IgG class (LOAEL
= 0.28, p<0.05)
Limitation: small number of animals tested
BML: 5.2 ug/g in kidney
Decreased thymus weight (LOAEL = 14.8)
Limitation: small number of animals tested
BML: 116-171 ug/g in kidney at 29 mg/kg-day
\
Reference
Bernaudin et al.
1981
Dieter et al. 1983
Hultman and
Johansson 1991
Hultman and
Enestrom 1992
NTP 1993
3.2.3.8 Developmental
No studies were located regarding the developmental toxicity of inorganic mercury in humans
after inhalation exposure.
The only information located regarding developmental toxicity in animals from inhalation
exposure to mercuric mercury comes from a study in which mice were exposed to aerosols containing
mercuric chloride during gestation (Selypes et al. 1984). Increases were observed in the incidence of
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delayed ossification and dead or resorbed fetuses; the statistical significance of these effects was not
reported. In addition, at the highest concentration, a significant increase in weight retardation was also
observed. Interpretation of this study is limited, however, because the aerosols were not well
characterized, and it is not known to what extent the droplets were respirable or were cleared from the
upper respiratory tract and swallowed.
Table 3-46
Developmental Toxicity of Inorganic Mercury in Animals: Inhalation Exposure
Species/
Strain/
No. per Sex
per Group
Mice/CFLP/
No. F NS
Exposure
Duration
4d
4hr/d
Gd9-12
Dose
(mg/m3)
«.
0, 0.17,
1.6
(Hgcy
Effects/Limitations/BML
Increased dead or resorbed fetuses; delayed ossification
(LOAEL = 0.17)
Limitations: Data were reported as number of embryos only,
not as number of affected litters; no statistical analysis;
aerosol exposure was not well characterized; maternal toxicity
was not evaluated
BML not reported
Reference
Selypes et al. 1984
-
Developmental effects have been reported in animals following oral exposure to inorganic
mercury. These efforts include an increased incidence of abnormal fetuses in hamsters (Gale 1974),
growth retardation in rats (Rizzo and Furst 1972) and decreased body weights in several rat studies.
Gale (1974) administered 0, 4, 8, 25, 35, 50, 75, or 100 mg mercuric acetate/kg (0, 2.5, 5, 16,
22, 32, 47, or 63 mg Hg/kg) to pregnant golden hamsters (10/exposed group; 3/control group) by
gavage in distilled water on the 8th day of gestation. The pregnant animals were sacrificed on
gestation day 12 or 14, and the uterine contents were examined. A statistically significant increase in
the incidence of abnormal fetuses (combined incidence of small, retarded, edematous, and/or
malformed fetuses) was observed at 16 mg Hg/kg. Statistically significant increases in the percentage
of resorbed fetuses was observed at 22 mg Hg/kg and in the percentages of small, retarded and
edematous fetuses observed at 32 mg Hg/kg. No treatment-related effects were observed on the
fetuses at 5 mg Hg/kg. Toxic effects observed in maternal animals included weight loss, diarrhea,
slight tremor, somnolence, tubular necrosis hi the kidneys and cvtoplasmic vacuolization of
hepatocytes.
Rizzo and Furst (1972) administered =7 mg Hg/kg as mercuric oxide to pregnant Long-Evans
rats (5/group) by gavage hi peanut oil on gestation day 5, 12, or 19 in a pilot study. On gestation day
20 or 21, the rats were sacrificed, and the uterine contents were examined. Rats administered mercury
on gestation day 5 had a higher percentage of fetuses with growth retardation and inhibition of eye
formation (statistical significance not reported). Similar increases in these effects were not observed
after administration on gestation day 12 or 19. No toxicity in maternal animals was reported.
McAnulty et al. (1982) administered 8, 12, 16, or 24 mg mercuric chloride/kg-day (6, 9, 12, or
18 mg Hg/kg-day) by gavage to pregnant rats (strain and number not specified) on gestation days 6-15
as reported in an abstract. The abstract did not report whether controls were used. Fetal and placenta!
weights were decreased at 9 mg Hg/kg-day and above. At 12 and 18 mg Hg/kg-day, increased
postimplantation losses were reported. These effects were attributed to maternal toxicity and decreased
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food intake. At 18 mg Hg/kg-day, increases in delayed ossification and malformations were reported.
Statistical analyses were not reported.
Pritchard et al. (1982a) administered 4, 8, or 16 mg mercuric chloride/kg-day (3, 6, pr 12 mg
Hg/kg-day) by the oral route to pregnant rats (number and strain not specified) from gestation day 15
until postpartum day 25, as reported in an abstract. The abstract did not state whether controls were
used. At 6 and 12 mg Hg/kg-day, pup weight was decreased on postpartum day 1. Subsequent weight
gain in these groups was also decreased. No other effects on development or behavior were observed
postpartum. Females at 6 and 12 mg Hg/kg-day had a decreased rate of weight gain, and gestation
time was slightly extended. Statistical analyses were not reported.
Pritchard et al. (1982b) administered 12, 16, or 24 mg mercuric chloride/kg-day (9, 12, 18 mg
Hg/kg-day) to female rats (strain and number not reported) by gavage before mating and during
gestation. The abstract did not report whether controls were used. At 12 mg Hg/kg-day and above,
females exhibited weight loss and appeared unhealthy, estrous cycles became irregular, and high
preimplantation losses were observed. No effects on ovulation, estrous cycles, implantation, and fetal
development were observed at 9 mg Hg/kg-day. Statistical analyses were not reported.
Table 3-47
Developmental Toxicity of Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/Long-
Evans/5 F
Rat/Strain
NS/no. F NS
Rat/Strain
NS/no. F NS
Rat/Strain
NS/no. F NS
Exposure
Duration
Once
Gd 5, 12, or
19
(gavage)
10 d
1 x/d
Gd6-15
(gavage)
Approx. 32 d
1 x/d
Gd 15-ppd
25
(gavage)
Before
mating and
during
gestation
(gavage)
Dose
(mg/kg-day)
0,2.0
(HgO)
6, 9, 12, 18
(Hgcy
3, 6, 12
(Hgcy
9, 12, 18
(HgCl2)
Effects/Limitations/BML
Growth retardation; inhibition of eye formation in group
treated on Gd 5, with some effect on Gd 12 group.
Limitations: No statistical analysis; small number of
litters in treated groups (and controls)
BML not reported
Decreased fetal and placenta! weights (LOAEL = 9);
malformations at 18.
Limitations: Reported as an abstract; few details
reported
BML not reported
Decreased pup weight and weight gain (LOAEL = 6);
no effect in an unspecified developmental and
behavioral testing battery.
Limitations: Reported as an abstract; few details
reported
BML not reported
High preimplantation loss (LOAEL = 12).
Limitations: Reported as an abstract; few details
reported
BML not reported
Reference
Rizzo and Furst
1972
McAnulty et al.
1982
Pritchard et al.
1982a
Pritchard et al.
1982b
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Table 3-47 (continued)
Developmental Toxicity of Inorganic Mercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Hamster/
Golden/10 F
exposed/3 F
controls
Exposure
Duration
Once
Gd 8
(gavage)
Dose
(mg/kg-day)
0, 2.5, 5, 16, 22,
32, 47, 63
[Hg
(CHjCooy
.
Effects/Limitations/BML
Increased incidence of abnormal (small, retarded,
edematous, and/or malformed— exencephaly.
encephalocele, ectrodactyly, etc.) fetuses (LOAEL = 16,
jxO.05); maternal toxicity: weight loss, diarrhea, slight
tremors, somnolence, tubular necrosis, hepatocellular
necrosis.
Limitation: Small sample size; smaller control group;
insufficient detail about number of animals sacrificed at
Gd 12 or Gd 14; single day of treatment; incomplete
examinations reported (no visceral, only partial skeletal)
BML not reported
Reference
Gale 1974
In addition to the oral and inhalation studies summarized above, several studies using other
routes of administration (i.p., s.c., i.v.) provide evidence of developmental toxicity associated with
exposure to mercury salts. These studies are summarized below.
Gale and Perm (1971) injected anesthetized pregnant golden hamsters (6-19/group)
intravenously with 0, 2, 3, or 4 mg mercuric acetate/kg (0, 1.3, 1.9, or 2.5 mg Hg/kg) on gestation day
8. Controls were injected with vehicle (demineralized water). Maternal animals were sacrificed on
gestation day 12 or 14, and the uterine contents were examined. A significantly increased incidence of
resorptions was observed at all doses. In addition, increased incidences of retarded and edematous
fetuses were observed at all doses (statistical significance not reported). Toxic effects observed in
maternal animals included weight loss, diarrhea, slight tremor, somnolence and kidney lesions;
however, the report did not specify at which doses the maternal effects were observed.
Gale (1974) compared the embryotoxicity of mercuric acetate administered by different routes
in pregnant golden hamsters (3-23/group). Subcutaneous administration of 0, 4, 8, 20, 35, or 50 mg
mercuric acetate/kg (0, 2.5, 5, 13, 22, or 32 mg Hg/kg) on gestation day 8 resulted in a significant
decrease in the percentage of normal embryos and a significant increase in the percentage of small
embryos at 2.5 mg Hg/kg. At 5 mg Hg/kg, significant increases in resorptions, abnormal, retarded,
edematous, and malformed fetuses were observed. Intraperitoneal administration of 0, 2, 4, or 8 mg
mercuric acetate (0, 1.3, 2.5, or 5 mg Hg/kg) on gestation day 8 resulted in significant increases in the
percentage of resorptions, abnormal, small, and edematous fetuses at 1.3 mg Hg/kg. Intravenous
administration of 0 or 4 mg mercuric acetate/kg (0 or 2.5 mg Hg/kg) on gestation day 8 resulted in
significant increases in resorptions, abnormal, small, retarded, edematous, and malformed fetuses at 2.5
mg Hg/kg. Comparison of the extent of the developmental toxicity demonstrated an effect of route of
administration: i.p. > i.v. > s.c. > oral.
Gale (1981) compared the embryotoxicity of mercuric acetate in 6 strains of hamsters
(LAK:LVG[SYR], CB/SsLak, LHC/Lak, LSH/SsLak, MHA/SsLak. PD4/Lak). Maternal animals of
the various strains (3-9/group) were injected subcutaneously with 0 or 15 mg/kg mercuric acetate (0 or
9.5 mg Hg/kg) on gestation day 8. Controls were injected with demineralized distilled water.
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Maternal animals were sacrificed on gestation day 12 or 15, and the uterine contents were examined.
The percentage of resorptions was significantly increased in all strains examined on days 12 and 15.
Strain-specific variations were observed in the incidences of abnormal, edematous and retarded fetuses
and in the incidences of ventral wall defects, distension of the pericardia! cavity, cleft palate,
hydrocephalus and cardiac abnormalities. Maternal toxicity was not described.
Kavlock et al. (1993) injected pregnant Sprague-Dawley rats (6-25/group) subcutaneously with
0, 1, 2, 3, or 4 mg mercuric chloride/kg (0, 0.7, 1.5, 2.2, or 3.0 mg Hg/kg) on gestation day 7, 9, 11,
or 13. On gestation day 21, rats were sacrificed, and the uterine contents were examined. No increase
in malformations was observed in fetuses from mercuric chloride-treated dams. Exposure on gestation
day 7 resulted hi a significant decrease in fetal weight and an increase in the number of supernumerary
ribs (statistical significance not reported) at 2.2 mg Hg/kg. Exposure on gestation day 9 resulted in
significantly decreased live fetuses/litter and increased resorptions at 3 mg Hg/kg. Exposure on
gestation days 11 or 13 resulted hi no significant differences in fetal parameters. Maternal toxicity
^increased mortality, decreased body weight, increased kidney weight, increased urine osmolality,
and/or increased serum urea or creatinine) were observed at 1.5 mg Hg/kg and above. No consistent
correlations were observed between maternal and fetal toxicity.
Kajiwara and Inouye (1986) injected Kud:ddY mice (10/group) intravenously with 0, 0.5, 1.0,
1.5, 2.0, and 2.5 mg Hg/kg as mercuric chloride on gestation day 0. Controls were injected with
vehicle (physiological saline). Maternal animals were sacrificed on gestation day 3.5, and the oviducts
and uterus were flushed to obtain preimplantation embryos. At 1.5 mg Hg/kg and above, the number
of abnormal embryos was significantly increased. Maternal animals at 1.5 mg Hg/kg and above
exhibited a decrease in body weight (statistical significance not determined). The study authors
suggested that fetal toxicity may have been related to maternal toxicity.
Kajiwara and Inouye (1992) injected Kud:ddY mice (5-15/group) intravenously with 0, 1, 2,
or 2.5 mg Hg/kg as mercuric chloride on gestation day 0. Controls were injected with vehicle
(physiological saline). Maternal animals were sacrificed on gestation day 5 or 12, and the oviducts
and uterine contents were examined. The animals sacrificed at gestation day 5 showed statistically
significant decreases hi the number of embryos at all doses and an increase in blastocysts without
decidua (delay of implantation) at 2 and 2.5 mg Hg/kg. The animals sacrificed at gestation day 12
showed a statistically significant decrease in the number of implants, number of living fetuses and
average fetal weight at 2 and 2.5 mg Hg/kg. Maternal toxicity was not well-described, but 7 females
at 2.5 mg Hg/kg and 2 females at 2 mg Hg/kg died. The study did not determine whether the failure
to implant was due to fetal toxicity or maternal uterine dysfunction.
A study by Bernard et al. (1992) was performed to assess whether prenatal and early postnatal
exposure to inorganic mercuric salts can produce nephrotoxic effects. They found that s.c. injection of
dams with just 1 mg/kg-day during pregnancy caused renal effects in the offspring. Of note is that
these effects did not appear to be significantly different hi the group of dams dosed throughout the
gestation period compared to dams dosed only during the last 8 days of gestation.
3.2.3.9 Reproductive
A single case study was located concerning reproductive toxicity in humans exposed to
inorganic mercury; however, it is not clear whether the effects were compound-related. No
information was identified regarding the reproductive toxicity of inorganic mercury following
inhalation exposure.
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Table 3-48
Reproductive Toxicity of Inorganic Mercury in Humans: Case Study
Species/
No. per Sex
Human/1 F
Exposure
Duration
Once
(tablet)
' Dose
(mg/kg-day)
30
(Hgcy
Effects/Limitations/BML
Spontaneous abortion 13 days after ingeslion of mercuric
chloride
Limitation: Case study; abortion may have been
unrelated to mercury exposure
BML not reported
Reference
Afonso and
deAlvarez 1960
In animals orally exposed to inorganic mercury compounds, changes in the estrous cycle and
ovulation and/or increased resorptions were reported (Rritchard et al. 1982b).
In male mice administered a single i.p. dose of 0.74 mg Hg/kg as mercuric chloride, fertility
decreased between days 28 and 49 post-treatment with no obvious histological effects noted in the
sperm (Lee and Dixon 1975). The period of decreased fertility indicated that spermatogonia and
premeiotic spermatocytes were affected. The effects were less severe than those noted after treatment
with a similar dose of methylmercury. A single i.p. dose of 1.5 mg Hg/kg as mercuric chloride
administered 1-5 days prior to mating in female mice resulted in a significant decrease in the total
number of implants, number of living embryos and a significant increase in the percentage of dead
implants (Suter 1975). These effects suggest that mercury may be a weak inducer of dominant lethal
mutations. In female golden hamsters administered 6.4 or 12.8 mg Hg/kg subcutaneously, there was
no observed increase hi chromosomal aberrations in metaphase n oocytes (Watanabe et al. 1982). At
the first estrous cycle post treatment, there was a significant increase in the number of degenerated
oocytes in animals at the high-dose group. At the second estrous cycle both treatment groups had
increased numbers of degenerated oocytes, suggesting an effect of mercuric chloride on ovulation.
Table 3-49
Reproductive Toxicity of Inorganic Mercury in Animals; Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/strain
NS/NS no. of F
Exposure
Duration
NS "before
mating and
during
gestation"
(gavage)
Dose
(mg/kg-day)
9, 12, 18
(HgCl2)
Effects/Limitations/BML
Irregular estrous cycles and high preimplantation loss
(LOAEL = 12); decreased ovulation (LOAEL = 18)
Limitations: Limited details; reported as an abstract; no
statistical analysis reported
BML not reported
Reference
Pritchard et al.
1982b
3.2.3.10 Genotoxicity
Two occupational studies (Anwar and Gabal 1991; Popescu et al. 1979) reported on workers
inhaling inorganic mercury; the data were inconclusive regarding the clastogenic activity of inorganic
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mercury. Workers involved in the manufacture of mercury fulminate (Hg[OCN]2) had a significant
increase in the incidence of chromosomal aberrations and micronuclei in peripheral lymphocytes when
compared to unexposed controls (Anwar and Gabal 1991). There was no correlation between urinary
mercury levels or duration of exposure to the increased frequency of effects; the study authors
concluded that mercury may not have been the clastogen in the manufacturing process. In a study by
Popescu et al. (1979), 18 workers exposed to a mixture of mercuric chloride, methylmercuric chloride
and ethylmercuric chloride had significant increases in the frequency of acentric fragments
(chromosome breaks). The findings, however, are suspect because the control group was not matched
for sex, smoking habits or sample size.
Table 3-50
Genotoxicity of Inorganic Mercury in Humans
Species/
No. per Sex
Human/29 M
exposed/ 29 M
control
Human/18 M
exposed/
10 control
Exposure
Duration
20.8 yr (avg)
(occup)
1.0.5 yr
(occup)
Dose
(mg/m3)
NS
Hg(OCN)2
0.15-0.44
(HgCy
Effects/Limitations/BML
Increased incidence of chromosomal aberrations (p<0.001)
and micronuclei (jxO.Ol) in lymphocytes of workers exposed
to mercury fulminate compared with age-matched controls;
no correlation between frequency of chromosome and
exposure duration or urinary mercury level.
BML: 123.2 ug/L in urine (avg)
Increased frequency of chromosomal breaks.
Limitations: Workers also exposed to methylmercuric
chloride and ethylmercuric chloride, and one worker had
history of benzene poisoning; control group was not
matched for sex, smoking habits, or sample size.
BML: -890 (ig/L in urine (avg)
Reference
Anwar and Gabal
1991
Popescu et al. 1979
Exposure to inorganic mercury may produce an increase in chromosomal aberrations in mice
following oral and inhalation exposure (Ghosh et al. 1991; Selypes et al. 1984). Mercuric chloride
administered to mice by gavage induced a dose-related increase in chromosome aberrations and
aberrant cells in the bone marrow (Ghosh et al. 1991); however, mice given i.p. doses of mercuric
chloride have shown no increase in chromosomal aberrations in bone marrow cells (Poma et al. 1981)
and no increase in aneuploidy in spermatogonia (Jagiello and Lin 1973). Similarly, an increased
incidence of chromosomal aberrations (primarily deletion and numeric aberrations) was observed in
livers of fetal mice exposed to mercury in utero as the result of maternal inhalation of aerosols of
mercuric chloride (Selypes et al. 1984). Female golden hamsters injected s.c. with mercuric chloride
were observed to have increased incidence of chromosome aberrations in bone marrow cells but not in
metaphase H oocytes (Watanabe et al. 1982). Mercuric chloride concentrations in the ovaries were
low but had an inhibiting effect on ovulatioa Verschaeve et al. (1984) reported that in vitro exposure
of human lymphocytes and muntjac fibroblasts to mercuric chloride resulted in segregation
abnormalities; namely, c-mitotic figures. The effects of mercuric chloride on genetic material has been
suggested to be due to the ability of mercury to inhibit of the formation of the mitotic spindle, which
can result in c-mitotic figures. Mercuric chloride has also been shown to inhibit nucleolus organizing
activity in human lymphocytes (Vershaeve et al. 1983).
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Positive dominant lethal results have been obtained in studies in which rats were administered
mercuric chloride orally (Zasukhina et al. 1983). Suter (1975) observed a small, but significant
increase in the number of non-viable implants when female mice were administered mercuric chloride
by intraperitoneal injection; this effect was not observed when males were treated. It was not clear
whether the increase in non-viable implants was due to maternal toxicity or to a true dominant lethal
effect of the treatment.
Sex-linked recessive lethal mutations were not observed as a consequence of exposure of male
Drosophila melanogaster by either feeding or injection (NTP 1993).
As summarized in NTP (1993) and U.S. EPA (1985), mercuric chloride has produced some
positive results for clastogenicity in a variety of in vitro and in vivo genotoxicity assays, but mixed
results regarding its mutagenic activity have been reported. Mercuric chloride was negative in gene
mutation tests with Salmonella typhimurium strains. TA1535, TA1537, TA98 and TA102 with or
without hepatic microsomal preparations (S9) (Arlauskas et al. 1985; Marzin and Phi 1985; Wong
1988). Mercuric chloride has shown evidence of DNA damage in the Bacillus subtilis rec assay
(Kanematsu et al. 1980) but did not induce lytic phage in a lysogenic E. coli strain (Rossman et al.
1984).
DNA damage (single strand breaks) has also been observed in assays using rat and mouse
embryo fibroblasts (Zasukhina et al. 1983) and Chinese hamster ovary (CHO) cells and human KB
cells (Cantoni and Costa 1983; Cantoni et al. 1982, 1984a,b; Christie et al. 1984, 1986; Robison et al.
1982, 1984; Williams et al. 1987). Mercuric chloride also produced chromosome aberrations and
sister chromatid exchange (SCE) in CHO cells (Howard et al. 1991) and SCE in human leucocytes
(Morimoto et al. 1982). Negative results for chromosomal aberrations were reported for FM3A cells
(from a mouse mammary carcinoma) (Umeda and Nishimura 1979) and for two human diploid lines,
WI38 and MRC5 (Paton and Allison 1972). Negative results for SCE were reported for don cells
(Ohno et al. 1982) and for P388D, mouse cells and CHO cells (Anderson 1983). Evidence of gene
mutations (considered weakly positive) was observed in L5178Y mouse lymphoma cells in the
presence of microsomal preparations (Oberly et al. 1982).
NTP (1993) reached the following conclusions from their in vitro testing of mercuric chloride:
not mutagenic for Salmonella typhimurium in preincubation protocols with and without rat and hamster
liver preparations; positive for L5178Y cells without addition of hepatic preparations; negative for
SCE in CHO cells without addition of S9 but weakly positive when rat S9 was added; positive for
chromosomal aberrations in CHO cells in the absence but not the presence of liver preparations; it was
not clear what role was played by cytotoxicity in the generation of these chromosomal aberrations.
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Table 3-51
Genotoxicity of Inorganic Mercury in Animals
Species/
Strain/
No. per Sex
per Group
Mice/CFLP
NS/No. F NS
Exposure
Duration
4d
4hr/d
Gd 9-12
Dose
(mg/m3)
0.17, 1.6
(Hgcy
Effects/Limitations/BML
Increased incidence of chromosomal aberrations in fetal
hepatocytes
Limitations: The number of mothers corresponding to the 10
fetuses examined was not reported; no statistical analysis
BML not reported
Reference
Selypes et al. 1984
3.3 - Methylmercury
Organic mercury compounds have been used as fungicides and as pharmaceutical agents
(diuretics). Organic mercurials including Metaphen, Merthiolate and Mercurochrome still find use as
topical antiseptics. Phenylmercury salts are used in pharmaceutical, ophthalmic and cosmetic
preparations to control growth of microbial organisms (Joklik et al. 1984). Other organic mercury
compounds include methylmercuric chloride (MMC), methylmercuric hydroxide (MMH) and
phenylmercuric acetate (PMA). Nearly all of the available toxicity studies for organic mercury
compounds, however, are for methylmercury. Unless otherwise noted, all studies summarized in tables
in this section are for methylmercury. All oral doses were converted to mg Hg/kg-day, and all
inhalation doses were converted mg Hg/m3 using the method shown hi Appendix A.
3.3.1 Critical Noncancer Data
This section provides descriptions of studies considered by U.S. EPA in evaluation of systemic
health endpoints, largely neurotoxicity in exposed adults and in children exposed in utero. Chapter 6
describes the derivation of an RfD for methylmercury based on developmental neurologic
abnormalities in human infants. For completeness some of these studies are also presented in
subsequent sections in tabular form.
3.3.1.1 Human Data
Several studies of methylmercury poisonings in humans have been reported (see discussion on
Neurologic Effects). CNS effects were observed in several studies summarized by Clarkson et al.
(1976), Nordberg and Strangert (1976), and WHO (1976). CNS effects including ataxia and
paresthesia have been observed hi subjects with blood mercury concentrations as low as 200 ug/L.
In 1971, a number of people in Iraq were exposed to methylmercury-treated seed grain that
was used in home-baked bread. Studies conducted on this population include that of Bakir et al.
(1973), Marsh et al. (1987) and others. Toxicity was observed in many adults and children who had
consumed this bread over a three-month period, but the population that showed greatest sensitivity
were offspring of pregnant women who ate contaminated bread during gestation. The predominant
symptom noted in adults was paresthesia, and it usually occurred after a latent period of from 16 to 38
days. In adults symptoms were dose-dependent, and among the more severely affected individuals
ataxia, blurred vision, slurred speech and hearing difficulties were observed. Signs noted in the infants
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lA
exposed during fetal development included cerebral palsy, altered muscle tone and deep tendon
reflexes, as well as delayed developmental milestones (e.g., walking before 18 months and talking
before 24 months). Some information indicated that male offspring were more sensitive than females.
The mothers experienced paresthesia and other sensory disturbances but at higher doses than those
associated with their children exposed in utero. Unique analytic features of mercury (analysis of
segments of hair correlated to specific time periods in the past) permitted approximation of maternal
blood levels that fetuses were exposed to in utero. The data collected by Marsh et al. (1987)
summarized clinical neurologic signs of 81 mother and child pairs. From x-ray fluorescent
spectrometric analysis of selected regions of maternal scalp hair, concentrations ranging from 1 to 674
ppm were determined, then correlated with clinical signs observed in the affected members of the 81
mother-child pairs. Among the exposed population there were affected and unaffected individuals
throughout the dose-exposure range. (See also Bakir et al. (1973) in sections on Death and
Neurological Effects).
McKeown-Eyssen et al. (1983) provided a report of neurologic abnormalities in four
communities of Cree Indians in northern Quebec. A group of 247 children first exhibited clinical
signs consistent with methylmercury exposure between 12 and 30 months of age. An attempt was
made to account for possible confounding factors; the interviewers determined alcohol and tobacco
consumption patterns among the mothers of affected childrea Age of the mothers and multiparity was
also taken into account in analysis of the data. The average indices of exposure were the same for
boys and girls (6 ug/g); only 6% of the population had exposure above 20 ug/g. The prevalence of
multiple abnormal neurologic findings was about 4% for children of both sexes. The most frequently
observed abnormality was delayed deep tendon reflexes. This was seen in 11.4% of the boys and
12.2% of the girls. Abnormality of muscle tone or reflexes showed a significant positive association
with maternal mercury exposure for boys, but not for girls. A consistent dose-response relationship
for this effect was not observed; however, the greatest prevalence of the effect in boys occurred for
aothers hi the highest exposure group (13.0-23.9 ppm mercury in hair). No other measure
abnormal w decreased neurologic function or development showed a significant positive association
i exposure. The discriminant analysis conducted for the boys to distinguish the
15 cases with abnormal muscle tone or reflexes from the 82 normal controls was unable to separate
differences between these groups based on confounding variables. The prevalence of abnormality of
muscle tone or reflexes was found to increase 7 times with each increase of 10 ug/g of the prenatal
exposure index. There was possible influence of alcohol consumption and smoking among mothers on
the effects observed'in their children.
Studies performed in New Zealand investigated the development of children who had prenatal
exposure to methylmercury (Kjellstrom et al. 1986a, 1989). A group of 11,000 mothers who regularly
ate fish was initially screened by survey; of these, about 1000 had consumed 3 fish meals per week
during pregnancy. Working from these two large groups, 31 matched pairs were established. For
proper comparison, a reference child matched for mother's ethnic group, age, child's birthplace and
birth date was located for each child in the high fish consumption group. Retrospective mercury
concentrations were determined from scalp hair of the mothers to match the period of gestation. The
average hair concentration for high exposure mothers was 8.8 ug/g and for the reference group was 1.9
ug/g. The children were tested at 4 and 6 years of age. At 4 years of age, the children were tested
using the Denver Developmental Screen Test (DDST). This is a standardized test of a child's mental
development and can be administered in the child's home. It consists of four major function sectors:
gross motor, fine motor, language and personal-social. A developmental delay in an individual item is
scored when the child has failed in his/her response and at least 90% of children can pass this item at
a younger age. The whole test is scored as abnormal, questionable, or normal. Standardized vision
tests and sensory tests were also performed to measure development of these components of the
June 1996
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nervous system. The prevalence for developmental delay in children was 52% for progeny of high
mercury mothers and 17% for progeny of mothers of the reference group. The hair mercury
concentrations of the mothers in this study were lower than those associated with CNS effects in
children exposed in Japan and Iraq. The results of the DDST demonstrated 2 abnormal scores and 14
questionable scores in the high mercury-exposed group and 1 abnormal and 4 questionable scores in
the control group. When the 8 pairs where ethnic group matching was not successful and twins were
excluded, the results remained statistically significant. Analysis of the DDST results by sector showed
that developmental delays were most commonly noted in the fine motor and language sectors, but the
differences between the experimental and control groups were not significant. There are questions as
to the ability of the DDST to identify putative effects of prenatal exposure to methylmercury. DDST
results are highly dependent upon the age of the child. The differences noted in performance of the
DDST between high mercury-exposed and referent children could be due to confounding variables.
Infants of the mercury-exposed group more frequently had low birth weights and were more likely to
be bom prematurely. The study is also limited by the fact that there was only a 44% participation
rate.
A second stage follow-up of the original Kjellstrom study was carried out when the children
were 6 years old (Kjellstrom et al. 1986b). In this later study the high exposure children were "
compared with 3 control groups with lower prenatal mercury exposure. During pregnancy, mothers in
two of these control groups had high fish consumption and average hair mercury concentrations of
3-6 ug/g and 0-3 ug/g, respectively. The high exposure group was matched with controls for
maternal ethnic group, age, smoking habits, residence, and sex of the child. For the second study, 61
of 74 high exposure children were available for study. Each child was tested with an array of
scholastic, psychological and behavioral tests which included Test of Language Development (TOLD),
the Wechsler Intelligence Scale for Children and McCarthy Scale of Children's Abilities. The results
of the tests were compared between groups. Confounding was controlled for by a modelling
procedure using linear multiple regression analysis. A principal finding was that normal results of the
psychological test variables were influenced by ethnic background and social class. High prenatal
methylmercury exposure did decrease performance in the tests, but it contributed only a small part of
the variation in test results. It was found that an average hair mercury level of 13-15 ug/g during
pregnancy was consistently associated with decreased test performance. Size of the experimental
groups limited the power of the study to determine if lower exposure levels might have had a
significant effect on test results. The Kjellstrom studies are limited for assessing methylmercury
toxicity because the intelligence tests used may not be the most appropriate for defining the effects of
methylmercury. Also, greater significance was seen in differences of cultural origins of the children
than the differences in maternal hair methylmercury concentrations.
The original epidemiologic report of methylmercury poisoning involved 628 human cases that
occurred in Minamata, Japan, between 1953 and 1960. The overall prevalence rate for the Minamata
region for neurologic and mental disorders was 59%. Among this group 78 deaths occurred, and hair
concentrations of mercury ranged from 50-700 ug/g. These hair mercury concentrations were
determined through the use of less precise analytic methods than were available for later studies. The
most common clinical signs observed in adults were paresthesia, ataxia, sensory disturbances, tremors,
impairment of hearing and difficulty in walking. Examination of the brains of severely affected
patients that died revealed marked atrophy of the brain (55% normal volume and weight) with cystic
cavities and spongy foci. Microscopically, entire regions were devoid of neurons, granular cells in the
cerebellum, golgi cells and Purkinje cells. Extensive investigations of congenital Minamata disease
were undertaken, and 20 cases that occurred over a 4-year period were documented. In all instances
the congenital cases showed a higher incidence of symptoms than did the cases wherein exposure
occurred as an adult Severe disturbances of nervous function were described, and the affected
»
June 1996 3-61 SAB REVIEW DRAFT
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and 26 months were done on groups of 738 and 736 individuals, respectively. Median maternal hair
mercury was 5.9 ppm and the range was 0.5 to 26.7 ppm. The overall conclusion of the studies
published to date is that it is yet unclear whether an association exists between low level mercury
exposure and neurologic deficits in children. The study does show a close correlation between
maternal hair mercury and neonatal levels of mercury in brain tissue (Myers et al. 1995d). The
authors cautioned in several papers that subtle neurologic and neurobehavioral effects are more likely
to be detected in older rather than younger children.
A large study was undertaken in the Faroe Islands in 1986 on neurologic developmental effects
of methylmercury and PCB exposure in utero (Grandjean et al. 1994a). Results have been published
in abstract. Subjects were a group of nearly 1000 children born between 1986 and 1987 and evaluated
at about 7 years of age. In addition to the DDST and other developmental tests that were performed at
a younger age, the test battery included the following: motor tests~the Neurobehavioral Evaluation
System (NES) finger tapping test and the NES Hand-Eye Coordination test; attention-NES Continuous
Performance Test and Wechsler Intelligence Scale for Children; verbal reasoning-WISC-r similarities;
language-Boston Naming Test; visuospatial~WISC-r Block Designs and Bender gestalt test; memory--
Tactual Performance Test and California Verbal Learning Test. The investigators placed the children
into 4 study groups based on maternal hair concentrations of 2.0 ppm, 3.9 ppm, 4.5 ppm and 8.1 ppm.
The authors indicate that Year 1 data suggest that some neurobehavioral dysfunction is related to
maternal seafood intake during pregnancy, particularly on WISC-R digital spans forward and the
Boston Naming Test Although the medians for the test scores are similar or identical across the study
groups, the upper exposure groups had more instances of scores in the lowest quartile. In addition to
these results, positive findings were seen on the NES continuous performance test. Potential
confounders are being investigated including the possibility of PCB exposure.
3.3.1.2 Animal Data
Rice (1989b) dosed five cynomolgus monkeys (Macaco, fascicularis) from birth to 7 years of
age with 0.05 mg Hg/kg-day as methylmercuric chloride and performed clinical and neurologic
examinations during the dosing period and for an additional 6 years. As a sensitive indicator of the
latent effects of methylmercury, neurologic examinations performed at the end of the observation
period revealed insensitivity to touch and loss of tactile response. In the later stages of the observation
period monkeys dosed with methylmercury were clumsier and slower to react when placed in the
. exercise cage than were unexposed monkeys.
Gunderson et al. (1986) administered daily doses of 0.04-0.06 mg Hg/kg as methylmercuric
hydroxide to 11 crab-eating macaques (Macaco fascicularis) throughout pregnancy, resulting in
maternal blood levels of 1080-1330 ug/L in mothers and 1410-1840 ug/L in the offspring. When
tested 35 days after birth, the infants exhibited visual recognition deficits.
Groups of 7 or 8 female crab-eating macaques (Macaco fascicularis) were dosed with 0.05 or
0.09 mg/kg-day of methylmercury through 4 menstrual cycles (Burbacher et al. 1984). They were
mated with untreated males, and clinical observations were made for an additional 4 months. Two of
7 high-dose females aborted, and 3 did not conceive during the 4-month mating period. The other two
females delivered live infants. Two of 7 females exposed to 0.05 mg/kg-day aborted; the remaining 5
females delivered live infants. All control females conceived, and 6 delivered live infants. These
reproductive results approached but did not reach statistical significance. Reproductive failure within
dose groups could be predicted by blood mercury levels. The dams did not show clinical signs of
methylmercury poisoning during the breeding period or gestation, but when females were dosed with
0.09 mg/kg-day for a year, 4 of 7 did show adverse neurologic signs.
June 1996 3-63 SAB REVIEW DRAFT
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Bornhausen et al. (1980) has reported a decrease in operant behavior performance in 4-month-
old rats whose dams had received methylmercuric chloride on gestation days 6-9. A statistically
significant effect was seen in offspring whose dams had received 0.01 and 0.05 mg/kg during
gestation. The authors postulated that more severe effects of in utero exposure would be seen in
humans since the biological half-time of mercury in the brain of humans is 5 times longer than the rat.
In addition, much longer in utero exposure to mercury would occur in humans since gestation is much
longer.
Groups of Wistar rats (50/sex/group) were administered daily doses of 0.002, 0.01, 0.05, and
0.25 mg Hg/kg-day as methylmercuric chloride for 26 months (Munro et al. 1980). Female rats that
received 0.25 mg/kg-day had reduced body weight gains and showed only minimal clinical signs of
neurotoxicity; however, male rats that received this dose did show overt clinical signs of neurotoxicity,
had decreased hemoglobin and hematocrit values, had reduced weight gains and showed increased
mortality. Histopathologic examination of rats of both sexes receiving 0.25 mg/kg-day revealed
demyelination of dorsal nerve roots and peripheral nerves. Males showed severe kidney damage, and
females had minimal renal damage. This study showed a NOAEL of 0.05 mg/kg-day and a LOAEL
of 0.25 mg/kg-day.
A 2-year feeding study of methylmercuric chloride was conducted in B6C3F1 mice
(60 mice/sex/group) at doses of 0, 0.4, 2, and 10 ppm (0, 0.03, 0.15, and 0.73 mg Hg/kg-day in males;
0, 0.02, 0.11, and 0.6 mg Hg/kg-day hi females) to determine chronic toxicity and possible
carcinogenic effects (Mitsumori et al. 1990). Mice were examined clinically during the study, and
neurotoxic signs characterized by posterior paralysis were observed in 33 males after 59 weeks and 3
females after 80 weeks in the 0.6-mg Hg/kg-day group. A marked increase in mortality and a
significant decrease in body weight gain were also observed in the high-dose males, beginning at 60
weeks. Post-mortem examination revealed toxic encephalopathy consisting of neuronal necrosis of the
brain and toxic peripheral sensory neuropathy in both sexes of the high-dose group. An increased
incidence of chronic nephropathy was observed in the 0.11- and 0.6-mg Hg/kg-day males. These
results indicated that B6C3F1 mice are more sensitive to the neurotoxic effects of methylmercury than
ICR mice.
Ultrastructural renal changes were also observed in rhesus monkeys treated with
0.08-0.12 mg/kg of methylmercury although clinical changes were not observed (Chen et al. 1983).
3.3.2 Cancer Data
3.3.2.1 Human Data
Three studies were identified that examined the relationship between methylmercury exposure
and cancer. No persuasive evidence of increased carcinogenicity attributable to methylmercury
exposure was observed in any of the studies. Interpretation of these studies, however, was limited by
poor study design and incomplete descriptions of methodology and/or results.
Tamashiro et al. (1984) evaluated the causes of death in 334 subjects from the Kumamoto
Prefecture who had been diagnosed with Minamata disease and died between 1970 and 1980.
Minamata disease was used as a surrogate for methylmercury exposure. The cases were fishermen and
their families who had been diagnosed with methylmercury poisoning (Minamata disease); thus,
Minamata disease was used as a surrogate for methylmercury exposure. The controls were selected
from all deaths that had occurred in the same city or town as had the cases and were matched on the
basis of sex, age at death (within 3 years) and year of death; two controls were matched to each case.
June 1996 3-64 SAB REVIEW DRAFT
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Malignant neoplasms were designated as the underlying cause of death in 14.7% (49/334) of the cases
and 20.1% (134/668) of the controls. For 47 cases in which Minamata disease was listed as the
underlying cause of death, the investigators reanalyzed the mortality data and selected one of the
secondary causes to be the underlying cause of death in order to allow examination of the cases and
controls under similar conditions and parameters. The three cases for which Minamata disease was
listed as the only cause of death were excluded from further analysis. Using the Mantel-Haenzel
method to estimate odds ratios, no significant differences were observed between the cases and
controls with respect to the proportion of deaths due to malignant neoplasms among males, females, or
both sexes combined. The estimated odds ratios and 95% confidence intervals were 0.84 (0.49-1.43),
0.58 (0.28-1.21), and 0.75 (0.50-1.11) for males, females, and both sexes combined Similarly, no
increases were observed among the cases relative to the controls when malignant neoplasms were
identified as a secondary cause of death or were listed on death certificates as one of multiple causes .
of death. These data suggest that cancer incidence is not increased in persons with overt signs of
methylmercury poisoning when compared to persons for whom no diagnosis of methylmercury
poisoning had been made. Interpretation is limited, however, by potential bias in designating the cause
of death among patients with known Minamata disease and by the uncertainty regarding the extent of
methylmercury exposure and undiagnosed Minamata disease among the controls.
In a subsequent study, Tamashiro et al. (1986) compared the mortality patterns (between 1970
and 1981) among residents of Fukuro and Tsukinoura-districts (inhabited mainly by fishermen and
their families) in the Kumamotd Prefecture with age-matched residents of Minamata city (also in the
Kumamoto Prefecture) who died between 1972 and 1978. In this study, high exposure to
methylmercury was inferred from residence in a district believed to have higher intake of local
seafood. By contrast, in the 1984 study described above, high methylmercury exposure was inferred
from a diagnosis of Minamata disease. A total of 416 deaths were recorded in the Fukuro and
Tsukinoura districts in 1970-1981, and 2,325 deaths were recorded in Minamata City in 1972-1978.
No statistically significant increase in the overall cancer mortality rate was observed; however, an
increase in the mortality rate due to liver cancer was observed (SMR, 207.3; 95% C.I. 116.0-341.9).
Analysis of mortality by sex showed a statistically significant increase in the rate of liver cancer only
among males (SMR, 250.5; 95% C.I. 133.4-428.4). Males also had statistically significantly higher
mortality due to chronic liver disease and cirrhosis. The authors note that these results should be
interpreted with caution because the population of Fukuro and Tsukinoura districts had higher alcohol
consumption and a higher prevalence of hepatitis B (a predisposing factor for hepatocellular cancer).
Interpretation of these results is also limited by an incomplete description of the methodology used to
calculate the SMRs; it is unclear whether the study authors used appropriate methods to compare
mortality data collected over disparate time frames (i.e. 12 years for exposed and seven years for
controls).
In a study from Poland, Janicki et al. (1987) reported a statistically significant (p<0.02)
increase in the mercury content of hair in leukemia patients (0.92 ±1.44 ppm; n=47) relative to that in
healthy unrelated patients (0.49 ± 0.41 ug/g; n=79). Similarly, the mercury content in the hair of a
subgroup of leukemia patients (0.69 ±0.75 ug/g; n=19) was significantly (p<0.05) greater than that in
healthy relatives who had shared the same residence for at least 3 years preceding the onset of the
disease (0.43 ± 0.24 ug/g; n=52). When patients with specific types of leukemia were compared with
the healthy unrelated subjects (0.49 ± 0.41 ug/g; n=79), only those with acute leukemia (type not
specified; 1.24 ± 1.93 ng/g; n=23) had a significantly increased hair mercury content. No significant
differences in hair mercury content were observed in 9 patients with chronic granulocytic leukemia or
15 patients with chronic lymphocytic leukemia when compared to the unrelated, healthy controls. This
study is of limited use for cancer risk assessment because of the following: small size of population
studied; inadequate description of the leukemia patients or healthy controls (e.g., age distribution,
June 1996 3-65 SAB REVIEW DRAFT
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length of residence in the region, criteria for inclusion in the study); uncertainty regarding the source
of mercury exposure (the authors presumed that exposure was the result of use of methylmercury-
containing fungicides); uncertainty regarding the correlation between the chronology of incorporation
of mercury in the hair and onset of the disease; and the failure to address exposure to other chemicals
or adjust for other leukemia risk factors. Furthermore, the variability of hair mercury content was
large, and the mean hair mercury levels were within normal limits for all groups. One cannot rule out
the likelihood that the observed correlation of leukemia incidence with mercury in hair is" due to
chance alone.
The carcinogenic effects of organomercury seed dressing exposure were investigated in a series
of case-control studies for incidence of soft-tissue sarcomas (Eriksson et al. 1981; Hardell and
Eriksson, 1988; Eriksson et al. 1990) or malignant lymphomas (Hardell et al. 1981). These studies
were conducted in Swedish populations exposed to phenoxyacetic acid herbicides or chlorophenols
(the exposures of primary interest in the studies), organomercury seed dressings, or other pesticides.
Exposure frequencies were derived from questionnaires and/or interviews. Control groups from the
same region of the country were matched to cases based on vital status. There were 402 total cases of
soft-tissue sarcoma, and (among persons not exposed to phenoxyacetic acid herbicides) there were 128
cases of malignant lymphoma. In each study, the odds ratio for exposure to organomercury in seed
dressings and sarcoma or lymphoma was either less than 1.0, or the range of the 95% confidence
interval for the odds ratio included 1.0; -therefore, no association was indicated for organomercury
exposure and soff-tissue sarcoma or malignant lymphoma. The conclusions from these studies are
limited, however, due to the study subjects' likely exposures to the other pesticides and chemicals. .
Table 3-52
Carcinogenic Effects of Methylmercury in Humans: Epidemiological Studies
Species/
No. per Sex
Human/334
exposed (M+F),
668 control
Human/412
exposed (M+F)
Human/47 w/
leukemia (sex
not specified)
control 79
Exposure
Duration
NS
NS
NS
Dose
(mg/kg-day)
NS
NS
NS
Effects/Liinitations/BML
No increase in cancer mortality among Minamata
exposure victims (i.e., with overt methylmercury
poisoning). Minamata disease was used as a surrogate for
methylmercury exposure.
Limitations: Exposure levels or number of undiagnosed
cases among controls not known.
Increased incidence of liver cancer in males living in the
vicinity of Minamata Bay.
Limitations: "Exposed" districts had higher alcohol
consumption and higher prevalence of hepatitis B.
Increased mercury in hair of leukemia patients; however,
mean hair mercury levels in the leukemia patients was
within the normal range.
Limitations: Small study population; source of
methylmercury exposure not clear; failure to address other
leukemia risk factors or exposure to other chemicals.
Reference
Tamashiro et al.
1984
Tamashiro et al.
1986
Janicki et al. 1987
3.3.2.2 Animal Data
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The results from three dietary studies in two strains of mice indicate that methylmercury is
carcinogenic. A fourth dietary study in mice, three dietary studies in rats and a dietary study in cats
failed to show carcinogenicity of methylmercury. Interpretation of two of the positive studies was
complicated by observation of tumors only at doses that exceeded the MTD. Interpretation of four
non-positive studies was limited because of deficiencies in study design or failure to achieve an MTD.
Methylmercuric chloride was administered in the diet at levels of 0, 0.4, 2, or 10 ppm (0, 0.03,
0.14 and 0.69 mg Hg/kg-day in males and 0, 0.03, 0.13, and 0.60 mg Hg/kg-day hi females) to
B6C3F1 mice (60/sex/group) for 104 weeks (Mitsumori et al. 1990). In high-dose males, a marked
increase in mortality was observed after 60 weeks (data were presented graphically; statistical analyses
not performed), Survival at study termination was approximately 50%, 60%, 60%, and 20% in
control, low-, mid-, and high-dose males, respectively, and 58%, 68%, 60%, and 60% in control, low-,
mid-, and high-dose females, respectively. The cause of the high mortality was not reported. At study
termination, the mean body weight hi high-dose males was approximately 67% of controls and in
high-dose females was approximately 90% of controls (data presented graphically; statistical analyses
not performed). Focal hyperplasia of the renal tubules was significantly (p<0.01) increased hi high-
dose males (14/60; the incidence was 0/60 hi all other groups). The incidence of renal epithelial
carcinomas (classified as solid or cystic papillary type) was significantly (p<0.01) increased in high-
dose males (13/60; the incidence was 0/60 hi all other groups). The incidence of renal adenomas
(classified as solid or tubular type) was also significantly (p<0.05) increased hi high-dose males; the
incidence was 0/60, 0/60, 1/60, and 5/60 hi control, low-, mid-, and high-dose males, respectively, and
0/60, 0/60, 0/60, and 1/60 hi control, low-, mid-, and high-dose females, respectively. No metastases
were seen hi the animals. The incidences of a variety of nonneoplastic lesions were increased hi the
high-dose rats including these: sensory neuropathy, neuronal necrosis hi the cerebrum, neuronal
degeneration hi the cerebellum, and chronic nephropathy of the kidney. Males exhibited tubular
atrophy of the testis (1/60, 5/60, 2/60, and 54/60 hi control, low-, mid-, and high-dose, respectively)
and ulceration of the glandular stomach (1/60, 1/60, 0/60, and 7/60 hi control, low-, mid-, and high-
dose males, respectively). An MTD was achieved hi mid-dose males and high-dose females. High
mortality hi high-dose males indicated that the MTD was exceeded hi this group.
Mitsumori et al. (1981) administered 0, 15, or 30 ppm of methylmercuric chloride (99.3%
pifre) hi the diet (0, 1.6 and 3.1 mg Hg/kg-day) to ICR mice (60/sex/group) for 78 weeks. Interim
sacrifices of up to 6/sex/group were conducted at weeks 26 and 52. Kidneys were microscopically
examined from all animals that died or became moribund after week 53 or were killed at study
termination. Lungs from mice with renal masses and renal lymph nodes showing gross abnormalities
were also examined. Survival was decreased in a dose-related manner; at week 78 survival was 24/60,
6/60 and 0/60 hi control, low- and high-dose males, respectively, and 33/60, 18/60 and 0/60, in
control, low- and high-dose females, respectively (statistical analyses not performed). The majority of
high-dose mice (51/60 males and 59/60 females) died by week 26 of the study. Examination of the
kidneys of mice that died or were sacrificed after 53 weeks showed a significant (p<0.001) increase in
renal tumors hi low-dose males (13/16 versus 1/37 hi controls). The incidence of renal epithelial
adenocarcinomas hi control and low-dose males was 0/37 and 11/16, respectively (p<0.001). The
incidence of renal epithelial adenomas hi control and low-dose males was 1/37 and 5/16, respectively
(p<0.01). No renal tumors were observed in females hi any group. No metastases to the lung or renal
lymph nodes were observed. Evidence of neurotoxicity and renal pathology was observed in the
treated mice at both dose levels. ^The high mortality hi both groups of treated males and in high-dose
females indicated that the MTD was exceeded hi these groups.
A follow-up study to the Mitsumori et al. (1981) study was reported by Hirano et al. (1986).
Methylmercuric chloride was administered in the diet to ICR mice (60/sex/group) at levels of 0, 0.4, 2,
June 1996 3-67 SAB REVIEW DRAFT
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or 10 ppm (0, 0.03, 0.15, and 0.73 mg Hg/kg-day in males and 0, 0.02, 0.11, and 0.6 mg Hg/kg-day in
females) for 104 weeks. Interim sacrifices (6/sex/group) were conducted at 26, 52, and 78 weeks.
Complete histopathological examinations were performed on all animals found dead, killed in extremis,
or killed by design. Mortality, group mean body weights and food consumption were comparable to
controls. The first renal tumor was observed at 58 weeks in a high-dose male, and the incidence of
renal epithelial tumors (adenomas or adenocarcinomas) was significantly increased in high-dose males
(1/32, 0/25, 0/29, and 13/26 in the control, low-, mid-, and high-dose groups, respectively). Ten of the
13 tumors hi high-dose males were adenocarcinomas. These tumors were described as solid type or
cystic papillary types of adenocarcinomas. No invading proliferation into the surrounding tissues was
seen. The incidence of renal epithelial adenomas was not significantly increased in males, and no
renal adenomas or adenocarcinomas were observed in any females. Focal hyperplasia of the tubular
epithelium was reported to be increased hi high-dose males (13/59; other incidences not reported).
Increases in nonneoplastic lesions in high-dose animals provided evidence that an MTD was exceeded.
Nonneoplastic lesions reported as increased in treated males included the following: epithelial
degeneration of the renal proximal tubules; cystic kidney; urinary cast and pelvic dilatation; and
decreased spermatogenesis. Epithelial degeneration of the renal proximal tubules and degeneration or
fibrosis of the sciatic nerve were reported in high-dose females.
Table 3-53
Carcinogenic Effects of Methylmercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Rat/strain NS/ 25
M.25F
Rat/
Sprague
Dawley/56 M, 56
F
Mice/Swiss/
54 M, 54 F
Mouse/ICR/
60M.60F
Mouse/ICR/
60 M, 60 F
Mouse/
B6C3F1/
60 M, 60 F
Exposure
Duration
2yr
ad lib in feed
130 wk
ad lib in
FEFD
weaning until
death in
drinking water
78 wk ad lib
in feed
104 wk ad lib
in feed
u
2yr
ad lib in feed
Dose
(mg/kg-day)
0, 0.004, 0.02,
0.1
0.011, 0.05,
0.28 (M); 0.014,
0.064, 0.34 (F)
0, 0.19, 0.19-
0.95
(MMA)
0, 1.6, 3.1
0, 0.02, 0.03,
0.11,0.15,
0.6, 0.73
0.03, 0.14, 0.69
(M); 0.03, 0.13,
0.6 (F)
Effects/Limitations/BML
Tumors at comparable incidence in all groups
Limitations: Small sample size; failure to achieve MTD
BML avg: 850 ug/L in blood at 0.004, 6,500 ug/L at
0.02, and 36,000-39,000 ng/L at 0.1
No increase in tumor incidence
No increase in gross tumor incidence
Limitation: Histological examination not performed.
Increased incidence renal adenomas and
adenocarcinomas in low-dose males.
Limitations: Very poor survival in both male dose
groups.
Incidence of renal epithelial adenocarcinoma
significantly increased in males at 0.73; not invasive.
Limitations: MTD exceeded (including severe renal
damage in high-dose males)
Renal epithelial carcinomas and adenomas in males at
0.69.
Limitation: MTD exceeded in high-dose males.
Reference
Verschuuren et al.
1976
Mitsumori et al.
1983, 1984
Schroeder and
Mitchener 1975
Mitsumori et al.
1981
Hirano et al. 1986
Mitsumori et al.
1990
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Table 3-53 (continued)
Carcinogenic Effects of Methylmercury in Animals: Oral Exposure
Species/
Strain/
No. per Sex
per Group
Mice/Swiss/
NS
Cat/domestic/
4-5 M, 4-5 F
Exposure
Duration
15 wk ad lib
in drinking
water
2yr
ad lib in feed
-
Dose
(mg/kg-day)
0, 0.03, 0.07,
0.27
(MMQ
0, 0.0084, 0.02,
0.046, 0.074,
0.176
Effects/Lunitations/BML
Number of lung adenomas/mouse and tumor size/mouse
increased with dose •
No increase in tumor incidence
Limitations: Small group size, short exposure duration,
no pathological data for 3 lowest doses.
Reference
Blakley 19S4
Charbonneau et al.
1976
No increase in tumor incidence was observed in a study using white Swiss mice (Schroeder
and Mitchener 1975). Groups of mice (54/sex/group) were exposed from weaning until death to
methylmercuric acetate in the drinking water at two doses. The low-dose group received 1 ppm
methylmercuric acetate (0.19 mg Hg/kg-day). The high-dose group received 5 ppm methylmercuric
acetate (0.95 mg Hg/kg-day) for the first 70 days and then 1 ppm, thereafter, due to high mortality
(21/54 males and 23/54 females died prior to the dose reduction). Survival among the remaining mice
was not significantly different from controls. Significant (p<0.001) reductions in body weight were
reported in high-dose males (9-15% lower than controls) and high-dose females (15-22% lower than
controls) between 2 and 6 months of age. Mice were weighed, dissected, gross tumors were detected,
and some sections were made of heart, lung, liver, kidney and spleen for microscopic examination.
No increase in tumor incidence was observed. This study is limited because complete histological
examinations were not performed, and pathology data other than tumor incidence were not reported.
Mitsumori et al. (1983, 1984) conducted a study in Sprague-Dawley rats. They administered
diets containing 0, 0.4, 2, or 10 ppm of methylmercuric chloride (0, 0.011, 0.05, and 0.28 mg Hg/kg-
day in males; 0, 0.014, 0.064 and 0.34 mg Hg/kg-day in females) to Sprague-Dawley rats (56
animals/sex/group) for up to 130 weeks. Interim sacrifices of 10/group (either sex) were conducted at
weeks 13 and 26 and of 6/group (either sex) at weeks 52 and 78. Mortality was increased in high-
dose males and females. At week 104, survival was approximately 55%, 45%, 75% and 10% in
control, low-, mid-, and high-dose males, respectively, and 70%, 75%, 75% and 30% in control, low-,
mid-, and high-dose females, respectively (data presented graphically). Body weight gain was
decreased in high-dose animals (approximately 20-30%; data presented graphically). No increase in
tumor incidence was observed in either males or females. Noncarcinogenic lesions that were
significantly increased (p< 0.05) in high-dose rats included the following: degeneration in peripheral
nerves and the spinal cord (both sexes); degeneration of the proximal tubular epithelium of the kidney
(both sexes); severe chronic nephropathy (females); parathyroid hyperplasia (both sexes); polyarteritis
nodosa and calcification of the abdominal arterial wall (females); bone fibrosis (females); bile duct
hyperplasia (males); and hemosiderosis and extramedullary hematopoiesis in the spleen (males). In
addition, mid-dose males exhibited significantly increased degeneration of the kidney proximal tubular
epithelium and hyperplasia of the parathyroid. An MTD was achieved in mid-dose males and in high-
dose females; the MTD was exceeded in high-dose males.
No increase in tumor incidence or decrease in tumor latency was observed in another study
using rats (strain not specified) (Verschuuren et al. 1976). Groups of 25 female and 25 male rats were
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administered methylmercuric chloride at dietary levels of 0, 0.1. 0.5 and 2.5 ppm (0, 0.004, 0.02 and
0.1 mg Hg/kg-day) for 2 years. No significant effects were observed" on growth or food intake except
for a 6% decrease (statistically significant) in body weight gain at 60 weeks in high-dose females.
Survival was 72%, 68%, 48% and 48% in control, low-, mid- and high-dose males, respectively; and
76%, 60%, 64% and 56% in control, low-, mid- and high-dose females, respectively (statistical
significance not reported). Increases in relative kidney weights were observed in both males and
females at the highest dose. No effects on the nature or incidence of pathological lesions were
observed, and tumors were reported to have been observed with comparable incidence and latency
among all of the groups. This study was limited by the small sample size and failure to achieve an
MTD.
No tumor data were reported hi a study using Wistar rats (Munro, 1980). Groups of 50 Wistar
rats/sex/dose were fed diets containing methylmercury; doses of 2, 10, 50, and 250 micrograms Hg/kg-
day were fed for 26 months. High-dose female rats-exhibited reduced body weight gains and showed «
minimal clinical signs of neurotoxicity; however, high-dose male rats showed overt clinical signs of
neurotoxicity, decreased hemoglobin and hematocrit values, reduced weight gains and significantly
increased mortality. Histopathologic examination of the high-dose rats of both sexes revealed
demyelination of dorsal nerve roots and peripheral nerves. Males showed severe dose-related kidney
damage, and females had minimal renal damage.
No increase in tumor incidence was observed in a multiple generation reproduction study using
Sprague-Dawley rats (Newberne et al. 1972). Groups of rats (30/sex) were given semisynthetic diets
supplemented with either casein or a fish protein concentrate to yield dietary levels of 0.2 ppm
methylmercury (0.008 mg Hg/kg-day). Another group of controls received untreated rat chow. Rats
that received diets containing methylmercury during the 2-year study had body weights and
hematology comparable to controls. Detailed histopathologic analyses revealed no lesions of the brain,
liver, or kidney that were attributable to the methylmercury exposure. Mortality data were not
presented. Interpretation of these data is limited by the somewhat small group sizes and failure to
achieve an MTD.
No increase hi tumor incidence was observed in a study using random-bred domestic cats •
(Charbonneau et al. 1976). Groups of cats (4-5/sex/group) were given doses of 0.0084, 0.020, 0.046,
0.074 or 0.176 mg Hg/kg-day either as methylmercury-contaminated seafood or as methylmercuric
chloride in the diet for up to two years. Controls were estimated to have received 0.003 mg Hg/kg-
day. Food consumption and body weight were not affected by treatment with methylmercury. Due to
advanced signs of neurotoxicity (loss of balance, ataxia, impaired gait, impaired reflexes, weakness,
impaired sensory function, mood change and tremor), cats at the highest dose tested were sacrificed
after approximately 16 weeks, and cats at the next highest dose were sacrificed after approximately
54-57 weeks. Cats at the next highest dose generally exhibited mild neurological impairment (altered
hopping reaction and hypalgesia). One cat at this dose was sacrificed after 38 weeks because of
neurotoxicity, and one cat died of acute renal failure after 68 weeks. Cats at the two highest doses
had pathological changes in the brain and spinal cord, but no histopathological changes were noted in
other tissues examined. Interpretation of the results of this study is limited because of the small group
sizes, early sacrifice of cats at the two highest dose levels and no available data regarding pathological
changes in cats at the three lowest dose levels. This study was also limited by its short duration when
compared to the lifespan of a cat
Blakley (1984) administered methylmercuric chloride to female Swiss mice (number/group not
specified) in drinking water at concentrations of 0, 0.2, 0.5 or 2.0 mg/L for 15 weeks. This
corresponded to approximately 0, 0.03, 0.07 and 0.27 mg Hg/kg-day. At the end of week 3, a single
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dose of 1.5 mg/kg of urethane was administered intraperitoneally to 16-20 mice/group. No effects on
weight gain or food consumption were observed. Lung tumor incidence in mice not administered
urethane (number/group not specified) was less than 1 tumor/mouse in all groups. Statistically
significant trends for increases in the number and size of lung adenomas/mouse with increasing
methylmercury dose were observed: the tumor number/mouse was 21.5, 19.4, 19.4 and 33.1 in control,
low-, mid- and high-dose mice, respectively, and the tumor size/mouse was 0.70, 0.73, 0.76 and 0.76
mm in control, low-, mid- and high-dose mice, respectively. The study authors suggest that the
increase in tumor number and size may have been related to immunosuppressive activity of
methylmercury. It should be noted that this is considered a short term assay and that only pulmonary
adenomas were evaluated.
