United States
Environmental Protection
Agency
EPA-452/R-96-001a
June 1996
Air
Mercury Study
Report to Congress
Volume I:
Executive Summary
SAB REVIEW DRAFT
xvEPA
Office of Air Quality Planning & Standards
and
Office of Research and Development
c66Q 11-2-1
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MERCURY STUDY REPORT TO CONGRESS
VOLUME I:
EXECUTIVE SUMMARY
SAB REVIEW DRAFT
June 1996
U.S. Environmental Protection Agency
Region 5, Library (PI-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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DEDICATION
"K^ The U.S. EPA scientists who authored this report dedicate their efforts to the memory of their
- ^ colleague, Terry Clark. Terry began his career at the U.S. EPA in 1975, where he became a national,
;;c and then an international expert in the atmospheric transport of acid rain and toxic trace gases. Terry
^ designed the initial long-range transport analysis for the Mercury Study. The energy and creativity he
o brought to his work sustained him even through the final months of his illness when he continued to
-J work daily on this report. His honesty, intelligence and generosity of spirit are greatly missed. Terry
v^ Clark died on January 28, 1994.
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TABLE OF CONTENTS
Page
U.S. EPA AUTHORS ii
SCIENTIFIC PEER REVIEWERS iv
WORK GROUP AND U.S. EPA REVIEWERS v
LIST OF TABLES AND FIGURES , vi
LIST OF SYMBOLS, UNITS AND ACRONYMS '. . .* vii
1. THE MERCURY STUDY REPORT TO CONGRESS 1-1
2. MERCURY IN THE ENVIRONMENT . .' 2-1
3. FINDINGS OF THE MERCURY STUDY REPORT TO CONGRESS r 3-1
Sources Contributing to Mercury in the Environment 3-1
Inventory Approach and Uncertainties 3-4
Anthropogenic Emissions Summary 3-5
Trends in Mercury Emissions ' 3-5
Trends in Mercury Consumption 3-7
Assessment of Exposure 3-8
Human Health Effects of Methylmercury 3-16
How Much Methylmercury is Harmful to Humans? . 3-19
Levels of Methylmercury Exposure Addressed by the U.S. Food and Drug
Administration, World Health Organization and State Recommendations 3-22
Characterization of Risk to Human Populations 3-24
How Much Methylmercury Exposure is Harmful to Wildlife and What Are the
Effects? 3-37
4. MANAGEMENT ALTERNATIVES 4-1
Possible Control Strategies 4-1
Clean Air Act Provisions Applicable to Mercury Control 4-9
Ongoing Activities 4-11
5. RESEARCH NEEDS 5-1
APPENDIX A: SUMMARY OF THE SCIENTIFIC PEER REVIEW A-l
June 1966 i SAB REVIEW DRAFT
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U.S. EPA AUTHORS
Principal Authors:
Martha H. Keating
Office of Air Quality Planning and Standards
Research Triangle Park, NC
Kathryn R. Mahaffey, Ph.D.
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
Contributing Authors:
Robert B. Ambrose, Jr., P.E.
Ecosystems Research Division
National Exposure Research Laboratory
Athens, GA
William G. Benjey, Ph.D.
Atmospheric Sciences Modeling Division
Air Resources Laboratory
National Oceanic and Atmospheric
Administration
Research Triangle Park, NC
on assignment to the U.S. EPA National Exposure
Research Laboratory
O. Russell Bullock
Atmospheric Sciences Modeling Division
Air Resources Laboratory
National Oceanic and Atmospheric
Administration
Research Triangle Park, NC
on assignment to the U.S. EPA National Exposure
Research Laboratory
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
Terry Clark, Ph.D.a
Atmospheric Sciences Modeling Division
Air Resources Laboratory
National Oceanic and Atmospheric
Administration
Research Triangle Park, NC
David H. Cleverly
National Center for Environmental Assessment
Office of Research and Development
Washington, DC
Chris Cubbison, Ph.D
National Center for Environmental Assessment-
Cincinnati
Office of Research and Development
Cincinnati, OH
Stanley Durkee
Office of Research and Science
Integration
Washington, DC
* Deceased
June 1966
11
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U.S. EPA AUTHORS (continued)
Anne Fairbrother, Ph.D., D.V.M.
Environmental Research Laboratory-Corvallis
Corvallis, OR
currently with:
ecological planning and toxicology, inc.
5010 S.W. Hout St.
Corvallis, OR 97333
Beth Hassett-Sipple
Office of Air Quality Planning and Standards
Research Triangle Park, NC
James D. Kilgroe, Ph.D.
National Environmental Research Laboratory
Office of Research & Development
Research Triangle Park, NC
William H. Maxwell, P.E.
Office of Air Quality Planning and
Standards
Research Triangle Park, NC
Debdas Mukerjee, Ph.D.
National Center for Environmental Assessment-
Cincinnati
Office of Research and Development
Cincinnati, OH
John W. Nichols, Ph.D.
Mid-Continent Ecology Division
Office of Research and Development
Duluth, MN
Warren D. Peters
Office of Air Quality Planning and Standards
Research Triangle Park, NC
Anne A. Pope
Office of Air Quality Planning and Standards
Research Triangle Park, NC
David J. Reisman
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
Michael Troyer
Office of Research and Science Integration
Cincinnati, OH
June 1966
in
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SCIENTIFIC PEER REVIEWERS
Brian J. Allee, 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
TimEder
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 1966
IV
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WORK GROUP AND U.S. EPA 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 Reviewers:
Robert Beliles, Ph.D., D.A.B.T.