3.3.3 Other Data
3.3.3.1 Death
Methylmercury is a potent toxin that causes impairment of the CNS and developmental
toxicity in humans. Ingestion of methylmercury from treated seed grain or contaminated fish has
resulted in death. An outbreak of methylmercury poisoning in Iraq caused deaths in people who
consumed methylmercury from bread made with grain treated with a fungicide (Al-Saleem and the
Clinical Committee on Mercury Poisoning 1976; Bakir et al. 1973). The deaths were attributed to
impaired CNS function. A syndrome known as Minamata disease has been characterized by nervous
system impairment following consumption of methylmercury-contaminated fish from Minamata Bay in
Japan. Symptoms of Minamata disease include the following: prickling; tingling sensation of
extremities; impaired peripheral vision, hearing, taste and smell; slurred speech; muscle weakness;
irritability; memory loss; depression; and sleeping difficulties (Kutsuna 1968; Takeuchi et al. 1962;
Tsubaki and Takahashi 1986). Deaths from Minamata disease can be broken into two categories:
deaths occurring from the beginning of the outbreak (1954) to 1969, and deaths occurring from 1970
to 1980 (Tamashiro et al. 1984). Over half of the deaths in the first group were attributed to
Minamata disease and/or noninflammatory disease of the central nervous system, or pneumonia,
whereas deaths in the second group were attributed to cerebrovascular disease with underlying
Minamata disease. The mean age at death for the first group was 45.4 years for males and 26.4 years
for females, and the mean age at death for the second group was 70.0 years for males and 72.7 years
for females (Tamashiro et al. 1984).
Table 3-54
Lethality of Methylmercury in Humans: Case Study of Oral Exposure
Species/
No. per Sex
Human/6,530
both sexes
Human/1,422
both sexes
Exposure
Duration
43-68 d
(feed)
NS
Dose
(mg/kg-day)
0.71-5.7 (est.)
NS
Effects/Limitations/BML
Of 6,350 cases admitted to hospitals, 459 died after
eating bread made from grain treated with methylmercury
fungicide
BML: <100-5,000 ng/L in blood
Of 1 ,422 patients from the Minamata disease outbreak in
1959, 378 died by 1980.
Limitation: exposure data limited
BML not reported
Reference
Bakir et al. 1973
Tamashiro et al.
1984
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Very little information regarding death after inhalation exposure to methylmercury was located.
One study reported a man who died after being exposed for three years to alkylmercury particles from
seed dressings £Hook et al. 1954). Prior to death, the man experienced increased symptoms of
neurotoxicity. A case study reported on the deaths of two women exposed to diethylmercury vapors
for 3-5 months (Hill 1943). Gastrointestinal effects and neurological symptoms occurred prior to
deaths. No other human studies were located regarding death after inhalation exposure to
methylmercury.
Table 3-55
Lethality of Methylmercury in Humans: Case Studies of Inhalation Exposure
Species/
No. per Sex
Human/2 F
Human/1 M
Exposure
Duration
3-5 mo
(occup)
Syr
(occup)
Dose
(mg/m3)
NS
NS
Effects/Limitations/BML
Death following exposure to diethylmercury vapors
Limitation: Case study; concomitant dermal exposure likely;
limited exposure data
BML not reported
Death following exposure to pesticide containing
methylmercury
Limitations: Case study; concomitant dermal exposure likely;
limited exposure data
Range: 500-640 ug/L in urine
Reference
Hill 1943
Hook et al. 1954
Mice given a single oral dose of methylmercury had an increased incidence of death compared
to controls (Yasutake et al. 1991). Male mice appear to be more sensitive to the effects of
methylmercury than females, possibly due to the effect of mercury on the male kidneys. Mice
exposed for 26 weeks to 3.1 mg Hg/kg-day as methylmercury in drinking water also showed an
increase in mortality compared to controls (51/60 males and 59/60 females of exposed group died
versus 1/60 males and 1/60 females in controls) (Mitsumori et al. 1981). Longer studies (78 and 104
weeks) confirm that methylmercury causes significantly increased mortality in mice compared to
controls (Mitsumori et al. 1981, 1990). No animal studies were located on death after inhalation
exposure to methylmercury.
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Table 3-56
Lethality of Methylmercury in Animals
Species/
Strain/ .
No. per Sex
per Group
Mouse/ICR/60
M.60F
Mouse/
B6C3Fj/60 M,
60F
Mouse/
C57BL/6 M, 6
F
Exposure
Duration
78 wk
ad lib in feed
104 wk
ad lib in feed
Once
Dose
(mg/kg-day)
0, 1.6, 3.1
(MMC)
0, 0.03, 0.13,
0.14, 0.60, 0.69
(MMC)
4, 8, 16, 24, 32,
40
Effects/Limitations/BML
51/60 males and 59/60 females receiving 3.1 mg/kg/d
died by week 26, vs 7 males and 6 females at 1.6, and
1 control males and 1 control female; death at 52 wk
was also elevated at 1.6
Limitation: No statistical analysis
BML not reported
50/60 males treated with 0.69 mg/kg/d died vs. 31/60
control males; survival of females and males at lower
doses was unaffected
BML not reported
4/6 males died (LOAEL = 16); LOAEL for females
was 40 (4/6 died); no statistical analysis or LD50
calculated
Limitation: small number of animals tested
BML: 2.45 ug/g in kidney of males at 16 mg/kg
Reference
Mitsumori et al.
1981
Mitsumori et al.
1990
Yasutake et al. 1991
3.3.3.2 Neurological
The nervous system is the primary target organ for methylmercury toxicity. Information from
the large-scale poisonings in Japan (Niigata and Minamata) and Iraq provide substantial information
regarding the neurotoxicity of methylmercury in humans (Bakir et al. 1973, 1980; Berglund et al.
1971; Harada 1978; Marsh et al. 1987; Rustam and Hamdi 1974). In Japan, poisonings occurred
between 1953 and 1960 when people consumed seafood that had been contaminated by methylmercury
released by a chemical plant into Minamata Bay and the Agano river near Niigata. In Iraq, poisonings
occurred in the winter of 1971 to 1972 when people ate bread made from seed grain that had been
treated with a mercury-containing fungicide. In all of these episodes, neurotoxicity was the most
prominent effect observed in the exposed populations. In the Iraqi incident, more than 6000 patients
were hospitalized, and more than 500 deaths occurred, usually due to CNS failure.
The least severely affected persons from the poisonings in Japan and Iraq experienced
numbness or tingling (paresthesia) of the extremities and/or perioral area. Additional symptoms
frequently experienced by more severely affected individuals included the following: ataxia (gait
impairment ranging from mild incoordination or unsteadiness to complete inability to walk); blurred
vision; constriction of visual fields (in extreme cases blindness); slurred speech; and hearing
difficulties (deafness in extreme cases). Less frequently observed symptoms associated with the
methylmercury poisonings included tremors, muscular weakness, abnormal reflexes, increased muscle
tone, and clouded memory or stuperousness. A long latent period (16-38 days in the Iraqi episode
and up to several years in the Japanese episodes) between exposure and onset of symptoms of
neurotoxicity was observed. The cause for the latent period is unknown. It is thought that latency
may be related to cellular repair mechanisms dealing with damage from lipid peroxidation. At the
point when repair processes are overwhelmed tissue damage and accompanying symptoms become
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apparent. The possible ameliorating effect of selenium in the diet has also been hypothesized to play a
part in latency.
Similar neurological symptoms have been observed in persons ingesting meat contaminated
with ethylmercuric chloride (CinCa et al. 1979). Two .boys who ultimately died from exposure
exhibited neurological signs including gait disturbance, ataxia, dysarthria, dysphagia, aphonia,
hyperactive tendon reflexes, hypotonia, mydriasis and agitation. In the surviving members of the
family, ataxia, gait impairment, spasticity, drowsiness, intention tremor, agitation, speech difficulties
and visual disturbances were reported. All effects except the narrowing of the visual fields
disappeared after exposure termination.
Histopathologic analyses of nervous system tissue taken from poisoning victims show neuronal
degeneration hi the cerebrum and cerebellum (Bakir et al. 1980; Swedish Expert Group 1971;
Takeuchi et al. 1962). In the cerebral cortex, the calcarine area was most regularly affected with
varying degrees of damage in the pre- and postcentral cortices, superior temporal gyms, and basal
ganglia. In the cerebellar cortex, granule cell loss predominated, but this was usually less severe than
cerebral damage. An autopsy of two boys who ingested ethyl mercury contaminated meat revealed
nerve cell loss and glial proliferation hi the cerebral cortex, demyelination, granule cell loss in the
cerebellum, and motor neuron loss in the ventral horns of the spinal cord (Cinca et al. 1979). Less
information is available regarding the histopathology of peripheral nerve involvement, but sural nerves
taken from two victims of the Minamata episode showed evidence-of peripheral nerve degeneration
and regeneration (Miyakawa et al. 1976). Fourteen Iraqi patients who developed ataxia and "pins and
needles" and could not perform heel-to-toe walk were examined for impaired peripheral nerve function
(Von Burg and Rustam 1974a, 1974b). Determinations of motor and sensory conduction velocities,
sensory threshold and latency, reflex of the tibial nerve and myoneural transmission were performed,
but there were no statistical significances between exposed and unexposed control groups; the mean
values of the experimental group, however, were somewhat lower than those of the controls. There
was also no consistent correlation between clinical or electrophysiological observation on the
peripheral nervous system and blood mercury levels. In two patients who were hospitalized 10 days
after ingestion of ethyl mercury-contaminated meat, sensory nerve conduction velocity was decreased
immediately after admission but was found to be normal six months later (Cinca et al. 1979).
Estimates of threshold levels for neurotoxicity have been performed by WHO (1990) using
data from the Niigata episode and the Iraqi poisoning. In the exposures in Japan, hair levels
associated with thresholds for neurotoxicity were estimated to be approximately 100 ppm. Estimates
of threshold levels associated with paresthesia in the Iraqi episode indicate that the threshold level for
parathesia is approximately 25 to 40 mg (total body burden). This corresponds to blood levels of
approximately 250 to 400 ug/L and hair levels of approximately 50 ug/g. Thresholds (total body
burden) estimated by Bakir et al. (1973) for other neurotoxic signs were 55 mg for ataxia, 90 mg for
dysarthria (difficulty with speech), 170 mg for deafness, and 200 mg for death.
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Table 3-57
Neurotoxicity of Methylmercury in Humans: Case Studies of Oral Exposure
Species/
No. per Sex
Human/14
cases
Human/No. NS
Human/2 M, 2
F
Human/6530
cases both
sexes
Human/81 F
Exposure
Duration
NS
NS
Once
43-68 d
<5 mo
Dose
(mg/kg-day)
NS
NS
NS
(ethyl mercury
chloride)
0.71-5.7 (est.)
NS
(MMQ
Effects/Limitations/BML
Ataxia; impaired heel-to-toe walk; complaints of "pins
and needles". Sensory and motor peripheral nerves were
not affected. Clinical and electrophysiological
observations did not correlate with blood concentration
Limitation: Exposure concentration and duration not
known
BML: blood Hg levels were 138-878 ng/L
Paresthesia/numbness; constriction of visual field;
incoordination; difficulty speaking; tremor in consumers
of contaminated fish
Limitation: Limited details reported
BML not reported
Gait disturbance, ataxia, dysarthria, speech difficulties,
visual disturbances, hyperactive tendon reflexes,
mydriasis, agitation, coma; nerve degeneration in cerebral
cortex, cerebellum, and ventral horns of spinal cord;
decreased sensory nerve conduction velocity. Ingestion
of ethyl mercury chloride-contaminated meat.
Limitation: Exposure concentration not known
BML: hair Hg levels of 152-542 ug/g
Paresthesia/numbness in extremities and perioral area;
ataxia; constriction of visual field or blindness; slurred
speech; hearing difficulties following ingestion of grain
contaminated with methylmercury. Incidence and
severity of effect correlated with blood concentration
BML: Total body burden >50 mg at time of onset
Paresthesia and "other neurological symptoms"
BML Range: 1-674 ug/g Hg in hair; one woman with 14
ug/g (maximum in strand) had paresthesia and a woman
widi 10 ug/g had other symptoms. However, others with
levels as high as 600 ug/g had no symptoms. (This is a
follow-up study to Bakir et al. 1980)
Reference
Von Burg and
Rustam 1974a,
1974b
Harada 1978
Cinca et al. 1979
Bakir et al. 1973,
1980
Marsh et al. 1987
There are two case studies that report neurotoxicity in humans following inhalation of
methylmercury (Hook et al. 1954; Hunter et al. 1940); however, no quantitative data were available.
The two studies described in Table 3-58 demonstrated the spectrum of neurotoxic effects that occur
following occupational exposure to methylmercury. Weiss and Simon (1975) have suggested that such
changes in function in the general population, particularly at relatively low doses, may not be
clinically detectable as a loss of function but may be unmasked by the normal processes of aging.
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Table 3-58
Neurotoxicity of Methylmercury in Humans: Case Studies of Inhalation Exposure
Species/
No. per Sex
Human/5 M
Human/1 M
Exposure
Duration
5 mo-2 yr
(occup)
3yr
(occup)
Dose
(mg/m3)
NS
NS
Effects/Limitation/BML
Tingling of limbs; unsteady gait; difficulty performing fine
movements; constricted visual field following exposure to
methylmercury nitrate, methylmercury iodide, of
methylmercury phosphate in chemical factories
Limitations: Case studies; concomitant dermal exposure and
exposure to other chemicals likely; limited exposure data
BML not reported
Weakness in arms and legs; irregular EEC; sensory and
speech disorders following exposure to pesticide containing
methylmercury
Limitations: Case study; concomitant dermal exposure likely;
limited exposure data
BML Range: 500-640 ug/L in unne
Reference
Hunter et al. 1940
Hook et al. 1954
As a result of the methylmercury poisonings in Japan and Iraq, substantial information on the
neurotoxicity of methylmercury has been generated from animal studies. Relatively brief, high level
exposures in rats have been shown to cause characteristic signs of neurotoxicity (flailing and hindlimb
crossing when the animal is lifted by the tail) and neuronal degeneration in the cerebellum, cerebral
cortex and dorsal root ganglia (Inouye and Murakami 1975; Leyshon and Morgan 1991; Magos et al.
1985; Yip and Chang 1981). As with humans there is a latency period; the effects frequently are not
observed or do not show maximal severity until several days after the cessation of dosing. In an acute
study, exposure of rats to a single gavage dose of 19.9 mg Hg/kg as methylmercuric chloride resulted
in impaired open-field tests such as decreases in standing upright, area traversed and activity compared
to the control group (Post et al. 1973). Animals were lethargic and ataxic initially, but symptoms
disappeared within 3 hours.
a
•
Longer-term, lower-level exposures revealed that evidence of neuronal degeneration may be
observed prior to the onset of overt signs of toxicity. Degeneration in the cerebellum was found in
rats given 10 mg Hg/kg as methylmercuric chloride once every 3 days for 15 days (Leyshon and
Morgan 1991) while severe degenerative changes in the dorsal root fibers were observed in rats given
1.6 mg Hg/kg-day as methylmercuric chloride for 8 weeks (Yip and Chang 1981). Munro et al.
(1980) observed demyelination of dorsal nerve roots and damage in sciatic nerves with oral exposure
to 0.25 mg Hg/kg-day as methylmercuric chloride for up to 26 months. In mice given 1.9 mg Hg/kg-
day as methylmercury, cerebellar lesions were observed as early as eight days after the start of dosing,
but changes in motor activity did not develop until 24 weeks of exposure (MacDonald and Harbison
1977). Similarly, cats receiving methylmercury in the diet for 11 months displayed degenerative
changes hi the cerebellum and cerebral cortex, but incoordination or weakness was observed in only a
small number of the animals with histopathological changes (Chang et al. 1974).
The molecular basis for methylmercury neurotoxicity is likely to be complex and
multifactorial. The broad affinity of mercury for -SH groups leads to membrane, enzyme and
cytoplasmic organelle interaction. Major mechanistic pathways have been proposed to include the
following: inhibition of macromolecular metabolism, especially that of protein translation and nucleic
acid biogenesis; oxidative injury; disturbance in Ca2+ hemostasis; aberrant protein phosphorylation.
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The mechanisms underlying inhibition of protein and RNA synthesis are multiple. Depending upon
the systems used with in vitro, in vivo or neuronal cell suspensions, evidence for inhibition of
translation associated with a change in ATP/ADP concentration has been found. On the other hand,
direct inhibition of elongation was documented secondary to the selective inhibition of certain
aminoacyl-tRNA synthetase (Cheung and Verity, 1985). Syversen (1977) investigated the effects of
methylmercury on protein synthesis in rats using techniques which allowed analysis of different cell
populations from the central nervous system. Results of this study indicated selective irreversible
damage to granule cells of the cerebellum, whereas damage to the other neurons, such as Purkinje cells
was reversible. Such selectivity of toxicity is a feature of the neuronal loss, seen in human and
experimental disease. Methylmercury has also been suggested to cause neuronal degeneration by
promoting the formation of reactive oxygen species (Ali et al. 1992; Le Bel et al. 1990, 1992; Verity
and Sarafian 1991). While contributory, such oxidative injury does not appear primary to the site of
toxicity as appropriate protective measures blocking oxidative stress and lipoperoxide formation are
only minimally cytoprotective.
A recent review by Atchison and Hard (1994) discusses several proposed mechanisms of
action of methylmercury on Ca2+ hemostasis and ion channel function. Individual studies have
demonstrated that the neuromuscular actions of methylmercury occur predominantly at the presynaptic
site (Atchison et al. 1984). Methylmercury may interfere with acetylcholine neurotransmitter release
and subsequently synaptic transmission (Atchison et al. 1986; Barrett et al. 1974; Schafer et al. 1990;
Schafer and Atchison 1989, 1991). Finally, Sarafian (1993) demonstrated that the methylmercury-
induced stimulation of protein phosphorylation in cerebellar granule cell culture is coupled to Ca2"1"
uptake, changed intracellular Ca2"1" hemostasis and inositol phosphate metabolism. These latter
observations invoke the activation-of the protein kinase C pathway.
Cats and monkeys appear to be more sensitive to the neurotoxic effects of methylmercury than
rodents. Long-term studies in primates and in cats have shown neurological impairment at doses as
low as 0.05 rag Hg/kg-day. In cats, mild impairment of motor activity and decreased pain sensitivity
was observed at 0.046 mg Hg/kg-day as methylmercury after 60 weeks of exposure (Charbonneau et
al. 1976). In cynomolgus monkeys given methylmercury from birth until approximately 7 years of
age, impairment of spatial visual function was observed after 3 years, and decreased fine motor
performance, touch and pinprick sensitivity and unpaired high frequency hearing were observed six to
seven years after cessation of dosing (Rice 1989b; Rice and Gilbert 1982, 1992). Exposure of
cynomolgus monkeys to 0.03 mg Hg/kg-day as methylmercury for approximately 4 months caused no
detectable changes in motor activity or effects on vision or hearing, but degenerative changes were
observed in neurons of the calcarine cortex and sural nerve when these were examined electron
microscopically (Sato and Ikuta 1975). At higher doses (0.08 mg Hg/kg-day), slight tremor, motor
incoordination and blindness were observed in Macaca fascicularis monkeys after four months of
exposure (Burbacher et al. 1988).
The developing organism is generally at higher risk of neurotoxicity than adults. The section
on developmental effects of methylmercury lists studies wherein animals were observed with
neurological or neurobehavioral deficits as a consequence of in utero or perinatal methylmercury
exposure.
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Table 3-59
Neurotoxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex
per Group
Rat/Wistar/
10 F
Rat/Wistar/
50 F, 50 M
Rat/Charles
River/6 M
• Rat/Wistar/24
M, 18F
Rat/Wistar/15
M
Swiss origin
Mouse
M
Cat/Breed
NS/15-16 both
sexes
Cat/Breed
NS/8-10 NS
Monkey/
Macaco
fascicularis/l-2
both sexes
Monkey/
Macaca
artoides,
Macaca
nemestrinaTi
both sexes
Exposure
Duration
0-12 or
12-20 d, Ix/d
(gavage)
up to 26 mo
ad lib in feed
8 wk
7d/wk
1 x/d
(gavage)
5d
1 x/d
(gavage)
5x/15d
(gavage)
28 wk
(ad lib
drinking
water)
11 mo
(ad lib in
feed)
2yr
7d/wk
(feed)
36-1 32 d
1 x/d
(feed)
90-270 d
1 x/wk
(gavage)
Dose
(mg/kg-day)
2,4
(MMQ*
0.002, 0.01,
0.05, 0.25
(MMQ*
0, 1.TS
(MMQ*
g
(MMQ*
0,10
(MMQ*
1.9, 9.5
(MMQ"
0, 0.015
(MM)
0.003, 0.008,
0.020, 0.046,
0.074, 0.176
(MMQ*
0.02, 0.03,
0.04, 0.07, 0.21
1 for 5 doses,
then
0.4, 0.5, 0.6
Effects/Limitations/BML
Hindlimb crossing (LOAEL = 4) after 0-12 days
BML not reported
Ruffled fur, loss of balance, hindhmb crossing, paralysis
(LOAEL = 0.25) after 6 mo (males more affected);
demyelination of dorsal nerve roots and damage in teased
sciatic nerves at 0.25
Avg. BML at 0.25: 1 15 ppm in blood
Degeneration of dorsal root fiber
BML not reported
Cerebellar granule cell and dorsal root ganglion cell
degeneration; flailing and hind leg crossing following
administration of methylmercuric chloride
Limitations: Only one level tested; no controls
Avg BML: 150,000 ug/L in blood
Granule cell degeneration in cerebellum
BML: 60 ug/g dry cerebellar weight
Ataxia; degenerative changes of Purkinje cells; granule
cell loss in cerebellum; (LOAEL = 1.9)
BML not reported
Degeneration of cerebellum and cerebral cortex; necrosis
of dorsal root ganglia of kittens fed mercury-
cpntaminated tuna
BML not reported
Impaired hopping reaction; decreased pain sensitivity;
degeneration of dorsal root ganglia (LOAEL = 0.046)
Avg BML: 9,000 ug/L in blood at 0.046 mg/kg-day
Atrophy of neurons in calcarine cortex; focal
degeneration in sural nerves (LOAEL=0.03); ataxic gait,
myoclonic seizures at 0.21 mg/kg-day
Limitation: small number of animals tested
BML: Maximal at 0.03 mg/kg-day of 460 ug/L in blood
and 62 ug/g in hair
Tremor; visual impairment (LOAEL = 0.5 mg/kg)
Limitations: Small number of animals tested, limited
description of effects
Avg BML: 2,900 ug/L in blood
Reference
Inouye and
Murakami 1975
Munro et al. 1980
Yip and Chang
1981
Magos et al. 1985
Leyshon and
Morgan 1991
MacDonald and
Harbison 1977
Chang et al. 1974
Charbonneau et al.
1976
Sato and Ikuta 1975
Evans et al. 1977
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Table 3-59 (continued)
Neurotoxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex
per Group
Monkey/
Macaco
fascicularis/5
exposed, 2
control (sex
NS)
Monkey/
Macaco
fascicularisri-%
F
Monkey/
Macaco
fascicularis/4
M, 1 F
exposed, 1 M,
2 F controls
Exposure
Duration
3-4 yr
7 d/wk
1 x/d
(NS)
-3yr
1 .x/d
(oral route
NS)
6.5-7 yr
7 d/wk
1 x/d
(capsule;
gavage)
Dose
(mg/kg-day)
0, 0.05
(MMQ*
0, 0.04, 0.06,
0.08
(MMQ*
0, 0.05
(MMQ*
Effects/Limitations/BML
Spatial visual impairment
Limitation: One dose level tested
BML: 600-900 ug/L in blood
Slight tremor, motor incoordination; blindness (LOAEL =
0.04) following administration of methylmercury
Hydroxide; time to onset was 177-395 d
Avg BML: 2,030 ug/L in blood at highest dose
Six years after end of dosing (follow-up study to Rice
and Gilbert 1982): decreased fine motor performance;
diminished touch and pinprick sensitivity, impaired high
frequency hearing (p<0.05)
Limitations: small number of animals tested; one dose
level tested
BML: Not detectable at time of testing
Reference
Rice and Gilbert
1982
Burbacher et al.
1988
Rice 1989b; Rice
and Gilbert 1992
*MMC = methyl mercuric chloride
3.3.3.3 Renal
No studies were located regarding the renal toxicity of methylmercury in humans following
oral exposure. Renal histopathology and decreased function have been observed following acute or
chronic oral exposure of rats and mice to methylmercury. Renal tubule vacuolation was observed in
rats receiving 8 mg Hg/kg-day for 5 days (Magos et al. 1985), and decreased phenolsulfonphthalein
excretion occurred in male mice receiving a single dose of 16 mg Hg/kg-day or greater and females at
32 mg Hg/kg-day or greater as methylmercuric chloride (Yasutake et al. 1991). Chronic nephropathy,
including epithelial degeneration of proximal tubules and interstitial fibrosis, was observed at longer
durations (Fowler 1972; Hirano et al. 1986; Mitsumori et al. 1990). Males were more sensitive than
females to renal effects (Mitsumori et al. 1990).
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Table 3-60
Renal Toxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex
per Group
Rat/Wistar/3
M,6F
exposed/16
controls (sex
NS)
Rat/Wistar/24
M, 18 F
Mouse/ICR/60
M, 60 F
Mouse/
B6C3FJ/60 M,
60F
Mouse/
C57BL/6 M, 6
F
Exposure
Duration
12 wk
ad lib in feed
5d
Ix/d
(gavage)
26 wk
ad lib in feed
104 wk
ad lib in feed
Once
(gavage)
•
Dose
(mg/kg-day)
0, 0.08 (M)
0, 0.09 (F)
(MMQ
8
0, 0.03, 0.15,
0.72 (M); 0.02,
0.11, 0.62 (F)
0, 0.03, 0.14,
0.68 (M); 0.03,
0.13, 0.6 (F)
(MMQ
4, 8, 16, 24, 32,
40 (MMQ
Effects/Limitations/BML
Cytoplasmic mass in proximal tubule cells
Limitation: Only one level tested; small number of
treated animals
BML not reported
Renal tubule vacuolation and dilation
Limitation: One level tested, no controls
Avg. BML: 150,000 ug/L in blood
Toxic epithelial degeneration of renal proximal tubules
(LOAEL = 0.62 F; 0.72 M)
BML not reported
Chronic nephropathy (epithelial cell degeneration,
regeneration of proximal tubules, interstitial fibrosis) in
males at >0.14 and in females at 0.60 (p<0.01)
BML not reported
Decreased phenolsulfonphthalein excretion and increased
serum creatinine in males (LOAEL = 16 in males, 32 in
females); swollen epithelial cells in proximal tubules
Limitation: No statistical analysis; small number of
treated animals
BML: 2.45 ug/g in kidneys of males and 1.9 ug/g in
kidneys of females at 16 mg/kg
Reference
Fowler 1972
Magos et al. 1985
Hirano et al. 1986
Mitsumori et al.
1990
Yasutake et al. 1991
3.3.3.4 Cardiovascular
Only one study was located regarding the cardiovascular toxicity of methylmercury in humans.
Hook et al. (1954) reported two men with elevated blood pressure after inhalation exposure to organic
mercury particulates from seed dressings. Other neurotoxic effects were also present at the time of
examination, and one man subsequently died.
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Table 3-61
Cardiovascular Toxicity of Methylmercury in Humans: Case Study
Species/
No. per Sex
•
Human/1 M
Exposure
Duration
3yr
(occup)
Dose
(mg/m3)
NS
Effects/Limitations/BML
Elevated blood pressure
Limitations: Case study; concomitant dermal exposure likely
BML Range: 500-640 (ig/L in urine
Reference
Hook et al. 1954
Very little information was located regarding the effects of oral methylmercury exposure on
the cardiovascular system. Rats given two daily doses of methylmercuric chloride exhibited decreases
in heart rates (Arito and Takahashi 1991). Rats treated with methylmercuric chloride for one month
had increased systolic blood pressures beginning 42 days after cessation of dosing (Wakita 1987).
This effect persisted for more than a year.
Table 3-62
Cardiovascular Toxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex
per Group
Rat/Wistar/10
(sex NS)
Rat/Sprague-
Dawley/5-6
(sex NS)
Exposure
Duration
23-28 d
7d/wk
(gavage)
2d
1 x/d
(gavage)
Dose
(mg/kg-day)
0.4, 1.2
(MMQ
12
(MMQ
Effects/Limitation/BML
Increased systolic pressure beginning 42 d after the end
of treatment (p<0.05)
BML not reported
Decreased heart rate (jxO.05)
Limitation: Only one dose tested for this parameter
BML: 10 ug/g in brain
Reference
Wakita 1987
Arito and Takahashi
1991
3.3.3.5 Gastrointestinal
No information was located regarding the gastrointestinal toxicity of methylmercury in
humans. Only one study was located regarding the gastrointestinal toxicity of methylmercury
following oral exposure in animals. Mitsumori et al. (1990) reported an increased incidence of
stomach ulceration in mice following a 2-year exposure to 0.69 mg Hg/kg-day as methylmercuric
chloride in drinking water.
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Table 3-63
Gastrointestinal Toxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex
per Group
Mouse/
B6C3Fj/60 M,
60F
Exposure
Duration
104 wk
ad lib in feed
Dose
(mg/kg-day)
0, 0.03, 0.14,
0.69 (M); 0.03,
0.13, 0.6 (F)
(MMQ
Effects/Limitations/BML
Stomach ulceration in males at 0.69 (p<0.05)
BML not reported
Reference
Mitsumori et al.
1990
3.3.3.6 Immunological
Suppression of the humoral and cellular immune responses have been observed in animals
after oral exposure to methylmercury or methylmercuric chloride. Both decreases in the production of
antibody-producing cells and decreased antibody titre in response to inoculation with immune-
stimulating agents (such as sheep red blood cells) have been observed (Blakley et al. 1980; Koller et
al. 1977; Ohi et al. 1976). Decreases hi natural killer T-cell activity have been observed in animals
after exposure to methylmercury (Dback 1991).
Table 3-64
Immunotoxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex per
Group
Rat/Brown Norway/6
both sexes exposed/22
both sexes/controls
Mouse/ICR/6 M
Mouse/Swiss/8-10 M
Exposure
Duration
NS x/wk-
2 mo
5d
1 x/d
(gavage)
3 wk
ad lib in
drinking water
Dose
(mg/kg-day)
0,4.8
(MMQ
0.27, 2.7
(MMQ
0.076, 0.3, 1.52
(MMQ
Effects/Limitations/BML
IgG deposits along the glomerular capillary wall
of the kidney, not in arteries, suggestive of an
autoimmune disease; no effect seen in controls.
Limitation: only one level tested
BML not reported
Decreased production of antibody-producing cells
(LOAEL = 2.7; pxO.Ol).
Limitation: small number of animals, only males
tested
BML not reported
Decreased production of antibody-producing cells
and decreased antibody liter (LOAEL = 0.076;
rxO.Ol).
Limitation: small number of animals, only males
tested
BML not reported
Reference
Bernaudin et al
1981
Ohi et al. 1976
Blakley et al. 1980
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Table 3-64 (continued)
Immunotoxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex per
Group
Mouse/Balb/c CUM/
8F
Rabbit/New Zealand
white/10 M, 10 F
Exposure
Duration •
12 wk
ad lib in feed
»
14 wk
Ix/d
in feed
Dose
(mg/kg-day)
0,0.5
0.04,0.4,0.8 ,
(MMQ
Effects/Limitations/BML
Reduced natural killer T-cell activity; decreased
thymus weight and cell number (pxO.Ol).
Limitation: small number of animals treated, only
females tested
BML not reported
Decreased antibody liter (LOAEL = 0.4) (26% of
the animals at 0.4 and no controls died by wk
14).
Limitations: No statistical analysis
BML: 2,240 ug/L in blood at 0.4 mg/kg/d at wk
14
Reference
Ilback 1991
Roller et al. 1977
3.3.3.7 Dermal
Al-Mufti et al. (1976) studied the effects of methylmercury in humans who ate contaminated
bread; a correlation between bread consumption and a history of rash was reported. No other
information was located regarding dermal effects of organic mercury following oral exposure.
Table 3-65
Dermal Toxicity of Methylmercury in Humans: Epidemiological Study
Species/
No. per Sex
Human/415
exposed/1012
controls (sex
NS)
Exposure
Duration
=1-3 mo
(feed)
Dose
(mg/kg-day)
NS
(MMQ
Effects/Limitations/BML
"History of rash" in 14% of exposed group, compared
with <1% of unexposed
Limitations: Effects poorly described; no statistical
analysis
BML not reported
Reference
Al-Mufti et al. 1976
3.3.3.8 Developmental
Methylmercury readily crosses the placenta! barrier, and marked developmental toxicity has
been observed in both humans and animals after gestational exposures. Infants exposed to
methylmercury through the mother's milk or during gestation had elevated blood mercury levels, as
did their mothers (Amin-Zaki et al. 1976). Human data from epidemic poisonings that occurred in
Japan (Harada 1978) and Iraq (Amin-Zaki et al. 1974), as well as isolated exposures (Snyder and
Seelinger 1976) indicate that methylmercury predominantly affects the developing nervous system.
Infants bom to mothers who ingested fish contaminated with methylmercury from Minamata Bay in
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Japan between 1953 and 1960 appeared normal at birth but within several months exhibited mental
retardation, retention of primitive reflexes, cerebellar symptoms, dysarthria, hyperkinesia,
hypersalivation, strabismus and pyramidal symptoms (Harada 1978). Similarly, infants born to
mothers who had ingested bread made with seed grain treated with methylmercury-containing
fungicides in Iraq during 1971 to 1972 exhibited symptoms ranging from delays in speech and motor
development to mental retardation, reflex abnormalities and seizures (Amin-Zaki et al, 1974, 1978).
Histopathologic analyses of-brain tissues from infants that died in the Iraqi (Choi et al. 1978) and
Minamata (Harada 1978) episodes showed atrophy and hypoplasia of the cerebral cortex, corpus
callosum and granule cell layer of the cerebellum; dysmyelination of the pyramidal tracts; and/or
abnormal neuronal cytoarchitecture characterized by ectopic cells and disorganization of cellular layers.
A number of studies have attempted to evaluate developmental neurotoxicity in populations
with elevated methylmercury exposure from consumption of fish as a major component of the diet but
for whom massive poisonings have not been reported. Kjellstrom et al. 1989) observed a higher
incidence of abnormal scoring 6n tests designed to assess intelligence and development among children
from New Zealand whose mothers had high levels of hair mercury. Also a study by McKeown-
Eyssen et al. (1983) of a Cree population from northern Quebec revealed a correlation between
maternal exposure (as determined using hair levels) and abnormal muscle tone or reflexes in male
children. A dose-response for this effect was not observed.
Dose-response analyses of human data from the Iraqi epidemic of 1971 to 1972 have indicated
correlations between maximal maternal hair levels during pregnancy and the severity of the
neurological deficits seen in the children (Cox et al. 1989; Marsh et al. 1981, 1987). An evaluation of
a calculated threshold for response is presented in Section 6.3.1 of this volume.
June 1996 3-84 SAB REVIEW DRAFT
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Table 3-66
Developmental Toxicity of Methylmercury in Humans: Case Studies
Species/
No. per Sex
Human/8 M, 7
F infants
Human/1 F
_
Exposure
Duration
-2 mo.
(feed)
'
6 mo.
3 mo.
postcoital-
term
(feed)
Dose
(mg/kg-day)
NS
NS
Effects/Limitations/BML
Assessment of 15 mother-infant pairs where the mothers
ate grain treated with methylmercury fungicide during
pregnancy. Motor and mental development were
impaired (blindness, impaired hearing) in 6 infants; there
were no congenital malformations.
BML: Affected infants: -3,000 ug/L in blood at 2
months; Affected mothers: >400 ug/L in blood
Severe neurological impairment (blindness, myoclonic
seizures, spastic quadriparesis) of male infant bom to a
mother eating meat from pigs that had eaten grain treated
with methylmercury •fungicide.
Limitation: Case report
BML not reported
Reference
Amin-ZaJd et al.
1974
Snyder and
Seelinger 1976
Table 3-67
Developmental Toxicity of Methylmercury in Humans: Epidemiologic Studies
Species/
No. per Sex
Human/220 F
Human/84
mother-child
pairs
Human/243
exposed (sex
NS) aged 12-
30 mo.
Exposure
Duration
NS
(food)
few days to
several mo.
(food)
Gestation and
lactation
(food)
Dose
(mg/kg-day)
NS
NS
NS
Effects/Limitations/BML
Mental retardation, atrophy of brain and degeneration of
cerebellum in offspring. Of 220 infants born in
Minamata (to mothers eating contaminated fish), 13 had
severe symptoms; the number with less severe symptoms
was not reported.
Limitations: Few details on methods or results
BML not reported
Assessment of mother-infant pairs where mothers ate
grain treated with methylmercury fungicide during
pregnancy (same Iraqi population as repotted by Amin-
Zaki et al. 1974). Severe psychomotor retardation in
infants.
BML Range: 37-293 ug/g in hair (maximum in segment
of maternal hair)
Abnormal tendon reflexes or muscle tone in male
offspring correlated with methylmercury exposure
(p<0.05). Conducted as a case-control study after
potential affected measures were identified.
Limitation: Author reported that the statistical method
could have led to an association by chance.
BML avg: 6 ug/g in maternal hair
i
Reference
Harada 1978
Marsh et al. 1981
McKeown-Eyssen et
al. 1983
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Table 3-67 (continued)
Developmental Toxicity of Methylmercury in Humans: Epidemiologic Studies
Species/
No. per Sex
Human/81
mother-child
pairs
Exposure
Duration
few days to
several mo.
(food)
Dose
(mg/kg-day)
NS
Effects/Limitations/BML
Assessment of mother-infant pairs where mothers ate
grain treated with methylmercury fungicide during
pregnancy (same Iraqi population as reported by Amin-
Zaki et al. 1974). Delayed walking and talking; seizures;
mental retardation.
BML Range: -18-598 ng/g (maximum in strand) in hair
of mothers of affected infants
Reference
Marsh et al. 1987
The developmental toxicity of oral exposure to methylmercury has been extensively studied in
animals. In rodents exposed in utero, a spectrum of effects has been observed ranging from decreases
in fetal weight and skeletal ossification and increases in skeletal variations and malformations (brain
lesions, hydrocephalus, cleft palate, micrognathia, edema, subcutaneous bleeding, hydronephrosis,
hypoplasia of the kidneys, dilation of the renal pelvis) to increased resorptions and fetal deaths (Fuyuta
et al. 1978, 1979; Inouye and Kajiwara 1988a; Inouye and Murakami 1975; Khera and Tabacova
1973; Nolen et al. 1972; Reuhl et al. 1981; Yasuda et al. 1985). The severity of the effects generally
increased with dose, and the incidence of malformations increased with exposures that occurred later
in gestation (Fuyuta et al. 1978; Inouye and Murakami 1975). Brain lesions have been observed in a
variety of areas including the brain mantle, corpus callosum, caudate putamen and cerebellum. In
guinea pigs, early gestational exposures (weeks 3-5 of pregnancy) resulted primarily in developmental
disturbances of the brain (smaller brains, dilated lateral ventricles and reduced size of caudate
putamen), whereas later gestational exposures (>week 6 of pregnancy) resulted in widespread neuronal
degeneration (Inouye and Kajiwara 1988b).
In addition to structural changes, functional changes have been observed in animals after
gestational exposures. Such functional effects include abnormal tail position during walking; flexion;
hindlimb crossing; decreased locomotor activity, responding in an avoidance task and righting
response; increased passiveness, startle-response and sensitivity to pentylenetetrazol-induced
convulsions; and impaired maze performance, operant behavior, swimming behavior, tactile-kinesthetic
function, visual recognition memory, temporal discrimination, and subtle learning deficits such as
insensitivity to changing reinforcement contingencies (Bornhausen et al. 1980; Buelke-Sam et al. 1985;
Burbacher et al. 1990; Eisner 1991; Geyer et al. 1985; Gunderson et al. 1988; Hughes and Annau
1976; Inouye et al. 1985; Musch et al. 1978; Olson and Boush 1975; Rice 1992; Rice and Gilbert
1990; Stoltenburg-Didinger and Markwort 1990; Suter and Schon 1986; Newland et al. 1994).
Overt neurological impairment is the endpoint used to document methylmercury poisonings;
however, as shown in animal studies, methylmercury may produce more subtle neurodevelopmental
effects such as impairment of sensory or cognitive systems. Schreiner et al. (1986) exposed rats to 0,
0.2 or 0.6 mg Hg/kg-day as methylmercuric chloride in utero and during lactation to evaluate pup
performance on visual discrimination reversal task. While no overt signs of neurotoxicity were
evident, subtle differences between the control and high-dose group were observed during more
difficult tasks. A stressful or highly demanding situation appears to be necessary for the expression of
these sensory effects, wherein the decreased ability to adapt to the altered conditions became manifest.
Spyker et al. (1972) reported that although no signs of neurological toxicity was observed in mouse
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pups exposed to methylmercury in utero, open field and swimming tests revealed subtle neurological
effects in the exposed "pups. Newland et al. (1994) administered methylmercury by gavage to pregnant
squirrel monkeys between weeks 11 and 14.5 of gestation. Doses were adjusted to maintain 0.7 to 0.9
ppm Hg in the maternal blood. There were three controls and three methylmercury-treated offspring.
Offspring were evaluated at 5-6 on a lever pressing test which required discrimination between degrees
of reinforcement. At steady state, monkeys exposed to methylmercury in utero were less sensitive to
differences in reinforcement rates. When reinforcement rates changed, exposed animals either changed
their behavior slowly in response to the altered reinforcement or not at all.
The developmental toxicity of methylmercury may be attributable to the ability of
methylmercury to bind to sulfhydryl-rich tubulin (a protein component of microtubules) and cause its
depolymerization (Falconer et al. 1994; Sager et al. 1983). Both cell division and cell migration
require intact microtubules for normal functioning. Disruption of microtubule function could result in
the derangement of cell migration (Choi et al. 1978; Falconer et al. 1994; Matsumoto et al. 1965) and
arrested cell division (Retihl et al. 1994; Sager et al. 1984). \
Table 3-68
Developmental Toxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex
per Group
Rat/Charles
River/20 F
Rat/Wistar/35 F
Rat/Wistar/10 F
Rat/Holtzman/5
F
Exposure
Duration
9d
Gd6-14
ad lib in
drinking
water
52 d
ad lib in feed
8, 12, or 20 d
1 x/d
Gd 12-20,
0-12, or 0-20
(gavage)
during gesta-
tion, during
lactation, or
postnatal
days 21 -30 in
drinking
water
Dose
(mg/kg-day)
0, 0.02, 0.2, 4
0, 0.002, 0.01,
0.05, 0.25
(MMC)
2,4
(MMC)
0,2.5
(MMC)
Effects/Umitations/BML
Increased number of fetuses with soft tissue variations
of the urinary system and incomplete ossification or
calcification (LOAEL = 4; p<0.05).
BML not reported
Increased incidence of eye defects (in harderian and
lachrymal glands) and salivary glands in fetuses
(LOAEL = 0.25); significant dose response (p = 0.01).
Mothers were treated from immaturity through weaning
or later.
Limitations: Incomplete reporting; of results
BML not reported
Increased brain lesions and generalized edema
(Gd 0-20) (LOAEL = 2).
Limitations: Limited data reporting; no statistical
analysis; small number of treated animals
BML not reported
Decreased visual evoked potential latencies for peaks
Nl (p<0.05), PI (p:£0.01) and P2 (pSO.Ol) in 30-day
old pups exposed during gestation, during lactation, or
during postnatal days 21-30.
BML not reported
Reference
Nolen et al. 1972
Khera and Tabacova
1973
Inouye and
Murakami 1975
Zenick 1976
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Table 3-68 (continued)
Developmental Toxicitv of Methylmercury in Animals
Species/
Strain/
No. per Sex
per Group
Rat/Charles
River CD/20 F
*
Rat/Long-
Evans/4 exposed,
6 control
Rat/Wistar/20 F
Rat/Wistar-
Neuherberg/
No. F. NS
Rat/Wistar/10 F
Rat/Sprague-
Dawley/No. F
NS
Rat/Sprague-
Dawley/No. F
NS
Rat/Sprague-
Dawley/
15-19 F
Exposure
Duration
47 d prior to
and during
gestation
ad lib in
drinking
water
Once
Gd7
(gavage)
Sd
1 x/d
Gd 7-14
(gavage)
4d
Gd6-9
(gavage)
4d
1 x/d
Gd6-9
(gavage)
Once
Gd 8
(gavage)
10 d
1 x/d
Gd6-15
(gavage)
4d
1 x/d
Gd6-9
(gavage)
Dose
(mg/kg-day)
0.42, 0.7, 1.4
(MMH)
0,4
(MMQ
0, 2, 4, 6
(MMQ
0, 0.04, 1.6
(MMQ
0, 0.004, 0.008,
0.035
(MMQ
0,6.3
(MMQ
0, 0.2, 1, 2, 4
(MMQ
0, 1.6, 4.8
(MMQ
Eifects/Limitations/BML
Ultrastructural changes, dose-related decrease in
biochemical activity in mitochondria of fetal
hepatocytes (p<0.01) following administration of
methylmercury hydroxide to mothers (LOAEL = 1.4).
BML: 40 ug/g (organic and inorganic) in liver of
fetuses at 1.4 mg/kg-day
V Increased P1-N1 amplitudes and decreased P2 and N2
latencies of cortically visual evoked potential (p<0.05).
BML not reported1
At 6 mg/kg-day, decreased maternal weight gain.
increased resorptions and fetal deaths (p<0.001);
decreased fetal body weight increased skeletal and
visceral malformations (hydrocephaly, wavy ribs).
(LOAEL = 4; p<0.01)
BML not reported
Impaired ability to perform operant conditioning
procedures (number of responses on lever required in
specified period of time) (LOAEL = 0.05).
Limitation: Statistical analyses not reported
BML not reported
Reduction in behavioral performance in offspring of
treated mice following operant conditioning
(LOAEL = 0.008; p<0.01).
BML not reported
Shorter avoidance latency in 60-day old offspring
(LOAEL = 6.3).
BML not reported
Delayed sexual development (vaginal patency and
testes descent), reduced pivoting, delayed surface
righting, partially retarded swimming development.
increased activity in center of open field, impaired
startle reflex response. Reduced maternal weight gain
and litter weight No live offspring were produced at 4
mg/kg-day (LOAEL = 2; p<0.05).
BML not reported
Delayed vaginal patency, delayed surface righting,
retarded swimming development, lower activity,
impaired complex water maze performance. Increased
mortality of pups at 1-21 days of age (LOAEL = 4.8;
p<0.05).
BML not reported
Reference
Fowler and Woods
1977
Dyer et al. 1978
Fuyuta et al. 1978
Musch et al. 1978
Bomhausen et al.
1980
Cuomo et al. 1984
Geyer et al. 1985
Vorhees 1985
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Table 3-68 (continued)
Developmental Toxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex
per Group
Rat/Wistar/
38 M, 38 F
Rat/HAN-
Wistar/10 F
Rat/Wistar/No. F
NS
Rat/Wistar/16 F
Rat/Sprague-
Dawley/No. and
sexNS
Mouse/SvSl/
No. F NS
Mouse/CFW/No.
FNS
Exposure
Duration
during
gestation and
lactation ad
lib in
drinking
water
13 days prior
to mating
until post-
natal day 21
in drinking
water
4d
1 x/d
Gd6-9
(gavage)
2 wk prior to
mating
through
weaning
ad lib in
drinking
water
Once
Gd 15
(gavage)
Once
Gd7or9
(i-p.)
Once
Gd8
(i.v.)
Dose
(mg/kg-day)
0, 0.2, 0.6
(MMQ
0, 0.2, 0.6, 1.7
(MMQ
0, 0.02, 0.04,
0.4,4
(MMQ
0, 0.08-0.38,
0.34-0.95
(MMQ
0,6.4
(MMQ
0,
0.16 mg
MMD/20g
0, 1, 2, 3, 5, 10
(MMH)
»
Effects/Limitations/BML
Increase in response latency in male (p<0.05) and
female pups (p<0.01) and in passiveness (p<0.05) in
visual discrimination reversal task at 0.6 mg/kg-day
(LOAEL = 0.6).
BML not reported
Reduced weight gain, ataxia and inability to give birth
in dams at 1.7. High mortality in pups at 1.7.
Impaired swimming behavior and righting reflex,
delayed sexual maturity (vaginal opening and testes
descent) at 0.2 and 0.6. (LOAEL = 0.2; p<0.05).
BML = 9,700-191,000 jig/L in dams and 10,000-
127,000 ug/L in pups at birth
Increased startle response; impaired swimming
behavior, decreased locomotor and nose-poking
behavior; alteration of dendritic spine morphology
(LOAEL = 4).
Limitations: Limited data reporting; no statistical
analysis
BML not reported
Impaired tactile-kinesthetic function (p<0.05) (LOAEL
= 0.08-0.38).
BML not reported
•
Increased GABAA receptors in prenatally exposed pups
sacrificed at 14 or 21 days postpartum; increased
behavioral depression after diazepam.
Limitations: Only one treatment level; no data on
number of animals
BML not reported
Impaired swimming ability and open-field behavior
(p<0.05) in 30-day old pups. Dose administered as
methylmercury dicyandiamide (MMD)
BML not reported
Increased number of trials to criterion (p<0.05) and
increased number that failed to attain criterion in 2- way
avoidance test conducted on 56-day old pups (LOAEL
= 3).
BML not repotted
Reference
Schreiner et al.
1986
Suter and Schon
1986
Stoltenburg-
Didinger and
Markwort 1990
Eisner 1991
Guidetu et al. 1992
Spyker et al. 1972
Hu fifis s And Ann&u
1976
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Table 3-68 (continued)
Developmental Toxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex
per Group
Mouse/
129/Svsl/
No. F NS
Mouse/C57BL/
10 F
Mouse/
DUB/ICR/8 F.
exposed, 7 F
controls
Mouse/
C3H/HeN/10 F
Guinea pig/
Hanley/5-9 F
Hamster/
Golden/10 F
Exposure
Duration
Once
GdlO
(s.c.)
8d
Ix/d
Gd6-13
(gavage)
Once
Gd 12
(gavage)
Once
Gd 13, 14,
15, 16, or 17
(gavage)
Once
Gd 21, 28,
35, 42, or 49
(gavage)
Once at
Gd 10, or
6d
1 x/d
Gd 10-15
(gavage)
»
Dose
(mg/kg-day)
0, 5, 7, 10
0, 2, 4, 4.8, 6
(MMC)
0,8
0, 16
(MMC)
9.4-15
7.5 ing/animal
(wt 500-800 g)
(MMC)
0, 1.6, 8
(MMC)
Effects/Limitations/BML
Longer center square latency at 10 (once) and 3.5 (3 d),
decreased rearings and increased backings at 3.5;
decreased locomotor activity at 7 and 10; postnatal
growth retardation at 7 and 10 (LOAEL = 7; p<0.05).
BML not reported
Increased resorptions and fetal deaths at 4.8 and 6
(rxO.Ol); increased malformations (cleft palate, fused
vertebrae) at 2 and higher (p<0.05); increased skeletal
variations; decreased maternal weight gain at 4.8
mg/kg-day (LOAEL = 2).
Limitation: small number of treated animals
BML not reported
Arrest of brain cells during mitosis (rxO.Ol).
Limitations: Only one dose tested; small number of
animals tested
BML not reported
Decreased neonatal survival and weight gain; impaired
righting response; decreased locomotor activity;
abnormal gait; crossing of hindlimbs; decreased brain
weight in groups treated on Gd 13 or 14 (p<0.01);
dilated lateral ventricles; slightly simplified cerebellar
pattern. Effects were seen in groups dosed on all days,
but somewhat stronger in those treated on Gd 13 or 14.
Limitations: Incomplete reporting of data; most
parameters were not analyzed statistically; only one
dose tested
BML: -20 |ig/g°in brain of fetuses
Aborted litters and retarded fetal brain development at
all treatment times.
Limitations: No statistical analysis; small number of
treated animals, only 1 day of dosing
Avg BML over treatment time: Fetal: 2,600 |i/L in
blood; Maternal: 1,800 jig/g in blood
Degeneration of cerebellar neurons in rats bom to
mothers treated with 1.6 mg/kg/d on Gd 10-15 or a
single dose of 8 mg/kg on Gd 10 and sacrificed
neonatally or as adults.
Limitation: small number of treated animals
BML not reported
Reference
Su and Okita 1976
Fuyuta et al. 1978
Rodier et al. 1984
Inouye et al. 1985
Inouye and
Kajiwara 1988b
Reuhl et al. 1981
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Table 3-68 (continued)
Developmental Toxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex
per Group
Monkey/Macaco
fascicularu/9 F
exposed, 8 F
control
Monkey/Afacaca
fascicularis/
12 F exposed, 13
F control
Monkey/Afecaca
fascicularis/5
mothers
Monkey/
Macaco
fascicuIaris/4 M,
1 F exposed, 1
M, 2 F controls
Monkey/Macaco
fascicularisf23 F
Monkey/Sawniri
sciureus/3 F
Exposure
Duration
approx. 1-3
yr 1 x/d prior
to mating
through
gestation (in
apple juice)
approx. 4 mo
to 2 yr 1 x/d
prior to
mating
through
gestation (in
apple juice)
4-4.5 yr
1 x/d in utero
and
postnataUy
(gavage)
6.5-7 yr
7d/wk
1 x/d
(capsule;
gavage)
unspecified
period prior
to mating
through
gestation
week 11 or
14.5 until
parturition
(gavage)
Dose
(mg/kg-day)
0, 0.04, 0.06
0..0.04
0, 0.01, 0.025,
0.5
(MMQ
0, 0.05
(MMQ
0.04, 0.06, 0.08
0.7 to 0.9 ppm
methylmercury
in maternal
blood
Effects/Limitations/BML
Impaired visual recognition memory (data pooled from
both groups of infants of exposed mothers) compared
to unexposed controls; test performed at 50-60 days of
age.
Limitation; small number of treatment animals
BML Range: 880-2,450 ug/L in blood of infants at
birth; 280-830 ug/L at testing
Decrease in social play behavior and concomitant
increase in nonsocial passive behavior compared to.
unexposed controls; tests performed at 2 weeks to
8 months of age.
Limitation: small number of treatment animals
BML Range: 1,565 ug/L in blood of infants at birth
Spatial visual impairment (LOAEL = 0.01).
Limitation: Small number of infants (5 high-dose; 2
mid-dose; 1 low-dose)
BML not reported
Six years after end of dosing (follow-up study to Rice
and Gilbert 1982); decreased fine motor performance;
diminished touch and pinprick sensitivity; impaired
high frequency hearing (p<0.05).
Limitations: small number of animals tested; one dose
level tested
BML: Not detectable at time of dosing
No effect on spatial memory of adult offspring of
animals treated with methylmercury hydroxide (data
pooled from 24 animals, all treated groups).
BML Range: 1,040-2,460 ug/L in blood of infants at
birth
Monkeys exposed in utero tested (on learned lever
pulling activity) at ages 5-6 yr. Methylmercury
treatment resulted in decreased sensitivity to degrees in
reinforcement; change in reinforcement degree resulted
in either no behavior change or slow change by
comparison to controls. Limitations: small number of
animals tested; incomplete reporting on treatment.
Reference
Gunderson et al.
1988
Gunderson et al.
1988
Rice and Gilbert
'1990
Rice 1989b; Rice
and Gilbert 1992
Gilbert et al. 1993
Newland et al. 1994
3.3.3.9 Reproductive
Although no data were located regarding the reproductive effects of oral exposure to
methylmercury in humans, animal data suggest that, at sufficiently high doses, methylmercury may
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adversely affect reproductive function in both males and females. When male rats were given
methylmercury for several days prior to mating, mated females were observed with increased
preimplantation losses (Khera 1973). Exposure of male monkeys to methylmercury for longer
durations has been shown to adversely affect sperm motility and speed and to result in increased
incidences of sperm tail defects (Mohamed et al. 1987). Decreases in spermatogenesis and tubular
atrophy of the testes have been observed upon histopathologicai analyses of the testes of mice exposed
to methylmercury chronically (Hirano et al. 1986; Mitsumori et al. 1990).
Less information is available regarding the effects of methylmercury on female reproductive
function. Exposure of female monkeys to methylmercury for 4 months prior to mating produced no
effects on the length of the menstrual cycle but resulted in decreased conceptions and increased early
abortions and stillbirths (Burbacher et al. 1988). Several studies have shown increased rates of
resorptions and abortions after exposure during gestation (Fuyuta et al. 1978; Hughes and Annau 1976;
Inouye and Kajiwara 1988a); however, it is unclear from these studies whether the effects observed are
the result of maternal reproductive failure or fetal toxicity.
Table 3-69
Reproductive Toxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex per Group
Rat/Wistar/10-20 M
Rat/Wistar/14-19 M
Mouse/Swiss Webster/1 0-20
M
Mouse/ICR/60 M, 60 F
Mouse/B6C3F!/60 M, 60 F
Monkey/Afacaca
fascicularis/3 M
Exposure
Duration
7d
Ix/d
(gavage)
95-125 d
1 x/d
5-7 d
1 x/d
(gavage)
104 wk
ad lib in feed
104 wk
ad lib in feed
20 wk
7d/wk
1 x/d
(gavage)
Dose
(mg/kg-day)
0, 1, 2.5, 5
(MMQ
0.1, 0.5, 1
(MMQ
0, 1, 2.5, 5
(MMQ
0, 0.03, 0.15,
0.72 (M); 0.02,
0.11, 0.62 (F)
(MMQ
0, 0.03, 0.14,
0.68 (M); 0.03,
0.13,
0.6 (F)
(MMQ
0, 0.047, 0.065
Effects/Limitations/BML
Reduced mean litter size after male exposure
(LOAEL = 5; rxO.Ol) in sequential mating trials
with unexposed females
BML not reported
Males were mated to unexposed females concurrent
with dosing. Reduced mean litter size (LOAEL =
0.5)
BML not reported
No effect on number of viable embryos, dead
embryos, or percent pregnancy (NOAEL = 5)
BML not reported
Significantly decreased spermatogenesis (LOAEL =
0.73; significance level not reported)
BML not reported
Tubular atrophy of the testes (LOAEL = 0.69;
p<0.01)
BML not reported
Decreased sperm motility and speed; increased
sperm tail defects (LOAEL = 0.065; p<0.05)
BML: -2200 ng/L in blood at 0.065 mg/kg-day,
approaching steady state
Reference
Khera 1973
Khera 1973
Khera 1973
Hirano et al.
1986
Mitsumori et
al. 1990
Mohamed et
al. 1987
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Table 3-69 (continued)
Reproductive Toxicity of Methylmercury in Animals
Species/
Strain/
No. per Sex per Group
Monkey/Macaco
fascicularisfJ-9 F
Exposure
Duration
4 mo prior to
mating
1 x/d
(gavage)
Dose
(mg/kg-day)
0, 0.04, 0.06,
0.08
(MMH)
Effects/Limitations/BML
Abortion; stillbirth; decreased conception in
exposed females (LOAEL = 0.06); no effect on
menstrual cyclicity
Avg. BML: 1,600 ug/L in blood at equilibrium at
0.06 mg/kg
Reference
Burbacher et
al. 1988
3.3.3.10 Genotoxicity
Data from several studies in humans suggest that ingesting methylmercury may cause
chromosomal aberrations and sister chromatid exchange (Skerrving et al. 1970, 1974; Wulf et al. 1986;
Franchi et al. 1994).
A study of nine Swedish subjects who consumed mercury-contaminated fish and 4 controls
showed a statistically significant rank correlation between blood mercury and percentage'of
lymphocytes with chromosome breaks (Skerrving et al. 1970). An extension of this study (Skerrving et
al. 1974) included 23 "exposed" (5 females and 18 males) and 16 "controls" (3 females and 13 males).
The authors reported a significant correlation between blood mercury level and frequency of chromatid
changes and "unstable" chromosome aberrations; there was no correlation with "stable" chromosome
aberrations.
The Wulf et al. (1988) study was of 92 Greenlander Eskimos. Subjects were divided into three
groups based on intake of seal meat (6 times/week; 2-5 times/week; once/week or no consumption of
seal meat). Higher frequency of SCE in lymphocytes was correlated with blood mercury
concentration; an increase of 10 ug Hg/L in blood was associated with an increase of 0.3 SCE/cell.
Positive correlations were also found for smoking, diet, living district and cadmium exposure.
Franchi et al. (1994) evaluated formation of micronuclei in peripheral blood lymphocytes of
Mediterranean fishers, a group with presumed high exposure to methylmercury. Fifty-one subjects
were interviewed on age, number of seafood-based meals/week and habits such as smoking and
alcohol consumption. Total blood mercury was measured; the range was 10.08 - 304.11 ng/g with a
mean of 88.97 + 54.09 ng/g. There was a statistically significant correlation between blood mercury
concentration and micronucleus frequency and between age and micronucleus frequency.
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Table 3-70
Genotoxicity of Methylmercury in Humans: Case Study
Species/
No. per Sex
per Group
Human/6 M, 3
F exposed; 3
M, 1 F control
Exposure
Duration
>5 yr
>3x/wk
Dose
(mg/kg-day)
NS
Effects/Limitations/BML
Correlation between blood mercury concentration and
chromosome breaks in lymphocytes cultured from people
who ate mercury-contaminated fish
Limitation: Small sample size; limited exposure data
BML Range: 4-650 ug/L in blood
Reference
Skerfving et al.
1970
Table 3-71
Genotoxicity of Methylmercury in Humans: Epidemiology Study
Species/
No. per Sex
Human/24-63
(both sexes)
Human / 51 M
Human/18M
exposed/10
control
Exposure
Duration
NS
measured as
seafood
meals/ week.
Range 2 - 14.
10.5 yr
(occup)
Dose
(mg/kg-day)
NS
NS
0.15-0.44
(HgCl2)
Effects/Limitations/BML
Incidence of sister chromatid exchanges (SCEs) in
cultured peripheral lymphocytes correlated with intake of
seal meat in an Eskimo population (as a surrogate for
mercury intake); p = 0.001. Other factors also correlated
with SCEs, but multiple regression analysis found that
some of the effect was attributable to mercury.
Limitation: Limited exposure data
BML not reported
Incidence of micronuclei positively correlated with blood
mercury concentration and with age. No correlation with
smoking or number of seafood meals /week. Limitation:
no control group.
BML range: 10.08 - 403.11 (ig/g blood.
Increased frequency of chromosomal breaks.
Limitations: Workers also exposed to mercuric chloride
and one worker had history of benzene poisoning; control
group was not matched for sex, smoking habits, or
sample size.
BML: »890 ug/L in urine (avg)
Reference
Wulf et al. 1986
Franchi et al. 1994.
Popescu et al. 1979
In a study with cats (Charbonneau et al. 1976), methylmercury did not induce dose-related
unscheduled DNA synthesis in lymphocytes or chromosomal aberrations in bone marrow cells after
oral exposure to methylmercury for up to 39 months (Miller et al. 1979). Statistically significant
decreases in unscheduled DNA synthesis and increases in chromosomal aberrations were observed, but
there was no dose-response.
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Table 3-72
Genotoxicity of Methylmercury in Cats-
Species/
Strain/
No. per Sex
per Group
Cat/Breed and
sexNS
,
Exposure
Duration
39 mo
7d/wk
Dose
(mg/kg-day)
0.008, 0.020,
0.046
Effects/Limitations/BML
No dose-related changes in unscheduled DNA synthesis
in cultured lymphocytes or frequency of chromosomal
aberrations in bone marrow of cats fed mercury-
contaminated fish or a fish diet supplemented with
methylmercunc chloride
Limitations: No positive' control; no assessment of
cytotoxicity
BML Range: 500-13,500 ug/L Hg in blood
Reference
Miller et al. 1979
Strain-specific differences exist with respect to the ability of methylmercury to produce
dominant lethal effects in mice (Suter 1975). When (SEC x C^Bl)?! males were injected with 10
mg/kg methylmefcury hydroxide, mere was a slight reduction in the total number of implantations and
a decrease in the number of viable embryos. This was not observed when (101 x C3H)F1 males were
exposed in a similar fashion. When female (10 x C3H)F! mice were treated with methylmercuric
hydroxide, no increase in the incidence of dead implants was observed (unlike the case for mercuric
chloride). Changes in chromosome number but no increase hi chromosome aberrations were observed
in oocytes of Syrian hamsters treated with one i.p injection of 10 mg/kg methylmercuric chloride
(Mailhes 1983). Methylmercury was administered s.c. to golden hamsters at doses of 6.4 mg or 12.8
mg Hg/kg/body weight. Polyploidy and chromosomal aberrations were increased in bone marrow
cells, but there was no effect on metaphase II oocytes. There was an inhibitory effect on ovulation
which the authors noted was not as severe as that induced by mercuric chloride in the same study
(Watanabe et al. 1982). Non-dysjunction and sex-linked recessive lethal mutations were seen in
Drosophila melanogaster treated with methylmercury in the diet (Ramel 1972).
As reviewed in WHO (1990), methylmercury is not a potent mutagen but is capable of causing
chromosome damage in a variety of systems. In vitro studies have generally shown clastogenic
activity but only weak mutagenic activity. Methylmercuric chloride and dimethylmercury were both
shown to induce chromosome aberrations and aneuploidy in primary cultures of human lymphocytes;
methylmercuric chloride was the mere potent clastogen at equally toxic doses (Betti et al. 1992). Both
methylmercury and mercuric chloride induced a dose dependent increase in SCE in primary human
lymphocytes and muntjac fibroblasts; methylmercury was about five time more effective in this regard
(Verschaeve et al. 1984; Morimoto et al. 1982).
Methylmercury has been shown to inhibit nucleolus organizing activity in human lymphocytes
(Verschaeve et al. 1983). Methylmercury can induce histone protein perturbations and has been
reported to interfere with gene expression in cultures of glioma cells (WHO 1990). Impaired growth
and development was noted in cultured mouse embryonic tissue treated in vitro with methylmercuric
chloride, but there was no increase hi SCE (Matsumoto and Spindle 1982). Costa et al. (1991)
showed that methylmercuric chloride caused DNA strand breaks in both V79 and rat glioblastoma cells
treated in vitro. Methylmercuric chloride produced more strand breaks than did mercuric chloride.
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Evidence of DNA damage has been observed in the Bacillus subtilis rec-assay (Kanematsu et
al. 1980). These authors reported negative results for methylmercury in spot tests for mutagenicity in
the following bacterial strains: E. coli B/r WP2 and WP2; and Salmonella typhimurium strains
TA1535, TA1537, TA1538, TA98 and TA100. Jenssen and Ramel (1980) in a review article indicated
that methylmercury acetate was negative in both micronucleus assays and in mutagenicity tests in
Salmonella; the article referred to Meddle, J.R. and W.R. Bruce (1977) and provided no experimental
details. Weak mutagenic responses for methyl mercuric chloride and methoxyethyl mercury chloride
were observed in Chinese-hamster V79 cells at doses near the cytotoxic threshold (Fiskesjo 1979), and
methylmercury produced a slight increase in the frequency of chromosomal nondisjunction in
Saccharomyces cerevisiae (Nakai and Machida 1973). Methylmercury, however, caused neither gene
mutations nor recombination in S. cerevisiae (Nakai and Machida 1973). Methylmercury retarded
DNA synthesis and produced single strand breaks in DNA in L5178Y cells (Nakazawa et al. 1975).
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4. POPULATIONS UNUSUALLY SUSCEPTIBLE TO MERCURY
A susceptible population is a group who may experience more severe adverse effects at
comparable levels or adverse effects at lower exposure levels than the general population. The greater
response of these sensitive subpopulations may be a result of a variety -of intrinsic or extrinsic factors.
Volume III describes populations that may be at increase risk because of higher exposure to mercury
and mercury compounds. Additional factors that may be important include, but are not limited to, the
following: an impaired ability of the detoxification, excretory, or compensatory processes in the body
to protect against or reduce toxicity; differences in physiological protective mechanisms (e.g., blood
brain barrier); or unique toxic reactions that are specific to the genetic makeup, developmental stage,
health status, gender or age of the individual.
The nervous and renal systems are the primary targets for mercury-induced toxicity. Data are
also available indicating some effects to the respiratory, cardiovascular, gastrointestinal, hematologic,
immune, and reproductive systems. The developing organism appears to be particularly sensitive to
methylmercury exposure. In addition, it is probable that individuals with preexisting damage or
disease hi target organs for mercury-induced toxicity may experience more severe effects upon
exposure to mercury. The populations listed below may be unusually susceptible to mercury toxicity.
• Developing Organisms. Data from epidemic poisonings in Japan (Harada 1978) and
Iraq (Marsh et al. 1987) indicate that infants exposed in utero to methylmercury
developed marked neurological development delays while their mothers experienced
little or no overt signs of toxicity. Data indicate that the developing fetus may be 5 to
10 times more sensitive than the adult (Clarkson, 1992). This difference in sensitivity
is believed to be due, in part, to the nigh sensitivity of developmental processes (i.e.,
cellular division, differentiation, and migration) to disruption by mercury (Choi et al.
1978; Sager et al. 1982). One factor that may account for this difference in sensitivity
is the presence of an incomplete blood brain barrier in the fetus. Another important
factor may be the lack of methylmercury excretion in the fetus (Grandjean et al.
1994b).
*
• Age - Infants and Other Age Groups. Available data indicate that neonates are at
increased risk to inorganic mercury and methylmercury. Both inorganic and organic
forms of mercury are excreted in breast milk (Sundberg and Oskarsson 1992; Yoshida
et al. 1992; Grandjean et al. 1994); thus, neonates in an exposed population may
experience increased mercury exposure. Animal data for rats indicate that suckling
infants retain a higher percentage of ingested inorganic mercury than do adults (Kostial
et al. 1978). The most significant difference in organ retention (neonates > adults) was
methylmercury in the brain following exposure to methylmercury (Yang et al. 1973;
Kostial et al. 1978) and inorganic mercury retained in the kidney following exposure
to elemental mercury (Yoshida et al. 1992). These differences may be associated with
an increased absorption of mercury with a milk diet, a decrease in excretion, or an
incomplete blood brain barrier (Kostial et al. 1978, Grandjean et al. 1994b).
Signs of toxicity may begin to be manifested several years after the cessation of
dosing, possibly related to subclinical effects being unmasked by aging. Rice (1989b)
dosed monkeys with methylmercury from birth to 6.5-7 years of age. Although there
were no overt signs of neurotoxicity during dosing, neurological deficits were observed
at 13 years of age, 6-7 years following cessation of exposure. Similarly, a small
June 1996 4-1 SAB REVIEW DRAFT
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human population with Minamata disease has been identified in Japan as experiencing
new or worsening neurological effects a few years following termination of mercury
exposure. This late-onset Minamata disease may be related to several factors including
aging (Igata 1993).
Gender. Sex-related differences in mercury toxicokinetics and sensitivity to mercury
have been observed, although data indicate that the more sensitive sex may differ by
species and strain. Using death as the critical endpoint, in one strain of mice,
C57BL/6N, males were less sensitive to methylmercury following daily dosing than
females while, in contrast, male mice were more sensitive than females in another
strain, BALB/cA (Yasutake and Hirayama 1988). In humans, although the ratio of
males to females with Minamata disease has been reported to be 1.2:1, the ratio of
deaths was recorded at 1.8:1 (Tamashiro et al. 1984).
Other studies are in general agreement that male rats (Thomas et al. 1986) and mice
(Nielsen and Andersen 199la, 1991b) eliminate mercury faster and have lower tissue
levels than females following dosing with methylmercury. Part of the difference in
whole-body retention of mercury in methylmercury-exposed mice has been associated
with varying degrees of deposition of mercury in the carcass, including the skin and
hair (Nielsen and Andersen 1991b). This difference is thought to be due in part to
differences in glutathione metabolism and renal excretion of mercury, which is affected
by the hormonal status of testosterone (Nielsen et al. 1994). Hirayama et al. (1987)
have reported that the toxicokinetics of methylmercury in castrated male mice was
very similar to that in female mice, and that the male pattern of methylmercury
toxicokinetics could be restored by testosterone treatment. Such differences were not
observed in a small set of similarly tested human volunteers (Miettinen et al. 1971).
Dietary Insufficiencies of Zinc. Glutathione. or Antioxidants. Mercury has been
suggested to cause tissue damage by increasing the formation of reactive oxygen
species and activation of lipoperoxidation, calcium-dependent proteolysis, endonuclease
activity, and phospholipid hydrolysis (Ali et al. 1992; LeBel et al. 1990, 1992;
Gstraunthaler et al. 1983; Verity and Sarafian 1991). Zinc, glutathione, and
antioxidant deficiencies would be expected to exacerbate mercury-induced damage by
limiting cellular defenses against the oxidative processes. Animal data support the
importance of zinc, glutathione, and antioxidants in limiting mercury-induced damage
(Fukino et al. 1992; Girardi and Elias 1991; Yamini and Sleight 1984) (see also
Section 5, Interactions).
Predisposition for Autoimmune Glomerulonephritis. Autoimmune glomeruloneph-ritis
is a form of renal toxicity characterized by proteinuria, deposition of immune material
(i.e., autoantibodies and complement C3) in the renal mesangium and glomerular blood
vessels and glomerular cell hyperplasia (Bigazzi 1992; Goldman et al. 1991; Mathieson
1992). Limited human data suggest that certain individuals may develop this
autoimmune response when exposed to inorganic or elemental mercury (Cardenas et al.
1993; Langworth et al. 1992b; Tubbs et al. 1982). While the etiology of this
syndrome has not been completely elucidated, data from susceptible and resistant
strains of animals indicate that susceptibility is governed by both major
histocompatibility complex (MHC) genes and non-MHC genes (Aten et al. 1991; Druet
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et al. 1978; Hultman and Enestrom 1992; Hultman et al. 1992; Michaelson et al. 1985:
Sapin et al. 1984).
Predisposition for Acrodynia. Acrodynia, also known as "pink disease," is a
hypersensitive response following exposure to elemental or inorganic mercury and is
characterized by the following signs and symptoms: irritability; marked mood swings:
restlessness; itching; flushing, swelling, and/or desquamation of the palms of the hands
and soles of the feet (the tip of the nose, ears, and cheeks may also be affected);
excessive perspiration; loss of appetite; tachycardia; hypertension; joint pains and
muscle weakness; photophobia; and sleeplessness. Acrodynia, which is more likely
related to exposure level rather than any inherent, genetic sensitivity, rarely occurs in
the general population.
Limited reports indicate that acrodynia has been almost exclusively observed in
children, affecting approximately 1 in 500 exposed children (Blondell and Knott 1993;
Warkany and Hubbard 1953). This disease was recently observed, in a 4-year-old
Michigan boy who was exposed to mercury vapor released from paint in which
mercury had been used as" a fungicide (Aronow et al. 1990). In this case, family
members (i.e., both parents and two siblings) were also exposed to the mercury vapors
but remained asymptomatic (Aronow et al. 1990). This case study supports the
hypothesis that there is no genetic predisposition to acrodynia.
Acrodynia was more frequently observed in the past when mercury-containing
laxatives, worming medications, teething powders and diaper rinses were widely used
(Gotelli et al. 1985; Warkany and Hubbard 1953). The physiological basis for this
hypersensitivity has not been identified. It does not appear, however, to be an allergic
reaction to mercury or to occur in the most highly exposed individuals (Warkany and
Hubbard 1953).
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5. INTERACTIONS
A number of interactions have been identified for chemicals that affect the pharmacokinetics
and/or toxicity of mercury compounds. Table 5-1 summarizes interactions in which potentiation or
protection from the toxic effects of mercury have been observed. Interactions that affect mercury
toxicokinetics are also shown. The effect on toxicity, however, can not be predicted based on changes
in distribution or excretion. For example, zinc pretreatment increases renal mercury levels but
decreases toxicity because it alters the distribution within the kidney (Zalups and Cherian 1992).
Only the interaction of selenium with mercury will be discussed in detail here. Selenium is
known to bioaccumulate in fish, so exposure to methylmercury in fish is associated with exposure to
increased levels of selenium. Where the main source pf dietary mercury is fish, the diet is naturally
enriched with selenium relative to mercury. Increased selenium has been suspected of providing some
degree of protection, either by preventing oxidative damage^ or by forming a methylmercury-selenium
complex-(Grandjean 1992a). It does not appear that the population in the Iraqi poisoning incident was
selenium deficient Animal studies have demonstrated that simultaneous ingestion of selenium may be
protective against toxicity of methylmercury based upon its antioxidant properties (see Table 5-1).
This may explain why the latent period in Japan, where the population was exposed to methylmercury
hi fish, was longer than that in Iraqi, where the exposure was to methylmercury in grain.
A common association between the metabolism of selenium and methylmercury is the thiol-
containing peptide glutathione (GSH). The metabolic cycling and oxidation-reduction of GSH are
integral processes coupled to the activation and metabolism of selenium (Hill and Burke 1982) and the
metabolism and detoxification of methylmercury (Ballatori and Clarkson 1982; Thomas and Smith
1982).
There are data to indicate that selenium co-administered with methylmercury can form
selenium-methyhnercury complexes (Magos et al 1987). The formation of these complexes appeared
temporarily to prevent methylmercury-induced tissue damage but also apparently delayed excretion of
the methylmercury in the urine. Thus, formation of selenium-methylmercury complexes may not
reduce methylmercury toxicity but may rather delay the onset of symptoms.
In support of the protective role of biological selenium, several investigators have found that a
diet supplemented with seafood high in selenium delayed the onset of methylmercury intoxication in
rats (Ganther 1980; Ohi et al. 1976). Ganther (1980) has observed that rats given selenium plus
methylmercury show increased body burdens of both selenium and methylmercury without signs of
toxicity. The accumulation of these elements may lead to mutual detoxification, but such
coaccumulation is not always linked to protection. Fair and associates (1985) have examined renal
ultrastructure changes along with changes in gamma glutamyl transferase activity in mice
coadministered both selenium and methylmercury in diet for 7 or 20 days or given a single i.p. dose.
The results of this study indicated that dietary selenium had only an initial protective effect against
mercury accumulation in the kidney; injected selenium offered longer protection.
Selenium has been shown to protect against the developmental toxicity of methylmercury in
mice (Nishikido et al. 1987; Satoh et al. 1985) and protects against oxidative damage by free radicals
(Cuvin-Aralar and Furness 1991; DiSimplicio et al. 1993; Ganther 1978). Further studies reported by
Fredricksson et al. (1993) indicated that dietary selenium supplementation during pregestation through
lactation in rats resulted in reduction of some adverse effects (hypoactivity) in neonates of the
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methylmercury administered to mothers by gavage during organogenesis period. Significant increases
in glutathione peroxidase activity were noticed in animals fed^ selenium supplemented diet.
It is possible that the protection afforded by selenium also applies to humans. Kosta et al.
(1975) have observed a coaccumulation of mercury and selenium in the organs and tissues of
mineworkers at an approximate molar ratio of 1:1. In these circumstances, the abnormally high
mercury levels detected in the tissues were without apparent adverse effects on the miners. After
exposure to mercury in the mines, several of the miners had been retired 10-16 years when the study
was conducted. The selenium intake from the diet was not reported but was said not to be abnormally
high, suggesting that the co-accumulation with mercury is a natural and autoprotective effect. It is
plausible that in areas naturally low in selenium, individuals would be at greater risk from
methylmercury poisoning than those in areas of high selenium concentration. One problem with
natural selenium is that its most abundant source (fish) is also the most abundant source of
methylmercury. The quantitative aspects of memylmercury-selenium interactions warrant further
studies in both humans and animals. % \
Table 5-1
Interactions of Mercury with Other Compounds
Compounds
Diethylmaleate and
inorganic mercury
Ethanol and
methylmercury
Ethanol and elemental
mercury
Ethanol and inorganic
mercury
Effects Observed
Increased renal toxicity
Potentiated toxicity
Increased mortality,
increased severity of
neurotoxicity, renal
toxicity
Decreased time to onset
of neurotoxicity
No data on toxicity
Decreased mercury
absorption
Increased mercury levels
in liver and in fetus
No data on toxicity
Increased mercury
exhalation
Proposed Underlying Mechanism(s)
Diethylmaleate causes depletion of nonprotein
sulfhydryls
Unknown; increased mercury concentrations were
observed in brain and kidneys, but changes in
mercury content were insufficient to fully explain
the potentiated toxicity
Inhibition of oxidation of metallic mercury to
mercuric mercury by catalase
The effect on toxicity can not be predicted, due
to the opposing effects.
Elemental mercury was exhaled, suggesting that
ethanol increased the activity of an unidentified
enzyme that reduces mercuric mercury to
elemental mercury.
Because elemental mercury, but not mercuric
mercury, can cross the blood brain barrier and
the placenta, toxicity to the brain and the
developing fetus may be increased
References
Girardi and Elias
1991
Rumbeiha et al.
1992
Tamashiro et al.
1986
Turner et al. 1981
Nielsen-Kudsk
1965
Magos and Webb
1979
Khayat and
Dencker 1982,
1984b
Dunn et al. 1981b
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Table 5-1 (continued)
Interactions of Mercury with Other Compounds
Compounds
Thiol compounds
[e.g., N-
acetylpenicillamine,
pemcillamine, 2-
mercapto propanol
(BAL)] and inorganic
mercury
Selenium and mercury
(simultaneous exposure)
Tellurium and elemental
or inorganic mercury
Potassium dichromate
and inorganic mercury
Zinc pre-treatment and
inorganic mercury
Zinc-deficiency and
inorganic mercury
Atrazine and
methylmercury
Vitamin C deficiency
and methylmercury
Vitamin E and
methylmercury
Effects Observed
Protection from renal
toxicity
Increased survival
Decreased or delayed
renal, developmental
toxicity
Decreased toxicity (effect
unspecified)
Retention in body
increased
Decreased renal function
(measured as inhibition of
p-aminohippurate
transport)
Some protection from
nephrotoxicity of
inorganic mercury
Exacerbation of renal
toxicity
Early onset of
neurotoxicity
Increased severity of
neurological damage
Increased survival and
decreased toxicity
Proposed Underlying Mechanism(s)
Competition for protein binding sites; subsequent
increases in urinary excretion of mercury
Mercuric mercury and selenium form a complex
with a high molecular weight protein
Methylmercury forms a bismethylmercury
selenide complex
Potential mechanisms for protection:
-redistribution from sensitive targets
-competition of selenium for mercury
binding sites associated with toxicity
-increased selenium available for selenium-
dependent glutathione peroxidase
(prevention of oxidative damage)
Complexation of tellurium with mercury, by
analogy to the chemically-related selenium
Unknown; both chemicals are toxic to the renal
proximal tubule
Zinc pretreatment induces metallothionein
binding in kidneys
Mercury binds preferentially to metallothionein,
so that less mercury is available to cause
oxidative damage in the proximal tubules
Zinc-deficiency and mercury both independently
increase renal oxidative stress
Together, the protective mechanisms of the
kidney are overwhelmed and oxidative damage is
compounded
Atrazine causes depletion of nonprotein
sulfhydryls
Antioxidant properties of Vitamin C and
protection against oxidative damage caused by
mercury
Protection is possibly related to antioxidant
properties of Vitamin E affording protection
against oxidative damage caused by mercury
References
Magos and Webb
1979
Parizek and
Ostadolva 1967
Satoh et al. 1985
Naganuma and
Imura 1981
Mengel and Karlog
1980
Cuvin-Aralar and
Furness 1991
Imura and
Naganuma 1991
Nylander and
Werner 1991
Magos and Webb
1979
Khayat and
Dencker 1984a
Baggett and Berndt
1984
Zalups and
Cherian 1992
Fukmo et al. 1992
Meydani and
Hathcock 1984
Yamini and Sleight
1984
Welsh 1979
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Table 5-1 (continued)
Interactions of Mercury with Other Compounds
Compounds
Potassium dichromate
and mercuric chloride
Effects Observed
Synergistic inhibition of
renal transport
Proposed Underlying Mechanism(s)
Mercuric chloride and potassium dichromate are
both toxic to renal proximal tubule
References
Baggett and Berndt i
1984
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6. HAZARD IDENTIFICATION AND DOSE-RESPONSE
ASSESSMENT
6.1 Background
Risk assessments done by U.S. EPA follow the paradigm established by the National Academy
of Sciences (NRC 1983). This entails a series of interconnected steps including hazard identification,
dose response assessment, exposure assessment and risk characterization. Two processes, hazard
identification and dose response are the focus of this chapter. Volume III of this Report presents the
exposure assessment for anthropogenic mercury emissions to the atmosphere, and Volume VI covers
the risk characterization.
Hazard identification poses the following questions: is the agent in question likely to pose a
hazard to human health; and what types of adverse effects could be expected as ^consequence of the
exposure to the agent Dose-response assessment uses available human, experimental animal and in
vitro data to estimate the exposure level or dose which is expected to produce and adverse health
effect. In accomplishing the aims of risk assessment U.S. EPA applies published Guidelines for Risk
Assessment.
6.1.1 Hazard Identification
U.S. EPA has published Guidelines for hazard identification in three areas: developmental
effects, germ cell mutagenicity, and carcinogenic effects. There are draft final Guidelines for
reproductive effects. The specific categorizations for each of those endpoints described in published
guidelines are discussed below. For general, systemic noncancer effects, there is no structured process
resulting in a categorization; instead, the hazard identification step is included in the dose-response
assessment process, wherein a critical effect is selected
6.1.1.1 Developmental Effects
Guidelines for hazard identification in the area of developmental effects were developed by
U.S. EPA in 1986 and subsequently revised (U.S. EPA 1989, 1991). The Guidelines direct that data
from all available relevant studies be considered, whether the studies indicate a potential hazard or not.
Preferred data are from human studies, when available, and animal studies. The revised guidelines do
not use an alphanumeric scheme such as that given in the carcinogenicity guidelines. Instead two
broad categories are used to characterize the health-related data base: Sufficient Evidence and
Insufficient Evidence. The Guidelines define Sufficient Human Evidence as follows:
"...data from epidemiologic studies (eg., case control and cohort) that provide
convincing evidence for the scientific community to judge that a causal relationship is
or is not supported. A case series in conjunction with strong supporting evidence may
also be used".
Sufficient Experimental Animal Evidence/Limited Human Data is described in the following way:
"The minimum evidence necessary to judge that a potential hazard exists generally
would be data demonstrating an adverse developmental effect in a single, appropriate,
well-conducted study in a single experimental animal species. The minimum evidence
needed to judge that a potential hazard does not exist would include data from
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appropriate, well-conducted laboratory animal studies (at least two) which evaluated a
variety of the potential manifestations of developmental toxicity, and showed no
developmental effects at doses that were minimally toxic to the adult."
6.1.1.2 Germ Cell Mutagenicity
The U.S. EPA (1986) has published Guidelines for classification of potential hazard of
mutagenic effects in human germ cells. Evidence from human and animal in vivo and in vitro systems
is considered in the judgement as to which of eight numerical classes of concern most clearly defines
the data on an environmental agent. In general, the hierarchy of preference for data type is the
following:
• Data on germ cells are preferred to data on somatic cells;
* \
• In vivo tests are preferred to in vitro',
• Data from tests in eukaryotes are preferred to data from prokaryotes.
The weight-of-evidence categories are these, presented in order of decreasing strength of evidence for
human germ cell mutagenicity.
1. Positive data derived from human germ cell mutagenicity studies.
2. Valid positive results from studies on heritable mutational events (any kind) in
mammalian germ cells.
3. Valid positive results from mammalian germ cell chromosome aberration studies that
do not include an intergeneration test
4. Sufficient evidence for a chemical's interaction with mammalian germ cells, together
with valid positive mutagenicity test results from two assays systems, at least one of
which is mammalian. The positive results may be both for gene mutations or both for
chromosome aberrations; if one is for gene mutations and the other for chromosome
aberrations, both must be from mammalian systems.
5. Suggestive evidence for a chemical's interaction with mammalian germ cells, together
with valid positive mutagenicity evidence from two assay systems as described under
4.
6. Positive mutagenicity test results of less strength than defined under 4, combined with
suggestive evidence for a chemical's interaction with mammalian germ cells.
7. Non-mutagenic. Although definitive proof of non-mutagenicity is not possible, a
chemical could be classified operationally as a non-mutagen for human germ cells, if it
gives valid negative results for all endpoints of concern.
8. Not classifiable based on inadequate evidence bearing on either mutagenicity or
chemical interaction with mammalian germ cells.
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These categories are intended as guidance in assessing a level of concern for an agent's
likelihood to be a germ cell mutagen. The three forms of mercury are discussed in Section 6.2.2 in
terms of level of concern rather than an assigned numerical category.
6.1.1.3 Carcinogenic effects
U.S. EPA categorizes the carcinogenic potential of a chemical, based on the overall weight-of-
evidence, according to the following scheme.
Group A: Human Carcinogen. Sufficient evidence exists from epidemiology studies
to support a causal association between exposure to the chemical and human cancer.
Group B: Probable Human Carcinogen. There is sufficient evidence of
carcinogenicity hi animals with limited (Group Bl) or inadequate (Group B2) evidence
in humans.
Group C: Possible Human" Carcinogen. There is limited evidence of
carcinogenicity hi animals in the absence of human data.
Group D: Not Classified as to Human Carcinogenicity. There is inadequate human
and animal evidence of carcinogenicity or no data are available.
Group E: Evidence of Noncarcinogenicity for Humans. There is no evidence of
carcinogenicity in at least two adequate animal tests in different species or in both
adequate epidemiologic and animal studies.
For specific guidance as to the use of human, animal and supporting data hi the above
categorization for cancer, consult the Risk Assessment Guidelines of 1986 (U.S. EPA 1987a).
U.S. EPA has been hi the process of revising its Guidelines for cancer risk assessment. The
revised Guidelines will implement the use of narrative categorization. The new^uidelines-atee^^
encourage greater use of mechanistic data, including information which can be gained from genetic
toxicology. Data which elucidate the mode of action of an agent will also have a direct impact on the
dose response assessment for carcinogenicity. In the past a default procedure for dose response
assessment was most often followed; that of linear low dose extrapolation using an upper bound on the
low dose term of a linearized multistage mathematical model. The revised Guidelines dictate that the
type of low dose extrapolation to be used, if any, be guided by information on the carcinogen's mode
of action. Evidence of genetic toxicity has now become key hi malting decisions about dose response
assessment.
The Mercury Study Report to Congress was prepared before the revised Carcinogen Risk
Assessment Guidelines were completed and approved. It was thus necessary to apply the existing
Guideline's alphanumeric categories; however, an expanded narrative was done, and the weight of
evidence judgement followed closely the revised format for expanded consideration of mechanistic
data.
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6.1.2 Dose-response Assessment
6.1.2.1 Systemic Noncancer Effects
In the quantification of systemic noncarcinogenic effects, an oral reference dose (RfD) or an
inhalation reference concentration (RfC) or both may be calculated. The oral RfD and inhalation RfC
are estimates (with uncertainty spanning perhaps an order magnitude) of a daily exposure to the human
population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious
health effects during a lifetime. The RfD and RfC are derived from a no-observed-adverse-effect level
(NOAEL), or lowest-observed-adverse-effect level (LOAEL), identified from a subchronic or chronic
study and divided by an uncertainty factor(s) times a modifying factor. The RfD or RfC is calculated
as follows:
RfD - _ (NOAEL or LOAEL)
[Uncertainty Factor (s) x Modifying Factor]
[Uncertainty Factor (s) x Modifying Factor]
The methodologies used to derive the RfD or inhalation RfC require the adjustment of
NOAEL and LOAEL values (whether from experimental animal or human studies) to lifetime
exposure conditions (i.e., 24 hours per day for a lifetime of 70 years). Inhalation RfC methods further
require conversion by dosimetric adjustment or the use of a physiologically-based pharmacokinetic
model from NOAELs and LOAELs observed in laboratory animal experiments to human equivalent
concentrations (HEC). Different default adjustments are made based on whether the observed toxicity
is in the upper or lower respiratory tract or at remote sites, and a NOAEL(HEC) or LOAEL(HEC) is
derived for use in the equation above (U.S. EPA 1990).
Selection of the uncertainty factor (UF^) to be employed in the calculation of the RfD/RfC is
based upon professional judgment which considers the entire data base of toxicologic effects for the
chemical. In order to ensure that UFs are selected and applied in a consistent manner, the U.S. EPA
(1994) employs a modification to the guidelines proposed by the National Academy of Sciences (NAS
1977, 1980), as shown in the box on the next page.
As noted in the box, the standard UF for extrapolating from animals to humans has been
reduced to three for the derivation of inhalation RfCs. A factor of three was chosen because,
assuming the range of the UF is distributed log normally, the reduction of a standard 10-fold UF by
half (i.e. 100'5) results in three. Other UFs can be reduced to three if the situation warrants, based on
the scientific judgement of the U.S. EPA RfD/RfC Work Group (an Agency peer review group).
Considerations in the selection of UFs and/or a modifying factor include, but are not limited to,
pharmacokinetics/pharmacodynamics, concomitant exposures, relevance of the laboratory animal
models, species sensitivity, severity of the effect, potential for recovery, slope and shape of the dose-
response curve, exposure uncertainties, quality of the critical study, and data gaps.
From the RfD, a Drinking Water Equivalent Level (DWEL) can be calculated. The DWEL
represents a medium-specific (i.e., drinking water) lifetime exposure at which adverse, noncarcinogenic
health effects are not anticipated to occur. The DWEL assumes 100% exposure from drinking water.
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Uncertainty Factors Used in RfDyRfC Calculations
Standard Uncertainty Factors (UFs)
Use a 10-fold factor when extrapolating from valid experimental results from studies using prolonged
exposure to average healthy humans. This factor is intended to account for the variation in sensitivity
among the members of the human population.
Use an additional 10-fold factor when extrapolating from valid results of long-term studies on
experimental animals when results of studies of human exposure are not available or are inadequate.
This factor is intended to account for the uncertainty in extrapolating animal data to risks for humans.
[10A] A 3-fold uncertainty factor is used for extrapolating from inhalation studies on experimental
animals to humans for the derivation of an inhalation RfC. This difference is because dosimetric
adjustments reduce the uncertainty associated with extrapolation between experimental animals and
humans.
Use an additional 10-fold factor when extrapolating from less than chronic results on experimental
animals when there are no useful long-term human data. This factor is intended to account for the
uncertainty in extrapolating from less than chronic NOAELs to chronic NOAELs. [10S]
Use an additional 10-fold factor when deriving an RfD from a LOAEL instead of a NOAEL. This
factor is intended to account for the uncertainty in extrapolating from-LOAELs to NOAELs. [10L]
Modifying Factor (MF)
Use professional judgment to determine another uncertainty factor (MF) that is greater than zero and
less than or equal to 10. The magnitude of the MF depends upon the professional assessment of
scientific uncertainties of the study and data base not explicitly treated above, e.g., the completeness of
the overall data base and the number of species tested. The default value for the MF is 1.
The DWEL provides the noncarcinogenic health effects basis for establishing a drinking water
standard. For ingestion data, the DWEL is derived as follows:
DWEL =
(RfD) x (Body weight in kg)
Drinking Water Volume in LJday
= — mgIL
where:
Body weight = assumed to be 70 kg for an adult
Drinking water volume = assumed to be 2 L/day for an adult
6.1.2.2 Developmental Effects
For agents considered to have sufficient evidence for developmental toxicity it is appropriate to
consider calculation of a quantitative dose-response estimate. In general, a threshold is assumed for
the dose-response curve for agents producing developmental toxicity. This is "based on the known
capacity of the developing organism to compensate for or to repair a certain amount of damage at the
cellular level. In addition, because of the multipotency of cells at certain stages of development,
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multiple insults at the molecular or cellular level may be required to produce an effect on the whole
organism" (U.S. EPA 1991).
Due to the paucity of human data, dose-response assessment of developmental toxicity is most
often done using animal data. The assessment includes the identification of dose levels associated
with observed developmental effects as well as those doses which apparently produce no adverse
effects. The critical effect is ascertained from the available data. A critical effect is defined as the
most sensitive developmental effect from the most appropriate and/or sensitive mammalian species;
LOAEL and NOAEL determinations are then made. The NOAEL is defined as "the highest dose at
which there is no statistically or biologically significant increase in the frequency of an adverse effect
in any of the possible manifestations of developmental toxicity when compared with the appropriate
control group in a data base characterized as having sufficient evidence for use in risk assessment"
(U.S. EPA 1991). The LOAEL is defined in the following manner: "The LOAEL is the lowest dose
at which there is a statistically or biologically significant increase in the frequency of adverse
developmental effects when compared with the appropriate control group in a data base characterized
as having sufficient evidence".
Because of the limitations associated with the use of the NOAEL/LOAEL approach, U.S. EPA
is investigating the use of alternative methods employing more data in a dose-response assessment.
One such approach is the estimation of a benchmark dose (BMD). This approach is based on the use
of a mathematical model to derive an estimate of an incidence level (e.g., 1%, 5%, 10%, etc). This is
done by applying a model to data in the observed range, selecting an incidence level at or near the
observed range (typically 10%), and then determining an upper confidence limit on the modelled
curve. The value of the upper limit, for a 10% incidence, is then used to derive the BMD, which is
the lower confidence limit on dose for that incidence level.
The last step in dose-response assessment is the calculation of a reference dose or reference
concentration for developmental toxicity (RfDDT or RfCDT). This is done by applying appropriate
uncertainty factors to the LOAEL, NOAEL, or BMD. Uncertainty factors generally include the
following:
• 10 for interspecies variation (animal to human)
• 10 for intraspecies variation
Additional factors may be applied to account for other areas of uncertainty, such as
identification of a LOAEL in the absence of a NOAEL. In this case, the factor may be as much as 10
fold, depending on the sensitivity of the endpoints evaluated in the data base. An uncertainty factor is
generally not used to account for duration of exposure when calculating the RfDDT. If developmental
toxicity is the critical effect for the chronic RfD, an additional uncertainty factor (for study duration)
may be used. Modifying factors may be used to deal with the degree of confidence in the data base
for the agent being evaluated. For a discussion of application of uncertainty factors to BMDs, see
U.S. EPA (1991).
6.1.2.3 Germ Cell Mutagenicity
According to U.S. EPA (1986), a dose-response assessment of an agent's potential for human
germ cell mutagenicity can presently be done using only data from in vivo heritable germ cell tests.
This will remain the case until such time as other assays are demonstrated to have an equivalent
predictability for human effects. The usable tests are, thus, limited to the following: morphological
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specific locus and biochemical specific locus assays: and heritable translocation tests. Data from such
assays are generated from exposures much higher than those expected for humans as a consequence of
environmental exposure. Estimation of extent of human risk is done by extrapolating the observed
mutation frequency or phenotypic effects downward to the expected human exposure range. Available
data and mechanistic considerations are used in the choice of the dose-response model and
extrapolation procedure.
6.1.2.4 Carcinogenic Effects
•
Mathematical models can be used, if data are sufficient, to calculate the estimated excess
cancer risk associated with either the ingestion or inhalation of the contaminant if toxicologic evidence
leads to the classification of the contaminant as one of the following: A, Known Human Carcinogen;
B, Probable Human Carcinogen; or C, Possible Human Carcinogen. The data used in these estimates
usually come from lifetime exposure studies using animals. In order to estimate the potential cancer
risk for humans from animal data, animal doses must be converted to equivalent human doses. This
conversion includes correction for noncontinuous exposure for less-than-lifetime exposure studies and
for differences in size. The factor to compensate for the size difference should be determined from
appropriate experimental data. In the absence of such data, a default value should be used, such as the
cube root of the ratio of the animal and human body weights. A default assumption is that the
average adult human body weight is 70 kg, that the average water consumption of an adult human is 2
L of water per day, and that the average adult breathes 20 m3 of air per day.
For contaminants with a carcinogenic potential, chemical levels are correlated with a
carcinogenic risk estimate by employing a cancer potency (unit risk) value together with the
assumption for lifetime exposure. The cancer unit risk has generally been derived by assuming low
dose linearity and applying a mathematical model such as a linearized multistage model with a 95%
upper confidence limit. Cancer risk estimates have also been calculated using other models such as
the one-hit, Weibull, logit and probit. There is little basis in the current understanding of the biologic
mechanisms involved in cancer to suggest that any one of these models is able to predict risk more
accurately than any other. Because each model is based upon differing assumptions, the estimates
derived for each model can differ by several orders of magnitude.
The scientific data base used to calculate and support the setting of cancer risk rate levels has
an inherent uncertainty that is due to the systematic and random errors in scientific measurement. In
most cases, only studies using experimental animals have been performed. Thus, there is uncertainty
when the data are extrapolated to humans. When developing cancer risk rate levels, several other
areas of uncertainty exist, such as the incomplete knowledge concerning the health effects of
contaminants in environmental media, the impact of the experimental animal's age, sex and species,
the nature of the target organ system(s) examined and the actual rate of exposure of the internal targets
in experimental animals or humans. Dose-response data usually are available only for high levels of
exposure and not for the lower levels of exposure closer to where a standard may be set. When there
is exposure to more than one contaminant, additional uncertainty results from a lack of information
about possible interactive effects.
6.2 Hazard Identification for Mercury
Because there are no U.S. EPA guidelines for hazard identification of systemic noncancer "
effects, this section does not include a discussion of systemic noncancer effects.
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6.2.1 Developmental Effects
6.2.1.1 Elemental Mercury
Data for developmental effects of elemental mercury are detailed in Section 3.1.3.11 (Tables 3-
25, 3-26 and 3-27). Human studies are inconclusive. The study by Mishinova et al. (1980) provided
insufficient experimental detail to permit evaluation of an exposure-response relationship. Sikorski et
al. (1987) found an increase in reproductive failure among 57 dental professionals by comparison to
controls. This reproductive failure (described as spontaneous abortions, stillbirths or congenital
malformations) was significantly correlated with exposure level. Maternal toxic signs were not
reported. The study was limited by the small population and the lack of description of the control
group. These findings were not reproduced in Ericson and Kallen's 1989 study of 8157 infants born
to dental professionals in Sweden. When compared to the general population, there was no increase in
malformations, abortions or stillbirths. Exposure data were limited in this study.
There are four animal studies evaluating potential developmental effects associated with
exposure to elemental mercury. In Baranski and Szymczyk (1973), female rats (strain not specified)
were exposed to 2.5 mg/m3 mercury vapor for 6 to 8 weeks before fertilization or for 3 weeks prior
to mating and on days 7-20 of gestation. In the first experiment, mortality among pups was increased
in the exposed group, and there were changes in pup organ weights (decreased kidney and liver weight
and increased ovary weight). In the second exposed group, mean number of live pups was decreased;
mortality among pups was 100% by day 6 post portion. There were signs of frank toxicity in the
dams including spasms, tremors and death. Information is taken from an English translation of this
Polish paper.
Steffek et al. (1987) is reported in abstract. Rats (strain not specified) were exposed to 0.1,
0.5 or 1.0 mg/m3 mercury for either the entire gestation period or for days 10-15. No effects on
resorption or gross abnormality were seen in the low-dose group. Exposure to the mid and high doses
for days 10-15 resulted hi increased numbers of resorption (5/41 and 7/71, respectively; denominators
are presumed to be numbers of litters - not specified in text); exposure for the entire gestational
period resulted in gross defects in 2/115 fetuses in the low dose and increased resorption (19/38) in the
high dose. Maternal and fetal weight was decreased in-the group exposed to 1.0 mg/m3 for the entire
gestation period. No statistical analyses were reported in the abstract
Two studies in rats focused on behavioral changes consequent to inhalation of elemental
mercury during development. In the first, Danielsson et al. (1993) exposed pregnant Sprague-Dawley
rats to mercury vapor at 1.8 mg/m3 for either 1 or 3 hours on gestation days 11-14 and 17-20. There
were no signs of toxicity in the dams and offspring of treated animals were no different from controls
on the following measures: body weight; clinical signs; pinna unfolding; surface righting reflex
development; tooth eruption; and results of a negative geotaxis test at days 7, 8 or 9 post partum.
Male rats exposed in utero were significantly hypoactive by comparison to controls at 3 months and
hyperactive at 14 months. Exposed males were unpaired in a test of habituation to novel
environments and showed decreased ability to learn a maze. They were not different from controls in
a circular swim test administered at 15 months of age. Females were tested only in the spontaneous
motor activity tests; treated females were no different from controls on this measure.
These results were similar to those reported by Fredriksson et al. (1992). In this instance rats
were exposed postnatally on days 11-17 of age to 0.05 mg Hg/m3 for either 1 or 3 hours/day. High-
dose rats showed increased activity (rearing) at 2 months but had decreased activity by comparison to
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controls at 4 months. Low-dose rats were no different from controls at 2 months; at 4 months this
group showed increased total activity and decreased rearing. In the spatial learning test administered
at 6 months low-dose rats showed increases in time to complete the task. High-dose animals were
observed to have increases in both time to complete the task and in numbers of errors. Data were not
reported on gender differences In behavior as a result of exposure to mercury vapor.
Both of these studies involved exposure during critical developmental periods, one pre-natal
and one post-natal prior to sexual maturity. Both showed differences from controls (by ANOVA) on
one of four major manifestations of developmental effects listed in the Guidelines for Developmental
Toxicity Risk Assessment (U.S. EPA 1991); namely, functional deficits, in this case in locomotion and
learning. In the Danielsson et al. (1993) paper, these, deficits were observed in male offspring in the
absence of maternal toxicity, which according to the Guidelines raises the level of concern. The studies
suggest that the observed effects are not reversible. Latency to reach a platform in the circular swim
maze was significant in the high-dose group at 15 months but not at 7 months, and total activity was
decreased in the low-dose group and increased in the high-dose group at 14 months.
The Guidelines specify that for a judgement of Sufficient Experimental Animal
Evidence/Limited Human Data the minimum data set is the following:
" The minimum evidence necessary to judge that a potential hazard exists generally
would be data demonstrating an adverse developmental effect in a single, appropriate,
well conducted study in a single experimental animal species."
As the data set for elemental mercury consists of two appropriate studies albeit with minimal
group sizes and two incompletely reported studies suggestive of effect, the judgement of Sufficient
Experimental Animal Evidence/Limited Human Data is the most appropriate.
6.2.1.2 Inorganic Mercury
Data on developmental effects of mercuric chloride are found in Section 3.2.3.8 (Tables 3-46
and 3-47). There is one study in mice of developmental effects of inhaled mercuric chloride and none
in humans. Selypes et al. (1984) reported increases in delayed ossification and dead or resorbed
fetuses as a consequence of exposure of CFLP/N mice to 0.17 and 1.6 mg Hg/m3 as mercuric chloride
in an aerosol for 4 Hours on days 9—12 of gestation. There were no statistical analyses, reporting of
blood mercury levels or evaluation of maternal toxicity.
Developmental effects following oral exposure to methylmercury are reported in five oral
studies in rats and hamsters (Table 3-47). McAnulty et al. '(1982) is reported in abstract. Oral (not
further specified) mercuric chloride was administered on days 6-15 of gestation at doses of 6, 9, 12, or
18 mg Hg/kg-day. This resulted in decreased fetal and placental weights in fetuses in the 6 mg/kg-day
group and malformation at the highest dose. The authors concluded that inorganic mercury was a
developmental toxicant only at doses which were maternally toxic.
Rizzo and Furst (1972) treated Long Evans rats with 0 or 2 mg Hg/kg-day as mercuric oxide
on gestation days 5, 12 or 19. Effects noted were growth retardation and inhibition of eye formation
in the group treated on day 5. No statistical analyses were reported, nor were blood mercury levels
given.
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Pritchard et al. (1982a) reported in abstract results of treating rats (strain not specified) with
mercuric chloride at 3.0, 6.0 or 12.0 mg Hg/kg-day for about 32 days (gestation day 15 until 25 days
postpartum). Effects included decreased pup weight and weight gain with a LOAEL of 6.0. In
another experiment reported in an abstract, Pritchard et al. (1982b) exposed female rats to mercuric
chloride doses of up to 18 mg Hg/kg-day before mating and during gestation. High implantation loss
was observed with exposure to 9 mg Hg/kg-day and higher. Embryonic and fetal development was
reported to be unaffected with doses up to 9 mg Hg/kg-day. The abstracts presented insufficient
details, and there was no reporting of statistical analyses.
Gale (1974) gavaged female hamsters (10/group) on gestation day 8 with mercuric acetate at
the following doses: 2.5, 5.0, 16.0, 22.0, 32.0, 47.0, or 63.0 mg Hg/kg-day. There were 3 control
animals. A variety of malformations and growth effects were noted hi animals treated with 16 mg/kg-
day or higher. The authors also treated hamsters via other routes. Their evaluation of efficacy hi
production of fetal effects was i.p. > i.v. > s.c. > oral. Maternal toxicity included weight loss,
diarrhea, slight tremor,, somnolence, tubular necrosis and hepatocellular necrosis (dose levels not
specified). There was insufficient detail reported for determination of a NOAEL for dams.
In addition to studies of oral or inhalation administration of inorganic mercury there are
several studies which indicate that mercury salts cause developmental toxicity when delivered i.p., s.c.
or i.v. routes (Bernard et al. 1992; Gale and Perm 1971; Gale 1974, 1981; Kajiwara and Inouye 1986,
1992; Kavlock et al. 1993). In Gale (1981), wherein exposure was of six strains of hamster to
mercuric acetate, s.c., there was no description of maternal toxicity. In Kavlock et al. (1993) (s.c.,
rats, mercuric acetate) fetal effects were noted at doses above the lowest observed maternally toxic
dose. Kajiwara and Inouye (1986) reported their opinion that in mice injected i.v. with mercuric
chloride, fetal toxicity was related to maternal toxicity. In their 1992 study, there was no determination
whether implantation loss in mercuric chloride exposed dams was due to fetal toxicity or to maternal
uterine dysfunction. The effects reported by Bernard et al. (1992) can be better characterized as a
transitory nephrotic effect rather than a developmental deficit
Each of these studies is limited in its usefulness for assessment of the risk of inorganic
mercury to cause human developmental toxicity. The data base as a whole suggests an effect of
inorganic mercury at doses as low as 2 mg Hg/kg-day. The data, however, are considered insufficient
for risk assessment based on any single study or on the database as a whole (Insufficient Evidence, in
the language of the Guidelines).
6.2.1.3 Methylmercury
Data for developmental effects of methylmercury are presented hi Section 3.3.3.8 (Tables 3-66,
3-67 and 3-68); studies are primarily by the oral route and none by the inhalation route. Human
studies of developmental effects include evaluation of children born to mothers exposed to
contaminated grain hi Iraq (Amin-Zaki et al. 1976; Marsh et al. 1981, 1987) and contaminated fish in
Japan (Harada 1978). Effects noted hi the Iraqi children included delays in speech and motor
development, mental retardation, reflex abnormalities and seizures. Infants born to mothers ingesting
fish from the contaminated Minamata Bay hi Japan appeared normal at birth. Within several months,
however, the following effects were noted: mental retardation, retention of primitive reflexes,
cerebellar symptoms, dysarthria, hyperkinesia, hypersalivation, strabismus and pyramidal symptoms.
Histologic examination of brain tissues of infants from both populations showed a number of signs of
pathology. Kjellstrom et al. (1989), in a study of a population in New Zealand, has observed an
inverse correlation between IQ in children and hair mercury levels in their mothers. In a group of
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Cree Indians in Quebec, maternal hair mercury level was correlated with abnormal muscle tone in
male children (McKeown-Eyssen et al. 1983).
Numerous animal studies have demonstrated a variety of developmental effects occurring an
rats, mice and monkeys exposed orally to methylmercury and are presented in Chapter 3 (Table 3-68).
Developmental effects have been observed in offspring of rats of three strains treated orally with
methylmercury. Developmental effects have also been seen in two strains of mice as well as in guinea
pigs, hamsters and monkeys.
In rodents exposed in utero, decreased fetal weight and increased fetal malformations and
deaths have been reported (Fuyuta et al. 1978, 1979; Inouye and Kajiwara 1988a; Inouye and
Murakami 1975; Khera and Tabacova 1973; Nolen et al. 1972; Reuhl et al. 1981; Yasuda et al. 1985).
Methylmercury exposure during gestation as well as during the lactation period produces
neurodevelopmental effects (structural and-functional alterations) in the exposed pups. Structural
effects include lesions in the brain mantle, corpus callosum, caudate putamen, and cerebellum. In
guinea pigs, early gestational exposures (weeks 3-5 of pregnancy) resulted primarily in developmental
disturbances of the brain (smaller brains, dilated lateral ventricles, and reduced size of caudate
putamen), whereas later gestational exposures (>week 6 of pregnancy) resulted in widespread neuronal
degeneration (Inouye and Kajiwara 1988b). Functional changes include abnormal tail position during
walking, flexion, hindlimb crossing, decreased locomotor activity, increased passiveness and startle-
response, impaired maze performance, operant behavior, swimming behavior, tactile-kinesthetic
function, visual recognition memory, and temporal discrimination (Bornhausen et al. 1980; Buelke-
Sam et al. 1985; Burbacher et al. 1990; Eisner 1991; Geyer et al. 1985; Gunderson et al. 1988;
Hughes and Annau 1976; Inouye et al. 1985; Musch et al. 1978; Olson and Boush 1975; Rice 1992;
Rice and Gilbert 1990; Stoltenburg-Didinger and Markwort 1990; Suter and Schon 1986).
While there are limitations to some of these studies (e.g., lack of information on BML, small
study size), the totality of the data base supports a judgment of Sufficient Human and Animal Data for
Developmental Toxicity of methylmercury.
6.2.2 Germ Cell Mutagenicity
6.2.2.1 Elemental Mercury
Data for genotoxicity of elemental mercury are described in Section 3.1.3.13 (Table 3-30).
Results for an association of somatic cell chromosomal effects with occupational exposure to elemental
mercury are variable. Popescu et al. (1979) and Verschaeve et al. (1976) reported increased incidence
of aberrations or aneuploidy. Most recently Barregard et al. (1991) showed a significant correlation
between cumulative exposure to elemental mercury and micronuclei induction in T-lymphocytes.
Negative results were reported by Verscheave et al. (1979) and Mabille et al. (1984). No studies of
mutagenic effect are reported.
Elemental mercury once absorbed is widely distributed throughout the body; there are no data,
however, on elemental mercury in gonadal tissue. Based on both positive and negative findings for
somatic cell chromosomal aberrations in workers, elemental mercury is placed in a group of low
confidence for potential as a human germ cell mutagen.
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6.2.2.2 Inorganic Mercury
Data for genotoxic effects of inorganic mercury are described in Section 3.2.3.10 (Table
3-50). There are no data on inorganic mercury from human germ cell mutagenicity studies or from
studies on heritable mutational events in animals. Anwar and Gabal (1991) reported a statistically
significant increase by comparison to age-matched controls in both chromosomal aberrations and
micronuclei in lymphocytes of workers exposed to mercury fulminate. There was a correlation
between frequency of aberrations and exposure duration. Elemental mercury has been shown to be
clastogenic both in vivo and in vitro. Results of tests for mutagenicity have been variable; generally
test results hi prokaryotes are negative for mutagenicity (but may be positive for DNA damage), and
results in eukaryotes are positive. Suter (1975) observed a small, but statistically significant increase
in non-viable implants when female mice were administered mercuric chloride intraperitoneally; the
authors were not certain whether this was a true dominant lethal effect or was attributable to maternal
toxicity.
Chromosome aberrations were observed in somatic cells in occupationally exposed humans
(Anwar and Gabal 1991), in somatic cells of mice exposed by gavage (Ghosh et al. 1991), and in
Chinese Hamster Ovary cells treated in vitro (NTP 1993; Howard et al. 1991). Sex-linked recessive
mutations were not observed in Drosophila (NTP 1993), and positive results in a dominant lethal test
were compromised by maternal toxicity (Suter 1975). There are other data for DNA damage and
limited data for gene mutation. Inorganic mercury is less well-distributed in the body than is
elemental mercury; it does not readily pass blood-brain or placenta! barriers. In one reported study
(Jagiello and Lin 1973), mice treated intraperitoneally were not shown to have an increased incidence
of aneuploidy in spermatogonia. Watanabe et al. (1982), however, showed that while hamsters
injected s.c with mercuric chloride had no increase in aberrations in metaphase II oocytes, there was
detectable mercuric chloride hi ovaries and some inhibition of ovulation.
The totality of available data indicates a moderate weight of evidence for germ cell
mutagenicity: sex-linked recessive and dominant lethal results were compromised, but there are
positive results for chromosomal aberrations in multiple systems (including in vivo exposure) and
evidence that mercuric chloride can reach female gonadal tissue.
6.2.2.3 Methylmercury
Summaries of data for genotoxicity of methylmercury are presented in Section 3.3.3.10 (Tables
3-70, 3-71 and 3-72).
Methylmercury appears to be clastogenic but not a potent mutagen. Methylmercury is widely
distributed hi the body, breaching both blood-brain and placenta! barriers in humans. There are data
indicating that methylmercury administered i.p. reaches germ cells and may produce adverse effects.
Suter (1975) observed a slight reduction in both numbers of implantations and viable embryos in (SEC
x C57Bl)Fj females which had been mated to treated males. This was not noted in (101 x C3H)?!
mice. When Syrian hamsters were treated intraperitoneally with methylmercury, aneuploidy but not
chromosomal aberrations was seen in oocytes. Sex-linked recessive lethal mutations were increased in
Drosophila melanogaster given dietary methylmercury. Watanabe et al. (1982) noted some decrease
in ovulation in hamsters treated s.c. with methylmercury, further indication that methylmercury is
distributed to female gonadal tissue.
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Studies have reported increased incidence of chromosome aberrations (Skerfving et al. 1970,
1974) or SCE (Wulf et al. 1968) in lymphocytes of humans ingesting mercury-contaminated fish or
meat. Chromosome aberrations have been reported in cats treated in vivo and in cultured human
lymphocytes in vitro. Evidence of DNA damage has been shown in a number of in vitro systems.
As there are data for mammalian germ cell chromosome aberration and limited data from a
heritable mutation study, methylmercury is placed in a group of high concern for potential human
germ cell mutagenicity. All that keeps methylmercury from the highest level of concern is lack of
positive results in a heritable mutation assay.
6.2.3 Carcinogenic Effects
This section presents the critical carcinogenicity studies evaluated by the U.S. EPA for the
weight-of-evidence classification of elemental, inorganic (mercuric chloride) and organic
(methylmercury) forms of mercury. These studies are discussed more completely in Chapter 3 and
summarized in Tables 3-1, 3-31, 3-32, 3-33, 3-52, and 3-53.
6.2.3.1 Elemental Mercury
Human data regarding the carcinogenicity of inhalation of elemental mercury are insufficient to
determine whether such exposures may result in increased cancer incidence. Several studies report
statistically significant increases in lung cancer mortality among groups of exposed workers (Amandus
and Costello 1991; Barregard et al. 1990; Buiatti et al. 1985; EUingsen et al. 1992). The interpretation
of these studies is limited by small sample sizes, probable exposure to other known lung carcinogens,
failure to consider confounders such as smoking and failure to observe correlations between estimated
exposure and the cancer incidence. A study of dental professionals found a significant increase in the
incidence of glioblastomas (Ahlbom et al. 1986). It is not known whether exposure to mercury, X-
rays, or other potential carcinogens in the workplace contributed to the effects observed. No increase
in cancer mortality was observed among workers exposed to mercury vapor in a nuclear weapons
facility (Cragle et al. 1984), but this study was also limited by the small sample size. No studies were
identified that examined cancer incidence in animals exposed chronically to elemental mercury vapor.
These studies are presented in greater detail in Section 3.1.2.
The overall findings from cytogenetic monitoring studies of workers occupationally exposed to
mercury by inhalation provide very limited evidence of genotoxic effects. Popescu et al. (1979)
compared four men exposed to elemental mercury vapor with an unexposed group and found an
increased number of chromosomal aberrations. Verschaeve et al. (1979) found an increased incidence
of aneuploidy after exposure to low concentrations.
In summary, human epidemiological studies failed to show a correlation between exposure to
elemental mercury vapor and increased cancer incidence, but the studies are limited by confounding
factors. Only one study in animals is reported (Druckrey et al. 1957); tumors were found only at
contact sites, and the study is incompletely reported as to controls and statistics. Animal data are, thus,
also inadequate. Findings from assays for genotoxicity are limited and provide no convincing
evidence that mercury exposure has an effect on the number or structure of chromosomes in human
somatic cells. The most appropriate category is, thus, Group D, not classifiable as to human
carcinogenicity.
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This classification was reviewed by the Carcinogen Risk Assessment Verification Endeavor
(CRAVE), an Agency Peer Review Work Group. The classification was accepted as appropriate on
March 3, 1994.
6.2.3.2 Inorganic Mercury
There are no data available on the carcinogenic effects of inorganic mercury (mercuric
chloride) in humans. In animals, there is equivocal evidence of carcinogenicity in rats and mice. In
rats gavaged with mercuric chloride for two years (NTP 1993), survival was significantly reduced in
males (17% and 8% survival in low and high-dose males versus 43% survival in controls), indicating
that the maximally tolerated dose (MTD) was exceeded. There was an increased incidence of
forestomach squamous cell papillomas (0/50, 3/50, 12/50 in control,-low, and high-dose males,
respectively; 0/50, 0/49 and 2/50 in control, low and high-dose females, respectively). Papillary
hyperplasia of the forestomach was also significantly elevated in both male dose groups and in high-
dose females. In addition, the incidence of thyroid follicular cell carcinomas in treated males (1/50,
2/50 and 6/50 in control, low- and high-dose males, respectively) showed a significantly positive trend.
There were, however, no increases in thyroid hyperplasia of adenomas; it is not clear that the increase
in thyroid carcinomas is a treatment-related effect. The NTP also considered the forestomach tumors
to be of limited relevance to humans; there was no evidence that these contact site tumors progressed
to malignancy.
In a companion study in mice (NTP 1993), there was a significantly increasing trend for renal
tubular cell tumors (adenomas and adenomacarcinomas). No dose groups were statistically
significantly different from the control by pair-wise comparison, although the incidence in the high-
dose group was elevated. There was a significant increase in severe nephropathy hi treated animals.
The NTP studies and two nonpositive bioassays are summarized in Section 3.2.2.
In summary, there are no data hi humans Unking mercuric chloride with carcinogenic effects.
Data in animals are limited. Focal hyperplasia and squamous cell papillomas of the forestomach as
well as thyroid follicular adenomas and carcinomas were observed in male rats gavaged with mercuric
chloride. In the same study, evidence for increased incidence of squamous cell forestomach
papillomas in female rats and renal adenomas and carcinomas in male mice were considered equivocal.