National Center for Environmental Assessment
Washington, DC
Eletha Brady-Roberts
National Center for Environmental Assessment
Cincinnati, OH
Dianne M. Byrne
Office of Air Quality Planning and Standards
Research Triangle Park, NC
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 1966
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LIST OF TABLES AND FIGURES
Page
Tables
3-1 Best Point Estimates of National Mercury Emission Rates by Category 3-6
3-2 Modeled Mercury Mass Budget in Metric Tons for 1989 Using the Specified Speciation
Profiles 3-11
3-3 WHO Data on Mercury in Hair 3-22
3-4 Body Weights and Fish Consumption Values Used in Exposure Modeling 3-25
3-5 Mercury Concentrations in the Top Ten Types of Fish Consumed by U.S. Residents . . . 3-29
3-6 Size of the Populations of Concern Consuming 100 Grams or More of Fi§h Per Day . . . 3-32
3-7 Wildlife Criteria for Mercury 3-38
4-1 Summary of Mercury Control Techniques for Selected Source Types 4-2
4-2 Potential Mercury Emission Reductions and Costs for Selected Source Categories 4-8
Figures
2-1 The Global Mercury Cycle 2-2
2-2 Cycling of Mercury in Freshwater Lakes 2-3
2-3 Example Aquatic Food Web ! . . 2-4
3-1 Comparison of Current and Pre-Industrial Mercury Budgets and Fluxes 3-2
3-2 Fate, Transport and Exposure Modeling Conducted in the Combined COMPDEP and
RELMAP Local Impact Analysis 3-9
3-3 Total Mercury Wet + Dry Deposition (Base Case) 3-10
3-4 Distribution of Mercury Concentrations in EPA-Sampled Fish Tissue Throughout the
U.S 3-26
3-5 Surface Water with pH < 5.5 and Anthropogenic Mercury Deposition 3-27
3-6 Distribution of Fish Consumption Rates of Various Populations 3-34
June 1966 vi SAB REVIEW DRAFT
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LIST OF SYMBOLS, UNITS AND ACRONYMS
ATSDR
BAF
bw
CAA
CH3Hg
(CH3)2Hg
CSFII
H.S
Hg°
Hg(II)
HgS
U.S. EPA
U.S. FDA
GACT
g
HAP
kg
km
MACT
Mg
ORD
Pg
ppm
RfC
RfD
Mg
WC
WHO
WHO/IPCS
Agency for Toxic Substances and Disease Registry
Bioaccumulation factor
Body weight
Clean Air Act as Amended in 1990
Monomethylmercury
Dimethylmercury
U.S. Department of Agriculture's Continuing Surveys of Individual Food
Consumption
Hydrogen sulfide
Elemental mercury
Mercuric ion (divalent mercury)
Mercuric sulfide
U.S. Environmental Protection Agency
U.S. Food and Drug Administration
Generally available control technology
Gram
Hazardous Air Pollutant
Kilogram (1,000 grams)
Kilometer (1,000 meters)
Maximum achievable control technology
Megagram (one million grams or one metric ton)
U.S. EPA Office of Research and Development
Picogram (10~12 gram)
Part per million
Reference concentration
Reference dose
Microgram (10~6 gram)
Wildlife criterion
World Health Organization
World Health Organization's International Programme for Chemical Safety
Year
June 1966
vu
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1. THE MERCURY STUDY REPORT TO CONGRESS
The Clean Air Act Amendments of 1990 (CAA) established section 112(n)(l)(B) which
requires the United States Environmental Protection Agency (U.S. EPA) to study the impacts of
mercury air pollution. In particular, section 112(n)(B) specifies the following:
The Administrator shall conduct, and transmit to the Congress not later than 4 years
after the date of enactment of the Clean Air Act Amendments of 1990, a study of
mercury emissions from electric utility steam generating units, municipal waste
combustion units, and other sources, including area sources. Such study shall consider
the rate and mass of such emissions, the health and environmental effects of such
emissions, technologies which are available to control such emissions, and the costs of
such technologies.
\
\
The U.S. EPA designed the Mercury Study to address many different (but linked) types of
information:
data on type, sources, and trends in emissions;
evaluation of the atmospheric transport of mercury to locations distant from emission
sources;
assessment of potential impacts of mercury emissions close to the source;
identification of major pathways of exposure to humans and non-human biota;
identification of the types of human health consequences of mercury exposure and the
amount of exposure likely to result in adverse effects;
evaluation of mercury exposure consequences for ecosystems and for non-human
species;
identification of populations especially at risk from mercury exposure due to innate
sensitivity or high exposure; and
estimates of control technology efficiencies and costs.
The Report used the above types of information to assess the impact of emissions to air of
mercury from a variety of sources. This assessment included judgments as to the potential hazard to
humans and wildlife of methylmercury exposure which (as is described in succeeding sections) is
largely through the consumption of contaminated fish.
There was no attempt in this Report to do a comparative risk/benefit analysis of fish as an
important source of protein and calories in the diet of U.S. populations. Such an analysis would be
beyond the scope of the CAA mandate. As emphasized in succeeding sections, the typical U.S.
consumer of fish is not in danger of consuming harmful levels of methylmercury and is not being
advised to reduce fish consumption.
This Mercury Study Report to Congress fulfills the mandate of section 112(n)(l)(B). The
report is in seven volumes:
June 1996 1-1 SAB REVIEW DRAFT
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Volume I: Executive Summary
Volume II: An Inventory of Anthropogenic Mercury Emissions in the United States
Volume III: An Assessment of Exposure from Anthropogenic Mercury Emissions in
the United States
Volume IV: Health Effects of Mercury and Mercury Compounds
Volume V: An Ecological Assessment of Anthropogenic Mercury Emissions in the
United States
Volume VI: Characterization of Human Health and Wildlife Risks from
Anthropogenic Mercury Emissions in the United States
Volume VII: An Evaluation of Mercury Control Technologies and Costs.