All increased tumor incidences were observed at what were considered high doses (in excess of the
MTD). In this context, the relevance of the thyroid tumor to human health evaluation has been
questioned; these tumors are considered to be secondary to the hyperplastic response. Results from in
vitro and in vivo tests for genotoxicity have been mixed with no clear indication of a strong somatic
cell genotoxic effect of mercuric chloride exposure.
Based on the absence of human data and limited data for carcinogenicity in animals, mercuric
chloride is classified as Group C, possible human carcinogen. This classification was reviewed by
CRAVE on March 3, 1994 and found to be appropriate.
6.2.3.3 Methylmercury
The available human data are inconclusive regarding the carcinogenicity of methylmercury in
humans exposed by the oral route. A study of leukemia patients from a rural area in Poland showed a
significantly higher mercury content in hair in the leukemia patients than in healthy unrelated patients
or healthy relatives (Janicki et al. 1987). The population studied was small, and the study did not
adjust for other leukemia risk factors. In addition, two studies of larger populations exposed to
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methylmercury during the Minamata incident failed to show increases in leukemia or total cancer
incidence (Tamashiro et al. 1984, 1986). Although one of these studies showed a significant increase
in liver cancer incidence, factors other than mercury exposure were likely contributors to the increase.
These epidemiological studies are presented in greater detail in Section 3.3.2.1.
Animal studies show some evidence of carcinogenicity in two strains of mice, but studies in
rats have not shown similar results. Male ICR mice given methylmercuric chloride in the diet for up
to two years had significantly increased incidences of renal epithelial adenomas and/or
adenocarcinomas (Hirano et al. 1986; Mitsumori et al. 1981). Similarly, male B6C3F1 mice given
methylmercuric chloride in the diet for up to two years had significantly increased incidences of renal
epithelial carcinomas and adenomas (Mitsumori et al. 1990). In contrast, Sprague-Dawley rats
administered methylmercury in the diet for up to 130 weeks exhibited no increase in tumor incidence
(Mitsumori et al. 1983, 1984). Although the dose was lower in the rats than in the mice, a maximally
tolerated dose was achieved in the rat study as evidenced by an approximately 20-30% decrease in
body weight gain and by significant increases in renal and neuronal toxicity in both male and female
rats at the highest dose tested. Other studies also failed to show increases in tumor incidence after
chronic exposure to methylmercury (Schroeder and Mitchener 1975; Verschuuren et al. 1976), but
these studies were limited by small sample sizes, failure to achieve a maximally tolerated dose and/or
incomplete histopathological examinations. These studies are presented more completely in Section
3.3.2.2.
In summary, data for carcinogenicity from human studies are considered inadequate. Three
studies that examined the relationship between methylmercury exposure in humans and increased
incidence of cancer were limited by poor study design or incomplete description of methodology or
results. Data from animal studies are considered to provide limited evidence of carcinogenicity. Male
ICR and B6C3F1 mice exposed to methylmercuric chloride in the diet were observed with increased
incidence of renal adenomas, adenocarcinomas and carcinomas. Tumors were observed at a single
site, in a single species and sex. Renal epithelial cell hyperplasia and tumors were observed only in
the presence of profound nephrotoxicity; tumors were suggested to be consequent to reparative changes
in the affected organs. Although genotoxicity test data suggest that methylmercury is clastogenic,
there are also negative tests.
The limited data in animals above support a categorization of Group C, possible human
carcinogen. The CRAVE Work Group accepted this weight-of-evidence judgment as appropriate at its
March 3, 1994 meeting.
6.3 Dose-Response Assessment For Mercury
6.3.1 Systemic Noncancer Effects
6.3.1.1 Oral Reference Doses (RfDs)
Elemental mercury
Metallic mercury is only slowly absorbed by the gastrointestinal tract (-0.01%) and because
of this is thought to be of no lexicological consequence (Klaassen et al. 1986) when ingested. Further
discussion of an RfD for this form of mercury is not presented.
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Inorganic mercury (mercuric chloride)
An RfD for inorganic mercury of 3x10~4 mg/kg-day has been verified by the RfD/RfC Work
Group. The critical effect serving as the basis for the RfD is kidney toxicity due to an auto-immune
disease caused by the accumulation of IgG antibodies in the glomerular region of the kidneys.
On October 26 and 27 of 1987, a panel of mercury experts met at a Peer Review Workshop
convened by U.S. EPA for the purpose of reviewing outstanding issues concerning the health effects
and risk assessment of inorganic mercury (U.S. EPA 1987). The panel participants are listed in
Appendix C. Five consensus conclusions and recommendations were agreed to as a result of this
workshop; these are presented in Table 6-1. The RfD was determined using data on autoimmune
glomerulonephritis observed in rats. Based on three studies using the Brown-Norway rat, a DWEL
value was determined using studies described below. The Brown-Norway rat is very sensitive to this
mercuric mercury-induced autoimmune effects, although this effect has also been demonstrated in
other strains of rats and other species of experimental animals (Andres 1984; Bernaudin et al. 1981;
Hultman and Enestrom 1992). The Brown-Norway rat is believed to be a good surrogate for the study
of mercury-induced kidney damage in sensitive humans (U.S. EPA 1987b). The glomerulonephritis is
characterized by deposition of anti-glomerular basement membrane antibodies (IgG) in renal glomeruli
and after prolonged exposure is often accompanied by proteinuria and, in some cases, nephrosis (Druet
et al. 1978).
LOAEL values were identified from three individual studies. In Druet et al. (1978), Brown-
Norway rats were exposed to mercuric chloride via subcutaneous injection, 3 times/week, for 8 weeks.
The dose levels administered were 0, 0.1, 0.25, 0.5, 1.0 and 2.0 mg Hg/kg, and there were
6-20 animals/group. An additional group of animals received 0.05 mg Hg/kg for 12 weeks. (The
number of animals/sex was not stated.) Druet and colleagues measured antibody formation (using a
fluoresceinated sheep anti-rat IgG antiserum) and urinary protein levels. Proteinuria occurred at doses
>0.1 mg/kg (LOAEL); the proteinuria was considered a highly deleterious effect, as it frequently led to
hypoalbuminemia and even death. A LOAEL for lifetime exposure was calculated to be 0.226 mg/kg-
day, using the following conversion:
0.05 mg/kg x 3 days/7 days x 0.739 [HgCl2 -» Hg2+] x 100% absorption/7%
= 0.226 mg Hg/kg-day
In a 60-day study conducted by Bernaudin et al. (1981), Brown-Norway rats (5/group) were
force-fed 0 or 3 mg/kg/week mercuric chloride. At the end of the 60 days, there were no classic
histological abnormalities in the kidneys of treated animals. Using immunofluorescence, however, IgG
deposition was evident in all of the treated rats, and weak proteinuria was noted in 3/5 dosed animals.
A lifetime LOAEL was calculated to be 0.317 mg Hg/kg-day. Dose conversion was done in the
following manner:
3 mg/kg x 1 day/7 days x 0.739 [HgCl2 -» Hg2+]
= 0.317 mg Hg/kg-day
June 1996 . • 6-16 SAB REVIEW DRAFT
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Table 6-1
Consensus Decisions of Peer Review Panel
The most sensitive adverse effect for mercury risk assessment is formation of mercuric
mercury-induced autoimmune glomerulonephritis. The production and deposition of IgG
antibodies to the glomerular basement membrane can be considered the first step in the
formation of this mercuric mercury-induced autoimmune glomerulonephritis.
The Brown-Norway rat should be used for mercury risk assessment. The Brown-Norway
rat is a good test species for the study of Hg2+-induced autoimmune glomerulonephritis.
The Brown-Norway rat is not unique hi this regard (i.e., this effect has also been observed
in rabbits).
The Brown-Norway rat is a good surrogate for the study of mercury-induced kidney
damage in sensitive humans. For this reason, the uncertainty factor (for interspecies
variability) used to~calculate criteria and health advisories (based on risk assessments using
the Brown Norway rat) should be reduced by 10-fold.
Hg2+ absorption values of 7% from the oral route and 100% from the subcutaneous route
should be used to calculate criteria and health advisories.
A DWEL of 0.010 mg/L was recommended based on the weight of evidence from the
studies using Brown Norway rats and limited tissue data.
Similar results were obtained by Andres (1984). Five Brown-Norway rats were exposed to
3 mg/kg mercuric chloride via gavage 2 times/week for 60 days. In this same study, Lewis rats (n=2)
were also exposed using the same dosing regimen. After 60 days, the kidneys of all treated animals
appeared normal histologically, and no proteinuria was reported in any treated animals; IgG deposition
in the renal glomeruli was demonstrated using immunofluorescence in Brown-Norway rats. No*
antibody deposition was noted in the Lewis rats. The lifetime LOAEL was determined to be 0.633 mg
Hg/kg-day. Dose conversion was done in the following manner:
3 mg/kg x 2 days/7 days x 0.739 [HgCl2 -» Hg2+]
= 0.633 mg Hg/kg-day
As the result of intensive review of these and other studies, as well as the discussions of the
panel of mercury experts convened for this purpose, a recommended DWEL of 0.01 mg/L was derived
from the LOAELs above, and, subsequently, the oral RfD value was back-calculated:
DWEL x 2 L/day
70 kg bw
RfD = 0.010 mg/L x 2L/day/70 kg bw
= 0.0003 mg/kg bw/day
June 1996 6-17 SAB REVIEW DRAFT
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The RfD for inorganic mercury was reviewed by the RfD/RfC Work Group which reached
consensus for verification on November. 16, 1988. The Work Group agreed to application of an
uncertainty factor of 1000 to the LOAELs above (which ranged from 0.23 to 0.63 mg Hg/kg-day).
The uncertainty factor was composed of a 10-fold each for subchronic to chronic and LOAEL to
NOAEL extrapolation, and an additional 10-fold factor for both animal to human and sensitive
populations. The resulting RfD of 3xlO'4 mg/kg-day was given high confidence based on the weight
of the evidence from the studies using Brown Norway rats and the entirety of the data base.
A literature search for the years 1988 to 1994 has been conducted and recently reviewed
(September 1994). The NTP (1993) study was among those considered/ A rat NOAEL of 0.23 mg
Hg/kg administered dose has been identified for renal effects for the 6-month portion of the study. A
description of the NTP gavage study has been included in the summary information for IRIS. U.S.
EPA concluded that no change in the RfD for inorganic mercury is needed at this time.
^ Methvlmercurv
U.S. EPA has on two occasions published RfDs for methylmercury which have represented the
Agency consensus for that time. These are described in the sections below. At the time of the
generation of the Mercury Study Report to Congress, it became apparent that considerable new data on
the health effects of methylmercury hi humans were emerging. Among these are large studies of fish
or fish and marine mammal consuming populations in the Seychelles and Faroes Islands. Smaller
scale studies are in progress which describe effects in populations around the U.S. Great Lakes. In
addition, there are new evaluations, including novel statistical approaches and application of
physiologically-based pharmacokinetic (PBPK) models, to published work described in'section 3.3.1.1
of this volume.
As the majority of these new data are either not yet published or have not yet been subject to
rigorous review, it was decided that it was premature for U.S. EPA to make a change in the
methylmercury RfD at this time. An inter-agency process, with external involvement will be
undertaken for the purpose of reviewing these new data, their evaluations, and the evaluations of
existing data. An outcome of this process will be an assessment by U.S. EPA of its RfD for
methylmercury to determine if a change is warranted.
Former RfD
A hazard identification and dose-response assessment was proposed for methylmercury in 1980
(U.S. EPA 1980) and later verified by the RfD/RfC Work Group on December 2, 1985. This
assessment was subsequently included on U.S. EPA's Integrated Risk Information System (IRIS). The
critical effects were multiple central nervous system (CNS) effects including ataxia and paresthesia in
populations of humans exposed to methylmercury through consumption of contaminated grain
(summarized by Clarkson et al. 1975, Nordberg and Strangert 1976 and WHO 1976); see study
descriptions in Section 3.3.
The RfD for methylmercury was determined to be 3x10~4 mg/kg-day, based on a LOAEL of
0.003 mg/kg-day (corresponding to 200 ug/L blood concentration) and an uncertainty factor of 10 used
to adjust the LOAEL to what is expected to be a NOAEL. An additional uncertainty factor of 10 for
sensidve individuals for chronic exposure was not deemed necessary at the time of the RfD's
verification, as the adverse effects were seen in what was regarded as a sensitive group of individuals,
namely adults who consumed methylmercury-contaminated grain.
June 1996 6-18 SAB REVIEW DRAFT
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Medium confidence was ascribed to the choice of study, data base and RfD. The blood levels
associated with the LOAEL were well supported by more recent data, but neither the chosen studies
nor supporting data base described a NOAEL. Medium confidence indicates that new data may
change the assessment of the RfD.
Since the time of verification, several submissions to IRIS have questioned the value of this
RfD, and, specifically, whether or not this RfD is protective against developmental effects.
Subsequent to the RfD verification, the effects in Iraqi children of in utero exposure to methylmercury
were reported by Marsh et al. (1987). Discussion of the methylmercury RfD by the RfD/RfC Work
Group was reported in 1992 and 1994. Consensus for verification of the RfD described below was
reached in January of 1995.
Current U.S. EPA RfD
\
Determination of critical effect
Marsh et al. (1987) was chosen as the most appropriate study for determination of an RfD
protective of a putative sensitive subpopulation; namely infants born to mothers exposed to
methylmercury during gestation. This paper describes neurologic abnormalities observed in progeny of
women who consumed bread prepared from methylmercury-treated seed grain while pregnant (See
Chapter 3 for study description). Among the signs noted in the infants exposed during fetal
development were cerebral palsy, altered muscle tone and deep tendon reflexes as well as delayed
developmental milestones (i.e., walking by 18 months and talking by 24 months). Each child in the
study was examined by two neurologists who scored observed effects on a scale for severity ranging
from 0 to 11. The data collected by Marsh et al. (1987) summarize clinical neurologic signs of 81
mother and child pairs. From x^ray fluorescent spectrometric analysis of selected regions of maternal
scalp hair, concentrations ranging from 1 to 674 ppm mercury were determined, then correlated with
clinical signs observed in the affected members of the mother-child pairs. Among the exposed
population there were affected and unaffected individuals throughout the exposure range.
Method employed for determination of critical dose
In order to quantitate an average daily mercury ingestion rate for the mothers, hair
concentrations were determined for periods-during gestation when actual methylmercury exposure had
occurred. This procedure is possible since hair grows an average rate of 1 cm/month (Al-Shahristani
et al. 1976) and since Iraqi women wear their hair very long; appropriate samples were, thus, available
for the period of gestation when exposure occurred.
A number of laboratory studies support a correlation between hair concentrations and
concurrent blood concentrations. Some variation in the ratio exists; a ratio of 250:1 (ug mercury/mg
in hair: jag mercury/ml of blood) was used to derive the RfD critical dose. A more complete discussion
for the choice of this ratio is provided hi the next section.
The hair concentration at a hypothetical NOAEL for developmental effects was determined by
application of a benchmark dose approach (see subsequent section for discussion of methods and data
used). The analysis used the combined incidence of all neurological effects in children exposed in
utero as reported in the Marsh et al. (1987) study. A Weibull model for extra risk was used to
determine the benchmark dose of 11 ppm mercury in maternal hair (11 mg/kg hair). This was
converted to 44 ng/L blood using the above 250:1 ratio.
June 1996 6-19 SAB REVIEW DRAFT
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11 mg/kg hair / 250 =44 fig/L blood
To obtain a daily dietary intake value of methylmercury corresponding to a specific blood
concentration, factors of absorption rate, elimination rate constant, total blood volume and percentage
of total mercury that is present in circulating blood were taken into account. Calculation was by use
of the following equation based on the assumptions that steady state conditions exist and that flrst-
order kinetics for mercury are being followed.
C x b x V
—-——
A xf
Whe^e:
d = daily dietary intake (expressed as ug of methylmercury)
C = concentration in blood (expressed as 44 ug/liter)
b = elimination constant (expressed as 0.014 days"1)
V = volume of blood in the body (expressed as 5 liters)
A = absorption factor (expressed as a unitless decimal fraction of 0.95)
f = fraction of daily intake taken up by blood (unitless, 0.05)
Solving for d gives the daily dietary intake of mercury which results in a blood mercury concentration
of 44 ug/L. To convert this to daily ingested dose (ug/kg-day) a body weight of 60 kg was assumed
and included in the equation denominator.
c xbx V
A xfx bw
44 (iglL x 0.014 days' x 5L
0.95 x 0.05 x 60 kg
1.1 fjglkg-day
The dose d (1.1 ug/kg-day) is the total daily quantity of methylmercury that is ingested by a 60 kg
individual to maintain a blood concentration of 44 ug/L or a hair concentration of 11 ppm.
The rationales for use of specific values for equation parameters follow.
Hair to blood concentration ratio. The hairblood concentration ratio for total mercury is
frequently cited as 250:1 expressed as ug mercury/g hair to ug mercury/ml of blood. Ratios reported
in the literature range from 140 to 416, a difference of about a factor of 3. Table 6-2 provides the
results of 12 recent studies in which hair to blood ratios were calculated for a variety of human
populations. Differences in the location of hair sampled (head versus chest and distance from scalp)
may contribute to the differences observed. Variability in the hair-blood relationship for mercury
concentration can also be attributed to the fact that unsegmented hair analysis gives a time-weighted
average of mercury exposure, while analysis of mercury in blood reflects a much shorter period
June 1996 6-20 - SAB REVIEW DRAFT
-------
average of exposure. As much as a 3-fold seasonal variation in mercury levels was observed in
average hair levels for a group of individuals with moderate to high fish consumption rates, with
yearly highs occurring in the fall and early winter (Phelps et al. 1980; Suzuki et al. 1992). The
relatively high ratio reported by Tsubaki (Table 6-2) may have reflected the fact that mercury levels
were declining at the time of sampling so that the hair levels reflect earlier, higher blood levels.
Cernichiari et al. (1995a) reported a maternal hairblood ratio of 416:1 for residents of the Seychelles
Islands. The authors remarked that while this ratio was high, statistical uncertainties do not permit a
judgement as to whether it is truly outside the range reported in WHO (1990) recapitulated in Table 6-
2. Phelps (1980) obtained multiple blood samples and sequentially analyzed lengths of hair from
individuals. Both hair and blood samples were taken for 339 individuals in Northwestern Ontario.
After reviewing the various reports for converting hair concentrations to blood concentrations, the
analysis in the Phelps (1980) paper was selected by the Agency RfD/RfC Work Group because of the
large sample size and the attention to sampling and analysis that was made. The ratio Phelps observed
between the total mercury concentration in hair taken close to the scalp and simultaneous blood
sampling for this group was 296:1. To estimate the actual ratio the authors assumed that blood and
hair samples were taken following complete cessation of methylmercury intake. They also assumed a
half-life of methyhnercury in blood of 52 days and a lag of 4 weeks for appearance of the relevant
level in hair at the scalp. Phelps also determined that 94% of the mercury in hair is methylmercury.
Based on these assumptions, they calculated that if the actual hair:blood ratio were 200:1, they would
have observed a ratio of 290. Based on these and other considerations, Phelps states that the actual
ratio is "probably higher than 200, but less than'the observed value of 296." As the authors point out,
one-third of the study population was sampled during the rising phase of seasonal variation ( and two-
thirds or more in the falling phase). Phelps et al. (1980) had assumed that all were sampled in the
falling phase. This fact would tend to result in a lower observed ratio; therefore, the actual average is
likely to be greater than 200. It was concluded by U.S. EPA that a midpoint value of 250 is
acceptable for the purpose of estimating average blood levels in the Iraqi population.
Fraction of mercury in diet that is absorbed (A). After administration of radiolabeled
methylmercuric nitrate in water to 3 healthy volunteers, uptake was reported to be >95%. (Aberg et al.
1969). This value is supported by experiments in human volunteers conducted by Miettinen et al.
(1971). These researchers incubated fish liver homogenate with radiolabeled methylmercury nitrate to
produce methylmercury proteinate. The proteinate was then fed to fish for a week; the fish were
killed, cooked and fed to volunteers after confirmation of methylmercury concentration. Mean uptake
exceeded 94%. Based upon these experimental results an absorption factor of 0.95 was used in these
calculations.
Fraction of the absorbed dose that is found in the blood (f). There are three reports of the
fraction of absorbed methylmercury dose distributed to blood volume hi humans. Kershaw et al.,
(1980) reported an average fraction of 0.059 of absorbed dose in total blood volume, based on a study
of 5 adult male subjects who ingested methylmercury-contaminated tuna. In a group of 9 male and 6
female volunteers who had received 203Hg-methylmercury in fish, approximately 10% of the total
mercury body burden was present in one liter of blood in the first few days after exposure; this
dropped to approximately 5% over the first 100 days (Miettinen et al. 1971) In another study, an
average value of 1.14% for the percentage of absorbed dose in one kg of blood was derived from data
on subjects who consumed a known amount of methylmercury in fish over a 3-month period (Sherlock
et al. 1984). Average daily intake in the study ranged from 43 to 233 ug/day, and there was a dose-
related effect on percentage of absorbed dose that ranged from 1.03% to 1.26% in one liter of blood.
Each of these values was multiplied by 5 to yield the total amount in the blood compartment, as there
are approximately 5 liters of blood in an adult human body (0.01 x 5 = 0.05). The value 0.05 has
June 1996 6-21 SAB REVIEW DRAFT
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Table 6-2
Available Data on HainBlood Ratio (total Hg)
*
Reference
Sumari et al., 19691
Sana et al., 1992
Tejning, 19671
Skerfving, 1974
Haxton et al., 1979
Tsubaki, 1971b2
Birke et al., 19722
Den Tonkelaar et al., 1974
Kershaw et al., 1980
Phelps et al., 1980
Sherlock et al., 1982
Tsubaki, 1971a'
Hair to
Blood
Ratio
140^
218
230
230
250
260
2803
280
292*
296
367
370
Number
of
Subjects
50
16
51
60
173
45
12
47
5
339
98
-25
Hg Range in
Whole Blood
(ug/L)
5-270
2.4-9.1
4-110
44-550
0.4-26
2-800
4-650
1-40.5
-
1-60
1.1-42.3
-
Hair Samples
Hg Range m
Hair (ppm)
1-57
0.15-20
1-30
1-142
0.1-11.3
20-325
1-180
<0.5-13.2
-
1-150
0.2-21
-
Length
(mm)
-
-
-
5
20
-
5
-
5
10
24
"longer tuft"1
Distance
to
Scalp
--
at scalp
axillary
at scalp
-
-
at scalp
--
at scalp
at scalp
-
-
1 As cited in Berglund et al. 1971
2 As cited in WHO, 1976
3 Ratio of methylmercury in hair to methylmercury in blood
4 Based on repeated measurements at different time points (3-8 ratios per individual) of the ratio of 5 mm hair segments to
corresponding 2-week average blood levels (assuming hair growth of 1.1 cm/month).
"—" = Not reported
been used for this parameter in the past by other groups; eg., Berglund et al. (1971) and WHO
(1990). A value of 0.05 was used for "f' in the above equation.
Elimination constant (b). Several studies reported clearance half-times for methylmercury
from blood or hair in the range of 35-189 days (Miettinen 1972; Kershaw et al. 1980; Al-Shahristani
et al. 1974; Sherlock et al. 1984). Two of these studies included the Iraqi population exposed during
the 1971-1972 incident. A value reported in Cox et al. (1989) was derived from the study group
which included the mothers of the infants upon which this risk assessment is based. The average
elimination constant of the 4 studies is 0.014; the average of individual values reported for 20
volunteers ingesting from 42 to 233 ug mercury/day in fish for 3 months (Sherlock et al. 1982) is also
0.014. A value of 0.014 days"1 was, thus, used for term "b" in the above equation.
Volume of blood in the body (V). That blood volume is 7% of body weight has been
determined by various experimental methods. There is an increase of 20% to 30% (to about 8.5 to
9%) during pregnancy (Best 1961). Specific data for the body weight of Iraqi women were not found.
June 1996
6-22
SAB REVIEW DRAFT
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Assuming an average body weight of 58 kg and a blood volume of 9% during pregnancy, a blood
volume of .5.22 liters was derived. In the equation on page 6-19, term "V" was taken to be 5 liters.
Body weight. While the critical endpoint for the RfD is developmental effects in offspring,
the critical dose is calculated using parameters specific to the mothers who ingested the mercury
contaminated grain. Data on body weights of the subjects were not available. A default value of 60
kg (rounded from 58) for an adult female was used.
Grouping of data
Data used in the U.S. EPA benchmark dose calculation were excerpted from the publication,
Seafood Safety (NAS 1991). The tables of incidence of various clinical effects in children that were
provided in this document readily lent themselves to the benchmark dose modeling approach. The
continuous data for the Iraqi population that were reported hi Marsh et al. (1987) were placed in five
dose groups, and incidence rates were provided for delayed onset of walking, delayed onset of talking,
mental symptoms, seizures, neurological scores above 3, and neurological scores above 4 for affected
children. Neurologic scores were determined by clinical evaluation for cranial nerve signs, speech,
involuntary movement, limb tone strength, deep tendon reflexes, plantar responses, coordination,
dexterity, primitive reflexes, sensation, posture, and ability to sit, stand and run. Table 6-3 shows the
input data for the modeling procedure for effects found in cju'ldren. Incidence data for each of the
adverse effects in children were taken directly from Table 6-11, Seafood Safety (NAS, 1991). The
effects of late walking, late talking, and neurologic scores greater than 3 were also combined for
additional analysis. Table 6-4 shows the incidence data for each of the effects observed in adults as
grouped in Table 6-13 of the Seafood Safety document.
Adjustments for background incidence
As an adjustment for background rates of effects, the benchmark dose estimates for
methylmercury were calculated to estimate the dose associated with "extra risk." Another choice
would have been to calculate based on "additional risk." Additional risk (AR) is defined as the added
incidence of observing an effect above the background rate relative to the entire population of interest,
AR = [P(d)-P(0)]/l. In the additional risk calculation, the background rate is subtracted off, but still
applied to the entire population, including those exhibiting the background effect, thus in a sense
"double counting" for background effects. It can be. seen that extra risk (ER) is always mathematically
greater than or equal to additional risk, ER = [P(d)-P(0)]/[l-P(0)], and is thus a more conservative
measure of risk (whenever the background rate is not equal to zero). Conceptually, extra risk is the
added incidence of observing an effect above the background rate relative to the proportion of the
population of interest that is not expected to exhibit such an effect. Extra risk is then more easily
interpreted than additional risk, because it applies the additional risk only to the proportion of the
population that is not represented by the background rate. Extra risk has been traditionally used in
U.S. EPA's cancer risk assessments (Anderson et al. 1983) and is discussed in detail in a report on the
benchmark dose by U.S. EPA's Risk Assessment Forum (U.S. EPA 1995).
Derivation of a benchmark dose
Benchmark dose estimates were made by calculating the 95% lower confidence limits on doses
corresponding to the 1%, 5% and 10% extra risk levels using a quanta! Weibull model (K.S.Crump
Division of ICF Kaiser International). The Weibull model was chosen for the benchmark dose
calculations for the methylmercury data as recent research suggests it may be the best model for
June 1996 6-23 SAB REVIEW DRAFT
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Table 6-3
Incidence of Effects in Iraqi Children By Exposure Group3
Effect
Late Walking
Late Talking
Mental Symptoms
Seizures
Neurological Scores >3
Neurological Scores >4
All Endpoints
N
1.37
0
2
1
0
3
0
4
27
10
2
1
0
0
1
1
3
14
52.53
2
3
1
1
4
2
6
13
163.38
3
4
3
2
»
3
2
8
12
436.60
12
11
4
4
9
6 *
14-
15
a From Table 6-11 of Seafood Safety; Dose is geometric mean in ppm maternal hair.
Table 6-4
Incidence of Effects in Iraqi Adults by Exposure Group3
Effect
Parasthesia
Ataxia
Visual Changes
Disarthna
Hearing Defects
Deaths
N
50
2
1
0
1
0
0
21
350
i
0
0
i "
0
0
19
750
g
2
4
1
1
0
19
1500
10
8
9
4
0
0
17
2500
20
15
14
6
3
0
25
3500
14
17
10
13
6
3
17
4500
7
7
6
6
5
2
7
a From Table 6-13 of Seafood Safety; Dose is geometric mean in ppb blood.
developmental toxicity data (Faustman et al., 1994). The form of the quantal Weibull that was used is
the following:
June 1996
6-24
SAB REVIEW DRAFT
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P(d) = AO + (l-AO)(l-exp[-Al * dA2~\,
where d is dose, AO is the background rate, Al is the slope, and A2 is a shape parameter. For each
endpoint and for the combined endpoints, the incidence of response was regressed on the dose. A
Chi-squared test of goodness-of-fit was used to test the null hypothesis (Ho) that the predicted
incidence was equal to the observed incidence, so that Ho would be rejected for p-values less than
0.05.
Results for individual effects and all effects combined for children exposed in utero are given
in Table 6-5; results for adults are given for comparison in Table 6-6. For calculation of the lower
bound on the 10% risk level, AO = 0.-12468, Al = 9.470230 x 10'3, A2 = 1.00000. The RfDVRfC
Work Group chose the benchmark (lower bound on the dose for 10% risk) based on modeling of all
effects in children. Recent research (Allen et al. 1994a, b) suggests that the 10% level for the
benchmark dose roughly correlates with a NOAEL for developmental toxicity data. Note that this
conclusion was based on controlled animal studies and on calculation of additional risk. Both the
polynomial and Weibull models place a lower 95% confidence limit on the dose corresponding to a
10% risk level at 11 ppm hair concentration for methylmercury. The benchmark dose rounded to 11
ppm was used hi the calculation of the RfD.
Dose groupings other than those used in Seafood Safety were also done and benchmark doses
run as above for comparison. Both density-based grouping and uniform concentration intervals were
used.
The local density of observations relative to the mercury level in hair was analyzed using a
density estimation algorithm (smooth function in S-PLUS for Windows, Ver. 3.1; S-PLUS Guide to
Statistical and Mathematical Analysis). The function estimates a probability density for the
distribution of a variable by calculating a locally-weighted density of the observations. That is, the
function estimates the probability that an observation will be near a specific value based on how the
actual values are clustered. In this case, the function was used to estimate the probability density for
an observation in the neighborhood of any given maternal hair mercury concentration. The density
plot is shown in Figure 6-1. The peaks represent relatively greater numbers of data points than the
troughs in the vicinity of the associated hair mercury concentrations.
The density distribution is characterized by four distinct peaks. Exposure dose groups were
defined as trough-to-trough intervals with the peak values taken as the nominal value for each interval.
The nominal dose-group value, concentration ranges, and incidence of combined developmental effects
are given in Table 6-7. A benchmark dose was calculated from the incidence of all effects as
grouped in Table 6-7. The lower 95% confidence interval on the benchmark dose for the 10%
response is 13 ppm, compared to the 11 ppm value used as the basis for the RfD.
The other alternative dose grouping approach was to divide the entire exposure range into
four equal log-dose intervals. The geometric midpoint of each interval was taken as the nominal value
for the interval. The nominal dose-group value, concentration ranges, and incidence of combined
developmental effects are given in Table 6-8. The benchmark calculated as the lower bound on the
10% incidence for all effects is 10.3 ppm, compared to the 11 ppm used for the RfD.
June 1996 6-25 SAB REVIEW DRAFT
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Table 6-5
Methylmercury Benchmark Dose Estimates (ppm hair)
Maximum Likelihood Estimates and 95% Lower Confidence Limits from Weibuli Model
Incidence of Effects in Children (Marsh et al. 1987)
Effect
Late Walking
Late Talking
Mental Symptoms
Seizures
Neuro Score >3
Neuro Score >4
All Endpoints
0.01
MLE
3.3
, 4.7
12.0
11.8
5.6
8.1
1.6
95% CL
2.1
2.4
6.4
6.7
3.3
4.6
1.1
0.05
MLE
16.7
22.1
61.0
60.4
28.8
41.4
8.3
95% CL
10.9
12.3
32.8
34:3
17.0
23.7
5.4
0.10
MLE
34.3
43.8 '
125.4
124.2
59.1
84.9
17.1
95% CL
22.4
25.3
67.5
70.5
34.9
48.7
11.1
G-O-Fa
P-Value
0.16
0.79
0.63
0.86
0.58
0.48
0.94
a Goodness-of-fit p-value for testing the null hypothesis. Ho: Predicted Incidence = Observed Incidence.
Table 6-6
Methylmercury Benchmark Dose Estimates (ppb blood)
Maximum Likelihood Estimates and 95% Lower Confidence Limits from Weibuli Model
Incidence of Effects in Adults (Bakir et al. 1987)
Effect
Paresthesia
Ataxia
Visual Changes
Disarthria
Hearing Defects
Deaths
0.01
MLE
45.3
330.4
64.1
728.9
1462.9
2226.3
95% CL
14.3
140.8
25.4
235.9
535.2
1106.8
0.05
MLE
169.0
652.9
249.1
1265.4
2137.5
3007.2
95% CL
73.2
369.7
129.9
62L8
1202.8
2167.3
0.10
MLE
30Z2
882.0
453.6
1614.4
2527.1
3434.2
95% CL
150.5
564.7
266.9
949.8
1705.6
2797.0
G-O-Fa
P-Value
0.36
0.22
0.26
0.41
0.53
0.83
1 Goodness-of-fit p-value for testing the null hypothesis, Ho: Predicted Incidence = Observed Incidence.
June 1996
6-26
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Figure 6-1
Density of Data Points Relative to Mercury
Concentration in Hair for Iraqi Cohort Data
a
o
10 50 100
ppmHginhwr
500
Table 6-7
Density-Based Dose Groupings
Nominal Dose (ppm)
1.18
10.6
78.8
381
Dose Range (ppm)
1 -4
5 -28
29 - 156
157 - 674
Incidence
5/27
3/16
10/17
18/21
Table 6-8
Uniform Dose Groupings
Nominal Dose (ppm)
2.25
11.5
58.6
299
Dose Range (ppm)
1 -5
6-25
26 - 132
133 - 674
Incidence
5/28
3/14
9/17
19/22
June 1996
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Uncertainty and modifying factors
A composite uncertainty factor of 10 was used. This uncertainty factor was applied for
variability in the human population, in particular the wide variation in biological half-life of
methylmercury and the variation that occurs in the hair to blood ratio for mercury. In addition, the
factor accounts for lack of a two-generation reproductive study and lack of data for possible chronic
manifestations of the adult paresthesia that was observed during gestation.
The default value of one was used for the modifying factor.
Calculation of the oral RfD for methylmercury
In this instance the RfD was calculated using the following equation:
a,
Benchmark Dose
UFxMF
1.1 ug/kg-day
10
1 x 10" mglkg-day
Confidence in the oral RfD for methylmercury
The principal study (Marsh et al. 1987) is a detailed report of human exposures with
quantitation of methylmercury by analysis of specimens from affected mother-child pairs. A strength
of this study is that the quantitative data are from the affected population and quantitation is based
upon biological specimens obtained from affected individuals. A threshold was not easily defined;
extended application of modeling techniques were needed to define the lower end of the dose-response
curve. This may indicate high variability of response to methylmercury in the human mother-child
pairs or misclassification of assigning pairs to the cohort. Recent concerns expressed in the research
community relate to the applicability of a risk assessment based upon a grain-consuming population
when the application of this risk assessment is likely to be for fish-consuming segments of a U.S.
population or populations from other developed countries. Confidence hi the supporting data base and
confidence in the RfD were considered medium by the RfD/RfC Work Group.
An analysis of uncertainty in an RfD based on the Iraqi data is found in Appendix D of this
volume. Discussions of areas of uncertainty can also be found hi Volume VI, Risk Characterization.
Two additional human epidemiologic studies of separate populations (Kjellstrom et al. 1986a,b,
1989; McKeown-Eyssen et al. 1983) generally support the dose range of the benchmark dose level for
perinatal effects. Both of these studies are described in section 3.3.1.1. A recent analysis of the
Kjellstrom data was published by Gearhart et al. (1995). In this analysis the authors used a PBPK
model which incorporated a fetal compartment. They calculated a benchmark dose on all 28 tests
included in the initial study design by Kjellstrom; this was done assuming values of 1 and 5% for
background deficiency in test scores. The range of benchmark doses calculated was 10 to 31 ppm
June 1996 6-28 SAB REVIEW DRAFT
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maternal hair mercury. The authors' preferred benchmark was 17 ppm, for an estimated background
incidence of 5% and the lower bound on the 10% risk level.
Chronic rodent (Bornhausen et al. 1980) and nonhuman primate studies (Burbacher et al. 1984;
Gunderson et al. 1986; Rice et al. 1989a,b) provide data to support LOAELs for other developmental
end points.
The most appropriate basis for and calculation of a RfD for methylmercury has been the
subject of much scientific discussion; several plausible alternatives to the U.S. EPA assessment have
been proposed. ATSDR used the analysis reported by Cox et al. (1989, see discussion below) of the
Iraqi developmental data in the derivation of an intermediate MRL (minimal risk level). Using
delayed onset of walking as the critical effect, a LOAEL of 14 ppm mercury in hair was determined.
A dose conversion from ppm hair to daily intake to maintain blood mercury levels in pregnant women
was done in a very similar manner to that employed by U.S. EPA. Values for parameters in the
equation on page 6-18 were consistent between the two Agencies with one exception; namely the use
of a blood volume of 4.1L by ATSDR compared to 5L by U.S. EPA. The methylmercury intake level
calculated by ATSDR to maintain a hair level of 14 ppm is 1.2 ug/kg-day compared to 1.1 ug/kg-day
to maintain a hair level of 11 ppm (used by U.S. EPA).
The state of New Jersey currently uses an RfD of O.TxlO"4 mg/kg-day (described in Stern
1993) compared to the U.S. EPA's RfD of IxlO'4 mg/kg-day. The critical effect chosen was
developmental endpoints in the Iraqi children exposed in utero including delayed onset of walking. A
recent discussion of this RfD was presented in the context of the external peer review of the Mercury
Study Report to Congress. Stern described the LOAEL as the mercury hair level equivalent to a
mercury blood level of 44 ^ig/L. To determine the intake level, the equation on page 6-18 was used
but with different values for two parameters; namely, b and f.
= 0.70 (Jg/kg-day
d- CxbxV
A xfx bw
- 44 tsg/L x 0.013 days'1 x 5L
0.95 x 0.077 x 60 kg
Choice of the value of 0.077 for f, fraction of daily intake taken up by blood was based on a
paper by Smith et al. (1994) which was not published at the time of the RfD/RfC work group
discussions and was not considered by them in choosing the parameter values. Smith et al. (1994)
(described briefly in chapter 2 of this volume) presents a study of methylmercury excretion kinetics
based on measurement of i.v. administered methylmercury (1.7-7.4 ug) in blood, urine and feces of 7
male volunteers. The authors claim that data from this study are superior to those from previous
studies in accounting for the portion of the labeled mercury which is metabolized to inorganic
mercury. Based on the linear extrapolation of the plot of blood concentration of methylmercury versus
time, the authors calculated that approximately 7.5% of the methylmercury remained in the blood
following rapid equilibration among tissue compartments. Based on fitting the experimental data to a
June 1996 6-29 . SAB REVIEW DRAFT
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five compartment pharmacokinetic model, they calculate that 7.7% (geometric mean) of the
methylmercury is found in the blood. It should be noted that the values for this parameter among the
seven subjects ranged from 6.5-9.5%.
Smith et al. (1994) taking conversion to inorganic mercury into account, reported an overall
estimate (geometric mean) of the half-life in blood (methylmercury-specific as per discussion in
previous paragraph) of 45 days (0.015 days"1). The half-lives (elimination constants) for the 7 subjects
ranged from 35 days (0.020 days"1) to 53 days (0.013 days'1). Stern (1993) notes the half-life reported
by Cox et al. (1989) was 48 days with a range of about 18-37. This corresponds to a value for b of
0.0144 day"1. The mean value is not reported by Cox, but a Monte Carlo simulation of the data
estimated a mean of about 47 days. The most frequently reported value (mode), however, was 55
days corresponding to a value for b of 0.013 day"1. Ultimately the value of b = 0.013 day"1 was
chosen by Stem as the most "typical" value.
In a recent publication, Gearhart et al. (1995) proposed a RfD in the range of 0.8 to 2.5 ug/kg-
day based on their analysis of effects in a population of children in New Zealand. This population
was assumed to be exposed in utero to methylmercury as a consequence of high fish consumption by
their mothers. Gearhart et al. (1995) estimated that a maternal intake of methylmercury in the range of
0.8 to 2.5 ug/kg-day corresponded to a NOAEL for developmental effects. These results support the
U.S. EPA estimate of maternal intake of 1 ug/kg-day methylmercury corresponding to a benchmark
dose of 11 ppm mercury in hair for developmental effects, prior to applying an uncertainty factor.
The primary area of disagreement between Gearhart et al. (1995) and the U.S. EPA is in the use of an
uncertainty factor. The authors felt that no uncertainty or modifying factors were needed as the
NOAEL was calculated on effects in a sensitive subpopulation. U.S. EPA applied a 10-fold
uncertainty factor to account for interindividual variation in the human population (particularly in hair
to blood mercury ratio) and for lack of certain types of data.
The NOAEL was implicitly defined by Gearhart et al. (1995) as the lower 95% confidence
interval on the benchmark dose associated with a 10% change in the test scores from the New Zealand
study (Kjellstrom et al. 1989). That is, the benchmark dose calculation was performed on continuous
variables by contrast to the binary variables from the Iraqi study (Marsh et al. 1987) used by the U.S.
EPA. The qualitative equivalence of the two kinds of benchmark doses has not been established; both
presumably represent a minimum risk level as does a NOAEL.
Details of the calculations used to derive the benchmark dose were not published by Gearhart
et al. (1995). The intake range was presumably based on benchmark doses of 17 to 50 ppm mercury
in hair estimated by Gearhart et al. (1995) from the New Zealand data (Kjellstrom et al. 1989) and
converted to oral intake rates by the use of a PBPK model (Gearhart et al. 1995). A dose-conversion
factor of about 0.05 [ppm mercury in hair/(ug/kg-day)J can be estimated, however, from Figure 5c in
Gearhart et al. (1995). Applying this factor to the benchmarks of 17 to 50 ppm mercury in hair yields
the reported intake range of 0.85 to 25 ug/kg-day. Although this dose-conversion factor is about half
of that estimated by the U.S. EPA, the two estimates are consistent when duration of exposure is
considered. The Gearhart et al. (1995) pharmacokinetic model predicts that equilibrium is not reached
until about 400 days. The dose conversion of 0.05 corresponds to hair mercury levels at equilibrium
and is the appropriate factor to apply to the New Zealand hair concentrations, which arose from
longer-term exposure. In contrast, the Iraqi exposure was only for a few months (Marsh et al. 1978).
A dose conversion of about 0.1, which is virtually the same as that used by the U.S. EPA, can be
estimated for a 3-month exposure from Figure 4 in Gearhart et al. (1995) and from Sherlock et al.
(1984) assuming a hair:blood mercury concentration ratio of 250:1.
June 1996 6-30 SAB REVIEW DRAFT
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Crump et al. (1995) reanalyzed data from the Iraqi methylmercury poisoning episode presented
in Marsh et al. (1981). Using a hockey stick parametric dose response analysis of these data, Cox et
al. (1989) concluded that the "best statistical estimate" of the threshold for health effects was 10 ppm
Hg in hair with a 95% range of uncertainty between 0 and 13.6. In their analysis, Crump et al. (1995)
reported that the statistical upper limit of the threshold could be as high as 255 ppm. Furthermore, the
maximum likelihood estimate of the threshold using a different parametric'model was presented as
virtually zero. These and other analyses demonstrated that threshold estimates based on parametric
models exhibit high statistical variability and model dependency, and are highly sensitive to the precise
definition of an abnormal response.
Using a statistical analysis for trend that does not require grouping of the data, Crump et al.
(1995) demonstrated that the association between health effects and methylmercury concentrations in
hair is statistically significant at mercury concentrations in excess of about 80 ppm. In addition,
Crump et al. calculated benchmark doses by applying dose-response models to each of the three
endpoints: late walking, late talking and neurological score. Unlike the benchmark calculations made
by the U.S. EPA (1994), these analyses did not involve grouping of the data into discrete dose groups,
nor did they require dichotomizing continuous responses like age first walked into "late walking" of
"no walking". Their calculation of the 95% lower bounds on the hair concentration corresponding to
an additional risk of 10% ranged from 54 ppm to 274 ppm mercury in hair. Crump et al. (1995)
concluded that the trend analyses and benchmark analyses provided a sounder basis for determining
RiDs than the type of hockey stick analysis presented by Cox et al. (1989). They felt that the acute
nature of the exposures, as well as other difficulties with the Iraqi data, present limitations in the use
of these data for a chronic RfD for methylmercury.
Cox et al. (1995) have published a recent analysis of the data on late walking in Iraqi children
exposed in utero to methylmercury. The authors indicate that dose-response analyses based on the
"late walking" endpoint are unreliable because of four influential observations hi the data set from
Marsh et al. (1978). The data points hi question are the only responders below 150 ppm (Hg in hair).
In particular, Cox et al. (1995) state that the four observations are isolated from the remainder of the
responders and would be expected to have considerable influence on threshold estimate. This
conclusion is based on a visual interpretation of a plot of the data (Figure 2 in Cox et al. 1995).
Based on visual inspection of the same figure, an argument could be made that the separation is not
that marked considering the first eight responders. No quantitative sensitivity analysis was performed
to investigate the effect of removing one or more of these data points. Cox et al. (1995) point out that
if the four points are assumed to represent background, then the threshold for late walking would be
greater than 100 ppm. It would seem unlikely, however, that these observations represent background
given that no responses were observed in the 37 individuals with lower levels of exposure. It should
be noted that the U.S. EPA benchmark dose was done on incidence of all effects, rather than on late
walking only.
The Cox et al. (1995) and Crump et al. (1995) analyses deal primarily with one endpoint,
namely, late walking. This appears to be the most sensitive of the endpoints described in Marsh et al.
(1978). Both Cox et al. and Crump et al., as well as the U.S. EPA analysis in Appendix D of
Volume IV, show considerable uncertainty in thresholds estimated from the data on late walking. The
peculiar nature of the uncertainty, in this case, makes it difficult to distinguish between 7 ppm
maternal hair mercury and 114 ppm as a best (maximum likelihood) estimate for the threshold. Cox et
al. (1995) attribute this bimodal uncertainty to four influential observations between 14 ppm and 60
ppm isolated from the remainder of the responders beginning at 154 ppm; they do not present
arguments, other than visual, for censoring these data. Crump et al. (1995) show that changing the
June 1996 6-31 SAB REVIEW DRAFT
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definition of late walking from greater than 18 months to 18 months or greater eliminates the bimodal
uncertainty with a best'threshold estimate of 230 ppm. The implication in both analyses is that the
background incidence of late walking, as reported in other studies, is not consistent with the lower
thresholds. While this is true, the use of historical controls for this analysis may not be appropriate,
given the relatively large number of observations at low exposure levels in the Iraqi cohort; 33% of
the observations were at hair mercury concentrations considered to be background levels (3 ppm or
less).
Late walking, as assessed in the exposed Iraqi population (Marsh et al. 1978), is almost
certainly a valid indicator of methylmercury toxicity but may well be unreliable as the sole basis for
detailed dose-response analysis. The primary reason for this may be the uncertainty in maternal recall
for both birth date and date of first walking. The uncertainty, in this particular case could be quite
large, given the lack of recorded information. The primary impact of this kind of uncertainty would
be on the response classification of individuals at the upper bound of normal (18 months for first
walking) and at the lower bound V>f abnormal. The lowest abnormal first walking time presented in
Marsh et al. (1978) was 20 months. The impact of assuming uncertainty in the classification of the
observations hi these two groups is large given the large number of observations in the two groups (19
data points at 18 months and 8 data points at 20 months). The analysis in Appendix D to Volume IV
of the Report to Congress shows that thresholds estimated for late walking are unstable when
classification uncertainty is considered. The same kind of subjective uncertainty is applicable to the
late talking endpoint, as well. The thresholds for late talking, however, are much more stable,
statistically, as there are fewer observations that are near the normal/abnormal threshold value of 24
months.
Birth date uncertainty also would have an impact on exposure uncertainty if correspondence of
exposure and gestation was estimated (Marsh et al. 1978) from birth date to any great extent. That is,
exposure may have occurred to a lesser extent (or not at all) than assumed during the critical period of
gestation. The result would be a lower exposure associated with the observation, depending on the
width of the critical time window during gestation and on the importance of duration of exposure in
the elicitation of the particular effect. If the exposure occurred after the critical period, any
observation of an effect would be attributed to causes other than methylmercury and be included in the
background.
Marsh et al. (1995) have published results of a study conducted between 1981 and 1984 in
residents of coastal communities of Peru. The prospective study was of 131 child-mother pairs; testing
for potential effects of fetal methylmercury exposure was patterned after the study of children exposed
in utero in Iraq. Peak maternal hair methylmercury ranged between 1.2 to 30 ppm with a geometric
mean of 8.3 ppm. These authors showed no effects of methylmercury on measures similar to those
performed on the Iraqi children (including time of first walking and talking). A NOAEL (in the
absence of a LOAEL) from this study would be 30 ppm maternal hair mercury. This is consistent
with the U.S. EPA benchmark dose of 11 ppm.
Concern has been raised by various scientists as to the impact that as yet unpublished studies
will have on the risk assessment for methylmercury. Reports have been delivered at scientific
meetings of results of studies of populations in the Faroes and Seychelles Islands known to consume
large amounts of seafood.
Data from the pilot (or cross sectional) study and testing of children from up to age 29 months
in the main or prospective study in the Seychelles have recently been published. The study design is
June 1996 6-32 SAB REVIEW DRAFT
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described in Section 3.3.1.1. The range of maternal hair mercury levels for the pilot study was 0.6 to
36.4 ppm; the range for the main study was 0.5 to 26.7 ppm. The overall conclusion of the authors is
that their results require careful interpretation, and that an association between relatively low level
mercury exposure in utero and neurologic deficits has not been conclusively demonstrated. The
authors cautioned that results from their 66-month tests of children need to be included in the
evaluation.
The interpretation by some risk assessors is that the effects noted in the Iraqi population
exposed to contaminated grain are not being seen at similar doses of methylmercury delivered in utero
via contaminated seafood. One assessment by a scientist at FDA is that the U.S. EPA RfD of l.OxlO"4
mg/kg-day for methylmercury is somewhat conservative and is certainly protective; a suggestion was
made that the uncertainty factor could be'decreased to 3, resulting in a RfD of 3.0xlO"4 mg/kg-day.
Several scientists have a^so suggested that a developmental toxicity RfD is needed for
methylmercury. This may not be necessary, however, if the critical effect is developmental toxicity
and the uncertainty factors used to estimate the lifetime RfD do not involve an adjustment for less than
lifetime exposure nor lack of complete data base.
6.3.1.2 Inhalation Reference Concentrations (RfCs)
Elemental mercury
The U.S. EPA has determined an RfC of 3xlO'4 mg/m3 for elemental mercury (U.S. EPA
1994). The inhalation RfC is based on neurologic toxicity observed in several human occupational
studies. The observed neurologic changes included hand tremor, increases in memory disturbances and
slight subjective and objective evidence of autonomic dysfunction. Fawer et al. (1983) measured
intention tremor (tremors that occur at the initiation of voluntary movements) in workers exposed to a
TWA concentration of 0.026 mg/m3 over an average of 15.3 years. It was noted, however, that the
tremors may have resulted from intermittent exposures to concentrations higher than the TWA.
Piikivi and colleagues conducted several studies in chloralkali workers on
electroencephalogram (EEG) abnormalities (Piikivi and Toulonen 1989); subjective measures of
memory disturbance and sleep disorders arid objective disturbances in psychological performance
(Piikivi and Hanninen 1989); and subjective and objective symptoms of autonomic dysfunction such as
induced pulse rate variations and blood pressure responses (Piikivi 1989). U.S. EPA extrapolated an
occupational exposure level associated with these neurological changes of 0.025-0.030 mg/m3 from
blood levels, based on a conversion factor calculated by Roels et al. (1987). The LOAEL (0.025
mg/m adjusted to 0.009 mg/m3 for continuous exposure of the general population) was divided by an
uncertainty factor of 30 (10 to protect sensitive individuals and for use of a LOAEL, and 3 for the
lack of reproductive studies in the database) to yield the RfC of 3x10~4 mg/m3.
The RfC was, thus, calculated in the following way.
June 1996 6-33 ' SAB REVIEW DRAFT
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= LOAEL mg Hgfm3
UF
_ 0.009 mg/m3
30
= 0.0003 mg/m3
This reference concentration was reviewed and verified by the RfD/RfC Work Group and was
verified on April 19, 1990. It was released under a special action by U.S. EPA (Jarabek 1992,
personal communication). •
0
Confidence in the critical study, the data base, and, thus hi the RfC were rated "medium" by
the Work Group/ Factors which were positive for confidence in the critical study were the use of a
sufficient number of human subjects, inclusion of appropriate controls, sufficient exposure duration
and that the LOAEL can be corroborated in other studies. It was noted, however, that for all but one
of the studies, exposure had to be extrapolated from blood mercury levels. The lack of human or
multispecies reproductive or developmental studies precluded higher confidence in the data base.
Inorganic (mercuric) mercury
Developmental toxicity (skeletal abnormalities and retarded growth) in mice (Selypes et al.
1984) and autoimmune disease in Brown-Norway rats (Bernaudin et al. 1981) have also been observed
following inhalation exposures. Due to the limitations of these inhalation studies and the inadequacy
of the remaining toxicologic and pharmacokinetic data bases, the RfD/RfC Work Group determined
the derivation of an RfC is not possible. The posting of this determination on IRIS is proceeding
concurrent with the finalization of this Report to Congress.
Methvlmercury
No estimate of risk from inhalation of methylmercury has been done by U.S. EPA.
6.3.1.3 Estimation of Risk from Dermal Exposure
The dermal contribution of the different mercury species to the total systemic exposure of each
of these mercury species may be important for a full characterization of risk to the potentially exposed
human populations. Many of the necessary data needed for conducting a dermal risk assessment are
currently lacking or not well enough understood to assess systemic exposure and risks from dermal
exposure to mercury species.
For any of these mercury species to be a dermal health hazard they must be absorbed across
the skin (epidermis and dermis) and be systemically distributed to the affected critical organ systems
(kidneys or CNS) via the circulatory system. The percutaneous absorption for each mercury species is
dependent on skin-specific factors (e.g., skin thickness, hydration, age, condition, circulation, and
temperature) and compound-specific factors (e.g., lipophilicity, polarity, chemical structure,, volatility,
and solubility), which are involved in determining the rate and amount of absorption by the cutaneous
route. Currently there are few known or agreed upon percutaneous absorption rates available for any
of the mercury species of interest. Some data on percutaneous absorption (Kp = Permeability
June 1996 6-34 SAB REVIEW DRAFT
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coefficient) for the mercuric forms of mercury in aqueous media have been reported in U.S. EPA
(1992).
• The media (aqueous, vapors or soil) where the mercury species are found must also be
considered in dermal risk assessments.. Each medium has its own set of factors that impact the
specific percutaneous absorption rates for each of the mercury species. For example, mercury
compounds found in aqueous media are dependent on factors such as solubility in water and increased
hydration of the skin. Mercury compounds associated with soil must be assessed for binding to the
particular soil type of concern, the adhesion of the soil of concern to skin, the desorption of the
mercury compound from the soil, and the absorption of the mercury compound across the skin and
into the circulatory system. Other aspects that must be considered with a dermal assessment are
binding or sequestration of the mercury species at the site of exposure or closely nearby, and the
metabolism of the mercury species in the skin that may result hi oxidation/reduction of the mercury
species to other valence states (thereby, potentially resulting hi different critical effects than from the
originally absorbed compound).
At present, many of the necessary mercury species/media factors have not been fully
ascertained and as a result credible dermal risk assessments cannot be accomplished at this time. A
more extensive discussion of dermal exposure assessment for risk assessment can be found in Dermal
Exposure Assessment: Principles and Applications (EPA/600/8-91/01 IB, January 1992).
6.3.2 Developmental Effects
6.3.2.1 Elemental Mercury
Elemental mercury was judged to have sufficient animal data for developmental toxicity. The
two studies which contribute most to the level of concern are Danielsson et al. (1993) and Fredicksson
et al. (1992). Both of these studies are limited as a basis for an RfDDT by the small numbers of
animals tested and by the very few dose groups. A further limitation is lack of data on gender
differences. No RfDDT for elemental mercury is available at this time.
6.3.2.2 Inorganic Mercury
There are no data from human studies which are suitable for derivation of an RfDDT.
Inspection of available animal studies indicates that there are five reports of developmental effects of
inorganic mercury given orally. In all of these, exposure was by gavage to pregnant animals, and
effects were monitored in progeny. Three papers were reported as abstracts giving few experimental
details.
Rizzo and Furst (1972) treated Long Evans rats (5/group) with a single gavage dose of 2 mg
Hg/kg as mercuric oxide on either day 5, 12, or 19 of gestation. Animals were sacrificed on day 20 or
21 of gestation. No effects of treatment on gestation day 12 or 19 were noted. According to the
authors treatment on day 5 resulted hi a higher percentage of growth retardation and inhibition of eye
formation, but no statistical analyses were done.
In Gale (1974), pregnant Golden hamsters were administered 0, 2.5, 5, 16, 22, 32, 47, or 63
mg Hg/kg-day mercuric acetate via gavage on gestation day 8. When the pregnant animals were
sacrificed on day 12 or 14, there was a significant increase in the incidence of abnormal fetuses
including small, retarded, or edematous (combined), and/or malformed fetuses. The NOAEL for
June 1996 6-35 SAB REVIEW DRAFT
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developmental toxicity was 5 mg mercuric chloride/kg or 2 mg Hg/kg. Maternal toxicky included
dose-related weight loss, diarrhea, slight tremor, somnolence, tubular necrosis and hepatocellular
vacuolization, but insufficient data were provided to allow determination of a LOAEL or NOAEL for
maternal toxicity.
The advantage of the Gale (1974) study as a basis for quantitation of potential risk is that
several doses of inorganic mercury were tested; the spacing of the doses was adequate for
identification of both a LOAEL and NOAEL. It is not recommended, however, that the NOAEL in
Gale (1974) serve as the basis for an RfDDT. There were relatively few animals tested (decreasing the
overall sensitivity of the assay) and not all endpoints were thoroughly evaluated. The test compound
was administered on only one day of gestation, and there is some question as to the suitability of the
golden hamster for developmental assays. The data base for developmental effects, while generally
supportive of the LOAEL is not adequate to determine if the measured endpoints were the most
sensitive for developmental effects of inorganic mercury.
6.3.2.3 Methylmercury
Weight of evidence for developmental toxicity indicates that a developmental toxicity RfD is
appropriate for methylmercury. A separate RfDDT may not be necessary as the critical effect for the
lifetime RfD is developmental toxicity. The current RfD (IxlO"4 mg/kg-day) was based on
developmental endpoints in offspring of women exposed during pregnancy; it may be taken as
protective against developmental toxicity. For less than chronic exposures it should be noted that the
RfDDT is not intended as a lifetime exposure value.
6.3.3 Germ Cell Mutagenicitv
Data do not support the generation of quantitative estimates for germ cell mutagenicity for any
form of mercury.
6.3.4 Carcinogenic Effects
6.3.4.1 Elemental Mercury
Elemental mercury is categorized as D, unable to classify as to human carcinogenicity. A
quantitative estimate for carcinogenic effect is, thus, inappropriate.
6.3.4.2 Inorganic Mercury
Quantification of the potential carcinogenic effects of mercuric chloride (classified as C,
possible human carcinogen) was not done. No increase in tumor incidence was observed in a
carcinogenicity study in which white Swiss mice were given 0.95 mg Hg/kg-day as mercuric chloride
in drinking water (Schroeder and Mitchener 1975). No statement regarding carcinogenicity was
reported in a 2-year feeding study in which rats were administered mercuric acetate in the diet at doses
of 0, 0.02, 0.1, 0.4, 1.7 and 6.9 mg Hg/kg-day (Fitzhugh et al. 1950).
The incidence of squamous cell papillomas of the forestomach and thyroid follicular cell
carcinomas from NTP (1993) was evaluated. No slope factor was based on the forestomach tumors
because this type of tumor is probably the result of irritation of the forestomach, cell death and
epithelial proliferation. The carcinogenic mechanism may be specific to irritation at the high doses
June 1996 6-36 SAB REVIEW DRAFT
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used in the bioassay: use of these tumors as a basis for human health assessment of low doses of
inorganic mercury is inappropriate.
Regarding the thyroid carcinomas, a variety of drugs, chemicals, and physiological
perturbations result in the development of thyroid follicular tumors in rodents. For a number of
chemicals, the mechanism of tumor development appears to be a secondary effect of longstanding
hypersecretion of thyroid-stimulating hormone by the pituitary (Capen and Martin 1989;
McClain 1989). In the absence of such long-term stimulatory effects, induction of thyroid follicular
cell cancer by such chemicals usually does not occur (Hill 1989). Use of the incidence of thyroid
tumors from NTP (1993) in low dose extrapolation is, thus, questionable.
6.3.4.3 Methylmercury
Quantification of the potential carcinogenic effects of methylmercury (classified as C, possible
human carcinogen) was not done. No increased incidence of tumors was seen in rats exposed to doses
of up to 0.34 mg Hg/kg-day for 130 weeks (Mitsumori et al. 1983, 1984) or in cats exposed to a diet
containing up to 0.176 mg Hg/kg-day for 2 years (Charbonneau et al. 1976).
No slope factor was calculated for methylmercury based on the incidence of renal epithelial
tumors in male mice. The two studies by Mitsumori et al. (1981, 1990) were limited by high
mortality in the high-dose males, the only group to exhibit a statistically significant increase in tumor
incidence. The study by Hirano et al. (1986) was not limited by survival problems, but the tumors
were observed in conjunction with nephrotoxicity and appear to be a high-dose phenomenon that may
not be linear at low doses. The tumors appeared to originate from focal hyperplasia of the tubular
epithelium induced as a reparative change. The hyperplasia was not observed in tubular epithelium
that was undergoing early degenerative changes; thus, the tumors may not occur where degenerative
changes do not occur. The appropriateness of deriving a quantitative risk estimate using the
assumption of linearity at low doses based on data for which a threshold may exist is questionable.
6.4 Risk Assessments Done By Other Groups
Quantitative estimates of hazards of oral exposure to methylmercury exposure have been
considered by the Food and Drug Administration, Agency for Toxic Substances and Disease Registry
(ATSDR), the Department of Energy and several State agencies. Several inhalation workplace
exposure limits are available in the United States and other countries.
6.4.1 Food and Drug Administration
In 1969, in response to the poisonings in Minamata Bay and Niigata, Japan, the U.S. FDA
proposed an administrative guideline of 0.5 ppm for mercury in fish and shellfish moving in interstate
commerce. This limit was converted to an action level in 1974 (Federal Register 39. 42738,
December 6, 1974) and increased to 1.0 ppm in 1979 (Federal Register 44; 3990, January 19, 1979) in
recognition that exposure to mercury was less than originally considered. In 1984, the 1.0 ppm action
level was converted from a mercury standard to one based on methylmercury (Federal Register 49,
November 19, 1984).
The action level takes into consideration the tolerable daily intake (TDI) for methylmercury, as
well as information on seafood consumption and associated exposure to methylmercury. The TDI is
the amount of methylmercury that can be consumed daily over a long period of time with a reasonable
June 1996 6-37 SAB REVIEW DRAFT
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certainty of no harm. U.S. FDA (and WHO) established a TDI based on a weekly tolerance of 0.3 mg
of total mercury per person, of which no more than 0.2 mg should be present as methylmercury.
These amounts are equivalent to 5 and 3.3 ^g, respectively, per kilogram of body weight. Using the
values of methylmercury, this tolerable level would correspond to approximately 230 ug/week for a 70
kg person or 33 ug/person/day. The TDI was calculated from data developed in parr by Swedish
studies of Japanese individuals poisoned in the episode of Niigata which resulted from the
consumption of contaminated fish and shellfish and the consideration of other studies of fish-eating
populations.
Based on observations from the poisoning event later in Iraq, U.S. FDA has acknowledged that
the fetus may be more sensitive than adults to the effects of mercury (Federal Register 44: 3990,
January 19, 179; Cordle and Tollefson, 1984, U.S. FDA Consumer, September, 1994). In recognition
of these concerns, U.S. FDA has provided advice to pregnant women and women of child-bearing age
to limit their consumption of fish known to have high levels of mercury (U.S. FDA Consumer, 1994).
U.S. FDA believes, however, that given existing patterns of fish consumption, few women (less than
-1%) eating such high mercury fish will experience slight reductions in the margin of safety. However,
due to the uncertainties associated with the Iraqi study, U.S. FDA has chosen not to use the Iraqi study
as a basis for revising its action level. Instead, the U.S. FDA has chosen to wait for findings of
prospective studies of fish-eating populations in the Seychelles Islands and in the Faroes Islands.
6.4.2 ATSDR
ATSDR has established Minimal Risk Levels (MRLs) for elemental, inorganic and
methylmercury (ATSDR 1994).
An acute inhalation MRL of 0.00002 mg/m3 has been derived for elemental mercury vapor
based on neurodevelopmental changes in rats. Specifically, the effects were changes hi locomotor
activity at 4 months of age and an increased time to complete a radial arm maze at 6 months of age
following exposure to 0.05 mg Hg/m3 for 1 hour during post-partum days 11-17 (Fredriksson et al.
1992). A chronic inhalation MRL of 0.000014 mg/m3 was derived for elemental mercury vapor based
on a significant increased in the average velocity of naturally occurring tremors in occupational
workers (Fawer et al. 1983).
Acute and intermediate oral MRLs were derived for inorganic mercury based on kidney effects
reported in the 1993 NTP study of mercuric chloride. The acute oral MRL was 0.007 mg Hg/kg-day
based on a 2-week study reporting a NOAEL of 0.93 mg Hg/kg-day for renal effects in rats (NTP
1993). At higher doses, an increased incidence of tubular necrosis was observed. The intermediate
oral MRL of 0.002 mg Hg/kg-day was established, based on a 6-month study reporting a NOAEL of
0.23 mg Hg/kg-day for renal effects (increased absolute and relative kidney weights) (NTP 1993).
An acute-intermediate oral MRL of 0.00012 mg Hg/kg-day was established for methylmercury.
ATSDR derived their assessment from the Marsh et al. (1981) and Cox et al. (1989) data; the MRL is
based on the lowest observed peak of total mercury concentration in maternal hair (0.0012 mg/kg-day
equivalent to a LOAEL of 14 ppm mercury in maternal hair) during pregnancy associated with a
delayed onset of walking in offspring in Iraqi children. This assessment is discussed in section 6.3.1.1
of this volume.
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6.4.3 Department of Energy
Brookhaven Laboratories has prepared a report for Office of Clean Coal Technology, DOE.
This report describes a probabilistic-based assessment which considered the potential increased health
risk for paresthesia in adults. Their estimate is based upon a yearly emission rate of 180 kg/year from
all fossil fuel power plants in the United States. This estimate represents less than 1% of the existing
global pool .of mercury that is introduced into the environment. Based upon the most sensitive adult
sign of paresthesia, the mercury emissions from power plants would result in an increased risk for
paresthesia of 0.004-0.007% with an upper 95th percentile risk of 0.013-0.017% (Lipfert et al. 1994).
6.4.4 National Institute of Environmental Health Sciences (NIEHS)
NIEHS, part of the National Institutes of Health, was required under section 301 of the CAA
"to conduct, and transmit to the Congress by November 15, 1993, a study ^o determine the threshold
level of mercury exposure below which adverse human health effects are not expected to occur." In
section 112 (n)(l)(C), NIEHS was encouraged to evaluate the health effects threshold for mercury in
the absence of specifics as to species of mercury but to consider mercury in fish. As mercury in fish
is primarily in the form of methylmercury, the NIEHS limited their consideration to this species.
The report was completed in 1993 and delivered to Office of Management and Budget for
clearance. It describes dose- response assessments for methylmercury done by WHO, FDA and U.S.
EPA and presents all three estimates as recommended for tolerable mercury concentrations. The
NIEHS report also describes estimates of fish consumption by the U.S. population.
*
6.4.5 Department of Labor
OSHA established a Permissible Exposure Limit (PEL), time-weighted average of 0.05 mg
Hg/m3 for mercury vapor, with a notation for skin exposure (U.S. Department of Labor 1989). A PEL
as a ceiling value of 0.1 mg Hg/m3, also with a notation for dermal exposure was set for aryl mercury
and inorganic mercury compounds.
NIOSH determined a Recommended Exposure Limit (REL), time-weighted average, of 0.05
mg Hg/m3 for mercury and 0.1 mg Hg/m3 for aryl and inorganic mercury compounds (NIOSH 1973,
1988).
6.4.6 Various States
A number of states have released fish consumption advisories based upon their independent
analysis of the available scientific literature for methylmercury. Most active among these states are
Michigan, New Jersey, Maine, Idaho, and Oregon. Generally, there is a trend to move to more
conservative values based upon developmental neurotoxicity defined in the Marsh et al. (1981) and
Cox et al. (1989) papers. The methylmercury RfD of OJxlO"4 mg/kg-day used by the state of New
Jersey is discussed in section 6.3.1.1. Some states are waiting for more specific guidance from U.S.
EPA.
6.4.7 World Health Organization
The International Programme on Chemical Safety (IPCS) of the World Health Organization
published a criteria document on mercury (WHO 1990). In that document, it was stated that " a daily
June 1996 6-39 SAB REVIEW DRAFT
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intake of 3 to 7 ug Hg/kg body weight would cause adverse effects of the nervous system, manifested
as an approximately 5% increase in the incidence of paraesthesias". The IPCS expert group also
concluded that developmental effects in offspring (motor retardation or signs of CNS toxicity) could
be detected as increases over background incidence at maternal hair levels of 10-20 ppm mercury.
These levels of concern were based on evaluation of data including the human poisoning incident in
Iraq described in Chapter 3.
6.4.8 ACGIH
The ACGIH has established Threshold Limit values (TLV) as eight-hour time-weighted
averages. They include the following:
Aryl mercury compounds 0.1 mg Hg/m3
"Mercury vapor 0.05 mg Hg/m3
Inorganic mercury 0.1 mg Hg/m3
No STEL is recommended at this time. The Biological Exposure Indices Committee has
recommended values for inorganic mercury in urine and blood of 35 ug/g creatinine and 15 ug/L
respectively.
The ACGIH classified inorganic mercury including elemental mercury as follows: A4- Not
classifiable as a Human Carcinogen: There are inadequate data on which to classify the agent in terms
of its carcinogenicity in humans and/or animals.
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7. ONGOING RESEARCH AND RESEARCH NEEDS
7.1 Ongoing Research
t
Table 7-1 lists ongoing research projects abstracted from the Federal Research in Progress
Data Base (FEDRIP, 1994).
Table 7-1
Ongoing Research
Investigator
T. Clarkson
P. Grandjean
W. Markesbery
M. Martin
R. Mitchell
G. Myers
T. Okabe
M. Owens
M. Rosenman
D. Savitz
P. Bigazzi
T. Burbacher
K. Mottet
Affiliation
University of Rochester, Rochester,
NY
Odense University, Odense,
Denmark
University of Kentucky, Lexington,
KY
University of Washington, Seattle,
WA
University of Kentucky, Lexington,
KY
University of Rochester, Rochester,
NY
Baylor College of Dentistry,
Dallas, TX
Science Applications International
Corp,
Falls Church, VA
Morehouse College, Atlanta, GA
University of North Carolina
Chapel Hill, Chapel Hill, NC
University of Connecticut,
Farmington, CT
University of Washington, Seattle,
WA
University of Washington, Seattle,
WA
Research Description
Human
Dose-response relationships in humans
exposed to methylmercury and prenatal
and early postnatal body burdens of
methylmercury.
Neurotoxicity risk from exposure to
methylmercury from seafood
Role of mercury and dental amalgams in
Alzheimer's disease
Epidemiology of mercury in dentists
Amalgam restorations and the relative
risk of adverse pregnancy outcome
Child development following prenatal
methylmercury exposure via fish
Establish maximum levels of exposure
from amalgams for dental patients and
personnel
Potential and adverse effects associated
with dental amalgam
Effect of mercury in amalgam and urine
to cognitive functioning in children
Mercury and reproductive health in
women dentists
Animal
Mercury induced auto-immune disease in
rats
Developmental effects of methylmercury
in monkeys and rats
Long-term toxicity associated with
inorganic mercury and methylmercury
Sponsor
National Institute of
Environmental Health
Sciences (NIEHS)
NIEHS
National Institute on
Aging
National Institute of
Dental Research
National Institute of
Dental Research
NIEHS
National Institute of
Dental Research
National Institute of
Dental Research
National Institute of
General Medical
Sciences
National Institute of
Dental Research
NIEHS
NIEHS
NIEHS
June 1996
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Table 7-1
Ongoing Research (continued)
Investigator
K. Pollard
B. Weiss
Affiliation
University of California, San
Diego, CA
University of Rochester, Rochester,
NY
.Research Description
Animal model of systemic autoimmunity
induced by mercury
Neurotoxicity throughout the lifespan of
mice exposed prenatally to
methylmercury
Sponsor
National Institute of
Arthritis and
Musculoskeletal and
Skin Diseases
NIEHS
Mechanistic
W. Atchison
D. Barfuss
T. Jensen
D. Lawrence
R. Noelle
K. Pollard
B. Rajanna
K. Ruehl
T. Sarafian
J. Stokes
R. Zalups
Michigan State University, East
Lansing, MI
Georgia State University, Atlanta,
GA
Herbert H. Lehman College, New
York, NY
Albany Medical College, Albany,
NY
Dartmouth Medical School,
Hanover, NH
Scripps Research Institute, San
Diego, CA
Selma University,
Selma, AL
Rutgers University,
New Brunswick, NJ
University of California, Los
Angeles, CA
Mount Desert Island Biological
Lab,
Salsbury Cove, ME
Mercer University School of
Medicine
Neurotoxic mechanism of chronic
methylmercury poisoning
Transport and toxicity of inorganic
mercury in the nephron
Effect on membrane structure and
organelle distribution
Effects of metals on the structure and
function of murine and human
lymphocytes
Effect of mercury on pMymphocyte
function
Mechanisms of autoantibody response
induced by mercury which target the
nucleohis
Biomechanisms of heavy metal toxicity
in rats
Mechanism of methylmercury
neurotoxicity during development in
mice
Effect of methylmercury on protein
phosphorylation in cerebellar granule
cells in brain
Effects of mercurials on transport
properties of the bladder
Cytotoxicity of mercuric chloride to
isolated rat proximal tubular cells
NIEHS
NIEHS
National Institute of
General Medical
Sciences
NIEHS
NIEHS
National Institute of
Allergy and Infectious
Diseases
National Institute of
General Medical
Sciences
NIEHS
NIEHS .
NIEHS
NIEHS
Two of these ongoing studies deserve further discussion because they may fill critical data
needs for the development of a reference dose for methylmercury. The first is the Seychelles Islands
Study led by Dr. T.W. Clarkson from the University of Rochester. The objective of this study is to
define the extent of human health risks from prenatal exposure to methylmercury. Dose-response
June 1996
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relationships in a human population with dietary exposure to methylmercury at levels believed to be in
the range of the threshold for developmental toxicity are being studied. Both prenatal and early
postnatal body burdens of methylmercury will be examined as well as transport to the brain.
This study is testing the hypothesis, developed in previous studies of prenatal exposure in the
Iraq population, that subtle psychological and behavioral changes in prenatally exposed children can be
quantitatively related using dose-response models to the mother's methylmercury exposure during
pregnancy. In the Seychelles, a group of islands off the coast of Africa near Madagascar, a group of
779 infants who were prenatally exposed to methylmercury through maternal fish consumption is
being studied with annual administration of neurodevelopmental, psychological and educational testing
of the children through 5.5 years of age. This population consumes a relatively large amount of
marine fish and marine mammals, both of which are likely to contain methylmercury. The study is
testing the hypothesis that methylmercury concentration in hair correlates with methylmercury in the
brain by using human autopsy data. Mechanisms of transport of methylmercury across the blood brain
barrier also are being studied to understand better the factors that limit the accuracy of hair mercury as
a biological marker for target tissue levels. Findings reported in recent publications are summarized in
section 3.3.1 J.
The second study is the Faroe Islands Study led by Dr. P.A. Grandjean from Odense
University in Denmark. The purpose of this study is to determine whether a neurotoxic risk is present
from methylmercury exposure from seafood and, if so, the threshold for such effects. This study is
examining a cohort of 1,000 children in the Faroe Islands, located in the North Atlantic between
Scotland and Iceland. As is the case in the Seychelles, this population consumes a relatively large
amount of seafood; consumption includes marine fish and marine mammals. Intrauterine exposures
were determined by mercury analysis of umbilical cord blood and maternal hair collected at
consecutive births during 21 months in 1986 and 1987. In 13 percent of the births, mercury levels
were greater than 10 ppm in maternal hair, and 25 percent of the cord blood samples had a mercury
concentration above the corresponding level of 40 ug/L. No cases of gross methylmercury poisoning
have been observed. The persistence of mercury in the body is being assessed from mercury hair
concentrations hi the children at one and six years of age, and dietary information is being collected.
A detailed pediatric examination and a test battery to identify possible subtle signs of neurobehavioral
dysfunction are being conducted. The test battery includes psychological tests and neurophysiological
measurement of evoked potentials; these methods are known from previous research to be particularly
sensitive to the types of neurotoxicity expected.
The Faroese population was chosen for this study because of the homogeneity and stability of
the population and the efficient coverage of the Danish health care system. The cohort includes 75%
of all births occurring during the sampling period. A high participation rate (about 80%) is expected
at the 6-year examination period. Alcohol use is minimal in Faroese women (75% were abstainers
during pregnancy), and 60% are nonsmokers. The lead exposure is low (median lead concentration in
cord blood was 1.7 ug/100 mL). Exposure to polychlorinated biphenyls (PCB), however, may be a
confounder, and alcohol intake of the fathers may have been high. Due to the high seafood intake,
selenium exposure is increased, and its possible protective action against mercury toxicity is being
examined. Findings reported at recent scientific meetings are summarized in section 3.3.1.1.
7.2 Research Needs
H
In addition to the ongoing studies described above, further research is necessary for refinement
of the U.S. EPA's risk assessments for mercury and mercury compounds. In order to reduce
June 1996 7-3 SAB REVIEW DRAFT
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uncertainties in the current estimates of the. oral reference doses (RfDs) and inhalation reference
concentrations (RfCs), longer-term studies with low-dose exposures are necessary. In particular,
epidemiological studies should emphasize comprehensive exposure data with respect to both dose and
duration of exposure. The current RfD and RfC values have been determined for the most sensitive
toxicity endpoint for each compound; that is, the neurological effects observed following exposure to
elemental or methylmercury, and the renal autoimmune glomerulonephritis following exposure to
inorganic mercury. For each of these compounds, experiments conducted at increasingly lower doses
with more sensitive measures of effect will improve understanding of the respective dose-response
relationships at lower exposure levels and the anticipated thresholds for the respective effects in
humans. Similar information from developmental toxicity studies would allow determination of RfDs
for developmental toxicity (RfDdt) for elemental and inorganic mercury. For inorganic mercury,
furthermore, the many ongoing studies in which mechanisms of action are being investigated will
greatly assist hi quantifying the risks posed by these compounds.
Well-conducted studies are also needed to clarify exposure levels at which toxic effects other
than those defined as "critical" could occur in humans. For all three forms of mercury, data are
inadequate, conflicting, or absent for the following: adverse reproductive effects (effects on function
or outcome, including multigeneration exposure); impairment of immune function; and genotoxic
effects on human somatic or germinal cells (elemental and inorganic mercury). Investigations that
relate the toxic effects to biomonitoring data will be invaluable in quantifying the risks posed by these
mercury compounds. In addition, work should focus on subpopulations that have elevated risk
because they are exposed to higher levels of mercury at home or in the workplace, because they are
also simultaneously exposed to other hazardous chemicals, or because they have an increased
sensitivity to mercury toxicity. Information on postnatal exposure without prenatal exposure is limited;
therefore, analyzing the potential risks associated with mercury exposure of young children is difficult.
There are data gaps in the carcinogenicity assessments for each of the mercury compounds.
The U.S. EPA's weight-of-evidence classification of elemental mercury (Group D) is based on studies
in workers who were also potentially exposed to other hazardous compounds including radioactive
isotopes, asbestos, or arsenic. There were no appropriate animal studies available for this compound.
Studies providing information on the mode of action of inorganic mercury and methylmercury
hi producing tumors will be of particular use in defining the nature of the dose response relationship.
The assessment of both noncarcinogenic effects and carcinogenic effects will be improved by
an increased understanding of the toxicokinetics of these mercury compounds. In particular,
quantitative studies that compare the three forms of mercury across species and/or across routes of
exposure are vital for the extrapolation of animal data when assessing human risk. For elemental
mercury there is a need for quantitative assessment of the relationship between inhaled concentration
and delivery to the brain or fetus; in particular the rate of elemental to mercuric conversion mediated
by catalase and the effect of blood flow. Such assessment is needed for evaluation of the impact of
mercury exposure from dental amalgam.
Work has been done on development of physiologically-based pharmacokinetic models. While
one of these has developed a fetal submodel, data on fetal pharmacokinetics are generally lacking.
The toxicokinetics of mercury as a function of various developmental stages should be explored.
Elemental mercury and methylmercury,appear to have the same site of action in adults; research is,
therefore, needed on the potential for neurotoxicity in newborns when the mother is exposed. This
work should be accompanied by pharmacokinetic studies and model development
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APPENDIX A
DOSE CONVERSIONS
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APPENDIX A
DOSE CONVERSIONS
All doses in the tables in Section 4 were adjusted for the amount of mercury in the compound.
For example, for animals administered 1 mg/kg/day mercuric chloride:
Molecular weight of mercuric chloride = 271.5
Molecular weight of mercury = 200.6
Dose of mercury = 1 mg Hg/kg/day x 200.6/271.5 = 0.74 mg Hg/kg/day
(1) To convert from ppm in feed to mg/kg body weight/day, the following equation was used:
mg toxicant (T)/kg body weight/day = mg T/kg food x food factor
where food factor = kg food per day intake/kg body weight
(2) To convert from ppm in water to mg/kg body weight/day, the following equation was used:
mg T/kg body weight/day = mg T/L water x L water per day intake/kg body weight
where L is liters of water intake per day
Species
Mouse
Rat
Rabbit
Sample Values Used
Water
intake/day
(Liter/day)
0.0057
0.049
0.41
Body weight
(kg)
0.03
0.35
3.8
Food factor.
(kg food/kg body
weight)
0.13"
0.05
0.049
(3) To convert from ppm in air to mg/m3 for a vapor, the following equation was used:
1 mg/m3 = 1 ppm x molecular weight/24.45
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APPENDIX B
SUMMARIES FOR THE
INTEGRATED RISK INFORMATION SYSTEM (IRIS)
-------
_I.B. REFERENCE CONCENTRATION FOR CHRONIC INHALATION EXPOSURE (RfC)
Substance Name - Elemental mercury (Hg)
CASRN -- 7439-97-6
Preparation date - 3/12/90
JLB.l. INHALATION RfC SUMMARY
Critical Effect Exposures* UF MF RfC
Hand tremor; increases NOAEL: None 30 1 . 3E-4
in memory disturbances; mg/cu.m
slight subjective and LOAEL: 0.025 mg/cu.m
objective evidence of (converted to LOAEL [ADJ]
autonomic dysfunction of 0.009 mg/cu.m
Human occupational
inhalation studies
Fawer et al., 1983;
Piikivi and Tolonen, 1989;
Piikivi and Hanninen, 1989;
Piikivi, 1989;
Ngim et al., 1992;
Liang et al., 1993
* Conversion Factors and Assumptions: This is an extrarespiratory effect of a vapor (gas). The
LOAEL is based on an 8-hour TWA occupational exposure. MVho = 10 cu.m/day, MVh = 20
cu.m/day. LOAEL(HEC) = LOAEL(ADJ) = 0.025 mg/cu.m x MVho/MVh x 5 days/7 days = 0.009
mg/cu.m. Air concentrations (TWA) were measured in the Fawer et al. (1983), Ngim et al. (1992),
and Liang et al. (1993) studies. Air concentrations were extrapolated from blood levels based on the
conversion factor of Roels et al. (1987) as described in the Additional Comments section for the
studies of Piikivi and Tolonen (1989), Piikivi and Hanninen (1989), and Piikivi (1989).
_I.B.2. PRINCIPAL AND SUPPORTING STUDIES (INHALATION RfC)
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Fawer et al. (1983) used a sensitive objective electronic measure of intention tremor (tremors .
that occur at the initiation of voluntary movements) in 26 male workers (mean age of 44 years)
exposed to low levels of mercury vapor in various occupations: fluorescent tube manufacture (n=7),
chloralkali plants (n=12), and acetaldehyde production (n=7). Controls (n=25; mean age of 44.6 years)
came from the same factories but were not exposed occupationally. Personal air samples (two per
subject) were used to characterize an average exposure concentration of 0.026 mg/cu.m. It should be
noted that it is likely that the levels of mercury hi the air varied during the period of exposure and
historical data indicate that previous exposures may have been higher. Exposure measurements for the
control cohort were not performed. The average duration of exposure was 15.3 years. The measures
of tremor were significantly increased hi the exposed compared to control cohorts, and were shown to
correspond to exposure and not to chronologic age. These findings are consistent with
neurophysiological impairments that might result from accumulation of mercury in the cerebellum and
basal ganglia. Thus, the TWA of 0.026 mg/cu.m was designated a LOAEL. Using the TWA and
adjusting for occupational ventilation rates and workweek, the resultant LOAEL(HEC) is 0.009
mg/cu.m.
Piikivi and Tolonen (1989) used EEGs to study the effects of long-term exposure to mercury
vapor in 41 chloralkali workers exposed for a mean of 15.6 +/- 8.9 years as compared with matched
referent controls. They found that the exposed workers, who had mean blood Hg levels of 12 ug/L
and mean urine Hg levels of 20 ug/L, tended to have an increased number of EEG abnormalities when
analyzed by visual inspection only. When the EEGs were analyzed by computer, however, the
exposed workers were found to have significantly slower and attenuated brain activity as compared
with the referents. These changes were observed hi 15% of the exposed workers. The frequency of
these changes correlated with cortical Hg content (measured hi other studies); the changes were most
prominent hi the occipital cortex less prominent hi the parietal cortex, and almost absent hi the frontal
cortex. The authors extrapolated an exposure level associated with these EEG changes of 0.025
mg/cu.m from blood levels based on the conversion factor calculated by Roels et al. (1987).
Piikivi and Hanninen (1989) studied the subjective symptoms and psychological performances
on a computer-administered test battery hi 60 chloralkali workers exposed to mercury vapor for a
mean of 13.7 +/- 5.5 years as compared with matched referent controls. The exposed workers had
mean blood Hg levels of 10 ug/L and mean urine Hg levels of 17 ug/L. A
statistically significant increase hi subjective measures of memory disturbance and sleep disorders was
found hi the exposed workers. The exposed workers also reported more anger, fatigue and confusion.
No objective disturbances hi perceptual motor, memory or learning abilities were found hi the exposed
workers. The authors extrapolated an exposure level associated
with these subjective measures of memory disturbance of 0.025 mg/cu.m from blood levels based on
the conversion factor calculated by Roels et al. (1987).
June 1996 B-2 SAB REVIEW DRAFT
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Both subjective and objective symptoms of autonomic dysfunction were investigated in 41
chloralkali workers exposed to mercury vapor for a mean of 15.6 +/- 8.9 years as compared with
matched referent controls (Piikivi, 1989). The quantitative non-invasive test battery consisted of
measurements of pulse rate variation in normal and deep breathing, in the Valsalva maneuver and in
vertical tilt, as well as blood pressure responses during standing and isometric work. The exposed
workers had mean blood levels of 11.6 ug/L and mean urine levels of 19.3 ug/L. The exposed
workers complained of more subjective symptoms of autonomic dysfunction than the controls, but the
only statistically significant difference was an increased reporting of palpitations in the exposed
workers. The quantitative tests revealed a slight decrease in pulse rate variations, indicative of
autonomic reflex dysfunction, in the exposed workers. The authors extrapolated an exposure level
associated with these subjective and objective measures of autonomic dysfunction of 0.030 mg/cu.m
from blood levels based on the conversion factor calculated by Roels et al. (1987).
Two more recent studies in other working populations corroborate the neurobehavioral toxicity
of low-level mercury exposures observed La the Fawer et al. (1983), Piikivi and Tolonen (1989),
Piikivi and Hanninen (1989), and Piikivi (1989) studies.
Ngim et al. (1992) assessed neurobehavioral performance in a cross-sectional study of 98
dentists (38 female, 60 male; mean age 32, range 24-49 years) exposed to TWA concentrations of
0.014 mg/cu.m (range 0.0007 to 0.042 mg/cu.m) versus 54 controls (27 female, 27 male; mean age 34,
range 23-50 years) with no history of occupational exposure to mercury. Air concentrations were
measured with personal sampling badges over typical working hours (8-10 hours) and converted to an
8-hour TWA. No details on the number of exposure samples or exposure histories were provided.
Blood samples from the exposed cohort were also taken and the data supported the correspondence
calculated by Roels et al. (1987). Based on extrapolation of the average blood mercury concentration
(9.8 ug/L), the average exposure concentration would be estimated at 0.023 mg/cu.m. The average
duration of practice of the exposed dentists was 5.5 years. Exposure measurements of the control
cohort were not performed. The exposed and control groups were adequately matched for age, amount
of fish consumption, and number of amalgam dental fillings. The performance of the dentists was
significantly worse than controls on a number of neurobehavioural tests measuring motor speed (finger
tapping), visual scaning, visumotor coordination and concentration, visual memmory, and visuomotor
coordination speed. These neurobehavioral effects are consistent with central and peripheral
neurotoxicity and the TWA is considered a LOAEL. Using the TWA and adjusting for occupational
ventilation rates and the reported 6-day workweek, the resultant LOAEL(HEC) is 0.006 mg/cu.m.
Liang et al. (1993) investigated workers in a fluorescent lamp factory with a
computer-adminstered neurobehavioral evaluation system and a mood inventory profile. The exposed
cohort (mean age 34.2 years) consisted of 19 females and 69 males exposed to ninterruptedly for at
least 2 years prior to the study. Exposure was monitored with area samplers and ranged from 0.008 to
0.085 mg/cu.m across worksites. No details on how the exposure profiles to account for time spent in
different worksites were constructed. The average exposure was estimated at 0.033 mg/cu.m. (range
0.005 to 0.19 mg/cu.m). The average duration was of working was 15.8 years for the exposed cohort.
Urinary excretion was also monitored and reported to average 0.025 mg/L. The control cohort (mean
age 35.1 years) consisted of 24 females and 46 males recruited from an embroidery factory. The
controls were matched for age, education, smoking and drinking habits. Exposure measurements for
the control cohort were not performed. The exposed cohort performed significantly worse than the
control on tests of finger tapping, mental arithmetic, two-digit searches, switiching attention, and visual
reaction time. The effect on performance persisted after the confounding factor of chronological age
was controlled. Based on these neurobehavioral effects, the TWA of 0.033 mg/cu.m is designated as
June 1996 B-3 SAB REVIEW DRAFT
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LOAEL. Using the TWA and adjusting for occupational ventilation rates and workweek, the resultant
LOAEL(HEC) is 0.012 mg/cu.m.
The above studies were taken together as evidence for a LOAEL based on neurobehavioral
effects of low-level mercury exposures. The LOAEL(HEC) levels calculated on measured air
concentration levels of the Ngim et al. (1992) and the Liang et al. (1993) studies bracket that
calculated based on the air concentrations measured by Fawer et al. (1983) as a median HEC level.
Extrapolations of blood levels, used as biological monitoring that accounts for variability hi exposure
levels, also converge at 0.025 mg/cu.m as a TWA which results in the same HEC level. Thus, the
TWA level of 0.025 mg/cu.m was used to represent the exposure for the synthesis of the studies
described above. Using this TWA and taking occupational ventilation rates and workweek into
account results hi a LOAEL(HEC) of 0.009 mg/cu.m.
_I.B.3. UNCERTAINTY AND MODIFYING FACTORS (INHALATION RfC)
UF — An uncertainty factor of 10 was used for the protection of sensitive
human subpopulations (including concern for acrodynia - see Additional
Comments section) together with the use of a LOAEL. An uncertainty factor of
3 was used for lack of data base, particularly developmental and reproductive
studies.
MF — None
_I.B.4. ADDITIONAL COMMENTS (INHALATION RfC)
Probably the most widely recognized form of hypersensitivity to mercury poisoning is the
uncommon syndrome known as acrodynia, also called erythredema polyneuropathy or pink disease
(Warkany and Hubbard, 1953). Infantile acrodynia was first described in 1828, but adult cases have
also since been reported. While acrodynia has generally been associated with short-term exposures
and with urine levels of 50 ug/L or more, there are some cases hi the literature in which mercury
exposure was known to have occurred, but no significant (above background) levels in urine were
reported. There could be many reasons for this, but the most likely is that urine levels are not a
simple measure of body burden or of target tissue (i.e., brain levels); however, they are the best means
available for assessing the extent of exposure. It was felt that the RfC level estimated for mercury
vapor based on neurotoxicity of chronic exposure in workers is adequate to protect children from risk
of acrodynia because such exposures of long duration would be expected to raise urine levels by only
0.12 ug/L against a background level of up to 20 ug/L (i.e., such exposures would not add
significantly to the background level of mercury in those exposed).
Roels et al. (1987) investigated the relationships between the concentrations of metallic
mercury hi air and levels monitored hi blood or urine in workers exposed during manufacturing of dry
alkaline batteries. Breathing zone personal samples were used to characterize airborne mercury vapors.
Total mercury in blood and urine samples were analyzed using atomic absorption. The investigation
controlled for several key factors including the use of reliable personal air monitoring, quality control
for blood and urine analyses, standardization of the urinary mercury concentration for creatinine
June 1996 B-4 SAB REVIEW DRAFT
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.concentration, and stability of exposure conditions (examined subjects were exposed to mercury vapor
for at least 1 year). Strong correlations were found between the daily intensity of exposure to mercury
vapor and the end of workshift levels in blood (r=0.86; n=40) or urine (r=0.81; n=34). These
relationships indicated a conversion factor of 1:4.5 (air:blood) and 1:1.22 (airurine as ug/g creatinine).
These factors were used to extrapolate blood or urine levels associated with effects in the reported
studies to airborne mercury levels.
Sensory and motor nerve conduction velocities were studied in 18 workers from a mercury cell
chlorine plant (Levine et al., 1982). Time-integrated urine Hg levels were used as an indicator of
mercury exposure. Using linearized regression analysis, the authors found that motor and sensory
nerve conduction velocity changes (i.e., prolonged distal latencies correlated with the time-integrated
urinary Hg levels in asymptomatic exposed workers) occurred when urinary Hg levels exceeded 25
ug/L. This study demonstrates that mercury exposure can be associated with preclinical evidence of
peripheral neurotoxicity. * *
a
Singer et al. (1987) studied nerve conduction velocity of the median motor, median sensor and
sural nerves in 16 workers exposed to various inorganic mercury compounds (e.g., mercuric oxides,
mercurial chlorides, and phenyl mercuric acid) for an average of 7.3 +/- 7.1 years as compared with an
unexposed control group using t-tests. They found a slowing of nerve conduction velocity in motor,
but not sensory, nerves that correlated with increased blood and urine Hg levels and an increased
number of neurologic symptoms. The mean mercury levels hi the exposed workers were 1.4 and 10
ug/L for blood and urine, respectively. These urine levels are 2-fold less than those associated with
peripheral neurotoxicity in other studies (e.g., Levine et al., 1982). There was considerable variability
in the data presented by Singer et al. (1987), however, and the statistical analyses (t-test) were not as
rigorous as those employed by Levine et al. (1982) (linearized regression analysis). Furthermore, the
subjects in the Levine et al. (1982) study were asymptomatic at higher urinary levels than those
reported to be associated with subjective neurological complaints in the workers studied by Singer et
al. (1987). Therefore, these results are not considered to be as reliable as those reported by Levine et
al. (1982).
Miller et al. (1975) investigated several subclinical parameters of neurological dysfunction in
142 workers exposed to inorganic mercury in either the chloralkali industry or a factory for the
manufacture of magnetic materials. They reported a significant increase in average forearm tremor
frequency in workers whose urinary Hg concentrations exceeded 50 ug/L as compared with unexposed
controls. Also observed were eyelid fasciculation, hyperactive deep-tendon reflexes and
dermatographia, but there was no correlation between the incidence of these findings and urinary Hg
levels.
Roels et al. (1985) examined 131 male and 54 female workers occupational^ exposed to
mercury vapor for an average duration of 4.8 years. Urinary mercury (52 and 37 ug/g creatinine for
males and females, respectively) and blood mercury levels (14 and 9 ug/L for males and females,
respectively) were recorded, but atmospheric mercury concentration was not provided. Symptoms
indicative of CNS disorders were reported but not related to mercury exposure. Minor renal tubular
effects were detected in mercury-exposed males and females and attributed to current exposure
intensity rather (urinary Hg >50 ug/g creatinine) than exposure duration. Male subjects with urinary
mercury levels of >50 ug/g creatinine exhibited preclinical signs of hand tremor. It was noted that
females did not exhibit this effect and that their urinary mercury never reached the level of 50 ug/g
creatinine. A companion study (Roels et al., 1987) related air mercury (Hg-air)levels to blood mercury
(Hg-blood) and urinary mercury (Hg-U) values in 10 workers in a chloralkali battery plant. Duration
June 1996 B-5 SAB REVIEW DRAFT
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of exposure was not specified. A high correlation was reported for Hg-air and Hg-U for preshift
exposure (r=0.70, p<0.001) and post-shift (r=0.81, p<0.001) measurements. Based on these data and
the results of their earlier (1985) study, the investigators suggested that some mercury-induced effects
may occur when Hg-U levels exceed 50 ug/g creatinine, and that this value corresponds to a mercury
TWA of about 40 ug/cu.m.
A survey of 567 workers at 21 chloralkali plants was conducted to ascertain the effects of
mercury vapor inhalation (Smith et al., 1970). Mercury levels ranged from <0.01 to 0.27 mg/cu.m and
chlorine concentrations ranged from 0.1 to 0.3 ppm at most of the working stations of these plants.
Worker exposure to mercury levels (TWA) varied, with 10.2% of the workers being exposed to <0.01
mg/cu.m, 48.7% exposed to 0.01 to 0.05 mg/cu.m, 25.6% exposed to 0.06 to 0.10 mg/cu.m and 4.8%
exposed to 0.24 to 0.27 mg/cu.m (approximately 85% were exposed to Hg levels less than or equal to
0.1 mg/cu.m). The duration of employment for the examined workers ranged from one year (13.3%)
to >10 years (31%), with 55.7% of the workers being employed for 2 or 9 Vears. A group of 600
workers not exposed to chlorine served as a control group for assessment of chlorine effects, and a
group of 382 workers not exposed to either chlorine or mercury vapor served as the reference control
group. A strong positive correlation (p<0.001) was found between the mercury TWAs and the
reporting of subjective neuropsychiatric symptoms (nervousness, insomnia), occurrence of objective
tremors, and weight and appetite loss. A positive correlation (p<0.001) was also found between
mercury exposure levels and urinary and blood mercury levels of test subjects. No adverse alterations
in cardiorespiratory, gastrointestinal, renal or hepatic functions were attributed to the mercury vapor
exposure. Additionally, biochemical (hematologic data, enzyme activities) and clinical measurements
(EKG, chest X-rays) were no different between the mercury-exposed and non-exposed workers. No
significant signs or symptoms were noted for individuals exposed to mercury vapor concentrations less
than or equal to 0.1 mg/cu.m. This study provides data indicative of a NOAEL of 0.1 mg Hg/cu.m
and a LOAEL of 0.18 mg Hg/cu.m. In a followup study conducted by Bunn et al. (1986), however,
no significant differences in the frequency of objective or subjective findings such as weight loss and
appetite loss were observed in workers exposed to mercury at levels that ranged between 50 and 100
ug/L. The study by Bunn et al. (1986) was touted, however, by the lack of information provided
regarding several methodological questions such as quality assurance measures and control of possible
confounding variables.
The mercury levels reported to be associated with preclinical and symptomatic neurological
dysfunction are generally lower than those found to affect kidney function, as discussed below.
Piikivi and Ruokonen (1989) found no evidence of glomerular or tubular damage in 60
chloralkali workers exposed to mercury vapor for an average of 13.7 +/- 5.5 years as compared with
their matched referent controls. Renal function was assessed by measuring urinary albumin and
N-acetyl-beta-glucosaminidase (NAG) activity. The mean blood Hg level hi the exposed workers was
14 ug/L and the mean urinary level was 17 ug/L. The authors extrapolated the NOAEL for kidney
effects based on these results of 0.025 mg/cu.m from blood levels using the conversion factor
calculated by Roels et al. (1987).
Stewart et al. (1977) studied urinary protein excretion in 21 laboratory workers exposed to
10-50 ug/cu.m of mercury. Their urinary level of mercury was about 35 ug/L. Increased proteinuria
was found in the exposed workers as compared with unexposed controls. When preventive measure
were instituted to limit exposure to mercury, proteinuria was no longer observed in the exposed
technicians.
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Lauwerys et al. (1983) found no change in several indices of renal function (e.g., proteinuria,
albuminuria, urinary excretion of retinol-binding protein, arninoaciduria, creatinine in serum,
beta-2-microglobulin in serum) in 62 workers exposed to mercury vapor for an average of 5.5 years.
The mean urinary Hg excretion in the exposed workers was 56 ug/g creatinine, which corresponds to
an exposure level of about 46 ug/cu.m according to a conversion factor of 1:1.22 (air.urine [ug/g
creatinine]) (Roels et al., 1987). Despite the lack of observed renal effects, 8 workers were found to
have an increased in serum anti-laminin antibodies, which can be indicative of immunological effects.
In a followup study conducted by Bernard et al. (1987), however, there was no evidence of increased
serum anti-laminin antibodies in 58 workers exposed to mercury vapor for an average of 7.9 years.
These workers had a mean urinary Hg excretion of 72 ug/g creatinine, which corresponds to an
exposure levels of about 0.059 mg/cu.m.
«
Stonard et al. (1983) studied renal function in 100 chloralkali workers exposed to inorganic
mercury vapor for an average of 8 years. No changes in the following urinary parameters of renal
function were observed at mean urinary Hg excretion rates of 67 ug/g creatinine: total protein,
albumin, alpha-1-acid "glycoprotein, beta-2-microglobulin, NAG, and gamma-glutamyl transferase.
When urinary Hg excretion exceeded 100 ug/g creatinine, a small increase in the prevalence of higher
activities of NAG and gamma-glutamyl transferase was observed.
The mercury levels reported to be associated with preclinical and symptomatic neurological
dysfunction and kidney effects are lower than those found to pulmonary function, as discussed below.
McFarland and Reigel (1978) described the cases of 6 workers who were acutely exposed (4-8
hours) to calculated metallic mercury vapor levels of 1.1 to 44 mg/cu.m. These men exhibited a
combination of chest pains, dyspnea, cough, hemoptysis, impairment of pulmonary function (reduced
vital capacity), diffuse pulmonary infiltrates and evidence of interstitial pneumonitis. Although the
respiratory symptoms resolved, all six cases exhibited chronic neurological dysfunction, presumably as
a result of the acute, high-level exposure to mercury vapor.
Lilis et al. (1985) described the case of a 31-year-old male who was acutely exposed to high
levels of mercury vapor in a gold-extracting facility. Upon admission to the hospital, the patient
exhibited dyspnea, chest pain with deep inspiration, irregular infiltrates in the lungs and reduced
pulmonary function (forced vital capacity [FVC]). The level of mercury to which he was exposed is
not known, but a 24-hour urine collection contained 1900 ug Hg/L. Although the patient improved
gradually over the next several days, 11 months after exposure he still showed signs of pulmonary
function abnormalities (e.g., restriction and diffusion impairment).
Levin et al. (1988) described four cases of acute high-level mercury exposure during gold ore
purification. The respiratory symptoms observed hi these four cases ranged from minimal shortness of
breath and cough to severe hypoxemia. The most severely affected patient exhibited mild interstitial
lung disease both radiographically and on pulmonary function testing. One patient had a urinary Hg
level of 245 ug/L upon hospital admission. The occurrence of long-term respiratory effects in these
patients could not be evaluated since all but one refused follow-up treatment.
Ashe et al. (1953) reported that there was no histopathological evidence of respiratory damage
in 24 rats exposed to 0.1 mg Hg/cu.m 7 hr/day, 5 days/week for 72 weeks. This is equivalent to a
NOAEL[HEC] of 0.07 mg/cu.m.
June 1996 . B-7 , SAB REVIEW DRAFT
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Kishi et al. (1978) observed no histopathological changes in the lungs of rats exposed to 3
mg/cu.m of mercury vapor 3 hours/day, 5 days/week for 12-42 weeks.
Beliles et al. (1967) observed no histopathological changes in the lungs of pigeons exposed to
0.1 mg/cu.m of mercury vapor 6 hours/day, 5 days/week for 20 weeks.
Neurological signs and symptoms (i.e., tremors) were observed in 79 workers exposed to
metallic mercury vapor whose urinary mercury levels exceeded 500 ug/L. Short-term memory deficits
were reported in workers whose urine levels were less than 500 ug/L (Langolf et al., 1978).
Impaired performance in mechanical and visual memory tasks and psychomotor ability tests
was reported by Forzi et al. (1978) in exposed workers whose urinary Hg levels exceeded 100 ug/L.
•
Decreased strength, decreased coordination, increased tremor, decreased sensation and
increased prevalence of Babinski and snout reflexes were exhibited by 247 exposed workers whose
urinaryHg levels exceeded 600 ug/L. Evidence of clinical neuropathy was observed at urinary Hg
levels that exceeded 850 ug/L (Albers et al., 1988).
Preclinical psychomotor dysfunction was reported to occur at a higher incidence in 43 exposed
workers (mean exposure duration of 5 years) whose mean urinary excretion of Hg was 50 ug/L.
Workers in the same study whose mean urinary Hg excretion was 71 ug/L had a higher incidence of
total proteinuria and albuminuria (Roels et al., 1982).
Postural and intention tremor was observed in 54 exposed workers (mean exposure duration of
7.7 years) whose mean urinary excretion of Hg was 63 ug/L (Roels et al., 1989).
Verbeck et al. (1986) observed an increase in tremor parameters with increasing urinary
excretion of mercury in 21 workers exposed to mercury vapor for 0.5-19 years. The LOAEL for this
effect was a mean urinary excretion of 35 ug/g creatinine.
Rosenman et al. (1986) evaluated routine clinical parameters (physical exams, blood chemistry,
urinalysis), neuropsychological disorders, urinary NAG, motor nerve conduction velocities and
occurrence of lenticular opacities in 42 workers of a chemical plant producing mercury compounds. A
positive correlation (p<0.05 to p<0.001) was noted between urinary mercury (levels ranged from
100-250 ug/L) and the number of neuropsychological symptoms, and NAG excretions and the decrease
in motor nerve conduction velocities.
Evidence of renal dysfunction (e.g., increased plasma and urinary concentrations of
beta-galactosidase, increased urinary excretion of high-molecular weight proteins and a slightly
increased plasma beta-2-microglobulin concentration) was observed in 63 chloralkali workers. The
incidence of these effects increased in workers whose urinary Hg excretion exceeded 50 ug/g
creatinine (Buchet et al., 1980).
Increased urinary NAG levels were found in workers whose urinary Hg levels exceeded 50
ug/L (Langworth et al., 1992).
An increase hi the concentration of urinary brush border proteins (BB-50) was observed in 20
workers whose mean urinary Hg excretion exceeded 50 ug/g creatinine (Mutti et al., 1985).
June 1996 B-8 . SAB REVIEW DRAFT
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Foa et al. (1976) found that 15 out of 81 chloralkali workers exposed to 60-300 ug/cu.m
mercury exhibited proteinuria.
An increased excretion of beta-glutamyl transpeptidase, indicative of renal dysfunction, was
found in 509 infants dermally exposed to phenylmercury via contaminated diapers (Gotelli et al.,
1985).
•
Berlin et al. (1969) exposed rats, rabbits and monkeys to 1 mg/cu.m of mercury vapor for 4
hours and measured the uptake and distribution of mercury in the brain as compared with animals
injected intravenously with the same doses of mercury as mercuric salts. Mercury accumulated hi the
brain following inhalation exposure to metallic mercury vapor at levels that were 10 times higher than
those observed following intravenous injection of the same dose of mercury as mercuric salts. These
results demonstrate that mercury is taken up by the brain following inhalation of the vapor at higher
levels than other forms of mercury and that this occurs in all species studied.
Limited animal studies concerning inhalation exposure to inorganic mercury are available. The
results of a study conducted by Baranski and Szymczyk (1973) were reported in an English abstract
Adult female rats were exposed to metallic mercury vapor at 2.5 mg/cu.m for 3 weeks prior to
fertilization and during gestation days 7-20. A decrease in the number of living fetuses was observed
in the dams compared with unexposed controls, and all pups born to the exposed dams died by the
sixth day after birth. However, no difference in the occurrence of developmental abnormalities was
observed between exposed and control groups. The cause of death of the pups in the mercury-exposed
group was unknown, although an unspecified percentage of the deaths was attributed by the authors to
a failure of lactation in the dams. Death of pups was also observed in another experiment where dams
were only exposed prior to fertilization (to 2.5 mg/cu.m), which supports the conclusion that the high
mortality in the first experiment was due at least hi part to poor health of the mothers. Without
further information, this study must be considered inconclusive regarding developmental effects.
The only other study addressing the developmental toxicology of mercury is the one reported
in abstract form by Steffek et al. (1987) and, as such, is included as a supporting study.
Sprague-Dawley rats (number not specified) were exposed by inhalation to mercury vapor at
concentrations of 0.1, 0.5 or 1.0 mg/ol.m throughout the period of gestation (days 1-20) or during the
period of organogenesis (days 10-15). The authors indicated the exposure protocols to be chronic and
acute exposure, respectively. At either exposure protocol, the lowest mercury level produced no
detectable adverse effect. At 0.5 mg/cu.m, an increase in the number of resorptions (5/41) was noted
for the acute group, and two of 115 fetuses exhibited gross cranial defects hi the chronic group. At
1.0 mg/cu.m, the number of resorptions was increased hi acute (7/71) and chronic (19/38) groups and
a decrease hi maternal and fetal weights also was detected hi the chronic exposure group. No
statistical analysis for these data was provided. A LOAEL of 0.5 mg/cu.m is provided based on these
data.
Mishinova et al. (1980) investigated the course of pregnancy and parturition hi 349 women
exposed via inhalation to metallic mercury vapors (unspecified concentrations) in the workplace as
compared to 215 unexposed women. The authors concluded that the rates of pregnancy and labor
complication were high among women exposed to mercury and that the effects depended on "the
length of service and concentration of mercury vapors." Lack of sufficient details preclude the
evaluation of dose-response relationships.
June 1996 B-9 SAB REVIEW DRAFT
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In a questionnaire that assessed the fertility of male workers exposed to mercury vapor,
Lauwerys et al. (1985) found no statistically significant change in the observed number of children
born to the exposed group compared with a matched control group. The urinary excretion of mercury
in the exposed workers ranged from 5.1 to 272.1 ug/g creatinine.
Another study found that exposure to metallic mercury vapor caused prolongation of estrus
cycles in animals. Baranski and Szymczyk (1973) reported that female rats exposed via.inhalation to
mercury vapor at an average of 2.5 mg/cu.m, 6 hours/day, 5 days/week for 21 days experienced longer
estrus cycles than unexposed animals. In addition, estrus cycles during mercury exposure were longer
than normal estrus cycles in the same animals prior to exposure. Although the initial phase of the
cycle was protracted, complete inhibition of the cycle did not occur. During the second and third
weeks of exposure, these rats developed signs of mercury poisoning including restlessness, seizures
and trembling of the entire body. The authors speculated that the effects on the estrus cycle were
caused by the action of mercury on the CNS (i.e., damage to the hypothalamic regions involved in the
control of estrus cycling).
Renal toxicity has been reported following oral exposure to inorganic mercury salts in animals,
with the Brown-Norway rat appearing to be uniquely sensitive to this effect These mercury-induced
renal effects in the Brown-Norway rat are the basis for the oral RfD for mercurial mercury. Several
investigators have produced autoimmune glomerulonephritis by administering HgC12 to
Brown-Norway rats (Druet et al., 1978).
The current OSHA standard for mercury vapor is 0.05 mg/cu.m. NIOSH recommends a TWA
Threshold Limit Value of 0.05 mg/cu.m for mercury vapor.
_I.B.5. CONFIDENCE IN THE INHALATION RfC
Study ~ Medium
Data Base — Medium
RfC - Medium
Due to the use of a sufficient number of human subjects, the inclusion of appropriate control
groups, the exposure duration, the significance level of the reported results and the fact that exposure
levels in a number of the studies had to be extrapolated from blood mercury levels, confidence in the
key studies is medium. The LOAEL values derived from these studies can be corroborated by other
human epidemiologic studies. The adverse effects reported in these studies are in accord with the
well-documented effects of mercury poisoning. The lack of human or multispecies
reproductive/developmental studies precludes assigning a high confidence rating to the data base and
inadequate quantification of exposure levels. Based on these considerations, the RfC for mercury is
assigned a confidence rating of medium.
_I.B.6. EPA DOCUMENTATION AND REVIEW OF THE INHALATION RfC
Source Document - U.S. EPA, 1995
June 1996 B-10 SAB REVIEW DRAFT
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This IRIS summary is included in The Mercury Study'Report to Congress which was reviewed
by OHEA and EPA's Mercury Work Group in November 1994. An interagency review by scientists
from other federal agencies took place in January 1995. The report was also reviewed by a panel of
non-federal external scientists in January 1995 who met in a public meeting on January 25-26. All
reviewers comments have been carefully evaluated and considered in the revision and finalization of
this IRIS summary. A record of these comments is summarized in the IRIS documentation files.
Other EPA Documentation - None
Agency Work Group Review « 11/16/89, 03/22/90, 04/19/90
Verification Date - 04/19/90
_I.B.7. EPA CONTACTS (INHALATION RfC)
Annie M. Jarabek / NCEA - (919)541-4847
William F. Sette / OPP - (703)305-6375
REFERENCES
Albers, J.W., L.R. Kallenbach, L.J. Fine, et al. 1988. Neurological abnormalities associated with
remote occupational elemental mercury exposure. Ann. Neurol. 24(5): 651-659.
Ashe, W.F., EJ. Largent, F.R. Dutra, D.M. Hubbard and M. Blackstone. 1953. Behavior of mercury
in the animal organism following inhalation. Ind. Hyg. Occup. Med. 17: 19-43.
Baranski, B. and I. Szymczyk. 1973. [Effects of mercury vapor upon reproductive functions of
female white rats]. Med. Pr. 24(3): 249-261. (Czechoslovakia^
Beliles, R.P., R.S. Clark, P.R. Belluscio, C.L. Yuile and L.J. Leach. 1967. Behavioral effects in
pigeons exposed to mercury vapor at a concentration of 0.1 mg/cu.m. Am. Ind. Hyg. J. 28(5):
482-484.
Berlin, M., J. Fazackerley and G. Nordberg. 1969. The uptake of mercury in the brains of mammals
exposed to mercury vapor and to mercuric salts. Arch. Environ. Health. 18: 719-729.
Bernard, A.M., H.R. Roels, J.M. Foldart and R.L. Lauwerys. 1987. Search for anti-laminin antibodies
in the serum of workers exposed to cadmium, mercury vapour or lead. Int. Arch. Occup. Environ.
Health. 59: 303-309.
Buchet, J.P., H. Roels, A. Bernard and R. Lauwerys 1980. Assessment of renal function of workers
exposed to inorganic lead, cadmium or mercury vapor. J. Occup. Med. 22(11): 741-750.
Bunn, W.B., C.M. McGill, T.E. Barber, J.W. Cromer and L.J. Goldwater. 1986. Mercury exposure in
chloralkali plants. Am. Ind. Hyg. Assoc. J. 47(5): 249-254.
June 1996 B-ll SAB REVIEW DRAFT
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Druet, P., E. Druet, F. Potdevin, et al. 1978. Immune type glomerulonephritis induced by HgC12 in
the Brown-Norway rat. Ann. Immunol. 129C: 777-792.
Fawer, R.F., Y. DeRibaupierre, M.P. Guillemin, M. Berode and M. Lob. 1983. Measurement of hand
tremor induced by industrial exposure to metallic mercury. J. Ind. Med. 40: 204-208.
Foa, V., L. Caimi, L. Amante, et al. 1976. Patterns of some lysosomal enzymes in the plasma and of
proteins in urine of workers exposed to inorganic mercury. Int. Arch. Occup. Environ. Health. 37:
115-124.
Forzi, M., M.G. Cassitto, C. Bulgheroni and V. Foa. 1978. Psychological measures in workers
occupationally exposed to mercury vapors: A validation study. In: Adverse Effects of Environmental
Chemicals and Psychotropic Drugs: Neurophysiological and Behavioral Tests, Vol. 2, H.J.
Zimmerman, Ed. Appleton-Century-Crofts, New York, NY. p. 165-171.
Gotelli, C.A., E. Astolfi, C. Cox, E. Cernichiari and T. Clarkson. 1985. Early biochemical effects of
an organic mercury funcigicide on infants: "Dose makes the poison". Science. 277: 638-640.
Kishi, R., K. Hashimoto, S. Shimizu and M. Kobayashi. 1978. Behavioral changes and mercury
concentrations hi tissues of. rats exposed to mercury vapor. Toxicol. Appl. Pharmacol. 46(3):
555-566.
Langolf, G.D., D.B. Chaffin, R. Henderson and H.P. Whittle. 1978. Evaluation of workers exposed
to elemental mercury using quantitative tests of tremor and neuromuscular functions. Am. Ind. Hyg.
Assoc. J. 39:976-984.
Langworth, S., C.G. Elinder, K.G. Sundquist and O. Vesterberg. 1992. Renal and immunological
effects of occupational exposure to inorganic mercury. Br. J. Ind. Med. 49: 394-401.
Lauwerys, R., A. Bernard, H. Roels, et al. 1983. Anti-laminm antibodies hi workers exposed to
mercury vapour. Toxicol. Lett. 17:113-116.
Lauwerys, R., H. Roels, P. Genet, G. Toussaint, A. Bouckaert and S. De Cooman. 1985. Fertility of
male workers exposed to mercury vapor or to manganese dust: A questionnaire study. Am. J. Ind.
Med. 7(2): 171-176.
Levin, M., J. Jacobs and P.O. Polos. 1988. Acute mercury poisoning and mercurial pneumonitis from
gold ore purification. Chest. 94(3): 554-558.
Levine, S.P., G.D. Cavender, G.D. Langolf and J.W. Albers. 1982. Elemental mercury exposure:
Peripheral neurotoxicity. Br. J. Ind Med 39: 136-139.
Liang, Y-X., R-K. Sun, Y. Sun, Z-Q. Chen and L-H. Li. 1993. Psychological effects of low exposure
to mercury vapor: Application of a computer-administered neurobehavioral evaluation system.
Environ. Res. 60: 320-327.
Lilis, R., A. Miller and Y. Lerman. 1985. Acute mercury poisoning with severe chronic pulmonary
manifestations. Chest. 88(2): 306-309.
June 1996 B-12 . SAB REVIEW DRAFT
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McFarland, R.B. and H. Reigel. 1978. Chronic mercury poisoning from a single brief exposure. J.
Occup. Med. 20(8): 532-534.
Miller, J.M., D.B. Chaffin and R.G. Smith. 1975. Subclinical psychomotor and neuromuscular
changes in workers exposed to inorganic mercury. Am. Ind. Hyg. Assoc. J. 36: 725-733.
Mishonova, V.N., P.A. Stepanova and V.V. Zarudin. 1980. Characteristics of the course of pregnancy
and labor in women coming in contact with low concentrations of metallic mercury vapors in
manufacturing work places. Gig Tr Prof Zabol. Issue 2: 21-23.
Mutti, A., S. Lucertini, M. Fornari, et al. 1985. Urinary excretion of a brush-border antigen revealed
by monoclonal antibodies in subjects occupationally exposed to heavy metals. Heavy Met Environ.
International Conference 5th. Vol.1, p. 565-567.
Ngim, C.H., S.C. Foo, K.W. Boey and J. Jeyaratnam. 1992. Chronic neurobehavioral effects of
elemental mercury in dentists. Br. J. Ind. Med. 49: 782-790.
Piikivi, L. 1989. Cardiovascular reflexes and low long-term exposure to mercury vapor. Int Arch.
Occup. Environ. Health. 61: 391-395.
Piikivi, L. and H. Hanninen. 1989. Subjective symptoms and psychological performance of
chlorine-alkali workers. Scand. J. Work Environ. Health. 15: 69-74.
Piikivi, L. and A. Ruokonen. 1989. Renal function and long-term low mercury vapor exposure. Arch.
Environ. Health. 44(3): 146-149.
Piikivi, L. and U. Tolonen. 1989. EEG findings in chlor-alkali workers subjected to low long term
exposure to mercury vapor. Br. J. Ind. Med. 46: 370-375.
Roels, H., R. Lauwerys, J.P. Buchet, et al. 1982. Comparison of renal function and psychomotor
performance in workers exposed to elemental mercury. Int Arch, Occup. Environ. Health. 50: 77-93.
Roels, H., J.P. Gennart, R. Lauwreys, J.P. Buchet, J. Malchaire and A. Bernard. 1985. Surveillance
of workers exposed to mercury vapor: validation of a previously proposed biological threshold limit
value for mercury concentration in urine. Am. J. Ind. Med. 7: 45-71.
Roels, H., S. Abdeladim, E. Ceulemans and R. Lauwreys. 1987. Relationships between the
concentrations of mercury in air and in blood or urine in workers exposed to mercury vapour. Ann.
Occup. Hyg. 31(2): 135-145.
Roels, H., S. Abdeladim, M. Braun, J. Malchaire and R. Lauwerys. 1989. Detection of hand tremor
in workers exposed to mercury vapor: A comparative study of three methods. Environ Res 49-
152-165.
Rosenman, K.D., J.A. Valciukas, L. Glickman, B.R. Meyers and A. Cinotti. 1986. Sensitive
indicators of inorganic mercury toxicity. Arch. Environ. Health. 41(4): 208-215.