The various analyses documented in this Report were designed and conducted in accordance
with accepted guidelines and procedures. For example, the human health risk assessment performed
for this Report follows published Guidelines foi;Risk Assessment (including guidelines on Exposure
Assessment, Developmental Toxicity, Carcinogenicity and Germ Cell Mutagenicity) and uses
established methodologies"for quantitative assessment of general systemic toxicity (e.g., in the
calculation of reference doses (RfDs) and reference concentrations (RfCs)). Moreover, the assessment
of ecological effects, presented in Volume V, follows U.S. EPA's Framework for Ecological Risk
Assessment. Criteria values for protection of piscivorous wildlife were developed using the
methodology developed for the Great Lakes Water Quality Initiative.
In 1994, the National Research Council of the National Academy of Sciences, in Science and
Judgment in Risk Assessment, recommended several areas in which U.S. EPA could improve its risk
assessment and risk characterization practices. These recommendations are listed below along with a
description of how they were implemented in this Report.
Provide an understanding of the type and magnitude of an adverse effect that a
specific chemical or emission could cause under particular circumstances. The Report
characterizes both the type and magnitude of health and ecological effects associated
with airborne emissions of mercury from anthropogenic sources.
Validate methods and models. All models used for the Report were critiqued by
scientific experts and model predictions were compared to measured mercury levels
using the most appropriate data available.
Describe the basis for default options. All assumptions are described and justified
based on available data. Where appropriate, exposure models were modified to
improve assumptions and to focus on areas of prediction where use of model
assumptions is most justified.
Articulate and prioritize data needs. The Report includes a section on Research Needs
hi each volume.
Distinguish between variability and uncertainty. The Report provides discussions that
attempt to make these distinctions for the risk results.
Perform formal uncertainty analyses. Uncertainty analyses were formally conducted
for the dose-response and exposure assessment steps of the study, and were implicit in
weight-of-evidence processes used in the hazard identification step of the human health
risk assessment and the problem formulation phase of the ecological risk assessment.
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Uncertainty also was analyzed quantitatively in other components of the study, such as
in the calculation of bioaccumulation factors and the RfD for methylmercury.
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2. MERCURY IN THE ENVIRONMENT
As a chemical element, mercury cannot be created or destroyed. The same amount has existed
on the planet since the earth was formed. Mercury, however, can cycle in the environment as part of
both natural and human (anthropogenic) activities. Measured data and modeling results indicate that
the amount of mercury mobilized and released into the biosphere has increased since the beginning of
the industrial age.
Several types of emission sources contribute to the total atmospheric loading of mercury. .
Once hi the air, mercury can be widely dispersed and transported thousands of miles from likely
emission sources. Studies indicate that the residence time of mercury in the atmosphere may be on the
order of a year, allowing its distribution over long distances, both regionally and globally, before being
deposited to the earth* Even after it deposits, mercury commonly is emitted back to the atmosphere
either as a gas or in association with particulates to be re-deposited elsewhere. Mercury undergoes a
series of complex chemical and physical transformations as it cycles among the atmosptiere, land, and
water. Humans, plants and animals are routinely exposed to mercury and accumulate it during this
cycle, potentially resulting in a variety of ecological and human health impacts.
Properties and Uses of Mercury
Elemental mercury metal is a heavy, silvery-white liquid at typical ambient temperatures and
atmospheric pressures. The vapor pressure of mercury metal is strongly dependent on temperature,
and it vaporizes readily under ambient conditions. Most of the mercury encountered in the atmosphere
is elemental mercury vapor.
Mercury can exist in three oxidation states: Hg° (metallic), Hg22"t" (mercurous) and Hg2+
(mercuric). The properties and behavior of mercury depend on the oxidation state. Most of the
mercury in water, soil, sediments, or biota (i.e., all environmental media except the atmosphere) is in
the form of inorganic mercury salts and organic forms of mercury.
Mercury is widely used because of its diverse properties. In very small quantities, mercury
conducts electricity, responds to temperature and pressure changes and forms alloys with almost all
other metals. Mercury serves an important role as a process or product ingredient in several industrial
sectors.
In the electrical industry, mercury is used in components such as fluorescent lamps, wiring
devices and switches (e.g., thermostats) and mercuric oxide batteries. Mercury also is used in
navigational devices, instruments that measure temperature and pressure and other related uses. It also
is a component of dental amalgams used in repairing dental caries (cavities).
In addition to specific products, mercury is used in numerous industrial processes. The largest
quantity of mercury used in manufacturing in the U.S. is the production of chlorine and caustic soda
by mercury cell chlor-alkali plants. Other processes include amalgamation, use in nuclear reactors,
wood processing (as an anti-fungal agent), use as a solvent for reactive and precious metals, and use as
a catalyst. Mercury compounds are also frequently added as a preservative to many pharmaceutical
products.
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The Role of Atmospheric Releases and Processes
A schematic of the most recent conceptualization of the current global mercury cycle is
presented in Figure 2-1. As indicated in this figure, mercury is emitted to the atmosphere by a variety
of sources, dispersed and transported in the air, deposited to the earth, and stored in or transferred
between the land, water, and air.