Singer, R., J.A. Valciukas and K.D. Rosenman. 1987. Peripheral neurotoxicity in workers exposed to
inorganic mercury compounds. Arch. Environ. Health. 42(4): 181-184.
June 1996 B-13 SAB REVIEW DRAFT
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Smith, R.G., AJ. Vorwald, L.S. Patil and T.F. Mooney, Jr. 1970. Effects of exposure to mercury in
the manufacture of chlorine. Am. Ind. Hyg. Assoc. J. 31(1): 687-700.
Steffek, A.J., R. Clayton, C. Slew and A.C. Verrusio. 1987. Effects of elemental mercury vapor
exposure on pregnant Sprague-Dawley rats (abstract only). Teratology. 35: 59A.
Stewart, W.K., H.A. Guirgis, J. Sanderson and W. Taylor. 1977. Urinary mercury excretion and
proteinuria in pathology laboratory staff. Br. J. Ind. Med. 34: 26-31.
Stonard, M.D., B.V. Chater, D.P. Duffield, Ai. Nevitt, JJ. O'Sullivan and G.T. Steel. 1983. An
evaluation of renal function in workers occupationally exposed to mercury vapor. Int Arch. Occup.
Environ. Health. 52: 177-189.
U.S. EPA. 1995. Mercury Study Report to Congress. Office of Research and Development,
Washington DC 20460. EPA/600/P-94/002Ab. External Review Draft.
Verbeck, M.M., H.J.A. Salle and C.H. Kemper. 1986. Tremor in workers with low exposure to
metallic mercury. Hyg. Assoc. J. 47(8): 559-562.
Warkany, J. and D.M. Hubbard. 1953. Acrodynia and mercury. J. Pediat. 42:365-386.
June 1996 B-14 SAB REVIEW DRAFT
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_II. CARCINOGENICITY ASSESSMENT FOR LIFETIME EXPOSURE
Substance Name -- Mercury, elemental
CASRN -- 7439-97-6
Preparation Date - 5/24/94
H.A. EVIDENCE FOR CLASSIFICATION AS TO HUMAN CARCINOGENICITY
_n.A.l WEIGHT-OF-EVIDENCE CLASSIFICATION
Classification - D; not classifiable as to human carcinogenicity
\
Basis - Based on inadequate human and animal data. Epidemiologic studies failed to show a
correlation between exposure to elemental mercury vapor and carcinogenicity; the findings in these
studies were confounded by possible or known concurrent exposures to other chemicals, including
human carcinogens, as well as lifestyle factors (e.g., smoking). Findings from genotoxicity tests are
severely limited and provide equivocal evidence that mercury adversely affects the number or structure
of chromosomes in human somatic cells.
_H.A.2 HUMAN CARCINOGENICITY DATA
Inadequate. A number of epidemiological studies were conducted that examined mortality
among elemental mercury vapor-exposed workers. Conflicting data regarding a correlation between
mercury exposure and an increased incidence of cancer mortalities have been obtained. All of the
studies have limitations that complicate interpretation of their results for associations between mercury
exposure and induction of cancer; increased cancer rates were attributable to other concurrent
exposures or lifestyle factors.
A retrospective cohort study examined mortality among 5663 white males who worked
between-195 3 and 1963 at a plant in Oak Ridge, Tennessee, where elemental mercury was used for
lithium isotope separation (Cragle et al., 1984). The workers were divided into three cohorts: exposed
workers who had been monitored on a quarterly basis for mercury levels in urine (n=2,133); workers
exposed in the mercury process section for whom urinalysis monitoring data were not collected
(n=270); and unexposed workers from other sections of the nuclear weapons production facility
(n=3260). The study subjects worked at least 4 months during 1953-1958 (a period when mercury
exposures were likely to be high); mortality data from death certificates were followed through the end
of 1978. The mean age of the men at first employment at the
facility was 33 years, and the average length of their employment was >16 years with a mean of 3.73
years of estimated mercury exposure. Ah- mercury levels were monitored beginning in 1955; during
1955 through the third quarter of 1956, air mercury levels were reportedly above 100 ug/cu.m in
30-80% of the samples. Thereafter, air mercury levels decreased to concentrations below 100 ug/cu.m.
The mortality experience (i.e., the SMR) of each group was compared with the age-adjusted mortality
experience of the U.S. white male population. Among exposed and monitored workers, no significant
increases in mortality from cancer at any site were reported, even after the level or length of exposure
was considered. A significantly lower mortality from all causes was observed. An excessive number
June 1996 B-15 ' SAB REVIEW DRAFT
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of deaths was reportedly due to lung cancer in the exposed and monitored workers (42 observed, 31.36
expected), but also in the unexposed workers (71 observed, 52.93 expected). The SMR for each group
was 1.34; the elevated incidence of lung cancer deaths was, therefore, attributed to some other factor at
the plant and/or to lifestyle factors (e.g., smoking) common to both the exposed and unexposed
groups. Study limitations include small cohort sizes for cancer mortality, which limited the statistical
stability of many comparisons.
Barregard et al. (1990) studied mortality and cancer morbidity between 1958 and 1984 hi 1190
workers from eight Swedish chloralkali plants that used the mercury cell process hi the production of
chlorine. The men included in the study had been monitored for urinary or blood mercury for more
than one year between 1946 and 1984. Vital status and cause of death were ascertained from the
National Population Register and the National Bureau of Statistics. The cancer incidence of the cohort
was obtained from the Swedish Cancer Register. The observed total mortality and cancer incidences
were compared with those of the general Swedish male population. Comparisons were not made
between, exposed and unexposed workers. Mean urinary mercury levels indicated a decrease in
exposure between the 1950s and 1970s; the mean urinary mercury level -was 200 ug/L during the
1950s, 150 ug/L during the 1960s and 50 ug/L in the 1970s. Mortality from all causes was not
significantly increased in exposed workers. A significant increase in deaths from lung tumors was
observed hi exposed workers 10 years or more after first exposure (rate ratio, 2.0; 95% CI, 1.0-3.8).
Nine of the 10 observed cases of lung cancer occurred among workers (457 of the 1190) possibly
exposed to asbestos as well as to mercury. No dose response was observed with respect to mercury
exposure and lung tumors. This study is limited because no quantitation was provided on smoking
status, and results were confounded by exposure to asbestos.
Ahlbom et al. (1986) examined the cancer mortality during 1961-1979 of cohorts of Swedish
dentists and dental nurses aged 20-64 and employed hi 1960 (3454 male dentists, 1125 female dentists,
4662 female dental nurses). Observed incidences were compared with those expected based on cancer
incidence during 1961-1979 among all Swedes employed during 1960 and the proportion of all
Swedes employed as dentists and dental nurses. Data were stratified by sex, age (5-year age groups)
and county. The incidence of glioblastomas among the dentists and dental ^nurses combined was
significantly increased compared to survival rates (SMR, 2.1; 95% CI, 1.3-3.4); the individual groups
had apparently elevated SMRs (2.0-2.5), but the 95%
confidence intervals of these groups included unity. By contrast, physicians and nurses had SMRs of
only 1.3 and 1.2, respectively. Exposure to mercury could not be established as the causative factor
because exposure to other chemicals and X-rays was not ruled out.
Amandus and Costello (1991) examined the association between silicosis and lung cancer
mortality between 1959 and 1975 in 9912 white male metal miners employed in the United States
between 1959 and 1961. Mercury exposures were not monitored. Exposures to specific metals among
the silicotic and nonsilicotic groups were analyzed separately. Lung cancer mortality in both silicotic
and nonsilicotic groups was compared with rates in white males in the U.S. population. Both silicotic
(n=l 1) and nonsilicotic mercury miners (n=263) had significantly increased lung cancer mortality
(SMR, 14.03; 95% CI, 2.89-40.99 for silicotics. SMR, 2.66; 95% CI, 1.15-5.24 for nonsilicotics).
The analysis did not focus on mercury miners, and confounders such as smoking and radon exposure
were not analyzed with respect to mercury exposure. This study is also limited by the small sample
size for non-silicotic mercury miners.
A case-control study of persons admitted to a hospital hi Florence, Italy, with lung cancer
between 1981-1983 was performed to evaluate occupational risk factors (Buiatti et al., 1985). Cases
June 1996 B-16 * SAB REVIEW DRAFT
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were matched with one or two controls (persons admitted to the hospital with diagnoses other than
lung cancer or suicide) with respect to sex, age, date of admission and smoking status. Women who
had "ever worked" as hat makers had a significantly increased risk of lung cancer. The duration of
employment as a hat maker averaged 22.2 years, and latency averaged 47.8 years. Workers in the
Italian hat industry were known to be occupationally exposed to mercury; however, the design of this
study did not allow evaluation of the relationship between cumulative exposure and cancer incidence.
In addition, interpretation of the results of this study is limited by the small sample size (only 6/376
cases reported this occupation) and by exposure of hat makers to other pollutants including arsenic, a
known lung carcinogen.
Ellingsen et al. (1992) examined the total mortality and cancer incidence among 799 workers
employed for more than 1 year in two Norwegian chloralkali plants. Mortality incidence between
1953 and 1988 and cancer incidence between 1953 and 1989 were examined. Mortality and cancer
incidence were compared with that of the age-adjusted general male Norwegian population. No
increase in total cancer incidence was reported, but lung cancer was significantly elevated in the
workers (rate ratio, 1.66; 95% CI, 1.0-2.6). No causal relationship can be drawn from the study
between mercury exposure and lung cancer because no correlation existed between cumulative mercury
dose, years of employment or latency time. Also, the prevalence of smoking was 10 20% higher in
the exposed workers, and many workers were also exposed to asbestos.
_H.A.3 ANIMAL CARCINOGENICITY DATA
Inadequate. Druckrey et al. (1957) administered 0.1 mL of metallic mercury to 39 male and
female rats (BD in and BD IV strains) via intraperitoneal injection. Among the rats surviving longer
than 22 months, 5/12 developed peritoneal sarcomas. The increase in the incidence of sarcomas was
observed only in those tissues that had been in direct contact with the mercury. Although severe
kidney damage was reported in all treated animals, no renal tumors or tumors at any site other than the
peritoneal cavity were observed.
_II.A.4 SUPPORTING DATA FOR CARCINOGENICITY
Cytogenetic monitoring studies of workers occupationally exposed to mercury by inhalation
provide very limited evidence that mercury adversely affects the number or structure of chromosomes
in human somatic cells. Popescu et al. (1979) compared four men exposed to elemental mercury
vapor with an unexposed group and found a statistically significant increase in the incidence of
chromosome aberrations in the WBCs from whole blood. Verschaeve et al. (1976) found an increase
in aneuploidy after exposure to low concentrations of vapor, but results could not be repeated in later
studies (Verschaeve et al., 1979). Mabille et al. (1984) did not find increases in structural
chromosomal aberrations of lymphocytes of exposed workers. Similarly, Barregard et al. (1991) found
no increase in the incidence or size of micronuclei and no correlation between micronuclei and blood
or urinary mercury levels of chloralkali workers. A statistically significant correlation was observed
between cumulative exposure to mercury and micronuclei induction in T lymphocytes in exposed
workers, suggesting a genotoxic effect.
June 1996 B-17 SAB REVIEW DRAFT
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_II.B QUANTITATIVE ESTIMATE OF CARCINOGENIC RISK FROM ORAL EXPOSURE
None.
JI.C QUANTITATIVE ESTIMATE OF CARCINOGENIC RISK FROM INHALATION
EXPOSURE
None.
_n.D EPA DOCUMENTATION, REVIEW, AND CONTACTS (CARCINOGENICITY
ASSESSMENT)
__n.D.l EPA DOCUMENTATION
Source document ~ U.S. EPA, 1995
This IRIS summary is included in The Mercury Study Report to Congress which was reviewed
by OHEA and EPA's Mercury Work Group in November 1994. An interagency review by scientists
from other federal agencies took place in January 1995. The report was also reviewed by a panel of
non-federal external scientists in January 1995 who met in a public meeting on January 25-26. All
reviewers comments have been carefully evaluated and considered in the revision and finalization of
this IRIS summary. A record of these comments is
summarized in the IRIS documentation files.
_II.D.2 REVIEW (CARCINOGENICITY ASSESSMENT)
Agency Work Group Review - 01/13/88, 03/03/94
Verification Date - 03/03/94
_H.D.3 U.S. EPA CONTACTS (CARdNOGENICITY ASSESSMENT)
Rita Schoeny / NCEA - (513)569-7544
REFERENCES
Ahlbom, A., S. Norell, Y. Rodvall and M. Nylander. 1986. Dentists, dental nurses, and brain
tumours. Br. Med. J. 292: 662.
Amandus, H. and J. Costello. 1991. Silicosis and lung cancer in U.S. metal miners. Arch. Environ.
Health. 46(2): 82-89,
June 1996 B-18 SAB REVIEW DRAFT
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Barregard, L., G. Sallsten and B, Jarvholm. 1990. Mortality and cancer incidence in chloralkali
workers exposed to inorganic mercury. Br. J. Ind. Med. 47(2): 99-104.
Barregard, L., B. Hogstedt, A. Schutz, A. Karlsson, G. Sallsten and G. Thiringer. 1991. Effects of
occupational exposure to mercury vapor on lymphocyte micronuclei. Scand. J. Work Environ. Health.
17: 263-268.
Buiatti, E., D. Kriebel, M. Geddes, M. Santucci and N. Pucci. 1985. A case control study of lung
cancer in Florence, Italy. I. Occupational risk factors. J. Epidemiol. Comm. Health. 39: 244-250.
Cragle, D.L., D.R. Hollis, J.R. Qualters, W.G. Tankersley and S.A. Fry. 1984. A mortality study of
men exposed to elemental mercury. J. Occup. Med. 26(11): 817-821.
Druckrey, H., H. Hamperl and D. Schmahl. 1957. Carcinogenic action of metallic mercury after
intraperitoneal administration hi rats. Z. Krebsforsch. 61: 511-519. (Cited in U.S. EPA, 1985)
Ellingsen, D., A. Andersen, H.P. Nordhagen, J. Efskind and H. Kjuus. 1992. Cancer incidence and
mortality among workers exposed to mercury hi the Norwegian chloralkali industry. 8th International
Symposium on Epidemiology in Occupational Health, Paris, France, September 10-12, 1991. Rev.
Epidemiol. Sante Publique. 40(1): S93-S94.
Mabille, V., H. Reels, P. Jacquet, A. Leonard and R. Lauwerys. 1984. Cytogenetic examination of
leucocytes of workers exposed to mercury vapor. Int. Arch. Occup. Environ. Health. 53: 257-260.
Popescu, H.I., L. Negru and L Lancranjan. 1979. Chromosome aberrations induced by occupational
exposure to mercury. Arch. Environ. Health. 34(6): 461-463.
U.S. EPA. 1980. Ambient Water Quality Criteria Document for Mercury. Prepared by the Office of
Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH
for the Office of Water Regulation and Standards, Washington, DC. EPA/440/5-80/058. NTIS PB
81-117699.
U.S. EPA. 1984a. Mercury Health Effects Update: Health Issue Assessment. Final Report.
Prepared by the Office of Health and Environmental Assessment, Environmental Criteria and
Assessment Office, Cincinnati, OH for the Office of Air Quality Planning and Standards, Research
Triangle Park, NC. EPA/600/8-84/019F. NTIS PB81-85-123925.
U.S. EPA. 1984b. Health Effects Assessment for Mercury. Prepared by the Office of Health and
Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH for the
Office of Emergency and Remedial Response, Washington, DC. EPA/540/1086/042. NTIS
PB86-134533/AS.
U.S. EPA. 1985. Drinking Water Criteria Document for Mercury. Prepared by the Office of Health
and Environmental Assessment Office, Cincinnati, OH for the Office of Drinking Water, Washington,
DC. EPA/600/X-84/178. NTIS PB86-117827.
U.S. EPA. 1988. Drinking Water Criteria Document for Inorganic Mercury. Prepared by the Office
of Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati,
OH for the Office of Drinking Water, Washington, DC. EPA/600/X-84/178. NTIS PB89-192207.
June 1996 B-19 SAB REVIEW DRAFT
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U.S. EPA. 1993. Summary Review of Health Effects Associated with Mercuric Chloride: Health
Issue Assessment (Draft). Prepared by the Office of Health and Environmental Assessment,
Environmental Criteria and Assessment Office, Cincinnati, OH for the Office of Air Quality Planning
and Standards, Research Triangle Park, NC. EPA/600/R-92/199.
U.S. EPA. 1995. Mercury Study Report to Congress. Office of Research and Development,
Washington, DC. External Review Draft. EPA/600/P-94/002Ab.
Verschaeve, L., M. Kirsch-Volders, C. Susanne et al. 1976. Genetic damage induced by
occupationally low mercury exposure. Environ. Res. 12: 303-316.
Verschaeve, L., J.P. Tassignon, M. Lefevre, P. De Stoop and C. Susanne. 1979. Cytogenetic
investigation on leukocytes of workers exposed to metallic mercury. Environ. Mutagen. 1: 259-268.
June 1996 B-20 SAB REVIEW DRAFT
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_I.A REFERENCE DOSE FOR CHRONIC ORAL EXPOSURE (RfD)
Substance Name - Mercuric chloride (HgC12)
CASRN -- 7487-94-7
Preparation Date - 11/01/88
_J.AJ ORAL RfD SUMMARY
Critical Effect Experimental Doses* UF MF RfD
Autoimmune effects NOAEL: None 1000 1 3E-4
mg/kg-day
Rat Subchronic * LOAEL: 0.226 mg/kg-day \
Feeding and *
Subcutaneous LOAEL: 0.317 mg/kg-day
Studies
LOAEL: 0.633 mg/kg-day
U.S. EPA, 1987
* Conversion Factors and Assumptions — Dose conversions in the three studies employed a 0.739
factor for HgC12 to Hg2+, a 100% factor for subcutaneous (s.c.) to oral route of exposure, and a
time-weighted average for days/week of dosing. This RfD is based on the back calculations from a
Drinking Water Equivalent Level (DWEL), recommended to and subsequently adopted by the Agency,
of 0.010 mg/L: (RfD = 0.010 mg/L x 2 L/day/70 kg bw = 0.0003 mg/kg bw/day). The LOAEL
exposure levels, utilized in the three studies selected as the basis of the recommended DWEL, are
from Druet et al. (1978), Bernaudin et al. (1981) and Andres (1984), respectively.
_I.A.2 PRINCIPAL AND SUPPORTING STUDIES (ORAL RfD)
U.S. EPA. 1987. Peer Review Workshop on Mercury Issues. Summary Report. Environmental
Criteria and Assessment Office, Cincinnati, OH. October 26-27.
On October 26-27, 1987, a panel of mercury experts met at a Peer Review Workshop on
Mercury Issues in Cincinnati, Ohio, and reviewed outstanding issues concerning the health effects and
risk assessment of inorganic mercury (U.S. EPA, 1987). The following five consensus conclusions
and recommendations were agreed to as a result of this workshop:
1) The most sensitive adverse effect for mercury risk assessment is formation of
mercuric-mercury-induced autoimmune glomerulonephritis. The production and
deposition of IgG antibodies to the glomerular basement membrane can be considered
the first step in the formation of this mercuric-mercury-induced autoimmune
glomerulonephritis.
2) The Brown Norway rat should be used for mercury risk assessment. The Brown
Norway rat is a good test species for the study of Hg2+-induced autoimmune
June 1996 B-21 SAB REVIEW DRAFT
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glomerulonephritis. The Brown Norway rat is not unique in this regard (this effect has
also been observed in rabbits).
3) The Brown Norway rat is a good surrogate for the study of mercury-induced kidney
damage hi sensitive humans. For this reason, the uncertainty factor used to calculate
criteria and health advisories (based on risk assessments using the Brown Norway rat)
should be reduced by 10-fold.
4) Hg2+ absorption values of 7% from the oral route and 100% from the s.c. route
should be used to calculate criteria and health advisories.
5) A DWEL of 0.010 mg/L was recommended based on the weight of evidence from the
studies using Brown Norway rats and limited human tissue data.
\
Three studies using the Brown Norway rat as the test strain were chosen.from a larger selection of
studies as the basis for the panel's recommendation of 0.010 mg/L as the DWEL for inorganic
mercury. The three studies are presented below for the sake of completeness. It must be kept in
mind, however, that the recommended DWEL of 0.010 mg/L and back calculated oral RfD of 0.0003
mg/kg-day were arrived at from an intensive review and workshop discussions of the entire inorganic
mercury data base, not just from one study.
In the Druet et al. (1978) study, the duration of exposure was 8-12 weeks; s.c. injection was
used instead of oral exposure. In this study the development of kidney disease was evaluated. In the
first phase the rats developed anti-GBM antibodies. During the second phase, which is observed after
2-3 months, the patterns of fixation of antisera changed from linear to granular as the disease
progressed. The immune response was accompanied by proteinuria and in some cases by a nephrotic
syndrome.
Both male and female Brown Norway rats 7-9 weeks of age were divided into groups of 6-20
animals each. The numbers of each sex were not stated. The animals received s.c. injections of
mercuric chloride (HgC12) 3 tunes weekly for 8 weeks, with doses of 0, 100, 250, 500, 1000 and 2000
ug/kg. An additional group was injected with a 50 ug/kg dose for 12 weeks. Antibody formation was
measured by the use of kidney cryostat sections stained with a fluoresceinated sheep anti-rat IgG
antiserum. Urinary protein was assessed by the biuret method (Druet et al., 1978).
Tubular lesions were observed at the higher dose levels. Proteinuria was reported at doses of
100 ug/kg and above, but not at 50 ug/kg. Proteinuria was considered a highly deleterious effect,
given that affected animals developed hypoalbuminemia and many died. Fixation of IgG antiserum
was detected in all groups except controls (Druet et al., 1978).
Bernaudin et al. (1981) reported that mercurials administered by inhalation or ingestion to
Brown Norway rats developed a systemic autoimmune disease. The HgC12 ingestion portion of the
study involved the forcible feeding of either 0 or 3000 ug/kg-week of HgC12 to male and female
Brown Norway rats for up to 60 days. No abnormalities were reported using standard histological
techniques in either experimental or control rats. Immunofluorescence histology revealed that 80%
(4/5) of the mercuric-exposed rats were observed with a linear IgG deposition in the glomeruli after 15
days of exposure. After 60 days of HgC12 exposure, 100% (5/5) of the rats were seen with a mixed
linear and granular pattern of IgG deposition hi the glomeruli and granular IgG deposition hi the
arteries. Weak proteinuria was observed in 60% (3/5) of the rats fed HgC12 for 60 days. The control
June 1996 B-22 SAB REVIEW DRAFT
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rats were observed to have no deposition of IgG in the glomeruli or arteries as well as normal urine
protein concentrations.
Andres (1984) administered HgC12 (3 mg/kg in 1 mL of water) by gavage to five Brown
Norway rats and two Lewis rats twice a week for 60 days. A sixth Brown Norway rat was given only
1 mL of water by gavage twice a week for 60 days. All rats had free access to tap water and pellet
food. After 2-3 weeks of exposure, the Brown Norway HgC12-treated rats started to lose weight and
hair. Two of the HgC12-treated Brown Norway rats died 30-40 days after beginning the study. No
rats were observed to develop detectable proteinuria during the 60-day study. The kidneys appeared
normal in all animals when evaluated using standard histological techniques, but examination by
immunofluorescence showed deposits of IgG present in the renal glomeruli of only the
mercuric-treated Brown Norway rats. The Brown Norway treated rats were also observed with
mercury-induced morphological lesions of the ileum and colon with abnormal deposits of IgA in the
basement membranes of the intestinal glands and of IgG in the basement membranes of the lomina
propria. All observations in the Lewis rats and the control Brown Norway rat appeared normal.
_I.A.3 UNCERTAINTY AND MODIFYING FACTORS (ORAL RfD)
UF - An uncertainty factor of 1000 was applied to the animal studies using Brown Norway rats as
recommended in U.S. EPA (1987). An uncertainty factor was applied for LOAEL to NOAEL
conversion: 10 for use of subchronic studies and a combined 10 for both animal to human and
sensitive human populations.
MF - None
_I.A.4 ADDITIONAL STUDIES / COMMENTS (ORAL RfD)
Kazantzis et al. (1962) performed renal biopsies hi 2 (out of 4) workers with nephrotic
syndrome who had been occupationally exposed to mercuric oxide, mercuric acetate and probably
mercury vapors. Investigators reported that the nephrotic syndrome observed in 3 of the 4 workers
may have been an idiosyncratic reaction since many other workers hi a factory survey had similarly
high levels of urine mercury without developing proteinuria. This conclusion was strengthened by
work in Brown Norway rats indicating a genetic (strain) susceptibility and that similar
mercury-induced immune system responses have been seen in affected humans and the susceptible
Brown Norway rats (U.S. EPA, 1987).
The only chronic ingestion study designed to evaluate the toxicity of mercury salts was
reported by Fitzhugh et al. (1950). In this study, rats of both sexes (20-24/group) were given 0.5, 2.5,
10, 40 or 160 ppm mercury as mercuric acetate in their food for up to 2 years. Assuming food
consumption was equal to 5% bw/day, the daily intake would have been 0.025, 0.125, 0.50, 2.0 and
8.0 mg/kg for the five groups, respectively. At the highest dose level, a slight depression of body
weight was detected in male rats only. The statistical significance of this body-weight depression was
not stated. Kidney weights were significantly (p<0.05) increased at .the 2.0 and 8.0 mg/kg dose levels.
Pathological changes originating hi the proximal convoluted tubules of the kidneys were also noted,
with more severe effects in females than males. The primary weaknesses of this study were (1) the
June 1996 B-23 * SAB REVIEW DRAFT
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lack of reporting on which adverse effects were observed with which dosing groups and (2) that the
most sensitive strain, the Brown Norway rat, was not used for evaluating the mercury-induced adverse
health effects.
NTP (1993) conducted subchronic and chronic gavage toxicity studies on Fischer 344 rats and
B6C3F1 mice to evaluate the effects of HgC12, and the kidney appeared to be the major organ
affected. In the 6-monlh study, Fischer 344 rats (10/sex /group) were administered 0, 0.312, 0.625,
1.25, 2.5 or 5 mg/kg-day of HgC12 (0.23, 0.46, 0.92, 1.9 and 3.7 mg/kg-day) 5 days/week by gavage.
Survival was not affected, although body-weight gains were decreased in males at high dose and in
females at or above the 0.46 mg/kg-day dose. Absolute and relative kidney weights were significantly
increased in both sexes with exposure to at least 0.46 mg/kg-day. In males, the incidence of
nephropathy was 80% in the controls and 100% for all treated groups; however, severity was minimal
hi the controls and two low-dose groups and minimal to mild in the 0.92 mg/kg-day group and higher.
In females, there was a significant increased incidence of nephropathy only in the high-dose group
(4/10 with minimal severity). Nephropathy was characterized by foci of tubular regeneration,
thickened tubular basement membrane and scattered dilated tubules containing hyaline casts. No
treatment-related effects were
observed in the other organs; however, histopathology on the other organs was performed only on
control and high-dose rats.
B6C3F1 mice (10/sex/group) were administered 0, 1.25, 2.5, 5, 10 or 20 mg/kg-day HgC12 (0,
0.92, 1.9,3.7, 7.4 or 14.8 mg/kg-day) 15 days/week by gavage for 6 months (NTP 1993). A decrease
in body-weight gain was reported in only the males at the highest dose tested. Significant increases
occurred in absolute kidney weights of male mice at 3.7 mg/kg-day or greater and relative kidney
weights of male mice at 7.4 and 14.8 mg/kg-day doses. The kidney weight changes corresponded to
an increased incidence of cytoplasmic vacuolation of renal tubule epithelium in males exposed to at
least 3.7 mg/kg-day. The exposed female mice did not exhibit any histopathologic changes in the
kidneys.
In the 2-year NTP study, Fischer 344 rats (60/sex/group) were administered 0, 2.5 and 5
mg/kg-day HgC12 (1.9 and 3.7 mg/kg-day) 5 days week by gavage (NTP, 1993). After 2 years,
survival was reduced hi only the treated male rat groups compared with the control. Mean body
weights were decreased hi both male and female treated groups. After 2 years, an increased incidence
of nephropathy of moderate-to-marked severity and increased incidence of tubule
hyperplasia was observed in the kidneys of exposed males compared with the controls. The control
males exhibited nephropathy, primarily of mild-to-moderate severity. Hyperparathyroidism,
mineralization of various tissues and fibrous osteodystrophy were observed and considered secondary
to the renal impairment No significant differences were found in renal effects between exposed and
control females. Other nonneoplastic effects included an increased incidence of forestomach
hyperplasia hi the exposed males and high-dose females.
NTP (1993) also administered to B6C3F1 mice (60/sex/group) daily oral gavage doses of 0, 5
or 10 mg/kg-day HgC12 (0, 3.7 and 7.4 mg/kg-day) 5 days/week by gavage for 2 years. Survival and
body weights of mice were slightly lower in HgC12-treated mice compared with controls. Absolute
kidney weights were significantly increased hi the treated males, while relative kidney weights were
significantly increased hi high-dose males and both low- and high-dose females. Histopathology
revealed an increase in the incidence and severity of .nephropathy in exposed males and an increase in
the incidence of nephropathy in exposed females. Nephropathy was defined as foci of proximal
convoluted tubules with thickened basement membrane and basophilic cells with scant cytoplasm.
June 1996 B-24 . SAB REVIEW DRAFT
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Some affected convoluted tubules contained syaline casts. Also, an increase in nasal cavity
inflammation (primarily infiltration of granulocytes in nasal mucosa) was observed in the exposed
animals.
Gale and Perm (1971) studied the teratogenic effects of mercuric acetate on Syrian golden
hamsters. Single doses of 2, 3 or 4 mg/kg were injected by the i.v. route on day 8 of gestation.
Growth retardation, increased resorption rates and edema of the fetuses were found at all three dose
levels, while an increase in the number of abnormalities was detected at the two higher doses. In a
more recent study, Gale (1981) compared the embryotoxic effects of a single s.c. dose of 15 mg/kg
mercuric acetate on the eighth day of gestation in five inbred strains and one noninbred strain of
Syrian hamsters. While strain differences were apparent, a variety of abnormalities were reported in
all the strains. Gale (1974) also compared the relative effectiveness of different exposure routes in
Syrian hamsters. The following sequence of decreasing efficacy was noted for mercuric acetate; i.p. >
i.v. > s.c. > oral. The lowest doses used, 2 mg/kg for i.p. and 4 mg/kg for the other three routes, were
all effective in causing increased resorption and percent abnormalities.
In male mice administered a single i.p.'dose of 1 mg/kg HgC12, fertility decreased between
days 28 and 49 post treatment with no obvious histological effects noted in the sperm (Lee and Dixon,
1975). The period of decreased fertility indicated that spermatogonia and premeiotic spermatocytes
were affected. The effects were less severe than following a similar dose of methyl mercury. A
single i.p. dose of 2 mg/kg HgC12 in female mice resulted hi a significant decrease in the total number
of implants and number of living embryos and a significant increase in the percentage of dead
implants (Suter, 1975). These effects suggest that mercury may be a weak inducer of dominant lethal
mutations.
_I.A.5 CONFIDENCE IN THE ORAL RfD
Study -- N/A
Data Base — High
RfD - High
No one study was found adequate for deriving an oral RfD; however, based on the weight of
evidence from the studies using Brown Norway rats and the entirety of the mercuric mercury data
base, an oral RfD of high confidence results.
I.A.6 EPA DOCUMENTATION AND REVIEW OF THE ORAL RfD
Source Document - U.S. EPA, 1988
This IRIS summary is included in The Mercury Study Report to Congress, which was
reviewed by OHEA and EPA's Mercury Work Group in November 1994. An interagency review by
scientists from other federal agencies took place in January 1995. The report was also reviewed by a
panel of non-federal external scientists in January 1995 who met in a public meeting on January
25-26. All reviewers comments have been carefully evaluated and considered in the revision and
finalization of this IRIS summary. A record of these comments is
June 1996 B-25 SAB REVIEW DRAFT
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summarized in the IRIS documentation files.
Other Docmentation - U.S. EPA, 1987
Agency Work Group Review - 08/05/85, 02/05/86, 08/19/86, 11/16/88
Verification Date - 11/16/88
»
_I.A,7 EPA CONTACTS (ORAL RfD)
W. Bruce Peirano / NCEA - (513)569-7540
Krishan Khanna / OST - (202)260-7588
REFERENCES
Andres, P. 1984. IgA-IgG disease in the intestine of Brown Norway rats ingesting mercuric chloride:
Clin. Immunol. Immunopathol. 30: 488-494.
Bernaudin, J.F., E. Druet, P. Druet and R. Masse. 1981. Inhalation or ingestion of organic or
inorganic mercurials produces auto-immune disease in rats. Clin. Immunol. Immunopathol. 20:
129-135.
Druet, P., E. Druet, F. Potdevin and C. Sapin. 1978. Immune type glomerulonephritis induced by
HgC12 in the Brown Norway rat Ann. Immunol. 129C: 777-792.
Fitzhugh, O.G., A.A. Nelson, E.P. Laug and F.M. Kunze. 1950. Chronic oral toxicants of
mercuric-phenyl and mercuric salts. Arch. Ind. Hyg. Occup. Med. 2: 433-442.
Gale, T.F. 1974. Embryopathic effects of different routes of administration of mercuric acetate in the
hamster. Environ. Res. 8: 207-213.
Gale, T.F. 1981. The embryotoxic response produced by inorganic mercury in different strains of
hamsters. Environ. Res. 24: 152-161.
Gale, T. and V. Perm. 1971. Embryopathic effects of mercuric salts. Life Sci. 10(2): 1341-1347.
Kazantzis, G., K.F.R. Schiller, A.W. Asscher and R.G. Drew. 1962. Albuminuria and the nephrotic
syndrome following exposure to mercury and its compounds. Q. J. Med. 31(124): 403-419.
Lee, I.D. and R.L. Dixon. 1975. Effects of mercury on spermatogenesis studied by velocity
sedimentation, cell separation and serial mating. J. Pharmacol. Exp. Ther. 194(1): 171-181.
NTP (National Toxicology Program). 1993. Toxicology and carcinogenesis studies of mercuric
chloride (CAS No. 7487-94-7) in F344 rats and B3C3F1 mice (gavage studies). NTP Technical
June 1996 B-26 SAB REVIEW DRAFT
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Report Series No. 408. National Toxicology Program, U.S. Department of Health and Human
Services, Public Health Service, National Institutes of Health, Research Triangle Park, NC,
Suter, K.E. 1975. Studies on the dominant lethal and fertility effects of he heavy metal compounds
methyl mercuric hydroxide, mercuric chloride, and cadmium chloride in male and female mice'.
Mutat. Res. 30: 365-374.
U.S. EPA. 1987. Peer Review Workshop on Mercury Issues. Environmental Criteria and Assessment
Office, Cincinnati, OH. Summary report. October 26-27.
U.S. EPA. 1988. Drinking Water Criteria Document for Inorganic Mercury. Prepared by the Office
of Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati,
OH for the Office of Drinking Water, Washington, DC. EPA/600/X-84/178. NTIS PB89-192207.
June 1996 B-27 SAB REVIEW DRAFT
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_II. CARCINOGENICITY ASSESSMENT FOR LIFETIME EXPOSURE
Substance Name — Mercuric Chloride
CASRN -- 7487-94-7
Preparation Date - 5/24/94
II.A EVIDENCE FOR CLASSIFICATION AS TO HUMAN CARdNOGENICITY
_H.A.l WEIGHT-OF-EVIDENCE CLASSIFICATION
Classification — C; possible human carcinogen
Basis — Based on the absence of data in humans and limited evidence of carcinogenicity in rats and
mice. Focal papillary hyperplasia and squamous cell papillomas in the forestomach as well as thyroid
follicular cell adenomas and carcinomas were observed in male rats gavaged with mercuric chloride
for 2 years. The relevance of the forestomach papillomas to assessment of cancer hi humans is
questionable because no evidence indicated that the papillomas progressed to malignancy. The
relevance of the increase in thyroid tumors has also been questioned because these tumors are
generally considered to be secondary to hyperplasia; this effect was not observed in the high-dose
males. It should also be noted that the authors considered the doses used in the study to exceed the
MTD for male rats. In the same study, evidence for increases hi squamous cell papillomas in the
forestomach of female rats was equivocal. In a second study, equivocal evidence for renal adenomas
and adenocarcinomas was observed hi male mice; there was a significant positive trend. This tumor
type is rare hi mice, and the increase hi incidence was statistically significant when compared with
historic controls. Two other nonpositive lifetime rodent studies were considered inadequate. Mercuric
chloride showed mixed results hi a number of genotoxicity assays.
_O.A.2 HUMAN CARCINOGENICITY DATA
None. No data are available on the carcinogenic effects of mercuric chloride in humans.
_E.A.3 ANIMAL CARCINOGENICITY DATA
Limited. The results from a dietary study hi rats and mice show equivocal evidence for
carcinogenic activity hi male mice and female rats and some evidence for carcinogenic activity hi male
rats. Two other dietary studies did not show any evidence for carcinogenicity, but these studies are
limited by inadequacies hi the data and experimental design, including the small number of
animals/dose and/or a lack of complete histopathological examinations.
Mercuric chloride (purity >99%) was administered by gavage hi water at doses of 0, 2.5 or 5
(mg/kg)/day (0, 1.9 and 3.7 (mg/kg)/day) to 60 F344 rats/sex/group, 5 days/week for 104 weeks (NTP,
1993). An interim sacrifice (10/sex/dose) was conducted after 15 months of exposure. Complete
June 1996 B-28 SAB REVIEW DRAFT
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histopathological examinations were performed on all animals found dead, killed in extremis, or killed
by design. Survival after 24 months was lower in low- and high-dose males at a statistically
significant rate; survival was 43, 17 and 8% in control, low-, and high-dose males, respectively, and
58, 47 and 50% in control, low-, and high-dose females, respectively. During the second year of the
study, body weight gains of low- and high-dose males were 91 and 85% of controls, respectively, and
body weight gains of low- and high-dose females were 90 and 86% of controls, respectively. At study
termination, nephropathy was evident in almost all male and female rats including controls, but the
severity was much greater in treated males. The incidence of "marked" nephropathy was 6/50, 29/50
and 29/50 in control, low- and high-dose males, respectively. Squamous cell papillomas of the
forestomach showed a statistically significant positive trend with dose by life table adjusted analysis;
the incidences were 0/50, 3/50 and 12/50 in control, low- and high-dose males, respectively. For
females, the incidence was 0/50, 0/49 and 2/50 hi control, low- and high-dose groups, respectively.
These neoplasms are rare in male rats and occurred in only 1/264 historical controls. The incidence of
papillary hyperplasia of the stratified squamous epithelium lining of me forestomach was elevated at a
statistically significant rate in all dosed males (3/49, 16/50 and 35/50 in control, low- and high-dose
males, respectively) and in high-dose females (5/50, 5/49 and 20/50 in control, low-and high-dose
females, respectively). The incidence of thyroid follicular cell carcinomas, adjusted for survival,
showed a significant positive trend in males; the incidence was 1/50, 2/50 and 6/50 in control, low-
and high-dose groups, respectively. The combined incidence of thyroid follicular cell neoplasms
(adenoma and/or carcinoma) was not significantly increased (2/50, 6/50 and 6/50 in control, low- and
.high-dose males, respectively). In female rats a significant decrease in the incidence of mammary
gland fibroadenomas was observed (15/50, 5/48 and 2/50 in control, low- and high-dose females,
respectively). The high mortality in both groups of treated males indicates that the MTD was
exceeded in these groups and limits the value of the study, for assessment of carcinogenic risk. NTP
(1993) considered the forestomach tumors to be of limited relevance to humans because the tumors did
not appear to progress to malignancy. NTP (1993) also questioned the relevance of the thyroid
carcinomas because these neoplasms are usually seen in conjunction with increased incidences of
hyperplasia and adenomas. In this study, however, no increases hi hyperplasia or adenomas were
observed. Hyperplasia incidence was 2/50, 4/50 and 2/50 in control, low- and high-dose males,
respectively; adenoma incidence was 1/50, 4/50 and 0/50 in control, low- and high-dose males,
respectively.
In the same study, mercuric chloride was administered by gavage hi water at doses of 0, 5 or
10 (mg/kg)/day (0, 3.7 and 7.4 (mg/kg)/day) to 60 B6C3F1 mice/sex/group 5 days/week for 104 weeks
(NTP, 1993). An interim sacrifice (10/sex/dose) was conducted after 15 months of exposure.
Terminal survival and body weight gain were not affected hi either sex by the administration of
mercuric chloride. It should be noted that survival of high-dose females was lower than controls;
female survival rates were 82, 70 and 62% hi control, low- and high-dose females, respectively.
Female mice exhibited a significant increase in the incidence of nephropathy (21/49, 43/50 and 42/50
in control, low- and high-dose females, respectively). Nephropathy was observed in 80-90% of the
males hi all groups. The severity of nephropathy increased with increasing dose. The Incidence of
renal tubular hyperplasia was 1/50, 0/50 and 2/49 in control, low- and high-dose males. The combined
incidence of renal tubular adenomas and adenocarcinomas was 0/50, 0/50 and 3/49 in control, low-
and high-dose males, respectively. Although no tumors were seen in the low-dose males, a
statistically significant positive trend for increased incidence with increased dose was observed. These
observations were considered important because renal tubular hyperplasia and tumors in mice are rare.
The 2-year historical incidence of renal tubular adenomas or adenocarcinomas in males dosed by
gavage with water was 0/205, and only 4 of the nearly 400 completed NTP studies have shown
increased renal tubular neoplasms in mice. Data from this study were not statistically compared with
June 1996 B-29 SAB REVIEW DRAFT
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historical control data'by NTP. EPA's analysis of the reported data with Fisher's Exact test showed
that the incidence of renal tubular adenomas or adenocarcinomas in the high-dose males was
significantly elevated when compared with historical controls (Rice and Knauf, 1994).
A 2-year feeding study in rats (20 or 24/sex/group; strain not specified) was conducted in
which mercuric acetate was administered in the diet at doses of 0, 0.5, 2.5, 10, 40 and 160 ppm (0,
0.02, 0.1, 0.4, 1.7 and 6.9 (mg Hg/kg)/day (Fitzhugh et al., 1950). Survival was not adversely affected
in the study. Increases in kidney weight and renal tubular lesions were observed at the two highest
doses. No statement was made in the study regarding carcinogenicity. This study was not intended to
be a carcinogenicity assay, and the number of animals/dose was rather small. Histopathological
analyses were conducted on only 50% of the animals (complete histopathology conducted on only
31% of the animals examined), and no quantitation of results or statistical analyses were performed.
No increase in tumor incidence was observed in a carcinogenicity study using white Swiss
mice (Schroeder and Mitchener, 1975). Groups of mice (54/sex/group) were exposed until death to
mercuric chloride in drinking water at 5 ppm Hg (0.95 (mg/kg)/day). No effects on survival or body
weights were observed. After dying, mice were weighed and dissected. The animals were examined
for gross tumors, and some sections were made of the heart, lung, liver, kidney and spleen for
microscopic examination. No toxic effects of mercuric chloride were reported in the study. No
statistically significant differences were observed in tumor incidences for treated animals and controls.
This study is of-limited use for evaluation of carcinogenicity because complete histological
examinations were not performed, only a single dose was tested, and the MTD was not achieved.
_H.A.4 SUPPORTING DATA FOR CARCINOGENICITY
The increasing trend for renal tubular cell tumors in mice observed in the NTP (1993) study
receives some support from similar findings in mice after chronic dietary exposure to methylmercury
(Hirano et al., 1986; Mitsumori et al., 1981, 1990). In these studies, dietary exposure to
methylmercuric chloride resulted in increases in renal tubular tumors at doses wherein substantial
nephrotoxicity was observed (see methylmercury file on IRIS).
As summarized in NTP (1993) and U.S. EPA (1985), mercuric chloride has produced some
positive results for clastogenicity in a variety of in vitro and in vivo genotoxicity assays; mixed results
regarding its mutagenic activity have been reported. Mercuric chloride was negative in gene mutation
tests with Salmonella typhimurium (NTP, 1993; Wong, 1988) but produced DNA damage as measured
in the Bacillus subtilis rec assay (Kanematsu et al., 1980). A
weakly positive response for gene mutations was observed in mouse lymphoma (L5178Y) cells in the
presence of microsomal activation (Oberly et al., 1982). DNA damage has also been observed in
assays using rat and mouse embryo fibroblasts (Zasukhina et al., 1983), CHO cells and human KB
cells (Cantoni and Costa, 1983; Cantoni et al., 1982, 1984a,b; Christie et al., 1984, 1986;
NTP, 1993; Williams et al., 1987). Mercuric chloride also produced chromosome aberrations and
SCEs in CHO cells (Howard et al., 1991) and chromosome aberrations in human lymphocytes
(Morimoto et al., 1982). Sex-linked recessive lethal mutations were not observed in male Drosophila
melanogaster (NTP, 1993).
Although mice given intraperitoneal doses of mercuric chloride have shown no increase in
chromosomal aberrations in bone marrow cells (Poma et al., 1981) and no increase in aneuploidy in
spermatogonia (Jagiello and Lin, 1973), mercuric chloride administered to mice by gavage induced a
June 1996 B-30 SAB REVIEW DRAFT
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dose-related increase in chromosome aberrations and aberrant cells in the bone marrow (Ghosh et al..
1991). Similarly, an increased incidence of chromosomal aberrations (primarily deletion and numeric
aberrations) was observed in livers of fetal mice exposed to mercury in utero as the result of maternal
inhalation of aerosols of mercuric chloride (Selypes et al., 1984). Positive dominant lethal results
(increased resorptions and post-implantation deaths in
untreated females) have been obtained in studies in which male rats were administered mercuric
chloride orally (Zasukhina et ak, 1983). A slight increase in post-implantation deaths and a decrease
in living embryos were also reported in treated female mice mated to untreated males (Suter, 1975);
however, it was not clear whether these effects were the result of germ cell
mutations or were secondary to maternal toxicity.
The effects of mercuric chloride on genetic material has been suggested to be due to the ability
of mercury to inhibit the formation of the mitotic spindle, an event known as c-mitosis (U.S. EPA,
1985).
_E.B QUANTITATIVE ESTIMATE OF CARCINOGENIC RISK FROM ORAL EXPOSURE
None. The incidences of squamous cell papillomas of the forestomach and thyroid follicular
cell carcinomas were evaluated. No slope factor was derived using the forestomach tumors because
these tumors are probably the result of doses of mercuric chloride above-MTD resulting in irritation of
the forestomach and subsequent cell death and epithelial proliferation. The carcinogenic mechanism
for mercuric chloride at the high doses observed may be specific to effects of irritation of the
forestomach.
Regarding the thyroid carcinomas, a variety of drugs, chemicals and physiological
perturbations result in the development of thyroid follicular tumors in rodents. For a number of
chemicals, the mechanism of tumor development appears to be a secondary effect of long-standing
hypersecretion of thyroid-stimulating hormone by the pituitary (Capen and Martin, 1989; McClain,
1989). In the absence of such long-term stimulatory effects, induction of thyroid follicular cell cancer
by such chemicals usually does not occur (Hill, 1989). The mechanism whereby thyroid tumors
developed hi the NTP (1993) assay is very unclear given that hyperplasia was not observed. The
study reviewers concluded that it was difficult to associate the increase in thyroid tumors with
mercuric chloride administration. Thus, it would be of questionable value to use the thyroid tumors in
rats as the basis for a quantitative cancer risk estimate for humans.
All tumors in rats were observed at doses equalling or exceeding the MTD. Kidney tumors in
mice were observed in only the high-dose males. The increased incidence was not statistically
significant in comparison to the concurrent controls, but was significant when compared with historical
controls. A linear low-dose extrapolation based on the male mouse kidney tumor data (three tumors in
the high-dose group only) is not appropriate.
JLC QUANTITATIVE ESTIMATE OF CARCINOGENIC RISK FROM INHALATION
EXPOSURE
None.
June 1996 B-31 ' SAB REVIEW DRAFT
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JI.D EPA DOCUMENTATION, REVIEW AND CONTACTS (CARCINOGENICITY
ASSESSMENT)
__ILD. 1 EPA 'DOCUMENTATION
Source Document - U.S. EPA, 1995
This IRIS summary is included in The Mercury Study Report to Congress which was reviewed
by OHEA and EPA's Mercury Work Group in November 1994. An interagency review by scientists
from other federal agencies took place in January 1995. The report was also reviewed by a panel of
non-federal external scientists in January 1995 who met in a public meeting on January 25-26. All
reviewers comments have been carefully evaluated and considered in the revision and finalization of
this IRIS summary. A record of these comments is summarized in the IRIS documentation files.
_H.D.2 REVIEW (CARCINOGENICITY ASSESSMENT)
Agency Work Group Review ~ 03/03/94
Verification Date - 03/03/94
_H.D.3 U.S. EPA CONTACTS (CARdNOGENICITY ASSESSMENT)
Rita Schoeny / NCEA - (513)569-7544
•
REFERENCES
Cantoni, O. and M. Costa. 1983. Correlations of DNA strand breaks and their repair with cell
survival following acute exposure to mercury (U) and X-rays. Mol. Pharmacol. 24(1): 84-89.
Cantoni, O., R.M. Evans and M. Costa. 1982. Similarity in the acute cytotoxic response of
mammalian cells to mercury (II) and X-rays: DNA damage and glutathione depletion. Biochem.
Biophys. Res. Commun. 108(2): 614-619.
Cantoni, O., N.T. Christie, A. Swann, D.B. Drath and M. Costa. 1984a. Mechanism of HgC12
cytotoxicity in cultured mammalian cells. Mol. Pharmacol. 26: 360-368.
Cantoni, O., N.T. Christie, S.H. Robinson and M. Costa. 1984b. Characterization of DNA lesions
produced by HgC12 in cell culture systems. Chem. Biol. Interact 49: 209-224.
Capen, C.C. and S.L. Martin. 1989. The effects of xenobiotics on the structure and function of
thyroid follicular and C-cells. Toxicol. Pathol. 17(2): 266-293.
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Christie, N.T., O. Cantoni, R.M. Evans, R.E. Meyn and M. Costa, 1984. Use of mammalian DNA
repair-deficient mutants to assess the effects of toxic metal compounds on DNA. Biochem.
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Christie, N.T., O. Cantoni, M. Sugiyama, F. Cattabeni and M. Costa. 1986. Differences in the effects
of Hg(II) on DNA repair induced in Chinese hamster ovary cells by ultraviolet or X-rays. Mol.
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Fitzhugh, O.G., A.A. Nelson, E.P. Lauge and P.M. Kunze. 1950. Chronic oral toxicities of
mercuric-phenyl and mercuric salts. Arch. Ind. Hyg. Occup. Med. 2: 433-442.
Ghosh, A.K., S. Sen, A. Sharma and G. Talukder. 1991. Effect of chlorophyllin on mercuric
chloride-induced clastogenicity in mice. Food. Chem. Toxicol. 29(11): 777-779.
Hill, R.N., L.S. Erdreich, O.V. Paynter, P.A. Roberts, S.L. Rosenthal and C.F. Wilkinson. 1989.
Review. Thyroid follicular cell carcinogenesis. Fund. Appl. Toxicol. 12: 629-697.
Hirano, M., K. Mitsumori, K. Maita and Y. Shiraso. 1986. Further carcinogenicity study on
methylmercury chloride in ICR mice. Jap. J. Vet. Sci. 48(1): 127-135.
Howard, W., B. Leonard, W. Moody and T.S. Kochhar. 1991. Induction of chromosome changes by
metal compounds in cultured CHO cells. Toxicol. Lett 56(1-2): 179-186.
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McClain, R.M. 1989. The significance of hepatic microsomal enzyme induction and altered thyroid
function hi rats: Implications for thyroid gland neoplasia. Toxicol. Pathol. 17(2): 294-306.
Mitsumori, K., K. Maita, T. Saito, S. Tsuda and Y. Shirasu. 1981. Carcinogenicity of methylmercury
chloride in ICR mice: Preliminary note on renal carcinogenesis. Cancer Lett. 12: 305-310.
Mitsumori, K., M. Hirano, H. Ueda, K. Maita and Y. Shirasu. 1990. Chronic toxicity and
carcinogenicity of methylmercury chloride hi B6C3F1 mice. Fund. Appl. Toxicol. 14: 179-190.
Morimoto, K., S. lijima and A. Koizumi. 1982. Selenite prevents the induction of sister-chromatid
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102: 183-192.
NTP (National Toxicology Program). 1993. NTP technical report on the toxicology and
carcinogenesis studies of mercuric chloride (CAS No. 7487-94-7) in F344 rats and B6C3F1 mice
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Poma, K., M. Kirsch-Volders and C. Susanne. 1981. Mutagenicity study of mice given mercuric
chloride. J. Appl. Toxicol. 1(6): 314-316.
Rice, G. and L. Knauf. 1994. Further Statistical Evaluation of the NTP Mercuric Chloride Mouse
Bioassay. Memorandum to the U.S. EPA CRAVE File for Mercuric Chloride, March 1.
Schroeder, H. and M. Mitchener. 1975. Life-time effects of mercury, methyl mercury, and nine other
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_I.A. REFERENCE DOSE FOR CHRONIC ORAL EXPOSURE (RfD)
Chemical -- Methylmercury (MeHg)
CASRN -- 22967-92-6
Preparation Date -- 2/10/95
_I.A.l ORAL RfD SUMMARY
Critical Effect Experimental Doses* UF MF RfD
Developmental Benchmark Dose: 11 ppm 10 1 1E-4
neurologic in hair; equivalent to mg/kg-day
abnormalities maternal blood levels
in human'infants 44 ug/L and body *
burdens of 69 ug or
Human epidemiologic daily intake of 1.1
studies ug/kg-day
Marsh et al., 1987; Seafood Safety, 1991
* Conversion Factors and Assumptions — Maternal daily dietary intake levels were used as the dose
surrogate for the observed developmental effects hi the infants. The daily dietary intake levels were
calculated from hair concentrations measured in the mothers. This conversion is explained hi the text
below. A benchmark dose approach was used rather than a NOAEL/LOAEL approach to analyze the
neurological effects hi infants as the response variable. This analysis is also explained hi the text
below.
_I.A.2 PRINCIPAL STUDIES (ORAL RfD)
Marsh, D.O., T.W. Clarkson, C. Cox, L. Amin-Zaki and S. Al-Trkiriti. 1987. Fetal methylmercury
poisoning: Relationship between concentration in a single strand of maternal hair and child effects.
Arch. Neurol. 44: 1017-1022.
Seafood Safety. 1991. Committee on Evaluation of the Safety of Fishery Products, Chapter on
Methylmercury: FDA Risk Assessment and Current Regulations, National Academy Press,
Washington, DC. p. 196-221.
In 1971-1972 many citizens hi rural Iraq were exposed to MeHg-treated seed grain that was
mistakenly used hi home-baked bread. Latent toxicity was observed hi many adults and children who
had consumed bread over a 2- to 3-month period. Infants born to mothers who ate contaminated bread
during gestation were the most sensitive group. Often infants exhibited neurologic abnormalities while
their mothers showed no signs of toxicity. Some information indicates that male infants are more
sensitive than females. Among the signs noted hi the infants exposed during fetal development were
cerebral palsy, altered muscle tone and deep tendon reflexes as well as delayed developmental
milestones, i.e., walking by 18 months and talking by 24 months. The neurologic signs noted in
June 1996 B-36 SAB REVIEW DRAFT
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adults included parestiiesia, ataxia, reduced visual fields and hearing impairment. Some mothers
experienced paresthesia and other sensory disturbances but these symptoms were not necessarily
correlated with neurologic effects in their children. Unique analytic features of mercury (Hg), that is,
analysis of segments of hair correlated to specific time periods in the past permitted approximation of
maternal blood levels that the fetuses were exposed to in utero. The data collected by Marsh et al.
(1987) summarizes clinical neurologic signs of 81 mother and child pairs. From x-ray fluorescent
spectrometric analysis of selected regions of maternal scalp hair, concentrations ranging from 1 to 674
ppm were determined and correlated with clinical signs observed in the affected members of the
mother-child pairs. Among the exposed population were affected and unaffected individuals
throughout the dose-exposure range.
While the purpose of the Seafood Safety publication was to critique the quantitative risk
assessment that FDA had performed for MeHg, this material is included in the EPA risk assessment
because the Tables of Incidence of various clinical effect^ in children that were provided in the FDA
assessment readily lend themselves to a benchmark dose approach. Specifically the continuous data
for the Iraqi population that was reported in Marsh et al. (1987) are placed in five dose groups and
incidence rates are provided for delayed onset of walking, delayed onset of talking, mental symptoms,
seizures, neurological scores above 3 and neurological scores above 4 for affected children.
Neurologic scores were determined by clinical evaluation for cranial nerve signs, speech, involuntary
movement, limb tone strength, deep tendon reflexes, plantar responses, coordination, dexterity,
primitive reflexes, sensation, posture, and ability to sit, stand and run. This paper provided groupings
of the 81 mother-infant pairs for various effects, and the authors present the data in Tables 6-11
through 6-16B.
EQUATION USED FOR CALCULATION OF DAILY DOSE: From the concentration of Hg present
hi maternal hair, a corresponding blood concentration value is determined. A hah- concentration of 11
ppm converts to a blood concentration of 44 ug/L; the following equation can then be used to
determine the daily dose that corresponds to that blood concentration of Hg. Use of this equation is
based on the assumption that steady-state conditions exist and that first-
order kinetics for Hg are being followed.
d = (C x b x V)/(A x f)
e
d (ug/day) = 44 ug/L multiplied by 0.014 multiplied by 5 liters divided by 0.95 then divided
by 0.05 yields 65 ug/day
where:
d = daily dietary intake (expressed as ug of MeHg)
C = concentration in blood (expressed as ug/L)
b = elimination constant (expressed as days-1)
V = volume of blood in the body (expressed as liters)
A = absorption factor (unitless)
f = fraction of daily intake taken up by blood (unitless)
June 1996 B-37 SAB REVIEW DRAFT
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The following sections provide the data and rationale supporting the choice of parameter
values used in the conversion equation. It should be noted that even if the upper or lower ranges ot
the parameter values were used, the conversion factor precision remains the same due to rounding
error. The Agency realizes that new pharmacokinetic data may become available that warrant a
change to some of these parameters.
HAIR TO BLOOD CONCENTRATION RATIO: The hairblood concentration ratio for total Hg is
frequently cited as 250. The following description provides a justification of why we have chosen to
use the ratio of 250:1. Ratios reported in the literature range from 140 to 370, a difference of more
than a factor of 2.5. Differences hi the location of hair sampled (head vs chest and distance from
scalp) may contribute to the differences observed. As much as a 3-fold seasonal variation in Hg levels
- was observed in average hair levels for a group of individuals with moderate to high fish consumption
rates, with yearly highs occurring in the fall and early winter (Phelps et al., 1980; Suzuki et al., 1993).
The high slope reported by Tsubaki and Inikayama (1977) may have reflected the fact that Hg levels
were declining at the time of sampling so that the hair levels reflect earlier, higher blood levels.
Phelps et al. (1980) obtained multiple blood samples an3 sequentially analyzed lengths of hair from
individuals. Both hair and blood samples were taken for 339 individuals in Northwestern Ontario.
After reviewing the various reports for converting hair concentrations to blood concentrations, the
Phelps paper was selected because of the large sample size and the attention to sampling and analysis.
The ratio Phelps observed between the total Hg concentration in hair taken close to the scalp and
simultaneous blood sampling for this group was 296. To estimate the actual ratio, the authors
assumed that blood and hair samples were taken following complete cessation of MeHg intake. They
also assumed a half-life of MeHg in blood of 52 days and a lag of 4 weeks for appearance of the
relevant level hi hair at the scalp. Phelps also determined that 94% of the Hg in hair was MeHg.
Based on these assumptions, they calculated that if the actual hainblood ratio was 200, they would
have observed a ratio of 290. Based on these and other considerations, Phelps states that the actual
ratio is "probably higher than 200, but less than the observed value of 296." As the authors point out,
2/3 of the study population were sampled during the falling phase of the seasonal variation (and 1/3 or
less hi the rising phase). This methodology would tend to result in a lower observed ratio; therefore,
the actual average is likely to be greater than 200.
In view of these limitations a value of 250 was considered acceptable for the purpose of
estimating average blood levels in the Iraqi population.'
CALCULATION OF DIETARY INTAKE FROM BLOOD CONCENTRATION: The first step in this
process is to determine the fraction of Hg in diet that is absorbed. Radio-labeled methyl-mercuric
nitrate (MeHgNO3) was administered in water to three healthy volunteers (Aberg et al., 1969). The
uptake was >95%. Miettinen et al. (1971) incubated fish liver homogenate with radio-labeled
MeHgNOS to produce methylmercury proteinate. The proteinate was then fed to fish that were killed
after a week and then cooked and fed to volunteers after confirmation of MeHg in the fish. Mean
uptake exceeded 94%. Based on these experimental results, this derivation used an absorption factor
of 0.95.
The next step involves determining the fraction of the absorbed dose that is found in the
blood. There are three reports on the fraction of absorbed MeHg dose distributed to blood volume hi
humans. Kershaw et al. (1980) report an average fraction of 0.059 of absorbed dose in total blood
volume, based on a study of five adult male subjects who ingested MeHg-contaminated tuna. In a
group of nine male and six female volunteers who had received 203 Hg-methylmercury hi fish
approximately 10% of the total body burden was present in 1 liter of blood hi the first few days after
June 1996 B-38 SAB REVIEW DRAFT
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exposure, dropping to approximately 5% over the first 100 days (Miettinen et al., 1971). In another
study, an average value of 1.14% for the percentage of absorbed dose in 1 kg of blood was derived
from subjects who consumed a known amount of MeHg in fish over a 3-month period (Sherlock et al.,
1982). Average daily intake in the study ranged from 43-233 ug/day and a dose-related effect on
percentage of absorbed dose was reported that ranged from 1.03-1.26% in 1 liter of blood (each of
these values should be multiplied by 5 [since there are approximately 5 liters of blood in an adult
human body] to yield the total amount in the blood compartment). The value 0.05 has been used for
this parameter hi the past (WHO, 1990).
ELIMINATION CONSTANT: Based on data taken from four studies, reported clearance half-times
from blood or hair ranged from 48-65 days. Two of these studies included the Iraqi population
exposed during the 1971-1972 outbreak. The value from the Cox study (Cox et al., 1989) is derived
from the study group that included the mothers of the infants upon which this risk assessment is based.
The average elimination constant of the four studies is 0.014; the average of individual values reported
for 20 volunteers ingesting from 42-233 ug Hg/day in fish for 3 months (Sherlock et al., 1982) is also
0.014.
VOLUME OF BLOOD IN THE BODY AND BODY WEIGHT: Blood volume is 7% of body weight
as has been determined by various experimental methods and there is an increase of 20 to 30% (to
about 8.5 to 9%) during pregnancy (Best and Taylor, 1961). Specific data for the body weight of Iraqi
women were not found. Assuming an average body weight of 60 kg. (Snyder et al., 1981) and a
blood volume of 9% of body weight during pregnancy, a blood volume of 5.4 liters is derived.
DERIVATION OF A BENCHMARK DOSE: Benchmark dose estimates were made for excess risk
above background based on a combination of all childhood neurologic end points. This method was
chosen since the Agency felt that any childhood neurologic abnormality is considered an adverse effect
and likely to have serious sequelae lasting throughout lifetime. In addition, grouping of all neurologic
endpoints provided a much better goodness of fit of the data than when any endpoint was used
individually. The endpoints that were grouped delayed the onset of walking and talking, neurologic
scores <3, mental symptoms, and seizures. Using these data sets taken from the Seafood Safety paper,
benchmark doses at the 1,5 and 10% incidence levels were constructed using both Weibull and
polynomial models. The Weibull model places the maximum likelihood estimate with corresponding
95% confidence level at 11 ppm of MeHg in maternal hair. The Agency decided to use the lower
95% confidence level for the 10% incidence rate. Recent research by Faustman et al. (1994) and
Allen et al. (1994a,b) suggests that the 10% level for the benchmark dose roughly correlates with a
NOAEL for quantal developmental toxicity data. The 95% lower confidence limits on doses
corresponding to the 1, 5, and 10% levels were calculated using both models and the values
determined using the polynomial model always fell within 3% of the Weibull values. For final
quantitative analysis the Weibull model was chosen because of goodness of fit of the data and because
this model has been used hi the past by the Agency for developmental effects. The experience of the
Agency indicates that this model performs well when modeling for developmental effects.
_I.A.3 UNCERTAINTY AND MODIFYING FACTORS (ORAL RfD)
UF ~ An uncertainty factor of 3 is applied for variability in the human population, in particular the
variation in the biological half-life of MeHg and the variation that occurs in the hair:blood ratio for
Hg. In addition, a factor of 3 is applied for lack of a two-generation reproductive study and lack of
June 1996 B-39 SAB REVIEW DRAFT
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data for the effect of exposure duration on sequelae of the developmental neurotoxicity effects and on
adult paresthesia. The total UF is 10.
MF — None
_I.A.4 ADDITIONAL STUDIES/COMMENTS (ORAL RfD)
9
McKeown-Eyssen et al. (1983) have provided a report of neurologic abnormalities in four
communities of Cree Indians in northern Quebec. A group of 247 children first exhibited clinical
signs consistent with MeHg exposure between 12 and 30 months of age. An attempt was made to
account for possible confounding factors; the interviewers determined alcohol and tobacco
consumption patterns among the mothers of affected children. Age of the mothers and multiparity was
also taken into account hi analysis of the data. The average indices of exposure were the same for
boys and girls at 6 ug/g; only 6% had exposure above 20 ug/g. The prevalence of multiple abnormal
neurologic findings was about 4% for children of both sexes. The most frequently observed
abnormality was delayed deep tendon reflexes; this was seen in 11.4% of the boys and 12.2% of the
girls. These investigators found that when there was a positive association between maternal Hg
exposure and abnormal neurologic signs in boys, the incidence rate was 7.2%. The incidence rate for
neurologic disorders in daughters was less and was found to be not statistically significant. Disorders
of muscle tone were usually confined to the legs. Persistence of the Babinski reflex and
incoordination due to delayed motor development were seen with equal frequency for both sexes. The
discriminant analysis conducted for the boys to distinguish the 15 cases with abnormal muscle tone or
reflexes from the 82 normal controls was unable to separate differences between these groups based on
confounding variables. The prevalence of abnormality of muscle tone or reflexes was found to
increase 7 times with each increase of 10 ug/g of the prenatal exposure index. Although this study
provides supportive data for the RfD, it is not included with the principal studies because it was
confounded by alcoholism and smoking among
mothers.
Studies performed in New Zealand investigated the mental development of children who had
prenatal exposure to MeHg (Kjellstrom et al., 1986, 1989). A group of 11,000 mothers who regularly
ate fish were initially screened by survey and of these about 1000 had consumed fish in three meals
per week during pregnancy. Working from this large population base, 31 matched pairs were
established. For proper comparison a reference child matched for ethnic group and age of mother,
child's birthplace and birth date was identified for each high Hg child. Retrospective Hg
concentrations were determined from the scalp hair of the mothers to match the period of gestation.
The average hair concentration for high-exposure mothers was 8.8 mg/kg and for the reference group
it was 1.9 mg/kg.
The children of exposed mothers were tested at 4 and 6 years of age. At 4 years of age the
children were tested using the Denver Developmental Screen Test (DDST) to assess the effects of Hg.
This is a standardized test of a child's mental development that can be administered in the child's
home. It consists of four major function sectors: gross motor, fine motor, language, and
personal-social. A developmental delay in an individual item is scored as
abnormal, questionable when the child has failed in their response and at least 90% of the children can
pass this item at a younger age. The results of the DDST demonstrated 2 abnormal scores and 14
questionable scores hi the high Hg-exposed group and 1 abnormal and 4 questionable scores in the
June 1996 B-40 SAB REVIEW DRAFT
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control group. Analysis of the DDST results by sector showed that developmental delays were most
commonly noted in the fine motor and language sectors but the
differences for the experimental and control groups were not significant. The investigators noted that
differences in performance of the DDST between high Hg-exposed and referent children could be due
to confounding variables and that DDST results are highly dependent upon the age of the child.
Standardized vision tests and sensory tests were also performed to measure
development of these components of the nervous system. The prevalence for developmental delay in
children was 52% from high Hg mothers and 17% from mothers of the reference group. In
comparison to other studied populations, the hair Hg concentration of the mothers hi this study were
lower than those associated with CNS effects in children exposed in Japan and Iraq. Results of the
DDST demonstrated 2 abnormal scores and 14 questionable scores hi the high
Hg-exposed group and 1 abnormal and 4 questionable scores in the control group. Analysis of the
DDST results by sector showed that developmental delays were most commonly noted in the fine
motor and language sectors but the differences for the experimental and control groups were not
significant The data obtained from this study is too limited for detailed dose-response analysis. The
differences in performance of the DDST between high Hg-exposed and referent children could be due
to confounding variables. DDST results are highly dependent upon the age of the child. Infants of
the Hg-exposed group more frequently had low birth weights and were more likely to be born
prematurely. Use of this study is also limited by the fact that there was only a 44% participation rate.
A second stage follow-up of the original Kjellstrom study was carried out when the children
were 6 years old to confirm or refute the developmental findings observed at age 4 (Kjellstrom et al.,
1989). In this later study the high exposure children were compared with three control groups with
lower prenatal Hg exposure. The mothers of children hi two of these control groups had high fish
consumption and average hair Hg concentrations during pregnancy of 3-6 mg/kg and 0-3 mg/kg,
respectively. For this study the high exposure group was matched for maternal ethnic group, age,
smoking habits, residence, and sex of the child. For this second study, 61 of 74 high-exposure
children were available for study. Each child was tested at age 6 with an array of scholastic,
psychological, and behavioral tests which included the Test of Language Development (TOLD), the
Wechsler Intelligence Scale for Children, and the McCarthy Scale of Children's Abilities. The results
of the tests were compared between groups. Confounding was controlled for by using linear multiple
regression analysis. A principal finding was that normal results of the psychological test variables
were influenced by ethnic background and
social class. The high prenatal MeHg exposure did decrease performance in the tests, but it
contributed only a small part of the variation in test results. The investigation found that an average
hair Hg level of 13-15 mg/kg during pregnancy was consistently associated with decreased test
performance. Due to the small size of the actual study groups it was not possible to determine if even
lower exposure levels might have had a significant effect on test
results. The Kjellstrom studies are limited for assessing MeHg toxicity because the developmental and
intelligence tests used are not the most appropriate tests for defining the effects of MeHg. Also,
greater significance was seen in differences of cultural origins of the children than the differences in
maternal hair MeHg concentrations.
The initial epidemiologic report of MeHg poisoning involved 628 human cases that occurred
in Minamata Japan between 1953 and 1960 (Tsubaki and frukayama, 1977). The overall prevalence
rate for the Minamata region for neurologic and mental disorders was 59%. Among this group 78
deaths occurred and hair concentrations of Hg ranged from 50-700 ug/g. Hair Hg concentrations were
determined through the use of less precise analytic methods than were available for later studies. The
specific values derived from these studies do not contribute directly to quantitative risk assessment for
June 1996 B-41 SAB REVIEW DRAFT
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MeHg. The most common clinical signs observed in adults were paresthesia, ataxia, sensory
disturbances, tremors, impairment of hearing and difficulty in walking. This particular group of
neurologic signs has become known as "Minimata disease." Examination of the brains of severely
affected patients that died revealed marked atrophy of the brain (55% normal volume and weight) with
cystic cavities and spongy foci. Microscopically, entire regions were devoid of neurons, granular cells
in the cerebellum, golgi cells and Purkinje cells. Extensive investigations of congenital Minamata
disease were undertaken and 20 cases that occurred over a 4-year period were documented. In all
instances the congenital cases showed a higher incidence of symptoms than did their mothers. Severe
disturbances of nervous function were described and the affected offspring were very late in reaching
developmental milestones. Hair concentrations of Hg in affected infants ranged from 10 to 100 ug/g.
Data on hair Hg levels for the mothers during gestation were not available.
Rice (1989) dosed five cynomolgus monkeys (Macaca fascicularis) from birth to 7 years of
age with 50 ug/kg-day and performed clinical and neurologic examinations during the dosing period
and for an additional 6 years. As an indicator of the latent effects of MeHg, objective neurologic
examinations performed at the end of the observation period revealed insensitivity to touch and loss of
tactile response. In addition, monkeys dosed with MeHg were clumsier and slower to react when
initially placed hi an exercise cage as well as in the later stages of the observation period.
Gunderson et al. (1986) administered daily doses of 50-70 ug/kg of MeHg to 11 crab-eating
macaques (Macaca fascicularis) throughout pregnancy which resulted in maternal blood levels of
1080-1330 ug/L in mothers and 1410-1840 ug/L in the offspring. When tested 35 days after birth the
infants exhibited visual recognition deficits.
In another study, groups of 7 or 8 female crab-eating macaques (Macaca fascicularis) were
dosed with 0.50 and 90 ug/kg-day of MeHg through four menstrual cycles (Burbacher et al., 1984).
They were mated with untreated males and clinical observations were made for an additional 4
months. Two of seven high-dose females aborted and three did not conceive during the 4-month
mating period; the other two females delivered live infants. Two of seven females of the 50 ug/kg-day
dose group aborted; the remaining females delivered live infants. All 8 females of the control group
conceived and 6 delivered live infants. These reproductive results approached but did not reach
statistical significance. Reproductive failure within dose groups could be predicted by blood Hg
levels. The dams did not show clinical signs of MeHg poisoning during the breeding period or
gestation but when females were dosed with 90 ug/kg-day for 1 year 4/7 did show adverse neurologic
signs.
Bomhausen et al. (1980) reported a decrease hi operant behavior performance in 4-month-old
rats whose dams had received 0.005 and 0.05 mg/kg-day of MeHg on days 6 through 9 of gestation.
A statistically significant effect (<0.05) was observed in offspring whose dams had received 0.01 and
0.05 mg/kg during gestation. The authors postulated that more severe effects of in utero exposure
would be seen in humans since the biological half-time of Hg in the brain of humans is 5 times longer
than the rat. In addition, much longer in utero exposure to Hg would occur in humans since gestation
is much longer in chronologic time.
In another investigation groups of Wistar rats (50/sex/dose) were administered daily doses of
2, 10, 50 and 250 ug/kg-day of MeHg for 26 months (Munro et al., 1980). Female rats that received
25 ug/kg-day had reduced body weight gains and showed only minimal clinical signs of neurotoxicity;
however, male rats that received this dose did show overt clinical signs of neurotoxicity, had decreased
hemoglobin and hematocrit values, had reduced weight gains, and showed increased mortality.
June 1996 B-42 SAB REVIEW DRAFT
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Histopathologic examination of rats of both sexes receiving 25 ug/kg-day revealed demyelination of
dorsal nerve roots and peripheral nerves. Males showed severe kidney damage and females had
minimal renal damage. This study showed a NOAEL of 5 ug/kg-day and a LOAEL of 25 ug/kg-day.
A 2-year feeding study of MeHg chloride was conducted in B6C3F1 mice (60 mice/sex/group)
at doses of 0, 0.4, 2 and 10 ppm (0, 0.04, 0.17, and 0.83 mg/kg-day) to determine chronic toxicity and
possible carcinogenic effects (Mitsumori.et al., 1990). The mice were examined clinically during the
study and neurotoxic signs characterized by posterior paralysis were observed in 33 males after 59
weeks and 3 females after 80 weeks in the 10 ppm group. A marked increase hi mortality and a
significant decrease hi body weight gain were also observed hi the 10 ppm male dose group, beginning
at 60 weeks. Post mortem examination revealed toxic encephalopathy consisting of neuronal necrosis
of the brain and toxic peripheral sensory neuropathy hi both sexes of the 10 ppm group. An increased
incidence of chronic nephropathy was observed hi the 2 and 10 ppm males. Based upon this study a
NOAEL of 0.04 mg/kg-day and a LOAEL of 0.17 mg/kg-day was determined. These results indicated
that B6C3F1 mice are more sensitive to the neurotoxic effects of MeHg than ICR mice.
KINETICS: MeHg in the diet is almost completely absorbed into the bloodstream. Animal studies
indicate (Walsh, 1982) that age has no effect on the efficiency of the gastrointestinal absorption, which
is usually hi excess of 90%. From the bloodstream MeHg is distributed to all tissues, and distribution
is complete within 4 days hi humans. The time necessary to reach peak brain levels from a single oral
dose is 1 or 2 days longer than other tissues and at this time the brain contains 6% of the total dose.
Also at this time the brain concentration is six times that of the blood.
Methylmercury is converted to inorganic Hg in various tissues at different rates in mammals.
The fraction of total Hg present as Hg++ depends on the duration of exposure and the time after
cessation of exposure. The percentages of total Hg present as inorganic Hg++ hi tissues of the Iraqi
population exposed for 2 months were: whole blood 7%, plasma 22%, breast milk 39% and urine
73%. Measurements hi the hepatic tissue of patients that had died was 16-40% of Hg++.
The fecal pathway accounts for 90% of the total elimination of Hg hi mammals after exposure
to MeHg. Essentially all Hg hi feces is hi the inorganic form. The process of fecal elimination begins
with biliary excretion with extensive recycling of both MeHg and Hg++ complexed with glutathione.
Inorganic Hg is poorly absorbed across the intestinal wall, but MeHg is readily reabsorbed such that a
secretion-resorption cycle is established. The intestinal microflora convert MeHg to inorganic Hg.
Whole body half-times determined hi human volunteers averaged 70 days with a range of
52-93 days. Observations of blood half-times is 50 days with a range of 39-70 days. Lactating
women have a significantly shorter whole body half-time of 42 days compared with 79 days hi
nonlactating women.
Selenium is known to bioconcentrate hi fish and it is thought that simultaneous ingestion of
selenium may offer a protective effect for the toxicity of MeHg based upon its antioxidant properties.
Selenium has been observed to correlate with Hg levels in blood (Granjean and Weihe, 1992).
June 1996 B-43 SAB REVIEW DRAFT
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_I.A.5 CONFIDENCE IN THE ORAL RfD
Study ~ Medium
Data Base — Medium
RfD -- Medium
The benchmark dose approach allowed use of the entire dose-response assessment and the
calculation of a value that was consistent with the traditional NOAEL/LOAEL approach. In addition,
the results of laboratory studies with nonhuman primates support the quantitative estimate of the
NOAEL/LOAEL range of the benchmark dose that was indicated by the human studies. The reported
literature covers detailed studies of human exposures with quantitation of MeHg by analysis of
specimens from affected mother-fetus pairs. A strength of the Marsh study is the fact that the
quantitative data came directly from the affected population and quantitation is based on biological
specimens obtained from affected individuals. Unfortunately, a threshold was not easily defined and
extended application of modeling techniques were needed to define the lower end of the dose-response
curve. This may indicate high variability of response to MeHg in the human mother-fetal pairs or
misclassification in assigning pairs to the cohort. Recent concerns expressed in the research
community relate to the applicability of a dose-response estimate based on a grain-consuming
population when the actual application is likely to help characterize risk for fish-consuming segments
of the population. Confidence in the supporting data base is medium. Confidence in the RfD is
medium.
_I.A.6 EPA DOCUMENTATION
Source Document - U.S. EPA, 1995
This IRIS summary is included in The Mercury Study Report to Congress, which was
reviewed by OHEA and EPA's Mercury Work Group in November 1994. An interagency review by
scientists from other federal agencies took place hi January 1995. The report was also reviewed by a
panel of non-federal external scientists in January 1995 who met hi a public meeting on January
25-26. All reviewers comments have been carefully evaluated and considered in the revision and
finalization of this IRIS summary. A record of these comments is summarized in the IRIS
documentation files.
Other EPA Documentation - U.S. EPA, 1980, 1984, 1987, 1988
Agency Work Group Review - 12/02/85, 03/25/92, 02/17/94, 08/04/94, 09/08/94,
09/22/94, 10/13/94, 11/23/94
Verification Date - 11/23/94
_I.A.7 EPA CONTACTS (ORAL RfD)
Rita Schoeny / OHEA - (513)569-7544
Bruce Mintz / OST ~ (202)260-9569
June 1996 B-44 SAB REVIEW DRAFT
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REFERENCES
Aberg, B., L. Ekman, R. Falk, U. Greitz, G. Persson and J. Snihs. 1969. Metabolism of methyl
mercury (203Hg) compounds in man. Arch. Environ. Health. 19: 478-484.
Allen B.C., R.J. Kavlock, C.A. Kimmel and E.M. Faustman. 1994a. Dose response assessment for
developmental toxicity. II. Comparison of generic benchmark dose estimates with NOAELS. Fund.
Appl. Toxicol. 23: 487-495.
Allen, B.C., R.J. Kavlock, C.A. Kimmel and E.M. Faustman. 1994b. Do§e response assessment for
developmental toxicity. in. Statistical models. Fund. Appl. Toxicol. 23: 496-509.
^
Best, C.H. and N.B. Taylor. 1961. A Text in Applied Physiology, 7th ed. The Williams and Wilkins
Co., Baltimore, MD. p. 19, 29.
Bornhausen, M., H.R. Musch and H. Greim. 1980. Operant behavior performance changes in rats
after prenatal methylmercury exposure. Toxicol. Appl. Pharmacol. 56:305-310.
Burbacher, T.M., C. Monnett, L.S. Grant and N.K. Mottet. 1984. Methylmercury exposure and
reproductive dysfunction in the nonhuman primate. Toxicol. Appl. Pharmacol. 75: 18-24.
Cox, C., T.W. Clarkson, D.O. Marsh and G.G. Myers. 1989. Dose-response analysis of infants
prenatally exposed to methylmercury: An application of a single compartment model to single-strand
hair analysis. Environ. Res. 49: 318-332.
Faustman, E.M., B.A. Allen, R.J. Kavlock and C.A. Kimmel. 1994. Dose-response assessment for
developmental toxicity. 1. Characterization of database and determination of no observed adverse
effect level. Fund. Appl. Toxicol. 23: 478-486.
Grandjean, P. and P. Weihe. 1993. Neurobehavioral effects of intrauterine mercury exposure:
Potential sources of bias. Environ. Res. 61: 176-183.
Gunderson, V.M., K.S. Grant, J.F. Fagan and N.K. Mottet. 1986. The effect of low-level prenatal
methylmercury exposure on visual recognition memory of infant crab-eating macaques. Child Dev.
57: 1076-1083.
Kershaw, T.G., T.W. Clarkson and P.H. Dhahir. 1980. The relationship between blood levels and the
dose of methylmercury in man. Arch. Environ. Health. 35(1): 28-36.
Kjellstrom, T., P. Kennedy, S. Wallis and C. Mantell. 1986. Physical and mental development of
children with prenatal exposure to mercury from fish. Stage 1: Preliminary test at age 4. National
Swedish Environmental Protection Board, Report 3080 (Solna, Sweden).
Kjellstrom, T., P. Kennedy, S. Wallis amd C. Mantell. 1989. Physical and mental development of
children with prenatal exposure to mercury from fish. Stage 2: Interviews and psychological tests at
age 6. National Swedish Environmental Protection Board, Report 3642 (Solna, Sweden).
June 1996 B-45 SAB REVIEW DRAFT
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Marsh, D.O., T.W. Clarkson, C. Cox et al. 1987. Fetal methylmercury poisoning: Relationship
between concentration in single strands of maternal hair and child effects. Arch. Neurol. 44:
1017-1022.
McKeown-Eyssen, G.E., J. Ruedy and A. Neims. 198 3-. Methylmercury exposure in northern Quebec.
II: Neurologic finds in children. Am. J. Epidemiol. 118(4): 470-479.
Miettinen, J.K., T. Rahola, T. Hattula, K. Rissanen and M. inlander. 1971. Elimination of 203-Hg
methylmercury in man. Ann. Clin. Res. 3: 116-122.
Mitsumori, K., M. Hiarano, H. Ueda, K. Maiata and Y. Shirasu. 1990. Chronic toxicity and
carcinogenicity of methylmercury chloride in B6C3F1 mice. Fund. Appl. Toxicol. 14: 179-190.
Munro, I.C., E.A. Nera, S.M. Charbonneau, B. Junkins and Z. Zawidzka. 1980. Chronic toxicity of
methylmercury in the rat J. Environ. Pathol. Toxicol. 3(5-6): 437-447.
Phelps, R.W., T.W. Clarkson, T.G. Kershaw and B. Wheatley. 1980. Interrelationships of blood and
hair mercury concentrations hi a North American population exposed to methylmercury. Arch.
Environ. Health. 35: 161-168.
Rice, D.C. 1989. Delayed neurotoxicity in monkeys exposed developmentally to methylmercury.
Neurotox. 10: 645-650.
Marsh, D.O., T.W. Clarkson, C. Cox et al. 1987. Fetal methylmercury poisoning: Relationship
between concentration hi single strands of maternal hair and child effects. Arch. Neurol. 44:
1017-1022.
Seafood Safety. 1991. Committee on Evaluation of the Safety of Fishery Products, Chapter on
Methylmercury: FDA Risk Assessment and Current Regulations, National Academy Press,
Washington, DC. p. 196-221.
Sherlock, J.C., D.G. Lindsay, J. Hislop, W.H. Evans and T.R. Collier. 1982. Duplication diet study
on mercury intake by fish consumers hi the United Kingdom. Arch. Environ. Health. 37(5): 271-278.
Snyder, W.S., M.T. Cook, L.R. Karhausen et al. 1975. International Commission of Radiological
Protection. No. 23: Report of a Task Group on Reference Man. Pergamon Press, NY.
Suzuki, T., T. Kongo, J. Yoshinaga et al. 1993. The hair-organ relationship hi mercury concentration
hi contemporary Japanese. Arch. Environ. Health. 48: 221-229.
Tsubaki, T.K. and K. Irukayama. 1977. Minamata Disease: Methylmercury Poisoning hi Minamata
and Niigata, Japan. Elsevier Science Publishers, New York. p. 143-253.
U.S. EPA. 1980. Ambient Water Quality Criteria Document for Mercury. Prepared by the Office of
Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH
for the Office of Water Regulation and Standards, Washington, DC. EPA/440/5-80/058. NTIS PB
81-117699.
June 1996 B-46 SAB REVIEW DRAFT
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U.S. EPA. 1984. Mercury Health Effects Update: Health Issue Assessment. Final Report. Prepared
by the Office of Health and Environmental Assessment, Environmental Criteria and Assessment
Office, Cincinnati, OH for the Office of Air Quality Planning and Standards, Research Triangle Park,
NC. EPA/600/8-84/019F. NTIS PB81-85-123925.
U.S. EPA. 1987. Peer Review Workshop on Mercury Issues. Environmental Criteria and Assessment
Office, Cincinnati, OH. Summary report. October 26-27.
U.S. EPA. 1988. Drinking Water Criteria Document for Inorganic Mercury. Prepared by the Office
of Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati,
OH for the Office of Drinking Water, Washington, DC. EPA/600/X-84/178. NTIS PB89-192207.
U.S. EPA. 1995. Mercury Study Report to Congress. Office of Research and Development,
Washington, DC. External Review Draft. EPA/600/P-94/002Ab.
Walsh, C.T. 1982. The influence of age on the gastrointestinal absorption of mercuric chloride and
methylmercury chloride in the rat. Environ. Res. 27: 412-420.
WHO (World Health Organization). 1990. Environmental Health Criteria 101: Methylmercury.
Geneva.
June 1996 ' B-47 SAB REVIEW DRAFT
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_II. CARCINOGENICITY ASSESSMENT FOR LIFETIME EXPOSURE
Substance Name ~ Methylmercury
CASRN -- 22967-92-6
Preparation Date -- 5/24/94
_H.A EVIDENCE FOR CLASSIFICATION AS TO HUMAN CARCINOGENICITY
_n.A.l WEIGHT-OF-EVIDENCE CLASSIFICATION
Classification - C; possible human carcinogen
Basis — Based on inadequate data in humans and limited evidence- of carcinogenicity hi animals.
Male ICR and B6C3F1 mice exposed to methylmercuric chloride in the diet had an increased
incidence of renal adenomas, adenocarcinomas and carcinomas. The tumors were observed at a single
site and in a single species and single sex. The renal epithelial cell hyperplasia and tumors were
observed only in the presence of profound nephrotoxicity and were suggested to be a consequence of
reparative changes in the cells. Several nonpositive cancer bioassays were also reported. Although
genotoxicity test data suggest that methylmercury is capable of producing chromosomal and nuclear
damage, there are also nonpositive genotoxicity data.
_II.A.2 HUMAN CARCINOGENICITY DATA
Inadequate. Three studies were identified that examined the relationship between
methylmercury exposure and cancer. No persuasive evidence of increased carcinogenicity attributable
to methylmercury exposure was observed La any of the studies. Interpretation of these studies,
however, was limited by poor study design and Incomplete descriptions of methodology and/or results.
Tamashiro et al. (1984) evaluated the causes of death in 334 subjects from the Kumamoto
Prefecture who had been diagnosed with Minamata disease (methylmercury poisoning) and died
between 1970 and 1980. The subjects involved fishermen and their families who had been diagnosed
with the disease; thus, Minamata disease was used as a surrogate for methylmercury exposure. The
controls were selected from all deaths that had occurred in the same city or town as the cases and
were matched on the basis of sex, age at death (within 3 years) and year of death; two controls were
matched to each subject. Malignant neoplasms were designated as the underlying cause of death in
14.7% (49/334) of the subjects and 20.1% (134/668) of the controls. For 47 subjects in which
Minamata disease was listed as the underlying cause of death, the investigators reanalyzed the
mortality data and selected one of the secondary causes to be the underlying cause of death in order to
allow examination of the subjects and controls under similar conditions and parameters. The three
subjects for which Minamata disease was listed as the only cause of death were excluded from further
analysis. Using the Mantel-Haenzel method to estimate odds ratios, no significant differences were
observed between the subjects and controls with respect to the proportion of deaths due to malignant
neoplasms among males, females or both sexes combined. The estimated odds ratios and 95%
confidence intervals were 0.84 (0.49-1.43), 0.58 (0.28-1.21) and 0.75 (0.50-1.11) for males, females
and both sexes combined. Similarly, no increases in odds ratio were observed among the subjects
June 1996 ' B-48 SAB REVIEW DRAFT
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relative to the controls when malignant neoplasms were identified as a secondary cause of death or
were listed on death .certificates as one of the multiple causes of death. These data suggest that cancer
incidence was not increased hi persons with overt signs of methylmercury poisoning when compared
with persons for whom no diagnosis of methylmercury poisoning had been made. Interpretation is
limited by potential bias in designating the cause of death among patients with known Minamata
disease and by the uncertainty regarding the extent of methylmercury exposure and undiagnosed
Minamata disease among the controls. In a subsequent study, Tamashiro et al. (1986) compared the
mortality patterns (between 1970 and 1981) among residents of the Fukuro and Tsukinoura districts in
the Kumamoto Prefecture (inhabited mainly by fishermen and their families) with that of age-matched
residents of Minamata City (also in the Kumamoto Prefecture) who died between 1972 and 1978. In
this study, high exposure to methylmercury was inferred from residence in a district believed to have
higher intake of local seafood. By contrast, in the 1984 study described above, high methylmercury
exposure was inferred from a diagnosis of Minamata disease. A total of 416 deaths were recorded hi
the Fukuro and Tsukinoura districts in 1970-1981, and 2325 deaths were recorded hi Minamata City in
1972-1978. No statistically significant increase hi the overall cancer mortality rate was observed;
however, an increase hi the mortality rate due to liver cancer was observed (SMR, 207.3; 95% CI,
116.0-341.9). Analysis of mortality by sex showed a statistically significant increase in the rate of
liver cancer only among males (SMR, 250.5; 95% CI, 133.4-428.4). Males also had statistically
significant higher mortality due to chronic liver disease
and cirrhosis. The authors note that these results should be interpreted with caution because the
population of the Fukuro and Tsukinoura districts had higher alcohol consumption and a higher
prevalence of hepatitis B (a predisposing factor for hepatocellular cancer). Interpretation of these
results is also limited by an incomplete description of the methodology used to calculate the SMRs; it
is unclear whether the study authors used appropriate methods to compare mortality data collected over
disparate time frames (12 years for exposed and 7 years for controls).
In a study from Poland, Janicki et al. (1987) reported a statistically significant increase in
mercury content of hair in leukemia patients (0.92 +/-1.44 ppm [sic]; n=47) relative to that in healthy
unrelated patients (0.49 +/-0.41 ppm; n=79). Similarly, the mercury content in the hair of a subgroup
of leukemia patients (0.69 +/- 0.75; n=19) was significantly greater than that in healthy relatives who
had shared the same residence for at least 3 years preceding the onset of the disease (0.43 +/- 0.24
ppm; n=52). When patients with specific types of leukemia were compared with the healthy unrelated
subjects (0.49 +/- 0.41 ppm; n=79), only those with acute leukemia (type not specified; 1.24 +/- 1.93
ppm; n=23) had a significantly increased hair mercury content No significant differences in hair
mercury content were observed hi 9 patients with chronic granulocytic leukemia or 15 patients with
chronic lymphocytic leukemia when compared with the unrelated, healthy controls. The authors
inferred that acute leukemia was associated with increased level of mercury hi hair. This study is of
limited use for cancer risk assessment because of the following: uncertainty regarding the correlation
between the chronology of incorporation of mercury hi the hair and onset of the disease; the small
population studied; the failure to describe adequately the characteristics of the leukemia patients or
healthy controls (age
distribution, length of residence in the region, criteria for inclusion in the study); uncertainty regarding
the source of mercury exposure (the authors presumed that exposure was the result of use of
methyhnercury-containing fungicides); and the failure to address exposure to other chemicals or adjust
for other leukemia risk factors. Furthermore, the variability of hair mercury
content was large, and the mean hair mercury levels were within normal limits for all groups. Thus,
the statistical significance may have been due to chance.
June 1996 B-49 SAB REVIEW DRAFT
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The carcinogenic effects of organomercury seed dressing exposure were investigated in a series
of case-control studies for incidence of soft-tissue sarcomas (Eriksson et al., 1981, 1990; Hardell and
Eriksson, 1988) or malignant lymphomas (Hardell et al., 1981). These studies were conducted in
Swedish populations exposed to phenoxyacetic acid herbicides or chlorophenols (the exposures of
primary interest in the studies), organomercury seed dressings, or other pesticides. Exposure
frequencies were derived from questionnaires and/or interviews. Control groups from the same region
of the country were matched to cases based on vital status. A total of 402 cases of soft-tissue sarcoma
and (among persons not exposed to phenoxyacetic acid herbicides) 128 cases of malignant lymphoma
were reported. In each study, the odds ratio for exposure to organomercury in seed dressings and the
incidence of sarcoma or lymphoma was either < 1.0 or the range of the 95% confidence interval for the
odds ratio included 1.0; therefore, no association was indicated for organomercury exposure and
soft-tissue sarcoma or malignant lymphoma. The study subjects were likely to have experienced
exposures to the other pesticides and chemicals.
_H.A.3 ANIMAL CARCINOGENICITY DATA
Limited. Three dietary studies in two strains of mice indicate that methylmercury is
carcinogenic. Interpretation of two of the positive studies was complicated by observation of tumors
only at doses that exceeded the MTD. A fourth dietary study in mice and four dietary studies in rats
failed to indicate carcinogenicity associated with methylmercury exposure. Interpretation of four of
the nonpositive studies was limited because of deficiencies hi study design or failure to achieve an
MTD.
Methylmercuric chloride (>99% pure) was administered in the diet at levels of 0, 0.4, 2 or 10
ppm (0, 0.03, 0.15 and 0.73 (mg/kg)/day in males and 0, 0.02, 0.11 and 0.6 (mg/kg)/day. in females) to
60 ICR mice/sex/group for 104 weeks (Hirano et al., 1986). Interim sacrifices (6/sex/group) were
conducted at 26, 52 and 78 weeks. Complete histopathological examinations were performed on all
animals found dead, killed in extremis or killed by design. Mortality, group mean body weights and
food consumption were comparable to controls. The first renal tumor was observed at 58 weeks in a
high-dose male, and the incidence of renal epithelial tumors (adenomas or adenocarcinomas) was
significantly increased in high-dose males (1/32, 0/25, 0/29 and 13/26 in the control, low-, mid- and
high-dose groups, respectively). Ten of the 13 tumors in high-dose males were adenocarcinomas.
These tumors were described as solid type or cystic papillary types of adenocarcinomas. No invading
proliferation into the surrounding tissues was observed. The incidence of renal epithelial adenomas
was not significantly increased hi males, and no renal adenomas or adenocarcinomas were observed in
any females studied. Focal hyperplasia of the tubular epithelium was reported to be increased in
high-dose males (13/59; other incidences not reported). Increases in non-neoplastic lesions in
high-dose animals provided evidence that an MTD was exceeded. Non-neoplastic lesions reported as
increased hi treated males included the following: epithelial degeneration of the renal proximal
tubules; cystic kidney; urinary cast and pelvic dilatation; and decreased spermatogenesis. Epithelial
degeneration of the renal proximal tubules and degeneration or fibrosis of the sciatic nerve was
reported in high-dose females.
Methylmercuric chloride (>99% pure) was administered hi the diet at levels of 0, 0.4, 2 or 10
ppm (0, 0.3, 0.14 and 0.69 (mg/kg)/day hi males and 0, 0.03, 0.13 and 0.60 (mg/kg)/day in females) to
60 B6C3F1 mice/sex/group for 104 weeks (Mitsumori et al., 1990). In high-dose males, a marked
increase in mortality was observed after week 60 (data presented graphically; statistical analyses not
June 1996 B-50 SAB REVIEW DRAFT
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performed by authors). Survival at study termination was approximately 50, 60. 60 and 20% in
control, low-, mid- and high-dose males, respectively, and 58, 68, 60 and 60% in control, low-, mid-
and high-dose females, respectively. The cause of the high mortality was not reported. At study
termination, the mean body weight in high-dose males was approximately 67% of controls and in
high-dose females was approximately 90% of controls (data presented graphically; statistical analyses
not performed by study authors). The incidence of focal hyperplasia of the renal tubules was
significantly increased in high-dose males (14/60; the incidence was 0/60 in all other groups). The
incidence of renal epithelial carcinomas (classified as solid or cystic papillary type) was also
significantly increased hi high-dose males (13/60; the incidence was 0/60 hi all other groups). The
incidence of renal adenomas (classified as solid or tubular type) was also significantly increased in
high-dose males; the incidence was 0/60, 0/60, 1/60 and 5/60 hi control, low-, mid- and high-dose
males, respectively, and 0/60, 0/60, 0/60 and 1/60 in control low-, mid- and high-dose females,
respectively. No metastases were seen in the animals. The incidences of a variety of non-neoplastic
lesions were increased in the high-dose mice including the following: sensory neuropathy; neuronal
necrosis in the cerebrum; neuronal degeneration in the cerebellum; and chronic nephropathy of the
kidney. Males exhibited tubular atrophy of the testis (1/60, 5/60, 2/60 and 54/60 in control, low-,
mid- and high-dose, respectively) and ulceration of the glandular stomach (1/60, 1/60, 0/60 and 7/60 in
control, low-, mid- and high-dose males, respectively). An MTD was achieved in mid-dose males and
high-dose females. High mortality in high-dose males indicated that the MTD was exceeded in this
group.
Mitsumori et al. (1981) administered 0, 15 or 30 ppm of methylmercuric chloride (>99% pure)
in the diet (0, 1.6 and 3.1 (mg/kg)/day) to 60 ICR mice/sex/group for 78 weeks. Interim sacrifices of
up to 6/sex/group were conducted at weeks 26 and 52. Kidneys were microscopically examined from
all animals that died or became moribund after week 53 or were killed at study termination. Lungs
from mice with renal masses and renal lymph nodes showing gross abnormalities were also examined.
Survival was decreased in a dose-related manner; at week 78 survival was 40, 10 and 0% in control,
low- and high-dose males, respectively, and 55, 30 and 0%, in control, low- and high-dose females,
respectively (statistical analyses not performed). The majority of high-dose mice (85% males and 98%
females) died by week 26 of the study. Examination of the kidneys of mice that died or were
sacrificed after 53 weeks showed a significant increase in renal tumors in low-dose males (13/16
versus 1/37 hi controls). The incidence of renal epithelial adenocarcinomas in control and low-dose
males was 0/37 and 11/16, respectively. The incidence of renal epithelial adenomas hi control and
low-dose males was 1/37 and 5/16, respectively. No renal tumors were observed in females in any
group. No metastases to the lung or renal lymph nodes were observed. Evidence of neurotoxicity and
renal pathology were observed in the treated mice at both dose levels. The high mortality in both
groups of treated males and in high-dose females indicated that the MTD was exceeded hi these
groups. (Note: Hirano et al. (1986) was a followup to this study.)
Mitsumori et al. (1983, 1984) administered diets containing 0, 0.4, 2 or 10 ppm of
methylmercuric chloride (0, 0.011, 0.05 and 0.28 (mg/kg)/day in males; 0, 0.014, 0.064 and 0.34
(mg/kg)/day in females) to 56/sex/group Sprague-Dawley rats for up to 130 weeks. Interim sacrifices
of 10/group (either sex) were conducted at weeks 13 and 26 and of 6/group (either sex) at weeks 52
and 78. Mortality was increased in high-dose males and females. At week 104, survival was
approximately 55, 45, 75 and 10% in control, low-, mid-and high-dose males, respectively, and 70, 75,
75 and 30% in control, low-, mid- and high-dose females, respectively (data presented graphically).
All males hi the high-dose group had died by week 119. Body weight gain was significantly
decreased hi high-dose males starting after week 44 and females
June 1996 B-51 SAB REVIEW DRAFT
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after 44 weeks (approximately 10-20%, data presented graphically). No increase in tumor incidence
was observed in either males or females. Noncarcinogenic lesions that were significantly increased in
high-dose rats included the following: degeneration in peripheral nerves and the spinal cord (both
sexes); degeneration of the proximal tubular epithelium (both sexes); severe chronic nephropathy
(females); parathyroid hyperplasia (both sexes); polyarteritis nodosa and calcification of arterial wall
(females); fibrosis of bone (females); bile duct hyperplasia (males); and hemosiderosis and
extramedullary hematopoiesis in the spleen (males). Mid-dose males exhibited significantly increased
degeneration of the proximal tubular epithelium and hyperplasia of the parathyroid. An MTD was
achieved in mid-dose males and exceeded in high-dose males and high-dose females.
No tumor data were reported hi a study using Wistar rats (Munro et al., 1980). Groups of 50
Wistar rats/sex/dose were fed diets containing methylmercury; doses of 2, 10, 50 and 250 (ug/kg)/day
were fed for 26 months. High-dose female rats exhibited reduced body weight gains and showed
minimal clinical signs of neurotoxicity; however, high-dose male rats showed overt clinical signs of
neurotoxicity, decreased hemoglobin and hematocrit values, reduced weight gains and significantly
increased mortality. Histopathologic examination of the high-dose rats of both sexes revealed
demyelination of dorsal nerve roots and peripheral nerves. Males showed severe dose-related kidney
damage, and females had minimal renal damage.
No increase in tumor incidence or decrease hi tumor latency was observed in another study
using rats of an unspecified strain (Verschuuren et al., 1976). Groups of 25 female and 25 male rats
were administered methylmercuric chloride at dietary levels of 0, 0.1, 0.5 and 2.5 ppm (0, 0.004, 0.02
and 0.1 (mg/kg)/day) for 2 years. No significant effects were observed on growth or food intake
except for a 6% decrease (statistically significant) hi body weight gain at 60 weeks hi high-dose
females. Survival was 72, 68, 48 and 48% hi control, low-, mid- and high-dose males, respectively,
and 76, 60, 64 and 56% hi control, low-, mid- and high-dose females, respectively (statistical
significance not reported). Increases hi relative kidney weights were observed in both males and
females at the highest dose. No effects on the nature or incidence of pathological lesions were
observed, and tumors were reported to have been observed with comparable incidence and latency
among all of the groups. This study was limited by the small sample size.
No increase hi tumor incidence was observed in a study using white Swiss mice (Schroeder
and Mitchener, 1975). Groups of mice (54/sex/group) were exposed until death to methylmercuric
acetate in the drinking water at two doses. The low-dose group received 1 ppm methylmercuric
acetate (0.19 (mg/kg)/day). The high-dose group received 5 ppm methylmercuric acetate (0.95
(mg/kg)/day) for the first 70 days and then 1 ppm thereafter, due to high mortality (21/54 males and
23/54 females died prior to the dose reduction). Survival among the remaining mice was not
significantly different from controls. Significant reductions hi body weight were reported in high-dose
males (9-15% lower than controls) and high-dose females (15-22% lower than controls) between 2 and
6 months of age. After dying, mice were weighed and dissected; gross tumors were counted, and
limited histopathologic sections were made of heart, lung, liver, kidney and spleen for microscopic
examination. This study is limited because complete histological examinations were not performed.
No increase hi tumor incidence was observed hi a multiple-generation reproduction study using
Sprague-Dawley rats (Newberne et al., 1972). Groups of rats (30/sex) were given semisynthetic diets
supplemented with either casein or a fish protein concentrate to yield dietary levels of 0.2 ppm
methylmercury (0.008 (mg/kg)/day). Another group of controls received untreated rat chow. Rats that
received diets containing methylmercury during the 2-year study had body weights and hematology
comparable to controls. Detailed histopathological analyses revealed no lesions of the brain, liver, or
June 1996 B-52 SAB REVIEW DRAFT
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kidney that were attributable to methylmercury exposure. Mortality data were not presented.
Interpretation of these data is limited by the somewhat small group sizes and failure to achieve an
MTD.
__H.A.4 SUPPORTING DATA FOR CARCINOGENICITY
Blakley (1984) administered methylmercuric chloride to female Swiss mice (number/group not
specified) in drinking water at concentrations of 0, 0.2, 0.5 or 2.0 mg/L for 15 weeks (approximately
0, 0.03, 0.07 and 0.27 (mg Hg/kg)/day). At the end of week 3, a single dose of 1.5 mg/kg of urethane
was administered intraperitoneally to 16-20 mice/group. No effects on weight gain or food
consumption were observed. Lung tumor incidence in mice not administered urethane (number/group
not specified) was less than one tumor/mbuse in all groups. Statistically significant trends for
increases in the number and size of lung adenomas/mouse with increasing methylmercury dose were
observed; the number of tumors/mouse was 21.5, 19.4, 19.4 and 33.1 in control, low-, mid- and
high-dose mice, respectively, and the tumor size/mouse was 0.70, 0.73, 0.76 and 0.76 mm in control,
low-, mid- and high-dose mice, respectively. The study authors suggest that the increase in tumor
number and size may have been related to the immunosuppressive activity of methylmercury. It should
be noted that this study is considered a short-term bioassay, and pulmonary adenomas were the only
tumor type evaluated.
Humans ingesting methyunercury-contaminated foods have been reported to experience
chromosomal aberrations (Skerfving et al., 1970, 1974) or SCE (Wulf et al., 1986); however,
interpretation of these studies is limited by methodological deficiencies.
As reviewed in WHO (1990), methylmercury is not a potent mutagen but appears to be
capable of causing chromosome damage and nuclear perturbations in a variety of systems. In Bacillus
subtilis, methylmercury produced DNA damage (Kanematsu et al., 1980). Methylmercury produced
chromosomal aberrations and aneuploidy in human peripheral lymphocytes (Betti et al., 1992), SCE in
human lymphocytes (Morimoto et al., 1982), and DNA damage in human nerve and lung cells as well
as Chinese hamster V-79 cells and rat glioblastoma cells (Costa et al., 1991).
Bone marrow cells of cats treated with methylmercury in a study by Charbonneau et al. (1976)
were examined by Miller et al. (1979). The methylmercury treatment resulted in an increased number
of nuclear abnormalities and an inhibition of DNA repair capacity. Methylmercury induced a weak
mutagenic response in Chinese hamster V-79 cells (Fiskesjo, 1979). Methylmercury also induced
histone protein perturbations and influenced factors regulating nucleolus-organizing activity (WHO,
1990). Moreover, methylmercury has been reported to interfere with gene expression in cultures of
glioma cells (WHO, 1990). Mailhes (1983) reported a significant increase in the number of
hyperploid oocysts in Lak:LVG Syrian hamsters fed methylmercury; however, no evidence of
chromosomal damage was reported. Suter (1975) concluded that strain-specific differences exist with
respect to the ability of methylmercury to produce dominant lethal effects hi mice. Nondisjunction
and sex-linked recessive lethal mutations were observed in Drosophila melanogaster treated with
methylmercury (Ramel, 1972 as cited hi U.S. EPA, 1985). Methylmercury produced single strand
breaks in DNA in cultured L5178Y cells (Nakazawa et al., 1975).
Negative studies have also been reported. Methylmercury acetate was reported to be negative
in a Salmonella typhimurium assay and a mouse micronucleus assay (Heddle and Bruce, 1977, as
June 1996 B-53 SAB REVIEW DRAFT
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reported in Jenssen and Ramel, 1980). Methylmercury was not mutagenic and did not cause
recombination in Saccharomyces cerevisiae but did slightly increase chromosomal nondisjunction
(Nakai and Machida, 1973). Matsumoto and Spindle (1982) reported no significant increase in SCE in
developing mouse embryos; they did report, however, that the developing mouse embryos were highly
sensitive to in vitro treatment with methylmercury.
_H.B QUANTITATIVE ESTIMATE OF CARCINOGENIC RISK FROM ORAL EXPOSURE
None. The two studies by Mitsumori et al. (1981, 1990) were limited by high mortality in the
high-dose males, the only group to exhibit a statistically significant increase in tumor incidence.
Tumors were observed only in those dose groups in which the MTD had been exceeded. The study
by Hirano et al. (1986) was not limited by low survival, but the tumors were observed in conjunction
with nephrotoxicity and, thus, their incidence may have been a high-dose phenomenon that would not
be expected to occur at low doses. The'tumors appeared to originate from focal hyperplasia of the
tubular epithelium induced as a reparative change. The hyperplasia was not observed in tubular
epithelium that was undergoing early degenerative changes. Thus, the tumors may not occur where
degenerative changes do not occur. The genotoxicity data indicate that methylmercury is not a potent
mutagen but may produce Chromosomal damage; these data do not support a hypothesis that
methylmercury is a genotoxic carcinogen. It appears, rather, that methylmercury exerts its
carcinogenic effect only at high dose, at or above an MTD. Because the linearized multistage
procedure is based on the assumption of linearity at low doses, the relevance of deriving a slope factor
based on data for which a threshold may exist is questionable.
It is likely that systemic non-cancer effects would be seen at methylmercury exposures lower
than those required for tumor formation. Long-term administration of methylmercury to experimental
animals produces overt symptoms of neurotoxicity at daily doses an order of magnitude lower than
those required to induce tumors in mice.
JI.C QUANTITATIVE ESTIMATE OF CARCINOGENIC RISK FROM INHALATION
EXPOSURE
None.
_H.D EPA DOCUMENTATION, REVIEW, AND CONTACTS (CARCINOGENICITY
ASSESSMENT)
_ILD.l EPA DOCUMENTATION
Source Documents - U.S. EPA, 1995
This IRIS summary is included in The Mercury Study Report to Congress which was reviewed
by OHEA and EPA's Mercury Work Group in November 1994. An interagency review by scientists
from other federal agencies took place hi January 1995. The report was also reviewed by a panel of
non-federal external scientists in January 1995 who met hi a public meeting on January 25-26. All
June 1996 B-54 SAB REVIEW DRAFT
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reviewers comments have been carefully evaluated and considered in the revision and rinalization of
this IRIS summary. A record of these comments is summarized in the IRIS documentation files.
__II,D.2 REVIEW (CARCINOGENICITY ASSESSMENT)
Agency Work Group Review -- 03/03/94
Verification Date - 03/03/94
_n.D.3* U.S. EPA CONTACTS (CARCINOGENICITY ASSESSMENT)
Rita Schoeny / OHEA - (513)569-7544
REFERENCES
Betti, C., T. Davini and R. Barale. 1992. Genotoxic activity of methyl mercury chloride and dimethyl
mercury in human lymphocytes. Mutat. Res. 281(4): 255 260.
Blakley, B.R. 1984. Enhancement of urethane-induced adenoma formation in Swiss mice exposed to
methylmercury. Can. J. Comp. Med. 48: 299 302.
Charbonneau, S.M., I.C. Munro, E.A. Nera et al. 1976. Chronic toxicity of methylmercury in the
adult cat. Toxicology. 5: 337-349.
Costa, M., N.T. Christie, O. Cantoni, J.T. Zelikoff, X.W. Wang and T.G. Rossman. 1991. DNA
damage by mercury compounds: An overview. In: Advances in Mercury Toxicity, T. Suzuki, N.
Imura and T.W. Clarkson, Ed. Plenum Press, New York, NY. p. 255-273.
Eriksson, M., L. Hardell, N.O. Berg, T. Moller and O. Axelson. 1981. Soft-tissue sarcomas and
exposure to chemical substances: A case-referent study. Br. J. Ind. Med. 38: 27 33.
Eriksson, M., L. Hardell and H.-O. Adami. 1990. Exposure to dioxins as a risk factor for soft-tissue
sarcoma: A population-based case-control study. J. Natl. Cancer Inst. 82(6): 486 490.
Fiskesjo, G. 1979. Two organic mercury compounds tested for mutagenicity in mammalian cells by
use of the cell line V 79-4. Hereditas. 90:103-109.
Hardell, L. and M. Erikssoa 1988. The association between soft-tissue sarcomas and exposure to
phenoxyacetic acids. A new case-referent study. Cancer. 62: 652 656.
Hardell, L., M. Eriksson, P. Lenner and E. Lundgren. 1981. Malignant lymphoma and exposure to
chemicals, especially organic solvents, chlorophenols and phenoxy acids: A case-control study. Br. J.
Cancer. 43: 169 176.
June 1996 B-55 SAB REVIEW DRAFT
-------
Heddle, J.R. and W.R. Bruce. 1977. Comparison of the micronucleus and sperm assay for
mutagenicity with the carcinogenic activities of 61 different agents. In: Origins of Human Cancer,
H.H. Hiatt, J.D. Watson, J.A. Winsten, Ed. Vol. 4. Cold Spring Harbor Conferences.
Hirano, M., K. Mitsumori, K. Maita and Y. Shirasu. 1986. Further carcinogenicity study on
methylmercury chloride in ICR mice. Jpn. J. Vet. Sci. 48(1): 127 135.
Janicki, K., J. Dobrowolski and K. Krasnicki. 1987. Correlation between contamination of the rural
environment with mercury and occurrence of leukemia in men and cattle. Chemosphere. 16: 253 257.
Jenssen, D. and C. Ramel. 1980. The micronucleus test as part of a short-term mutagenicity test
program for the prediction of carcinogenicity evaluated by 143 agents tested. Mutat. Res. 75: 191
202.
Kanematsu, N., M. Kara and T. Kada. 1980. Rec assay and mutagenicity studies on metal
compounds. Mutat Res. 77: 109-116.
Mailhes, J.B. 1983. Methylmercury effects on Syrian hamster metaphase II oocyte chromosomes.
Environ. Mutagen. 5: 679-686.
Matsumoto, N. and A. Spindle. 1982. Sensitivity of early mouse embryos to methylmercury toxicity.
Toxicol. Appl. Pharmacol. 64: 108-117.
Miller, C.T., Z. Zawidska, E. Nagy and S.M. Charbonneau. 1979. Indicators of genetic toxicity in
leukocytes and granulocytic precursors after chronic methylmercury ingestion by cats. Bull. Environ.
Contain. Toxicol. 21:296-303.
Mitsumori, K., K. Maita, T. Saito, S. Tsuda and Y. Shirasu. 1981. Carcinogenicity of methylmercury
chloride in ICR mice: Preliminary note on renal carcinogenesis. Cancer Lett. 12: 305 310.
•
Mitsumori, K., K. Takahashi, O. Matano, S. Goto and Y. Shirasu. 1983. Chronic toxicity of
methylmercury chloride in rats: Clinical study and chemical analysis. Jpn. J. Vet Sci. 45(6):
747-757.
Mitsumori, K., K. Maita and Y. Shirasu. 1984. Chronic toxicity of methylmercury chloride in rats:
Pathological study. Jpn. J. Vet Sci. 46(4): 549-557.
Mitsumori, K., M. Hirano, H. Ueda, K. Maita and Y. Shirasu. 1990. Chronic toxicity and
carcinogenicity of methylmercury chloride in B6C3F1 mice. Fund. Appl. Toxicol. 14: 179 190.
Morimoto, K., S. lijima and A. Koizumi. 1982. Selenite prevents the induction of sister-chromatid
exchanges by methyl mercury and mercuric chloride in human whole-blood cultures. Mutat. Res.
102: 183-192.
Munro, I., E. Nera, S. Charbonneau, B. Junkins and Z. Zawidzka. 1980. Chronic toxicity of
methylmercury in the rat. J. Environ. Pathol. Toxicol. 3: 437-447.
June 1996 B-56 SAB REVIEW DRAFT
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Nakai, S. and I. Machida. 1973. Genetic effect of organic mercury on yeast. Mutat. Res. 21(6): 348.
Nakazawa, N., F. Makino and S. Okada. 1975. Acute effects of mercuric compounds on cultured
mammalian cells. Biochem. Pharmacol. 24: 489-493.
Newberne, P.M., O. Glaser and L. Friedman. 1972. Chronic exposure of rats to methyl mercury in
fish protein. Nature. 237: 40-41.
Ramel, C. 1972. Genetic effects. In: Mercury in the Environment - An Epidemiological and
Toxicological Appraisal, L. Friberg and J. Vostal, Ed. CRC Press, Cleveland, OH. p. 169-181. (Cited
in U.S. EPA, 1985).
Schroeder, H. and M. Mitchener. 1975. Life-time effects of mercury, methylmercury, and nine other
trace metals hi mice. J. Nutr. 105: 452 458.
Skerfving, S., K. Hansson and J. Lindsten. 1970. Chromosome breakage in humans exposed to
methyl mercury through fish consumption. Arch. Environ. Health. 21:133-139.
Skerfving, S., K. Hansson, C. Mangs, J. Lindsten and N. Ryman. 1974. Methylmercury-induced
chromosome damage hi man. Environ. Res. 7: 83-98.
Suter, K.E. 1975. Studies on the dominant-lethal and fertility effects of the heavy metal compounds .
methylmercuric hydroxide, mercuric chloride, and cadmium chloride in male and female mice. Mutat.
Res. 30: 365-374.
Tamashiro, H., M. Arakaki, H. Akagi, M. Futatsuka and L.H. RohL 1984. Causes of death in
Minamata disease: Analysis of death certificates. Int. Arch. Occup. Environ. Health. 54: 135-146.
Tamashiro H., Arakaki M., Futatsuka M. and E.S. Lee. 1986. Methylmercury exposure and mortality
hi southern Japan: A close look at causes of death. J. Epidemiol. Comm. Health. 40: 181-185.
U.S. EPA. 1980. Ambient Water Quality Criteria Document for Mercury. Prepared by the Office of
Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH
for the Office of Water Regulation and Standards, Washington, DC. EPA/440/5-80/058. NTIS
PB81-117699.
U.S. EPA. 1984a. Mercury Health Effects Update: Health Issue Assessment. Final Report. Prepared
by the Office of Health and Environmental Assessment, Environmental Criteria and Assessment
Office, Cincinnati, OH for the Office of Air Quality Planning and Standards, Research Triangle Park,
NC. EPA/600/8-84/019F. NTIS PB81-85-123925.
U.S. EPA. 1984b. Health Effects Assessment for Mercury. Prepared by the Office of Health and
Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH for the
Office of Emergency and Remedial Response, Washington, DC. EPA/540/1086/042. NTIS
PB86-134533/AS. ,
June 1996 B-57 SAB REVIEW DRAFT
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U.S. EPA. 1988. Drinking Water Criteria Document for Inorganic Mercury. Prepared by the Office
of Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati,
OH for the Office of Drinking Water, Washington, DC. EPA/600/X-84/178. NTIS PB89-192207.
U.S. EPA. 1993. Summary Review of Health Effects Associated with Mercuric Chloride: Health
Issue Assessment (Draft). Prepared by the Office of Health and Environmental Assessment,
Environmental Criteria and Assessment Office, Cincinnati, OH for the Office of Air Quality Planning
and Standards, Research Triangle Park, NC. EPA/600/R-92/199.
U.S. EPA. 1995. Mercury Study Report to Congress. Office of Research and Development,
Washington, DC. External Review Draft. EPA/600/P-94/002Ab.
Verschuuren, H.G., R. Kroes, E.M. Den Tonkelaar et al. 1976. Toxicity of methylmercury chloride in
rats. HI. Long-term toxicity study. Toxicology." 6: 107 123.
WHO (World Health Organization). 1990. Methyl mercury. Vol. 101. Geneva, Switzerland: World
Health Organization, Distribution and Sales Service, International Programme on Chemical Safety.
Wulf, H.C., N. Kromann, N. Kousgaard, J.C. Hansen, E. Niebuhr and K. Alboge. 1986. Sister
chromatid exchange (SCE) in Greenlandic eskimos: Dose-response relationship between SCE and seal
diet, smoking, and blood cadmium and mercury concentrations. Sci.'Total Environ. 48: 81-94.
June 1996 B-58 SAB REVIEW DRAFT
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APPENDIX C
ATTENDEES OF U.S. EPA PEER REVIEW WORKSHOP
ON MERCURY ISSUES
-------
LIST OF WORKSHOP ATTENDEES
Giuseppe Andres
State University of New York
206 Farber Hall
Main Street Campus
Department of Pathology
Buffalo, NY 14214
716-831-2846
Karen Blackburn
Environmental Criteria and Assessment Office
U.S. EPA
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7569
Harlal Choudhury
Environmental Criteria and Assessment Office
U.S. EPA
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7536
Tom Clarkson
University of Rochester
P.O. Box EHSC
University of Rochester Medical School
Rochester, NY 14642
716-275-3911
Michael Dieter
NTP, NIEHS, NIH
P.O. Box 12233
Research Triangle Park, NC 27709
919-541-3368
Michael Dourson
Environmental Criteria and Assessment Office
U.S. EPA
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7544
Ernest Foulkes
University of Cincinnati College of Medicine
Department of Environmental Health
Cincinnati, OH 45267-0056
513-872-5769
Kris Khanna
Office of Drinking Water
U.S. EPA
4Q1 M Street, S.W.
Washington, DC 20460
202-382-7588 ,
Loren D. Koller
Oregon State University College of Veterinary
Medicine
Corvallis, OR 97331
503-754-2098
W. Bruce Peirano
Environmental Criteria and Assessment Office
U.S. EPA
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7540
David Reisman
Environmental Criteria and Assessment Office
U.S. EPA
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7588
Paul L. Richter
New Jersey Department of
Environmental Protection
CN413
401 East State Street - Sixth Floor
Trenton, NJ 08625
609-984-9759
June 1996
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Linda Saunders
Eastern Research Group, Inc.