" Figure 2-1
The Global Mercury Cycle
Anthropogenic
Evasion
(Re-emitted
Anthropogenic
&'
Natural)
Re-emitted
Anthropogenic
&
Natural
Local & Regional
Deposition
Global Terrestrial
Deposition
Global Marine
Deposition
Source: Adapted from Mason, R.P., Fitzgerald, W.F., and Morel, M.M. 1994. The Biogeochemical Cycling of
Elemental Mercury: Anthropogenic Influences. Geochim. Cosmochim. Acta, in press.
Mercury deposits on the earth in different ways and at different rates, depending on its
physical and chemical form. Mercuric species are subject to much faster atmospheric removal than
elemental mercury. Mercuric mercury bound to airborne particles and in a gaseous form is readily
scavenged by precipitation and is also dry deposited (that is, deposited in the absence of precipitation).
In contrast, elemental mercury vapor has a strong tendency to remain airborne and is not susceptible to
any major process resulting hi direct deposition to the earth's surface. Although much uncertainty still
exists, several studies indicate that the relative contribution of mercury loadings to land and water
from atmospheric deposition can be substantial.
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Numerous studies of elevated mercury levels in remote locations, where atmospheric transport
and deposition appears to be the primary mechanism for contamination, provide further evidence of the
importance of the atmospheric pathway.
Fate and Transport of Mercury in the Environment
The movement and distribution of mercury in the environment can be confidently described
only in general terms. There has been increasing consensus on many, but not all, of the detailed
behaviors of mercury in the environment. The depiction of the mercury cycle in Figure 2-2 illustrates
the major transfer and transformation processes expected to occur. These processes include a number
of infinite and/or indefinite loops.
Figure 2-2
Cycling of Mercury in Freshwater Lakes
CH3HgCH3
CH3HgCH3X
xxxxxxxxxxx
xxxxxxxxxxx
xxxxxxxxxxx
xxxxxxxxxxx
xxxxxxxxxxx
Source: Adapted from Winfrey, M.R. and J.W.M. Rudd. 1990. Review -- Environmental Factors Affecting the
Formation of Methylmercury in Low pH Lakes. Environ. Toxicol. Chem. 9:853-869.
Mercury cycling and partitioning in the environment are complex phenomena that depend on
numerous environmental parameters. The following points generally describe the key factors that
affect the fate and transport of mercury in the environment.
The form of mercury in air affects both the rate and mechanism by which it deposits
11 to earth.
Wet deposition apparently is the primary mechanism for transporting mercury from the
atmosphere to surface waters and land.
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Once in aquatic systems, mercury can exist in dissolved or particulate forms and can
undergo a number of chemical transformations (see Figure 2-2).
Contaminated sediments at the bottom of surface waters can serve as an important
mercury reservoir, with sediment-bound mercury recycling back into the aquatic
ecosystem for decades or longer.
Mercury has a long retention time in soils. As a result, mercury that has accumulated
in soils may continue to be released to surface waters and other media for long periods
of time, possibly hundreds of years.
Potential Exposure Pathways
Plants, animals and humans can-be exposed to mercury by direct contact with contaminated
environmental media or ingestion of mercury-contaminated water and food.
Generally, mercury accumulates up aquatic food chains so that organisms in higher trophic
levels have higher mercury concentrations. An example aquatic food web is shown in Figure 2-3. At
the top trophic levels are piscivores, such as humans, bald eagles, cormorants, herring gulls and other
fish-eating species. The larger wildlife species (e.g., bald eagle, otter) can prey on fish that occupy
high trophic levels, such as trout and salmon, which in turn feed on smaller "forage" fish. Smaller
piscivorous wildlife (e.g., kingfishers, ospreys) tend to feed on the smaller forage fish, which in turn
feed on zooplankton or benthic invertebrates. Zooplankton feed on phytoplankton and the smaller
benthic invertebrates feed on algae and detritus. Thus, mercury is transferred and accumulated through
several trophic levels.
Figure 2-3
Example Aquatic Food Web
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Mercury Methylation and Bioaccumulation
Methylation of mercury is a key step in the entrance of mercury into food chains. The
biotransformation of inorganic mercury species to methylated organic species in water bodies can
occur in the sediment and the water column. All mercury compounds entering an aquatic ecosystem,
however, are not methylated; demethylation reactions as well as volatilization of dimethylmercury
decrease the amount of methylmercury available in the aquatic environment There is a large degree
of scientific uncertainty regarding the rate at which these reactions take place. There is general
scientific agreement however that there is significant variability between waterbodies concerning the
environmental factors that influence the methylation of mercury.
Nearly 100% of the mercury that bioaccumulates in fish tissue is methylated. A relationship
exists between the methylmercury content in fish and lake pH, with higher methylmercury content in
fish tissue typically found in more acidic lakes. Numerous factors in addition to low pH can influence
the bioaccumulation of mercury in aquatic biota. These include the length of the aquatic food chain,
temperature and dissolved organic material. Physical and chemical characteristics of a watershed
affect the amount of mercury that is translocated from soils to water bodies. Interrelationships
between these factors are poorly understood, however, and there is no single factor (including pH) that
has been correlated with mercury bioaccumulation in all cases examined.
Mercury accumulates in an organism when the rate of uptake exceeds the rate of elimination.
Although all forms of mercury can accumulate to some degree, methylmercury generally accumulates
to a greater extent than other forms of mercury. Inorganic mercury can also be absorbed but is
generally taken up at a slower rate and with lower efficiency than is methylmercury. Elimination of
methylmercury takes place very slowly resulting in tissue half-lives (i.e., the time in which half of the
mercury in the tissue is eliminated) ranging from months to years. Elimination of methylmercury from
fish is so slow that long-term reductions of mercury concentrations in fish are often due mainly to
growth of the fish. In comparison, other mercury compounds are eliminated relatively quickly
resulting in reduced levels of accumulation.