6 Whittemore Street
Arlington, MA 02174
617-648-7800 '
Heidi Schultz
Eastern Research Group, Inc.
6 Whittemore Street
Arlington, MA 02174
617-648-7800
Cindy Sonich-Mullin
Environmental Criteria and Assessment Office
U.S. EPA
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7523
Bob Vanderslice
Office of Drinking Water
U.S. EPA
401 M Street, S.W.
Washington, DC 20460
202-475-6711
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APPENDIX D
HEALTH EFFECTS OF MERCURY AND MERCURY COMPOUNDS
UNCERTAINTY ANALYSIS OF THE METHYLMERCURY RfD
-------
D.I Introduction and Background
The purpose of the analysis in this appendix is two-fold: first, to determine plausible bounds
on uncertainty associated with the data and dose conversions used to derive the methylmercury
Reference Dose (RfD); second, to compare the RfD to estimated distributions of human population
thresholds for adverse effects. The analysis presented in this appendix is a modeled estimate of the
human threshold for specific health effects attributable to methylmercury exposure. The basis for the
analysis and the RfD is the data from the 1971 Iraqi methylmercury poisoning incident, specifically
the data from the Marsh et al. (1987) study. The analysis also includes studies pertinent to the
conversion of mercury concentrations in hair to estimated ingestion levels. The population studied in
Marsh et al. (1987) is hereafter referred to as the Iraqi cohort. The methylmercury RfD was based on
a benchmark dose calculated from the combined developmental effects of late walking, late talking,
mental effects, seizures and neurological effects (scores greater than 3 on a test) in children of women
exposed during pregnancy; benchmark doses for the individual developmental effects and for adult
paresthesia were also calculated. All the benchmark doses for developmental endpoints were
calculated from the Iraqi cohort data. The adult paresthesia benchmark dose was calculated from data
-presented hi Bakir et al. (1973). The studies and their use in the calculation of the RfD for
methylmercury are described in detail in chapters 3 and 6 of Volume IV of this Report.
The approach used in this analysis and the EPA's RfD methodology presuppose the existence
of thresholds for certain health effects. The RfD is defined by the U.S. EPA (U.S. EPA, 1995) as
an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily
exposure to the human population (including sensitive subgroups) that is likely to be
without an appreciable risk of deleterious effects during a lifetime.
This definition implies that the RfD is an exposure level that is below the threshold for adverse effects
in a sensitive subpopulation. For purposes of this analysis, the human population threshold is defined
as the threshold for the most sensitive individual of an identified sensitive subpopulation. The
definition of sensitive subpopulations excludes hypersensitive individuals whose susceptibilities fall far
outside the normal range. A threshold is defined as the level of exposure to an agent or substance
below which a specific effect is not expected to occur. The definition of threshold does not include
concurrent exposure to other agents eliciting the same effect by the same mechanism of action. In
other words, there is an assumption that the induced response is entirely a result of exposure to a
single agent The adverse health endpoints for the methylmercury RfD as determined by the RfD/RfC
Workgroup are the specific clinically-observed endpoints reported hi Marsh et al. (1987). The
uncertainty analysis was confined to "those endpoints. The 81 pregnant female/offspring pairs
comprising the Iraqi cohort were taken as a surrogate for the most sensitive subpopulation expected in
the general U.S. fish consuming population. The sensitive subpopulation was specifically identified as
humans exposed to methylmercury in utero.
Other analyses of the Iraqi cohort data are available hi the literature but are not directly
applicable to the estimation of threshold distributions. An analysis presented hi the Seafood Safety
report (NAS, 1991) groups the Iraqi cohort observations by ranges of measured mercury concentrations
hi hair in order to estimate the cumulative response distribution. The response data grouped by hair
mercury concentrations groupings were used to calculate the benchmark dose levels on which the
methylmercury RfD was based. As any grouping of data introduces an additional level of uncertainty,
this threshold analysis was based on the ungrouped observations.
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Cox et al. (1989) presented estimates of thresholds based on the ungrouped observations of
the Iraqi cohort for two of the five developmental endpoints considered by the U.S. EPA in the
derivation of the methylmercury RfD. Cox et al. (1989) used a threshold model that included the
threshold as a parameter. The value of the threshold parameter was estimated by a statistical method
that optimized the likelihood at different values of the threshold. The estimated threshold for late
walking in offspring (first walking after 18 months) was 7.3 ppm mercury in hair with an upper 95%
confidence limit of 14 ppm. This threshold value was based on the best (optimized likelihood)
estimate for background incidence of late walking of 0%. The upper 95% confidence limit was highly
sensitive to the value of the background parameter, increasing to 190 ppm mercury in hair for a
background of 4%. The optimized likelihood threshold for neurological effects (neurological scores >
3) based on a background incidence of 9% was 10 ppm mercury in hair with an upper 95% confidence
limit of 287 ppm.
The analysis examined the major sources of uncertainty explicitly and implicitly inherent to the
methylmercury RfD and attempted to bound them quantitatively. There are a number of sources of
uncertainty in the estimation of either a human threshold or an RfD from the Iraqi cohort data and
from the dose conversion used to estimate ingestion dose levels from hair mercury concentrations.
The principal uncertainties arise from the following sources: the variability of susceptibilities within
the Iraqi cohort; population variability in the pharmacokinetic processes reflected in the dose
conversion; response classification error; and exposure classification error.
The data show a very broad range of susceptibilities in the 81 individuals, of the Iraqi cohort.
An analysis of the response rates based on hair mercury concentrations showed up to a 10,000-fold
span between the 5th and 95th percentiles when projected to the general population (Hattis and Silver,
1994). Uncertainty in threshold estimates arising from the variability in individual susceptibilities was
estimated by calculating a distribution of thresholds from a regression model for repeated bootstrap
samples of the original Iraqi cohort data set The bootstrap procedure and regression model are
described in section D.2.1. The bootstrap procedure results in a distribution of population thresholds
for specific effects hi units of ppm mercury in hair.
The methylmercury RfD used a dose conversion formula (section 6.3.1.1 of Volume IV of this
report) to estimate the ingestion dose in mg methylmercury per kg body weight per day (mg/kg-day)
that would result in a specified mercury concentration in hair. This formula comprises a number of
variables that are associated with biological processes. There are measured ranges for each variable
which can be attributed to interindividual variability in pharmacokinetics and to experimental variation.
The response "classification is the assignment of an individual observation to one of two
categories — responder or nonresponder. The response classification for each of the developmental
endpoints reported in Marsh et al. (1987) is based on a fixed value (response decision point) that
constitutes a response when exceeded. It is possible that some observations, particularly those that
represent responses in the immediate vicinity of the response decision point, were misclassified; a
responder may have been classified as a nonresponder or vice versa. The response classifications for
late walking and late talking are particularly susceptible to this type of error. The response estimates
were based on subject recall in members of a population that does not traditionally record these events.
The classification of neurological test battery scores is more objective but still susceptible to some
degree of investigator interpretation and misclassification.
Exposure classification error is the inclusion of individuals in the exposure group who had
been exposed outside a critical period. This type of error is a source of uncertainty for all
developmental endpoints that have a critical period of exposure combined with uncertainty about the
June 1996 D-2 SAB REVIEW DRAFT
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actual timing of the gestational period. The result of this type of error is the misclassification of an
unexposed individual as an exposed individual. The consequence of this misclassification is an
overestimation of the exposure level associated with a given response or nonresponse and subsequent
overestimation of population variability. For example, in the Iraqi cohort it is noted that an individual
with the highest estimated mercury exposure is a non-responder for developmental effects on the
nervous system. This may be due to differences in individual susceptibility to methyl mercury
toxicity, or it may be a consequence of misclassification; the individual may have been exposed during
a period of time which is not critical to development. There is potential for misclassification as the
determination of the correspondence of gestational period and exposure was dependent on subject
recall. Data pertaining to this type of uncertainty are not yet available.
Other areas of uncertainty are those directly related to the RfD methodology. Specifically, it
was concluded by an Agency Work Group that there were no adequate chronic or reproductive studies.
An uncertainty factor of 10 is generally applied when chronic studies are not available. This
uncertainty factor is based on an assumption inherent to the RfD methodology that increased exposure
duration will lower the dose required for observation of the effect. Support for this assumption has
been published (Weil and McCollister, 1963; Dourson and Stara, 1989) and is discussed in section
D.2.2.2 of this Appendix. An uncertainty factor of 3 is generally applied if reproductive studies are
not available. No-Observed-Adverse-Effect Levels (NOAELs) for reproductive studies are generally 2-
fold to 3-fold higher than NOAELs for chronic studies and are not expected to be the basis for the
RfD more than 5% of the time (Dourson, Knauf and Swartout, 1992).
D.2 Methods
Thresholds were estimated in a two-stage process. The first stage was the estimation of
threshold distributions based on hair mercury concentrations, which was accomplished by applying a
regression model to successive bootstrap samples of the observations in Marsh et al. (1987). This
process is detailed in section D.2.1. The second stage was the conversion of the thresholds expressed
as ppm mercury in hair to mg methylmercury per kg body weight per day (mg/kg-day); this involved a
Monte Carlo analysis of the variability of the underlying biological processes. The dose-conversion
model is described in section D.2.2.
For the uncertainty analysis thresholds for four of the six endpoints evaluated for the
methylmercury RfD and for combined developmental effects were estimated. The developmental
effects included in the threshold analysis were late walking, late talking and neurological effects.
Thresholds for seizures and mental symptoms were not estimated because these effects occurred at 5-
fold higher hair-mercury concentrations than did the other effects. As the resulting thresholds would
be much higher than the others they would not be expected to contribute significantly to the combined
"developmental effects threshold distribution as defined for this analysis (the lowest of the individual
effect thresholds for each bootstrap sample). Response rates for seizures and mental symptoms would
be expected to influence the benchmark dose, however, as the benchmark dose is a function of all
responses. The Iraqi cohort data are summarized hi Table D-l. A plus (+) in Table D-l indicates a
positive response. A positive response for neurological effects was a neurological score greater than 3
as defined in Marsh et al. (1987). Positive responses for late walking and late talking were 18 months
and 24 months (after birth), respectively (Marsh et al., 1987).
June 1996 D-3 SAB REVIEW DRAFT
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Table D-l
Incidence of Developmental and Adult Effects as reported in Marsh, et al., 1987
max ppm
mercury in
hair
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
3
3
5
6
6
7
8
9
10
10
12
12
14
16
18
1Q
oeuro test
scores > 3
+
+
+
+
late
walking a
+
•h
late
talking b
+
+
+
adult
pares-
thesia
+
max ppm
mercury in
hair
23
26
38
45
48
52
59
60
62
72
74
75
78
86
98
104
114
118
154
196
202
242
263
269
294
336
339
357
362
376
399
404
405
418
443
468
557
568
598
674
neuro test
scores > 3
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
late
walking
•f
+
+
+
•f
•f
+
+
+
+
+
+
+
+
•f
late
talking
+
+
+
+
+
+
+
•f
+
+
+
+
+
+
+
•f
+
+
adult
pares-
thesia
-h
-f
+
-(•
•t-
-f
•(-
+
+
+
+
+
+
a defined as first walking after 18 months
b defined as first talking after 24 months
All threshold calculations and Monte Carlo simulations were performed in S-PLUS (ver 3.2)
for Microsoft® Windows® (ver 3.1) on several microprocessors based on the Intel® 486DX2/66
microprocessors. Sensitivity analyses were performed in Crystal Ball® (ver 3.0) and Excel® (ver 4.0)
for Windows®.
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D.2.1 Estimation of Thresholds Based on ppm Mercury in Hair
Hair mercury concentrations at the thresholds for adult paresthesia, three developmental
endpoints and combined developmental effects were estimated from the Iraqi cohort data. Threshold
estimation was accomplished by applying a regression model to successive bootstrap samples of the 81
observations. The bootstrap method was based on the assumption that the observed sample was a
random sampling of a larger population and that each observation was equally likely to occur in
additional samples (Efron 1986; Efron and Tibshirani, 1991). The bootstrap approach was applied in
order to allow estimation of confidence intervals on the dose associated with a given response. The
bootstrap process consisted of taking a random sample of the same size as the observed1 sample
distribution from the original sample distribution. The sampling was conducted with replacement of
selected observations prior to the next random selection such that individual observations may have
appeared more than once in any given sample. For this analysis, 5000 bootstrap samples were
generated. Thresholds, fti units of ppm mercury in hair, for each endpoint were calculated for each
bootstrap sample. The output was a distribution of 5000 thresholds for each endpoint representing the
variability hi the population-threshold as estimated from the Iraqi cohort data. The threshold
distribution for combined effects was defined as the minimum of the single-effect threshold hair-
mercury concentrations calculated at each bootstrap iteration; this definition is based on the assumption
that the endpoints were independent The stability of the bootstrap was evaluated by determining the
change in the 5th and 95' percentiles, and their ratio, for each doubling of the number of iterations.
The bootstrap was considered to be stable if successive estimates were within 5% of each other.
The threshold model used in this analysis treated background incidence and response related
to exposure (induced response) independently. The procedure is illustrated in Figure D-l, which
shows the Marsh et al., 1987 data and regression lines for developmental neurological effects
(neurological score > 3). Figure D-l is an example of a threshold determination from a single
iteration of the bootstrap procedure. All of the response data were binary; that is, individuals were
either responders or nonresponders for a given effect The binary responses associated with each hair
mercury concentration are indicated by a "+" at the top and bottom of the chart for responders and
nonresponders, respectively. The mean background and fitted regression lines for induced response are
shown. The threshold was defined in the regression model as the concentration of mercury in hair
corresponding to the fitted response equal to the background incidence. This is equivalent to the point
of intersection of the background and induced response line indicated as point A in Figure D-l. This
model was chosen so that the threshold estimate was a consequence of, rather than a contribution to,
the estimation of background and induced response. Other threshold models that could have been
used, such as the one used by Cox et al. (1989), include the threshold as a parameter to be
simultaneously estimated with all other model parameters. Also the maximum likelihood approach for
parameter estimation was not used because of the apparent extreme sensitivity of the upper 95%
confidence limit on the threshold to variation hi the background estimate (Cox et al., 1989).
June 1996 D-5 SAB REVIEW DRAFT
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Figure D-l
Regression Model for Determination of Bootstrap Thresholds
I.CH
0.8-
0.6-
0.4-
0.3-
0.0-
+ » observations
background incidence
background • 0
log-lin«az fit
log-pzobit fit
i i
5 10
50 100
500
ppm Micuxy in hair
lognonnal (GM = 250, GSD = 1.35)
pd = probability density
•
In the regression model, response was regressed on the logarithm of dose using the probit
function (log-probit model). In those cases where the log-probit model-predicted responses were
always greater than background, a log-linear regression model was used to determine the threshold
(point B in Rgure D-l). The log-probit and log-linear fitted regression lines are shown for one
bootstrap sample hi Figure D-l.
Hair mercury concentrations of 1 ppm were assumed to represent background exposure levels
(Katz and Katz, 1992). All other observations, the first of which was at 14 ppm for any effect, were
included hi the estimation of background and induced response rate as follows. Background
incidence for each effect was estimated directly from each bootstrap sample by performing repeated
regressions of response on hah- mercury concentrations, starting with the assumed background range
and successively adding data points at the next higher hair mercury concentration until the regression
slope was near zero and was least statistically significant. Background incidence was defined as the
mean of the fitted values of the resulting regression. The induced response regression slope was
calculated in a similar fashion, starting with all observations above concentrations of 12 ppm hair-
mercury and successively adding data points at the next lower hair mercury concentration until the
regression slope was maximized and statistically significant (p < 0.05).
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D.2.2 Estimation of In.gestion Dose Levels in mg/k.g-day
D.2.2.1 Estimation of Dose Conversion Uncertainty
The uncertainty arising from the calculation of ingestion dose levels, in mg/kg-day, corresponding
to measured concentrations of mercury in hair was estimated through analysis of the dose conversion
formula. The formula, which estimates ingested dose levels corresponding to the measured
methylmercury concentration in hah", incorporates the formula used hi the derivation of the RfD with
the inclusion of an additional term to account for the hair to blood concentration ratio for
methylmercury and the conversion of elimination constants to their equivalent half-lives. The latter
was done as a matter of convenience as most of the studies reported half-lives rather than elimination
constants. The formula used in the derivation of the RfD is described in Chapter 6 (section 6.3.1.1) of
Volume IV of this report and is reproduced here as equation 1.
.. CxbxV (i)
A x f x bw
where
d is the daily dietary intake in mg/kg-day,
C is the concentration of methylmercury in the blood in ug/Iiter,
b is the elimination constant (of methylmercury from the blood) in days"1,
V is the volume of blood in the body in liters,
A is the fraction of mercury in the diet that is absorbed,
f is the fraction of absorbed mercury that is found in the blood.
bw is body weight hi kg.
Variable C hi formula 1 can be related to the concentration of mercury hi hah" by the formula given in
equation 2.
(2)
hb
where
Hgh is the concentration of mercury hi hair hi ppm (ug mercury/g hair),
hb is the hair to blood concentration ratio for methylmercury hi ug mercury/g hair/(ug
mercury/ml blood).
Variable b in formula 1, which is a first-order elimination rate constant, and the clearance half-life are
related by the formula given in equation 3.
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b =
(3)
log. 2
fte
where
b is the elimination constant,
loge 2 is the natural logarithm of 2 (= 0.693),
t% is the half-life of methylmercury in the blood.
Substituting for C and b in equation 1 from equations 2 and 3, respectively, gives the formula for
ingestion levels based on mercury concentrations in hair (equation 4).
log, 2xHghxV
hbxt^xfxAxbw
Dividing both sides of equation 4 by Hgh gives the dose conversion factor, which when multiplied
by a hair mercury concentration gives the corresponding ingestion level in mg/kg-day (equation 5).
DCF -- « _ (5)
hbxt^xfxAxbw
where
DCF is the dose conversion factor in ppm mercury in hair/(mg/kg-day).
Lower levels of exposure to methylmercury were expected to be associated with the observation of
effects in adults for exposure durations longer than those observed for the Iraqi cohort (U.S. EPA,
1995; Barnes and Dourson, 1988). The potential effect of exposure duration on the dose eliciting
chronic effects is given in equation 6.
where
DCFeda is the exposure-duration adjusted dose conversion factor in ppm mercury in
hair/(mg/kg-day),
DCF is the dose conversion factor (from equation 5),
UD is a unitless adjustment for uncertainty arising from limited exposure duration.
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Monte Carlo simulations were conducted for equations 5 anfl 6. The output of these
simulations were used to calculate a family of ingestion threshold distributions (in mg/kg-day) for each
endpoint. This was done by multiplying the bootstrap threshold distribution for a given endpoint by
specific percentiles of the appropriate dose conversion distribution. Each member of a family of
distributions was associated with a specific probability dependent on the relative likelihood of the
DCF.
D.2.2.2 Input Variable Distributions
•
Distributions were assigned to each variable in equations 5 and 6 based on the data available
in the literature. The general form of the distribution, whether triangular, normal or lognormal, was
determined by examination of the shape of the distribution of empirical data and by consideration of
the underlying biological and physical processes. A triangular distribution was used when a judgement
was made that the value of the variable fell within identifiable absolute limits. Many of the variables
reflect underlying exponential processes and would be distributed as the logarithm of the nominal
values. Such variables were described as being distributed hi log space. In these cases a lognormal,
or log-triangular distribution was chosen to represent the variable.
Determination of the distribution parameters focused on identifying the median (50th
percentile) and extreme percentiles from the available data. The focus was on the median, rather than
the mean, in order to specify percentiles of the distribution. In general, the mean value of the Monte
Carlo output is more closely related to the median, rather than the mean, of the inputs. The extreme
percentiles were those corresponding to the lowest and highest observations and were determined by
multiplying the rank order of the observation by 100/(n +1), where n is the total number of
observations. For example, the lowest and highest observations in a sample size of 9 define the 10th
and 90th percentiles (80% confidence interval), respectively. Calculating the percentiles on the basis of
n + 1, rather than n, allowed for the possibility of obtaining more extreme values in additional
samples. The median and extreme percentiles were preserved in the final distribution whenever
possible by adjusting the distribution parameters accordingly. The distribution assigned to each
variable is given in Table D-2.
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Table T>-2
Dose Conversion Monte Carlo Simulation Input Variable Distributions
Variable
hbc
t*
yd
fe
A
bw
Form
lognormal
log triangular
triangular
log triangular
triangular
lognormal
Nominal a
Value
250 :
53 days
•5 liters
0.059 g
0.95 g
-55kg
Parameters "
GM =250, GSD = 1.5
min = 1.455, mode = 1.676, max = 2.085 (Iog10)
min = 3.63, mode = 5.0, max = 6.37
min = -1.41, mode = -1.30, max = -0.934 (Iog10)
min = 0.90, mode = 0.95, max = 1.0
GM = 55, GSD = 1.13
a median or geometric median
b Key: GM - geometric mean, GSD - geometric standard deviation; min - absolute minimum, mode
- most likely value, max = absolute maximum
c correlated with ti^ [correlation coefficient (r) = -0.5]
d correlated with bw (r = -0.47).
e correlated with bw (r = +0.57)
f ug Hg/g hair/mg Hg/1 blood
8 unitless ratio
Hair to Blood Concentration Ratio for methybnercury (hb)
This variable represents variation in a population of the ratio of the concentration of
methylmercury in hair to the concentration of methylmercury in blood. The distribution for this
variable was based on the EPA RfD/RfC Work Group's analysis of the available data, which is
presented in Chapter 6 (section 6.3.1.1) of Volume IV of this report. The EPA RfD/RfC Work Group
has judged that the most appropriate value for this variable lies between 200 and 300 based on results
published by Phelps et al. (1980), with 250 selected as the point estimate. The value of 250 was used
as the geometric mean of the distribution for hb. The data given in Phelps et al. (1980) were not
detailed enough to allow determination of the shape or variance of the distribution. The lognormal
form for the distribution was chosen as most representative of the empirical data reported in Sherlock
et al. (1982). The geometric standard deviation (GSD) of 1.5 was estimated hi this analysis from the
same data (Sherlock et al., 1982). The distribution is shown in Figure D-2.
A correlation coefficient (r) of -0.5 was assumed between hb and t^ in the Monte Carlo
simulation of equation 5. The amount of mercury in the hair should be at least partially dependent on
how quickly rnethylmercury is eliminated from the blood; that is, the faster that methylmercury is
eliminated from the blood, the greater would be the difference between the concentration of mercury
hi the hair and mercury in the blood. The relationship between t^ and hb would be expected to be
inverse (negative correlation); high values of t^ would be associated with low values of hb. The
magnitude of the correlation coefficient was judged by the U.S. EPA to be at least as strong as -0.5.
The data available for the calculation of the correlation between t% and hb are extremely limited. A
correlation coefficient between hairblood concentration and half-life of about -0.3 was calculated in
this analysis from data on four individuals (Kershaw et al., 1980).
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Figure D-2
Probability Density Function for Hair-to-Blood Concentration Ratio (hb)
3«0 30O
hmlnhlood ratio
lognormal (GM = 250, GSD = 1.35)
pd = probability density
Half-Life of Methylmercury in the Blood (t,A)
Several human studies reported clearance half-lives for methylmercury from blood in the
range of 32-105 days with averages of 45-70 days (Miettinen et al., 1971; Bakir et al., 1973; Kershaw
et al., 1980; Smith et al., 1994). An average elimination constant for methylmercury from the blood
of 0.014 with a range of 0.0099 to 0.0165 was reported by Sherlock, et al. (1984) corresponding to an
average half-life of 50 days with a range of 42-70 days. Table D-3 gives the average and range of
reported half-lives or half-lives calculated from equation 3 for each of the five studies. A histogram of
the combined data was highly skewed and roughly triangular in shape. The log-triangular distribution
(Figure D-3) was chosen as best representative of the empirical data. The median value of this
distribution was 53 days; this was slightly higher than that used in the derivation of the methylmercury
RfD (50 days), which would result hi slightly lower dose conversion values.
Table D-3
Half-Life of Methylmercury in the Blood (days)
Reference
Smith et al. 1994
Sherlock et al. 1984
Kershaw et al. 1980
Bakir et al. 1973 .
Greenwood et al. 1978
Low
31.9
42
46.7
40
49
Average
45.3
49.5
51.9
65
70
High
60
70
66.5
105
95
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Figure D-3
Probability Density Function for the Half Life of Methylmercury in the Blood (t,/2)
50 «« TO •« H 100
half-lit* Idiy.)
log triangular {min = 1.455, mode = 1.676, max = 2.085 (Iog10)}
pd = probability density
Volume of Blood in the Body (V)
This variable represents the variation in a population of the total volume of blood in the
body. The distribution was based on published values of estimated whole blood volumes for a cohort
of 20 pregnant Nigerian women (Harrison, 1966). Whole blood volumes in the third trimester of
pregnancy ranged from 4.0 to 6.0 liters; the mean and median values were both 5.0 liters (Harrison,
1966). The distribution of empirical data was roughly triangular and symmetrical. The minimum and
maximum values were adjusted so that the range of observed values fell within the 90% confidence
interval (n = 20). The distribution is shown hi Figure D-4.
Blood volume was assumed to be positively correlated with body weight; larger blood
volumes would be associated with higher body weights. For this analysis a correlation coefficient of
0.57 between V and bw from the data given in Harrison (1966) was calculated. This correlation
coefficient was assumed for the standard Monte Carlo simulation of equation 5.
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Figure D-4
Probability Density for Blood Volume (V)
i— 1 1 —i 1 1 ~~i
0.035 0.040 a.ait 0.050 0.055 0.0(0 0.0(5
blood volun.
triangular {min = 3.5 liters, mode = 5.0 liters, max = 6.5 liters}
pd = probability density
Fraction of Absorbed methylmercury in the Blood (f)
This variable reflects the distribution and dilution of the absorbed methylmercury in all
compartments of the body. The distribution on f was based on several human studies showing values
in the range of 5-10% of the absorbed dose of methylmercury in the blood (Miettinen et al., 1971;
Kershaw et al., 1980; Sherlock et al., 1984). All of the measured values have been adjusted for an
assumed total blood volume of 5 liters. The studies are summarized in Chapter 6 (section 6.3.1.1) of
Volume IV of this report. The distribution is shown in Figure D-5. The median value of this
distribution of 5.9% was higher than that used in the derivation of the methylmercury RfD (5%),
which would result in lower dose conversion values.
Sherlock et al. (1984) reported that the fraction of methylmercury in the blood was
negatively correlated with body weight; smaller fractions of methylmercury hi the blood were
associated with larger body weights. For this analysis the U.S. EPA calculated a correlation of -0.47
between f and bw from the data given in Sherlock et al. (1984). This correlation was assumed for the
Monte Carlo simulation of equation 5.
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Figure D-5
Probability Density for Fraction of Absorbed Methylmercury in the Blood (f)
0.04 O.fll O.OC O.OT 0.04 _ t.10
rnmiim mm l> am blood
log triangular {min = -1.41, mode = -1.30, max = -0.934 (Iog10)}
pd = probability density
Fraction of methylmercury in the Diet that is Absorbed (A)
This distribution was based on the results of two human studies showing uptake of radio-
labeled methylmercury of 95% and greater (Aberg et al., 1969) and 94% and greater (Miettinen et al.,
1971) and animal studies showing 90% or greater absorption (summarized in Walsh, 1982). The
distribution reflects the expectation that this value is close to 100 and will not vary much. The
distribution is shown in Figure D-6.
Figure D-6
Probability Density for Fraction of Methylmercury Absorbed from the Gut (A)
O.tO O.M O.M «.M
triangular {min = 0.90, mode = 0.95, max = 1.0}
pd = probability density
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Body Weight (bw)
The distribution for body weight was based on Harrison (1966), previously described for the
definition of V. The observed body weights during the third trimester of pregnancy ranged from 49.5
kg to 73.9 kg with a geometric mean of 55 kg (Harrison, 1966). A lognormal distribution was
visually fitted to the data. The distribution is shown in Figure D-7. The median value of this
distribution of 55 kg was lower than that used in the derivation of the methylmercury RfD (60 kg).
Use of the lower value for bw which would result in higher dose conversion values.
Figure D-7
Probability Density Distribution for Body Weight (bw)
lognormal (GM = 55 kg, GSD = 1.13)
pd = probability density
Uncertainty Arising from Limited Exposure Duration (UD)
This factor is an adjustment for the uncertain effects of exposure duration on the magnitude
of the effective dose. It is based on the assumption that continuing exposure will result in the
appearance of effects at exposure levels wherein there were no effects observed following shorter
exposure durations. The U.S. EPA commonly applies an uncertainty factor of 10 when calculating a
chronic RfD from a study of subchronic duration (U.S. EPA, 1995). In concept, the value of 10 for
this uncertainty factor represents a high estimate of the uncertainty hi order to maintain the protective
nature of the RfD. An empirical analysis of the Weil and McCollister (1963) data by Dourson and
Stara (1983) supports the use of an uncertainty factor of 10 as protective. About 50% of ratios of
subchronic NOAELs to chronic NOAELS for rats exposed to a variety of substances other than
methylmercury (as reported by Weil and McCollister, 1963) were below 3.5 and 95% were below 10
(Dourson and Stara, 1983).
The published data were insufficient for the estimation of a distribution for UD. A point
estimate of 4.7 was made for UD from a few studies of methylmercury toxicity in nonhuman primates.
These studies are summarized hi Table D-4. Table D-4 gives NOAELs and Lowest-Observed-
Adverse-Effect Levels (LOAELs) for studies of short-term and long-term duration in monkeys. The
neurologic endpoints were limited to clinically-observable effects in order to maintain approximate
equivalence of effects across exposure durations. UD was estimated by dividing the short-term
LOAEL of 0.21 mg/kg-day (Sato and Ikuta, 1975) by 0.045, the average of the two long-term
exposure LOAELS of 0.04 and 0.05 mg/kg-day (Burbacher et al., 1988; Rice and Gilber, 1992).
June 1996
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UD was used in equation 6 to adjust the dose conversion factor for the estimation of the
exposure level associated with chronic effects. Specifically, the exposure duration adjusted dose
conversion factor (DCFeda) from equation 6 was multiplied by the adult paresthesia bootstrap
threshold distribution to obtain an ingestion threshold distribution for chronic neurologic effects. The
precise nature of the chronic effects was not specified because the effects observed in the monkey
studies used to define UD included a number of different neurologic effects. In this case the
paresthesia observed in the Iraqi cohort was used as a surrogate for all possible adult neurologic
effects that might occur following short-term exposure to methylmercury.
Table D-4
Methylmercury Toxicity in Animals
Reference
Sato & Ikuta 1975
Burbacher et al.,
1988
Rice & Gilbert,
1992
Exposure
Duration
36-132 days
3 years
6.5-7 years
Effects
ataxic gait, myoclonic
seizures
slight tremor, motor
incoordination,
blindness
decreased fine motor
performance, other
NOAEL LOAEL
0.07 0.21
none 0.04
none 0.05
D.2.2.3 Correlation of Input Variables
Apart from the assumptions of correlation between individual input variables described
previously (standard correlations), a simplifying assumption was that the susceptibility of any
individual was independent of the value of the dose-conversion factor. It is very likely, however, that
susceptibility and ti/4 are correlated. Longer residence times of methylmercury hi the blood,
corresponding to longer half-lives, should have a direct effect on toxicity. Thus, there would be some
likelihood that the susceptibility of the individual at the population threshold (the most sensitive
individual) would be related to larger values of t^. Monte Carlo analyses of equation 5 limiting t% to
values greater than 53 days (the median of the t,^ distribution) or greater than 84 days (the 90th
percentile of the t^ distribution) were also conducted. The latter simulation was included only to
determine the sensitivity of the output to changes in the assumption and was not considered to be a
realistic scenario. Standard input variable correlations were assumed for these simulations. Results of
this simulation are presented in section D.3.
A sensitivity analysis was conducted to examine the effect of different correlation
assumptions on the relative contribution of each input variable distribution to the variance of the
Monte Carlo simulation output The sensitivity analysis was performed for standard correlations, no
correlations and for the alternate half-life (t^ > 53 days or t^ > 84 days) scenarios.
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D.2.3 Estimation of Uncertainty Arising from Response Classification Error
A variable-response model was constructed to assess response classification error. The
variable-response model is identical to the general threshold model except that responses presumed to
be uncertain were given fractional values rather than 0 or 1 (for nonresponders and responders,
respectively). Values of 0 or 1 were generated at each bootstrap sampling by comparison of the
fractional value with random numbers drawn from a uniform distribution between 0 and 1. The
uncertain observations were defined as those that fell close to the defined minimum response. For late
walking the observations that fell into this category were those of 18 - 20 months (late walking = not
walking by 18 months). For late talking the uncertain observations were 24 - 26 months (late talking
= not talking by 24 months). A value of 0.5 was assigned to each of the observations for late walking
and late talking that were designated uncertain; this represented the largest possible uncertainty in
classification (50% classification error). A 50% classification error was judged to be plausible, given
the highly variable factors involved in the original classifications hi Marsh el al. (1987). A separate
analysis was conducted for late walking assuming a 25% classification error. This was done to allow
for the possibility that the large number of observations at exactly 18 months (22), was a result of 18
months being used as an upper bound, rather than an exact estimate. This could have occurred for
observations that were uncertain but judged by the authors (Marsh et al., 1987) to be 18 months or
less.
The determination of neurological scores hi Marsh et al. (1987) was considered to be more
objective than the determination of late walking or late talking. There is, however, a possibility that
the distinction between adjacent scores is not absolute. The uncertain observations for neurological
effects were scores of 3 or 4. Although there was no clear basis for determining classification error
for this endpoint, the error was judged likely to be considerably less than for late walking and late
talking. Simulations were run assuming a 10% or 20% classification error. A classification error rate
of 20% was considered to be an upper bound.
There was no basis on which to determine the extent of classification error for adult
paresthesia. In addition, there was no way to determine which responses (or nonresponses) were
marginal. A 5% classification error was assumed for all observations to determine the sensitivity of
the threshold simulation to small error rates.
D.3 Results
D.3.1 Bootstrap Analysis
Figures D-8 to D-10 sho_w the frequency distributions of thresholds for the individual
developmental endpoints resulting from the bootstrap analysis. Figure D-ll is the threshold
distribution for the occurrence of any developmental effect The figures are histograms of the
frequency of occurrence of calculated threshold mercury concentrations resulting from 5,000 iterations
of the bootstrap procedure. The threshold values on the abscissa are given in Iog10 units. These
distributions represent uncertainty in the estimation of population thresholds calculated from hair
mercury concentrations. The small separate peaks at about 600 ppm in Figures D-8 to D-10 represent
bootstrap samples that result hi a nonsignificant (p > 0.05) log-probit regression slope. A
nonsignificant slope implies that there was no relationship between hair mercury concentrations and
observed response for that bootstrap sample. In these cases the threshold was defined as the largest
hair mercury concentration in the sample. The frequency with which nonsignificant slopes occurred
was interpreted as a measure of the reliability of the endpoint as a measure of methylmercury toxicity.
This occurred in 0.4% of the bootstrap samples for neurological effects, 0.1% of the late walking
June 1996 D-17 SAB REVIEW DRAFT
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bootstrap samples and in 0.2% of the late walking bootstrap samples. These percentages effectively
limit the upper end of the bootstrap confidence intervals that can be defined meaningfully. As an
example, the upper limit on the bootstrap confidence interval for the neurological effects threshold was
99.6%. The maximum two-tailed symmetrical confidence interval on the median threshold for this
endpoint that excluded nonsignificant slopes was 99.2%.
At the other end of the distributions, the log-probit model fails to estimate a threshold when
the lowest log-probit response is greater than the background incidence (log-probit regression line fails
to intersect background in Figure D-l). This would occur hi all bootstrap samples where a zero
background incidence was estimated or when the calculated log-probit response at 3 ppm mercury in
hair was greater than the background incidence for that sample. The threshold was calculated from
the log-linear regression model in these cases (point B in Figure D-l); this occurred in 6%, 13.5% and
100% of the bootstrap samples for neurological effects, late talking and late walking, respectively.
The average background incidence, as estimated from the Iraqi cohort data for each bootstrap sample,
was 10.7% for neurological effects and 8.6% for late talking. Background incidence for late walking
was 0% for all samples.
Figure D-8
Bootstrap Threshold Distribution for Developmental Neurological Effects
5000 itantiou
SOfri
300-
fr«qu«ocy
1 a
i Hi in h»ir (logic)
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Figure D-9
Bootstrap Threshold Distribution for Late Walking
5000
500-
400-
300-
fr«qu«ncy
012
ppa Hj in hair UoglO)
Figure D-10
Bootstrap Threshold Distribution for Late Talking
5000 it«r»tion»
400-
fr«qu«BCir
in hair (10910)
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Figure D-ll
Bootstrap Threshold Distribution for Combined Developmental Effects
5000 iterations
in hair I logic)
Figure D-12 shows the distribution of bootstrap thresholds for adult paresthesia . The
distribution is a result of 5000 iterations of the bootstrap procedure. Nonsignificant regression slopes
occurred hi 7.5% of the samples as shown by the peak at around 2.8 (600 ppm) in Figure D-12. The
largest confidence interval for the adult paresthesia threshold that excluded nonsignificant slopes was
85%. Background incidence for adult paresthesia was 0% for all samples.
Figure D-12
Bootstrap Threshold Distribution for Adult Paresthesia
5003 iteration*
i Hg in hair (loglO)
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Table D-5 gives selected percentiles from the cumulative bootstrap threshold distributions for
the developmental endpoints and adult paresthesia. The threshold values given in Table D-5 are given
in units of ppm mercury in hair. The values given for the 5th and 95th percentiles in Table D-5 define
the 90% bootstrap confidence interval for each threshold. The adult paresthesia thresholds were the
lowest of all the endpoints modeled but showed the greatest variability. The late walking threshold
was the lowest and most variable of the individual developmental endpoint thresholds. The combined-
effects threshold was the least variable of all the thresholds as would be expected from the method of
calculation (minimum of the three individual endpoint thresholds). For the combined developmental
effects, the late walking threshold was the lowest of the three thresholds most often (45%) with
neurological effects and late talking contributing the lowest threshold 31% and 24% of the time,
respectively.
Table D-5
Bootstrap-Threshold Distributions in ppm Mercury in Hair for All Effects
Endpoint
neurological effects a
late walking b
late talking c
combined developmental effects 24
adult paresthesia
5th
3.8
3.6
5.5
2.5
0.64
Bootstrap Percentile
25th
10
8.0
13
5.3
1.5
50th
19
14
20
8.7
2.8
75th
33
25
31
14
5.9
95th
63
58
57
24
>500
a neurological test scores > 3 in children exposed in utero
b walking after 18 months
c talking after 24 months
d threshold for the occurrence of any developmental effect
All bootstraps stabilized within 4000 iterations as measured by the change in the 5th and 95th
percentiles and the ratio of those percentiles. The largest change from 2000 to 4000 iterations in any
of the stability measurements was 3.5%.
D.3.2 Response Classification Uncertainty
Table D-6 gives percentiles of the cumulative bootstrap threshold distributions resulting from
the consideration of response classification error for each of the endpoints. The distributions were a
result of 5000 iterations of the bootstrap procedure. Frequency plots for these distributions are shown
in Figures D-13 and D-14 for late walking and adult paresthesia, respectively.
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Table D-6
Bootstrap Threshold Distributions in ppm Mercury in Hair with Inclusion of
Response Classification Error
Endpoint
neurological effects
(CEa = 10%)
neurological effects
(CE = 20%)
late walking
(CE = 25%)
late walking
(CE =-50%)
late talkingb
adult paresthesia0
5th
2.6
2.3
0.74
0.79
2.1
0.50
Bootstrap Percentile
25th
8.4
7.4
2.5
2.6
5.9
1.5
50th
16
15
5.7
5.0
12
3.3
75th
30
30
15
12
25
15
95lh
71
>600
>600
>600 "
99
>600
a classification error assumption for responses at boundary of minimum value defining a positive response
b 50% classification error (boundary responses)
c
5% classification error assumed for all responses above background
Figure D-13
Bootstrap Threshold Distribution for Late Walking with Response-Classification Error
5000 iterations
0 1
pt« H» in h»ir (loglO)
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Figure D-14
Bootstrap Threshold Distribution for Adult Paresthesia with Response-Classification Error
5000 it»z«tion»
500-,
0 1
Sim HB in hair (logio)
The primary result of the assumptions of response classification error was an increase in the
number of bootstrap samples resulting hi a nonsignificant log-probit regression slope as shown in
Table D-7. Late walking and adult paresthesia, for which over 20% of the regression slopes were
nonsignificant, were the most sensitive to classification error. Bootstrap confidence intervals of less
than 60% were the largest that excluded nonsignificant log-probit slopes for both endpoints. 6.1% of
the slopes for neurological effects were nonsignificant when a 20% classification error was assumed;
there was little effect when the error estimate was reduced to 10%. Only 4.6% of the slopes for late
walking were nonsignificant with a classification error of 50%; the width of the 90% confidence
interval, however, increased by a factor of 4. Eliminating late walking from the combined
developmental effects resulted in a 53% increase in the median bootstrap threshold to 12 ppm; the 5th
and 95th percentiles were increased to 2.7 and 32 ppm, respectively.
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Table D-7
Percentage of Bootstrap Thresholds Resulting from
Nonsignificant Log-Probit Regression Slopes
Endpoint
neurological effects
late walking
late talking
adult paresthesia
standard1
regression
0.4
0.8
0.2
7.5
classification error
regression
CE2 = 10% 2.1
CE = 20% 6.1
CE = 25% 10.9
CE = 5,0% 21.9
CE = 50% 4.6
CE = 5%3 20.9
a no classification error
b classification error assumption
c all observations
D.3.3 Dose Conversion Monte Carlo Simulation and Sensitivity Analysis
Table D-8 shows the results of the Monte Carlo simulation of the dose conversion factor
(DCF in equation 5) for different correlation assumptions. Standard assumptions (scenario 1, Table
D-8) were the distribution assignments and correlations described in section D.2.2.2 and summarized
in Table D-2. Scenario 2 hi Table D-8 assumed that all the variables hi equation 5 were independent.
Scenarios 3 and 4 included the standard assumptions and an additional assumption that an increased
residence time of methylmercury hi the blood contributed to the susceptibility of the most sensitive
individual. Scenario 3 restricted t% to the upper half of the standard distribution, representing a
moderate association of half-life and susceptibility, while scenario 4 represented a stronger association,
restricting tiA to the upper 10% of the standard distribution.
Figure D-15 shows the dose conversion frequency distributions for the standard dose
conversion factor distribution (scenario 1, Table D-8). The values on the abscissa are given in ppm
mercury hi hair/(mg/kg-day) hi Iog10 units. The distributions hi Table D-8 and Figure D-15 represent
the uncertainty hi the ratio of the exposure level (hi mg/kg-day) to hair mercury concentration for the
most sensitive individual of the exposed population. The nominal dose conversion factor is defined
here as the median value of the standard simulation (scenario 1, Table D-8). The median value for the
standard simulation was 8.0 x 10"5 with a 90% confidence interval spanning a 3.57-fold range. The
corresponding dose conversion value used hi the derivation of the methylmercury RfD was 9.8 x 10"5.
That is, the methylmercury RfD would change very little if calculated using the median of the
simulated dose conversion distribution. Using the nominal dose conversion factor, an exposure level
of 1 x 10"4 mg/kg-day corresponds to a hair mercury concentration of 1.25 ppm, with a 90%
confidence interval of 0.69 ppm to 2.36 ppm.
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Table D-8
Dose Conversion Factor Monte Carlo Simulation Output for
Different Correlation Assumptions in mg/kg-day
Scenario
1) standard correlations'1
2) no correlations
3) t^ > 53 daysb (std. correlations)
4) t^ > 84 days0 (std. correlations)
Percentile
5th . 25th 50th
4.2 x ID'5
2.6 x 1(T5
3.0 x 10'5
2.2 x 10'5
6.2 x 10"5
5.0 x 10"5
4.7 x lO'5 .
3.4 x 10'5
8.0 x 10'5
7.9 x ID"5
6.2 x lO'5
4.6 x lO'5
75th
1.0 x lO'4
1.2 x 10'4
8.3 x 10'5
6.1 x 10'5
95th
1.5 x 10'4
2.3 x 10'4
1.3 x 10'4
9.4 x ID'5
a hb correlated with t^ (r = -0.5); f correlated with bw (r = -0.47); V correlated with bw (r = +0.57)
b 50th percentile of t^
c 90th percentile of tVl
Figure D-15
Dose Conversion Distribution (standard assumptions)
-4.0
flo»10)
Monte Carlo simulation of equation 4 with HgH = 1
Standard assumptions as in Table D-2
The result of assuming correlations between input variables in equation 5 (Table D-8, scenario
1) is a 60% reduction of the width of the 90% confidence interval compared to assuming total
independence of inputs (scenario 2). Conversely, restricting t1/4 to the upper half of the distribution
(correlating susceptibility and t^) resulted in increased uncertainty around the DCF and lower dose
conversion estimates. Reductions in the median dose conversion estimate were 22% and 42% for a
moderate (tt/4 > 53 days) and a strong (t% > 84 days) association of ty2 and susceptibility, respectively;
June 1996
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the increase in the width of the 90% confidence interval, as measured by the ratio of the 95th and 5th
percentiles, was about 20% in both cases.
Table D-9 shows the relative contribution of each dose conversion input variable to the
variance of the Monte Carlo simulation output for selected scenarios from Table D-8". It can be seen
from Table D-9 that hb contributed the most to the variance of the output across the scenarios, while
bw, V and A contributed relatively little. The relative contribution to the output variance of t% and f
was highly sensitive to the correlation assumptions.
Table D-9
Sensitivity Analysis for Dose Conversion Monte Carlo Simulation:
Contribution of Each Input Variable to Output Variance (%)
Input Variable
hb
t*i
f
V
bw
A
Scenario
no correlations standard alternate t^
correlations21
47.9
26.7
16.3
4.7
4.3
0
46.5
7.9
33.3
9.6
2.4
0.3
60.4
0.0
29.0
8.3
1.9
0.4
hb correlated with t^ (r=-0.5); f correlated with bw (r=-0.47); V correlated with bw
(r=+0.57)
' t^ > 53 days
The variability of the dose conversion simulation was somewhat less than the contribution
from the bootstrap procedure. The widths of the 90% bootstrap confidence intervals on the thresholds
(in ppm mercury in hair) ranged from 1.1 to 1.3 orders of magnitude (12-20 fold difference in the 5th
and 95th percentiles from Table D-5). The width of the 90% confidence interval for the standard dose
conversion simulation spanned 0.55 orders of magnitude (Table D-8), or about 18-30% of that for the
bootstrap confidence intervals.
D.3.4 Estimation of Ingestion Thresholds
The distributions given in Table D-7 were used to obtain dose-conversion confidence intervals
for specific threshold estimates. Table D-10 gives values at selected percentiles for the distribution of
dose-conversion uncertainty around the median ingestion threshold estimates. The values in Table
D-10 are given in units of 10"4 mg/kg-day as a convenience for comparison with the RfD of 1 x 10"4
mg/kg-day. The distributions in Table D-10 were determined by multiplying the appropriate dose
conversion distribution from Table D-7, as noted hi Table D-10, by the median bootstrap threshold
estimates for each of the endpoints given in Table D-5. That is, the distributions in Table D-10
represent the output of equations 5 or 6 with Hgh equal to the median of the indicated bootstrap
June 1996
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distribution. As an example, the distribution for developmental neurological effects in Table D-10 was
a result of multiplying the 5th, 25th, 50th, 75th and 95th percentiles of the standard DCF distribution by
the median bootstrap threshold (19 ppm mercury in hair) for developmental neurological effects. The
duration-adjusted adult paresthesia distribution was the dose conversion distribution for adult
paresthesia given in Table D-ll divided by 4.7 (UD). Table D-ll is the equivalent- of Table D-8 for
the 5th percentile bootstrap threshold estimates from Table D-5.
Table D-10
Dose Conversion Distributions for Median Ingestion Threshold Estimates
in mg/kg-day (x 1 53 days)b
adult paresthesia*
duration-adjusted adult paresthesia0
Percentile
5m 25m 50m 75th 95th
7.9
6.1
8.4
3.7
2.6
1.2
0.26
12
9.0
12
5.4
4.0
1.7
0.36
15
12
16
7.0
5.4
2.2
0.47
19
15
20
8.7
7.2
2.8
0.60
27
21
29
13
11
4.0
0.86
a standard assumptions (scenario 1, Table D-9)
b scenario 3, Table D-9
0 adult paresthesia distribution divided by UD
Table D-ll
Dose Conversion Distributions for 5th Percentile Ingestion Threshold Estimates
in mg/kg-day (x 104)
Endpoint
neurological effects*
late walking*
late talking*
combined developmental effects*
combined developmental effects'1
(t% > 53 days)
adult paresthesia3
duration-adjusted adult paresthesia0
standard assumptions (scenario 1, Ta
b scenario 3, Table D-9
Percentile
5th 25th 50th 75th 95th
1.6
1.5
2.3
1.0
0.74
0.27
0.058
2.4
2.2
3.4
1.5
1.1
0.40
0.086
3.1
2.9
4.4
2.0
1.5
0.52
0.11
3.8
3.6
5.5
2.5
2.0
0.65
0.14
ble D-9)
5.6
5.2
8.0
3.6
3.1
0.94
0.20
adult paresthesia distribution divided by UD
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The 5% and 95% columns in Tables D-10 and D-ll represent the 90% confidence intervals for
specific percentiles of the thresholds expressed as ingestion levels in mg/kg-day. For example, with
respect to the neurological effects distribution in Table D-10, there is 90% confidence that the true
median threshold for neurological effects is between 7.8 x 10"4 and 2.7 x 10"3 mg/kg-day. Similarly,
there is 90% confidence that the true 5th percentile of the neurological effects threshold distribution is
between 1.4 x 10"4 and 4.8 x 10"4 mg/kg-day (Table D-ll). The median ingestion threshold for
duration-adjusted adult paresthesia was 1 x 10"4 mg/kg-day, with a 90% confidence interval of 2.3 x
10"5 mg/kg-day to 4 x 10"5 mg/kg-day.
Figure D-16 is a plot of the cumulative bootstrap threshold distribution for combined
developmental effects multiplied by values of the dose conversion distribution at selected percentiles.
The plots from left to right in Rgure D-16 represent different realizations of the distribution of
ingestion thresholds based on the relative likelihood of specific values of the dose conversion factor
(5 , 50th and 95th percentiles). The horizontal box and whisker plot corresponds to the dose
conversion distribution multiplied by the median of the bootstrap threshold distribution for combined
developmental effects as given in Table D-10; the box is the interquartile range (25th to 75th-
percentiles) and the whiskers are the 5th and 95th percentiles. Figure D-17 is the same plot for
combined developmental effects with the assumption that t^ is greater than 53 days (Table D-8,
scenario 3). Figures D-18 to D-22 are the equivalent plots for the individual-effect thresholds.
Ingestion threshold distributions for adult effects are shown in Figures D-21 and D-22. Figure D-21 is
the ingestion threshold distribution for the adult paresthesia observed in the Iraqi cohort. Figure D-22
is the ingestion threshold distribution for duration-adjusted adult paresthesia resulting from dividing the
adult paresthesia ingestion threshold distribution by UD. That is, Figure D-22 is the distribution in
Figure EK21 shifted to the left by a factor of 4.7.
June 1996 D-28 SAB REVIEW DRAFT
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Figure D-16
Cumulative Combined Developmental Effects Ingestion Threshold Distribution
0.8-
0.*-
0.0-1
0.00005 0.0001 0.00050 0.001
threshold (ng/kg-day)
0.00500
p = cumulative probability
Figure D-17
Cumulative Combined Developmental Effects Ingestion Threshold Distribution
____^__ (^> 53 days)
1.0-
0.4
O.J-
o.oJ
chz««bol
-------
Figure D-18
Cumulative Neurological Effects Ingestion Threshold Distribution
0.4
0.0-1
ing«acloa th£«*bold diflcrlbutloa
dM* oonv»r«ian distribution
IfD
0.00005 0.0001 0.00050 0.001
thzMhold (ng/kg-day)
p = cumulative probability
Figure D-19
Cumulative Late Walking Ingestion Threshold Distribution
o.o-i
0.00005 0.0001
0.00050 0.001
thiBabold (ng/kg-day
0.00500
p = cumulative probability
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Figure D-20
Cumulative Late Talking Ingestion Threshold Distribution
O.I
0.+
0.3-
threshold distribution
—ffl— doM enav*»ioa dUtxitoution
B£D
0.00005 0.0001 0.00050 0.001
threshold (ag/kg-day)
p = cumulative probability
Figure D-21
Cumulative Adult Paresthesia Ingestion Threshold Distribution
(no exposure duration adjustment)
0.4.
O.J-
o.o-i
0.00001 0.00005 0.0001 0.00050 0.001
thiMhold (as/kg- day)
p = cumulative probability
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Figure D-22
Cumulative Duration-Adjusted Adult Paresthesia Threshold Distribution
0.4
ehzMhold dijcrtbucion
5«-S 5«-S
threshold (ing/kg-day)
p = cumulative probability
The uncertainty around the location of the RfD within each of the distributions shown in
Figures D-17 to D-22 is indicated by the vertical line at 1 x 10"4 mg/kg-day; this uncertainty came
from the dose conversion variability. As an example, the RfD fell between the 0.035 and 4.5
percentiles of the combined developmental effects threshold distribution with 90% confidence as
determined by the intersection of the RfD line with the 5% and 95% ingestion threshold curves
(Rgure D-16). The median estimate of the location of the RfD in this distribution was the 0.25
percentile. Similarly, the RfD fell between the 39th and 91st percentile of the duration-adjusted adult
paresthesia threshold distribution with the median at the 75th percentile (Figure D-22). The RfD was
at the 18th percentile of the adult paresthesia threshold distribution and below the 1st percentile for all
of the other threshold distributions.
D.4 Discussion of Uncertainty Analysis
Because the Iraqi cohort is considered to be a sensitive subgroup, as defined in the RfD
methodology, the output distributions of the analysis are meant to reflect the uncertainty around an
estimate of the thresholds for effects in humans including sensitive individuals. The results for each
endpoint should be interpreted as the distribution of the uncertainty around the human population
threshold. The results should not be interpreted as the distributions of individual thresholds within the
population. Estimates of risk above the threshold cannot be obtained from this analysis.
This analysis has attempted to incorporate all areas of uncertainty involved in the derivation of
the methylmercury RfD in Chapter 6 of Volume IV of this report. The 10-fold uncertainty factor (UF)
includes a 3-fold (10°-5) factor for human variability and a 3-fold factor for the lack of reproductive
and chronic studies. The UF for human variability includes a consideration of both susceptibility and
variation in methylmercury blood half-lives. The bootstrap threshold confidence intervals incorporate
the former, and the latter is explicitly modeled (t^) in the dose conversion Monte Carlo analysis.
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Uncertainty arising from the lack of chronic data is estimated by UD; this uncertainty was a point
estimate only, as the data were inadequate for defining a distribution for UD. Because UD was derived
as a scaling factor for adult effects it is not directly comparable to the UF for chronic effects used in
the derivation of the RfD, which was based on developmental endpoints. The only uncertainty
included in the RfD and not addressed here is the uncertainty attributed to the lack of a reproductive
study; there are no appropriate data for the estimation of this uncertainty. In general, reproductive
NOAELs are slightly lower than developmental NOAELs for other substances, but much higher than
chronic NOAELs (Dourson, Knauf and Swartout, 1992). That is, the uncertainty in the thresholds is
expected to be much less for lack of a reproductive study than for lack of a chronic study.
The uncertainty analysis presented in this appendix was limited to only those data and
formulae directly related to the derivation of the methylmercury RfD. Other data sets or models were
not considered. A few sources of uncertainty in the data used to derive the methylmercury RfD have
not been included in this analysis. Exposure classification error arising from uncertainty as to the
correspondence of actual exposure and critical exposure period cannot be estimatea from the data as
published in Marsh et al., 1987. This source of uncertainty could be a major contributor to the "
apparent extreme variability of susceptibilities in the Iraqi cohort. Variability in the interpretation of
the definition of a response was not estimated in this analysis. That is, there would have been some
differences in how individuals interpreted what constituted first walking or first talking, probably more
so for the latter. The classification errors assumed for this analysis only account for uncertainty in the
timing of the event given an unequivocal positive response. Also, the response decision points
defining an adverse effect were accepted uncritically. For example, changing the definition of late
walking to either greater than 16 months or greater than 20 months would have a significant effect on
the analysis. Measurement error for hair mercury concentrations has not been estimated for this
analysis; the necessary data are unavailable in the published reports (Marsh et al., 1987; Cox et al.,
1989). In addition, the results of this analysis are conditional on a specific representation of
population variability in the parameters of the dose conversion variables. That is, the choice of the
form, and parameters for the distributions assigned to each of the variables is largely a matter of
judgement. The particular set of parameters chosen for each distribution is only one option of a
number of possible choices; uncertainty as to the value of the parameters is not included in the
analysis.
The threshold analysis shows that adult paresthesia was the most sensitive individual effect
observed for the Iraqi cohort, particularly when adjusted for the effects of continuing exposure. That
is, in this analysis, paresthesia in adults was estimated to be observable at a lower exposure than the
developmental endpoints. The absence of an observed background incidence for paresthesia in the
Iraqi cohort partially contributed to the low threshold estimates. A background incidence for
paresthesia would be expected in the general population. The adult paresthesia bootstrap thresholds
were also the most unstable as measured by the frequency of nonsignificant slopes. The RfD fell
between the 39th and 91st percentiles of the duration-adjusted adult paresthesia threshold distribution,
a considerably larger range than that for any of the developmental effects. On the average, the RfD
fell below the 1st percentile for all developmental effects, with only a 5% chance that it was as high as
the 16th percentile.
The response-classification uncertainty analyses were based on hypothetical classification error
rates. Assumptions of 50% response-classification error for late walking and late talking were worst-
case for those values immediately adjacent to the response decision point value for any given effect.
That is, for late walking, the values of 18 or 20 months for first walking and 24 or 26 months for first
talking were assumed to be equally likely, resulting in misclassification 50% of the time. This would
require an uncertainty in recall of these events of at least 2 months, which is not unlikely in this
June 1996 D-33 SAB REVIEW DRAFT
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particular situation. The actual classification error was likely to be somewhat less than 50%,
particularly as the large number-of observations for late walking at 18 months (22 of the 81
individuals) suggests that 18 months may have been used as an upper bound in some cases. The
response-classification error assumptions for late walking and late talking were best-case for all other
values as no error is assumed. Even with a 25% classification error, however, the results of the
response-classification uncertainty analysis indicate that the late walking endpoint was unreliable as a
measure of methylmercury toxicity. The exclusion of this endpoint would not have a very large
impact on the combined developmental effects threshold distribution, increasing the thresholds by
about 50%. Although the late talking threshold distribution is not grossly affected by response-
classification error, variability in interpretation of the definition of the endpoint (first talking) likely
would have been greater than that for walking; this uncertainty was not estimated in this analysis. The
neurological effects thresholds were least sensitive to classification error, assuming that the true error
was closer 10% than 20%. The assumption seems reasonable given the much greater objectivity of the
measurement of, the effect Adult paresthesia was the most sensitive to classification error, showing
extreme variability in the threshold estimates with a classification error rate as low as 5% (all
observations). These results suggest that strong conclusions based on the late walking and adult
paresthesia endpoints are unwarranted.
Results of the alternate scenarios (Table D-8) show that the primary effect of the correlation
assumptions among the dose conversion input variables was a fairly large reduction in the variance of
the Monte Carlo simulation output. The assumption of correlation of individual susceptibility and
half-life of methylmercury in the blood did not have a marked effect on the simulation except for a
42% reduction in the median when a strong correlation was assumed (tJ/4 > 84 days). The latter
scenario probably represented a worst-case situation although no data were found that directly address
the magnitude of the hypothetical correlation.
The sensitivity analysis indicates that the variables that contribute the most to the dose
conversion simulation variability are the hair.blood ratio (hb), the half-life of methylmercury in the
blood (ti/4) and the fraction of absorbed methylmercury found in the blood (f). There is very little that
can be done to reduce the uncertainty in these variables because appropriate data directly applicable to
the Iraqi cohort are not available. These results could be of use in the experimental design and
collection of data for estimates of ingestion levels from hair concentrations in the future.
D.5 Conclusions of Analysis of Uncertainty Around Human Health Effects of Methylmercury
A major source of the variability was in the estimation of bootstrap thresholds from the Iraqi
cohort data as evidenced by the 12-20 fold difference in the 5th and 95th percentiles of the bootstrap
threshold distributions. The uncertainty arising from limited exposure duration contributed almost as
much, with a 12.5-fold difference in the 5th and 95th percentiles. The corresponding spreads in the
dose conversion distributions were 2.4-4.2 fold. Correlations between variables were important with
respect to the variance of the Monte Carlo simulations but were not well-defined by empirical data.
Additional areas of uncertainty remain to be modeled.
Of the developmental endpoints, the neurological effects, which are determined by a battery of
tests and do not depend on subject recall, would seem to be the most objective measure of
methylmercury toxicity. Late walking was not a reliable endpoint because of sensitivity to
classification error.
The RfD of 1 x 10"4 mg/kg-day is very likely well below the threshold for developmental
effects but may be above the threshold for exposure duration-adjusted adult paresthesia. Strong
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conclusions based on the latter result are not warranted because of the sensitivity of the.adult
paresthesia threshold to classification error and the general lack of data addressing the effects of
exposure duration.
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