Methylmercury production and accumulation in the freshwater ecosystem is an efficient
process for accumulating mercury which can then be ingested by piscivores including birds, non-
human mammals and people. In addition, methylmercury generally comprises a relatively greater
percentage of the total mercury content at higher trophic levels. Accordingly, mercury exposure and
accumulation is of particular concern for animals at the highest trophic levels in aquatic food webs and
for animals and humans that feed on these organisms.
Human Exposure Pathways and Health Effects
Humans are most likely to be exposed to methylmercury through fish consumption. Exposure
may occur through other routes as well (e.g., the ingestion of methylmercury-contaminated drinking
water and food sources other than fish, and dermal uptake through soil and water); however, the fish
consumption pathway dominates these other pathways for people who eat fish.
There is a great deal of variability among individuals in fish-eating populations with respect to
food sources and fish consumption rates. The populations most highly exposed are those located in
areas where the concentration of methylmercury in freshwater fish is elevated, in part as the result of
anthropogenic releases from industrial and combustion sources. Methylmercury exposure rates among
children who consume fish are predicted to be higher than for adults who consume fish because of
their lower body weight. Humans could also be exposed to inorganic mercury through inhalation, or
June 1996 2-5 SAB REVIEW DRAFT
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consumption of contaminated water or food. Inhalation exposure to elemental mercury is largely
confined to humans whose occupations put them in contact with elemental mercury vapors.
Mercury is a known human toxicant which has been associated with occupational exposure
(for example, "Mad Hatters' Disease") and with exposure through consumption of contaminated food.
Studies in humans and in experimental animals are described in Volume IV of the Mercury Study
Report to Congress. Generally, the effect seen at the lowest exposure level for elemental and
methylmercury is neurotoxicity. The range of neurotoxic effects can vary from subtle decrements in
motor skills and sensory ability to tremors, inability to walk, convulsions and death.
Environmental Impacts
Effects of mercury on fish include death, reduced reproductive success, impaired growth and
development and behavioral abnormalities. Exposure to mercury can also cause adverse effects in
plants, birds and mammals. Reproductive effects are the primary concern for avian mercury poisoning
and can occur at dietary concentrations well below those which cause overt toxicity. Sublethal effects
of mercury on birds include liver damage, kidney damage, and neurobehavioral effects. Effects of
mercury on plants include death and sublethal effects. Sublethal effects on aquatic plants can include
plant senescence, growth inhibition and decreased chlorophyll content. Sublethal effects on terrestrial
plants can include decreased growth, leaf injury, root damage, and inhibited root growth and function.
Although clear causal links between mercury contamination and population declines in various
wildlife species have not been established, mercury may be a contributing factor to population declines
of the endangered Florida panther and the common loon. Some researchers have concluded, however,
that mercury levels in most areas are not nigh enough to adversely affect bird populations.
The National Mercury Problem
Current levels of mercury in freshwater fish in the United States are such that advisories have
been issued in 35 states that warn against the consumption of certain amounts and species of fish that
are contaminated with mercury. Six states (based on 1994 data) have statewide advisories (i.e.,
advisories posted on every freshwater body in that state). These advisories are based on the results of
sampling surveys that measure mercury levels in representative fish species collected from water
bodies. The.advisories are intended for people who catch or eat fish from those waterbodies. The
States have the discretion of establishing action levels which are different from those of the FDA.
Fish in commerce are under the jurisdiction of the FDA which issues action levels for
concentration of mercury in fish and shellfish. The current action level is 1 ppm mercury based on a
consideration of health impacts. The typical U.S. consumer of commercial seafood is not in danger of
ingesting harmful levels of methylmercury in seafood. The existing FDA management and consumer
advice is protective of food in commerce. In some areas, freshwater fish can have mercury levels
which exceed the U.S. FDA action limit of 1 ppm. The concentration of methylmercury in
commercially important marine species is, on the average, lower than the FDA action level.
June 1996 2-6 SAB REVIEW DRAFT
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3. FINDINGS OF THE MERCURY STUDY REPORT TO
CONGRESS
Sources Contributing to Mercury in the Environment
In the CAA, Congress directed U.S. EPA to examine sources of mercury emissions, including
electric utility steam generating units, municipal waste combustion units and other sources, including
area sources. The U.S. EPA interpreted the phrase "... and other sources..." to mean that a
comprehensive examination of mercury sources should be made and to the extent data were available,
air emissions should be quantified. Volume II of this report describes in some detail various source
categories that emit mercury. In many cases, a particular source category is identified as having the
potential to emit mercury, but data are not available to assign a quantitative estimate of emissions.
The U.S. EPA's intent was to identify as many sources of mercury emissions to the air as possible and
to quantify those emissions where possible.
The mercury emissions data that are available vary considerably in quantity and quality among
different source types. Not surprisingly, the best available data are for source categories that U.S.
EPA has examined in the past or is currently studying.
Sources of mercury emissions in the United States are ubiquitous. To characterize these
emissions, the types are defined in the following way:
Natural mercury emissions - the mobilization or release of geologically bound
mercury by natural processes, with mass transfer of mercury to the atmosphere;
Anthropogenic mercury emissions -- the mobilization or release of geologically bound
mercury by human activities, with mass transfer of mercury to the atmosphere; or
Re-emitted mercury - the mass transfer of mercury to the atmosphere by biologic and
geologic, processes drawing on a pool of mercury that was deposited to the earth's
surface after initial mobilization by either anthropogenic or natural activities.
Contemporary anthropogenic emissions of mercury are only one component of the global
mercury cycle. Releases from human activities today are adding to the mercury reservoirs that already
exist in land, water, and air, both naturally and as a result of previous human activities. Given the
present understanding of the global mercury cycle, the flux of mercury from the atmosphere to land or
water at any one location is comprised of contributions from the following:
The natural global cycle,
The global cycle perturbed by human activities,
Regional sources, and
Local sources.
Local sources could also include direct water discharges in addition to air emissions. Past uses
of mercury, such as fungicide application to crops are also a component of the present mercury burden
in the environment.
Understanding of the global mercury cycle (shown schematically in Figure 3-1) has improved
significantly with continuing study of source emissions, mercury fluxes to the earth's surface, and the
June 1996 3-1 SAB REVIEW DRAFT
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Figure 3-1
Comparison of Current and Pre-Industrial
Mercury Budgets and Fluxes
Current Mercury
Budgets and
Fluxes
Anthropogenic
4,000
Hg(p) S8% Hg(gssaous) Hg(0)
I 2% Hg(particul»te)
Local & Regional
Deposition
2,000
Global Terrestrial
Deposition
3,000
Global Marine
Deposition
2,000
Paniculate
Removal
200
All Fluxes irMO'Kg/y
All Pools in 10* Kg
Pre-Industrial
Mercury Budgets
and Fluxes
Air
1.600
98% Hg(gattous) Hg(0)
2% Hg(particulate)
Global Terrestrial
Deposition
1,000
Global Manne
Deposition
. 600
Paniculate
Removal
60
All Fluxes in 101 Kg/y
All Pools in 101 Kg
Source: Adapted from Mason, R.P. Fitzgerald, W.F. and Morel, M.M. 1994. The
Biogeochemical Cycling of Elemental Mercury: Anthropogenic Influences. Geochem. Cosmochim.
Acta, in press.
June 1996
3-2
SAB REVIEW DRAFT
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magnitude of mercury reservoirs that have accumulated in soils, watersheds and ocean waters.
Although considerable uncertainty still exists, it has become increasingly evident that anthropogenic
emissions of mercury to the air rival or exceed natural inputs. Recent estimates place the annual
amounts of mercury released into the air by human activities at between 50 and 75 percent of the total
yearly input to the atmosphere from all sources. Recycling of mercury at the earth's surface,
especially from the oceans, extends the influence and active lifetime of anthropogenic mercury
releases.
A better understanding of the relative contribution of mercury from anthropogenic sources is
limited by substantial remaining uncertainties regarding the level of natural emissions as well as the
amount and original source of mercury that is re-emitted to the atmosphere from existing reservoirs.
Recent estimates indicate that of the approximately 200,000 tons of mercury emitted to the atmosphere
since 1890, about 95 percent resides in terrestrial soils, about 3 percent in the ocean surface waters,
and 2 percent in the atmosphere. More study is needed before it is possible to accurately differentiate
natural fluxes from these reservoirs from re-emissions of mercury originally released from
anthropogenic sources. For instance, approximately one-third of total current global mercury
emissions are thought to cycle from the oceans to the atmosphere and back again to the oceans, but a
major fraction of the emissions from oceans consists of recycled anthropogenic mercury. It is believed
that much less than 50 percent of the oceanic emission is from mercury originally mobilized by natural
sources. Similarly, a potentially large fraction of terrestrial and vegetative emissions consists of
recycled mercury from previously deposited anthropogenic and natural emissions.
Comparisons of contemporary (within the last 15-20 years) measurements and historical
records indicate that the total global atmospheric mercury burden has increased since the beginning of
the industrialized period by a factor of between two and five (see Figure 3-1). For example, analysis
of sediments from Swedish lakes shows mercury concentrations in the upper layers that are two to five
times higher than those associated with pre-industrialized times. In Minnesota and Wisconsin, an
investigation of whole-lake mercury accumulation indicates that the annual deposition of atmospheric
mercury has increased by a factor of three to four since pre-industrial times. Similar increases have
been noted in other studies of lake and peat cores from this region, and results from remote lakes in
southeast Alaska also show an increase, though somewhat lower than found in the upper midwest U.S.
While the overall trend in the global mercury burden since pre-industrial times appears to be
increasing, there is some evidence that mercury concentrations in the environment in certain locations
have been stable or decreasing over the past few decades. For example, preliminary results for eastern
red cedar growing near industrial sources (chlor-alkali, nuclear weapons production) show peak
mercury concentrations in wood formed in the 1950s and 1960s, with stable or decreasing
concentrations in the past decade. Some results from peat cores and lake sediment cores also suggest
that peak mercury deposition occurred prior to 1970. Data collected over 25 years from many
locations in the United Kingdom on liver mercury concentrations in two raptor species and a fish-
eating grey heron indicate that peak concentrations occurred prior to 1970. The sharp decline in liver
mercury concentrations in the early 1970s suggests that local sources, such as agricultural uses of
fungicides, may have led to elevated mercury levels two to three decades ago. Similar trends have
been noted for mercury levels in eggs of the common loon collected from New York and New
Hampshire. This downward trend in mercury concentrations observed in the environment over the last
few decades generally tracks with mercury use and consumption patterns over the same timeframe
(discussed below).
While selected studies provide some evidence of declining mercury concentrations on a very
localized level, there does not appear to be a decrease in the global mercury burden. For example, a
June 1996 3-3 SAB REVIEW DRAFT
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1992 national study by U.S. EPA found mercury residues in fish at 92 percent of more than 314
surface water bodies tested in the U.S.. This study found mercury levels above 1 part per million
(ppm), the level used by the Food and Drug Administration as a basis for banning the sale of fish, at 2
percent of the sites surveyed, and above 0.5 ppm, a consumption advisory level used in many states, at
15 percent of the sites surveyed. More recent and complete data are not available to evaluate the
current trend in mercury concentrations in fish tissue. As of 1994, however, sixty percent of the
almost 1,300 consumption advisories issued in the U.S. due to toxic contamination in fish were for
mercury contamination. As of 1994, 35 states had at least one waterbody under mercury advisory,
including six states with statewide mercury advisories.
Given the considerable uncertainties regarding the levels of natural and re-emitted mercury
emissions, the emissions inventory focused only on the nature and magnitude of mercury emissions
from anthropogenic sources. The U.S. EPA recognizes, however, that an assessment of the relative
public health and environmental impact that can be attributed to current anthropogenic emissions is
greatly complicated by both natural mercury emissions, previous emissions of mercury that have
subsequently deposited and other sources such as water discharges and other previous uses (e.g.,
fungicide application). Further study is needed to determine the importance of natural and re-emitted
mercury, and the contribution of water discharges relative to atmospheric deposition.
Inventory Approach and Uncertainties
For most anthropogenic source categories, an emission factor-based approach was used to
develop both facility-specific estimates for modeling purposes and nationwide emission estimates.
This approach requires an emission factor, which is a ratio of the mass of mercury emitted to a
measure of source activity. It also requires an estimate of the annual nationwide source activity level.
Examples of measures of source activity include total heat input for fossil fuel combustion and total
raw material used or product generated for industrial processes. Emission factors are generated from
emission test data, from engineering analyses based on mass balance techniques, or from transfer of
information from comparable emission sources. Emission factors reflect the "typical control" achieved
by the air pollution control measures applied across the population of sources within a source category.
The emission factor-based approach does not generate exact emission estimates. Uncertainties
are introduced in the estimation of emission factors, control efficiencies and the activity level
measures. Ideally, emission factors are based on a substantial quantity of data from sources that
represent the source category population. For trace pollutants like mercury, however, emission factors
are frequently based on limited data that may not have been collected from representative sources.
Changes in processes or emission measurement techniques over time may also result in biased
emission factors. Emission control estimates are also generally based on limited data; as such, these
estimates are imprecise and may be biased. Further uncertainty in the emission estimates is added by
the sources of information used on source activity levels, which vary in reliability.
Once emitted to the environment, the fate and transport of mercury is greatly influenced by
the chemical form of mercury. The data collected for the emissions inventory was essentially all
reported as total mercury with the exception of utility boilers for which there were limited speciated
samples. In the exposure analysis described below, estimates were made of speciation profiles for
modeling purposes.
To improve the emissions estimates, a variety of research activities are needed. These are
listed in Chapter 5 of this volume.
June 1996 3-4 SAB REVIEW DRAFT
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Anthropogenic Emissions Summary
Table 3-1 summarizes the estimated national mercury emission rates by source category.
While these emission estimates for anthropogenic sources have limitations, they do provide insight into
the relative magnitude of emissions from different groups of sources. All of these emissions estimates
should be regarded as best point estimates given available data.
Of the estimated 220 Megagrams (Mg) (243 tons) of mercury emitted annually into the
atmosphere by anthropogenic sources in the United States, approximately 85 percent is from
combustion point sources, 13 percent is from manufacturing point sources, 1 percent is from
miscellaneous sources and 1 percent is from area sources. Four specific source categories account for
approximately 83 percent of the total anthropogenic emissions medical waste incineration
(27 percent), municipal waste combustion (23 percent), utility boilers (21 percent), and commercial/
industrial boilers (12 percent). It should be noted that the U.S. EPA has finalized mercury emission
limits for*municipal waste combustors, and has proposed merfcury emission limits for medical waste
incinerators. These emission limits will reduce mercury emissions from those sources by 90 percent.
All four of the most significant sources represent high temperature waste combustion or fossil
fuel processes. For each of these operations, the mercury is present as a trace contaminant in the fuel
or feedstock. Because of its relatively low boiling point, mercury is volatilized during high
temperature operations and discharged to the atmosphere with the exhaust gas.
Trends in Mercury Emissions
It is difficult to predict with certainty the temporal trends in mercury emissions for the U.S.,
although there appears to be a trend toward decreasing total mercury emissions from 1990 to 1995.
This is particularly true for the combustion sources wherein mercury is a trace contaminant of the fuel.
Also, as previously noted, there are a number of source categories where there is insufficient data to
estimate current emissions let alone potential future emissions. Based on available information,
however, a number of observations can be made regarding mercury emission trends from source
categories where some information is available about past activities and projected future activities.
e
There has been a real success in the U.S. in the dramatic drop in mercury emissions from
manufacturing over the past decade. Current emissions of mercury from manufacturing sources are
generally low (with the exception of chlor-alkali plants using the mercury cell process). The emissions
of mercury are more likely to occur when the product is broken or discarded. Therefore, in terms of
emission trends, one would expect that if the future consumption of mercury remains consistent with
the 1993 consumption rate, emissions from most manufacturing sources would remain about the same.
For industrial or manufacturing sources that use mercury in products or processes, the overall
consumption of mercury is generally declining. Industrial consumption of mercury has declined by
about two thirds between 1988 (1508 Mg) and 1993 (558 Mg). Much of this decline can be attributed
to the elimination of mercury as a paint additive (20 percent) and the reduction of mercury in batteries
(36 percent). Use of mercury by other source categories remained about the same between 1988 and
1993.
Secondary production of mercury (i.e., recovering mercury from waste products) has increased
significantly over the past few years. Of the 558 Mg of mercury used in industrial processes in 1993,
63 percent was provided by secondary mercury producers. This is a two-fold increase since 1991.
The number of secondary mercury producers is expected to increase as more facilities open to recover
June 1996 3-5 SAB REVIEW DRAFT
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Table ES-3
Best Point Estimates of National Mercury Emission Rates by Category
Source of mercury3
Area sources
Lamp breakage
General lab use
Dental prep and use
Mobile sources
Paint use
Agricultural burning
Landfills
Point sources
Combustion sources
MWI/
MWCs
Utility boilers v
Coal
Oil
Natural gas
Commercial/industrial boilers
Coal
Oil
Residential boilers
Coal
Oil .
SSIs
Crematories
Wood-fired boilers'1
Hazardous waste combustors'
Manufacturing sources
Primary lead
Secondary Hg production
Chlor-alkali
Portland cement
Primary copper1
Lime manufacturing
Electrical apparatus
Instruments
Carbon black
Fluorescent lamp recycling
Batteries
Primary Hg production
Mercury compounds
Byproduct coke
Refineries
Miscellaneous sources
Geothermal power
Turf products
Pigments, oil, etc.
TOTAL
1990-1993
Mg/yrb'c
2.8
1.4
0.7
0.7
d
e
d
d
217.3
186.9
58.8
50
46.5
(46.3)8
(0.23) .
(0.002)
26.3
(20.7)
(5.5)
3.2
(0.5)
(2.7)
1.7
0.4
0.3
d
29.1
8.2
6.7
5.9
5.9
0.6
0.6
0.42
0.5
0.23
0.005
0.02
d
d
d
d
13
1.3
e
e
220.1
1990-1993
tons/yrb'c
3.1
1.5
0.8
0.8
d
e
d
d-
239.4
205.9
64.7
55
51.3
(51)
(0.25)
(0.002)
29
(22.8)
(6.0)
3.5
(0.6)
(3.0)
1.8
0.4
0.3
d
32
9.0
7.4
6.5
6.5
0.7
0.7
0.46
0.5
0.25
O.OOfc
0.02
d
d
d
d
1.4
1.4
e
e
2425
% of Total
Inventory
13
0.6
0.3
0.3
d
e
d
d
98.7
84.9
26.7
22.7
21.2
(21.0)
(0.1)
(0.0)
12.0
(9.4)
(2.5)
1.4
0.2
(1.2)
0.7
0.2
0.1
d
13.2
3.7
3.1
2.7
2.7
0.3
0.3
0.2
0.2
0.1
0.002
0.0
d
d
d
d
0.6
0.6
e
e
100.0
1 MWC = Municipal waste combustor, MWI = medical waste incinerator, SSI = sewage sludge incinerator.
b Numbers do not add exactly because of rounding.
c Where available, emissions estimates for 1995 are discussed in the text However, these 1995 estimates were not used in any of the modeling analyses.
d Insufficient information to estimate 1990 emissions.
* Mercury has been phased out of use.
f In the course of an MWI rulemaking, with the receipt of new data, U.S. EPA expects to revise the mercury emission estimate for MWIs downward.
8 Parentheses denote subtotal within a larger point source category.
h Includes boilers only; does not include residential wood combustion (wood stoves).
1 In 1995 incinerators and lightweight aggregate kilns (not cement kilns) were estimated to emit 5.0 tons of mercury.
1 1990 emissions are estimated for only one source, which ceased operations m February 1995. The nationwide estimate for 1995 is 0.08 tons.
June 1996
3-6
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mercury from fluorescent lamps and other mercury-containing products (e.g., thermostats). As a result
there is potential for mercury emissions from this source category to increase.
The largest identifiable sources of mercury emissions currently are municipal waste combustors
and medical waste incinerators. Emissions from these source categories are expected to decline
significantly by the year 2000 due to regulatory action the U.S. EPA is taking under the statutory
authority of section 129 of the CAA. The U.S. EPA has finalized rules for municipal waste
combustors and proposed rules for medical waste incinerators that will reduce mercury emissions from
both of these source categories by about 90 percent. In addition to this federal action, a number of
states (including Minnesota, Florida and New Jersey) have implemented mandatory recycling programs
to reduce mercury-containing waste, and some states have regulations that impose emission limits that
. are lower than the federal regulation. These factors will reduce national mercury emissions from these
source categories even further.
After municipal solid waste and medical waste incinerators have been controlled, the largest
remaining identified source of mercury emissions will be fossil fuel combustion by utility boilers,
particularly coal combustion. Future trends in mercury emissions from this source category are largely
dependent on both the nation's future energy needs and the fuel chosen to meet those needs. Another
factor is the nature of actions the utility industry may take in the future to meet air quality
requirements under the Clean Air Act.
Trends in Mercury Consumption
Data on Industrial demand for mercury show a general decline in domestic mercury use since
demand peaked in 1964. Domestic demand fell by 74 percent between 1980 and 1993, and by more
than 50 percent since 1988. The rate of decline, however, has slowed since 1990. Further evidence of
the declining need for mercury hi the U.S. is provided by the general decline in imports since 1988
and the fact that exports have exceeded imports since at least 1989. Approximately 78 percent of the
net U.S. exports of mercury during the last five years has come from federal sales, with a steadily
increasing portion of the federal sales coming from the National Defense Stockpile managed by DLA.
Federal sales accounted for 97 percent of the U.S. demand in 1993.
Most recently, there has been a sharp drop in Federal sales. In July 1994, DLA suspended
future sales of mercury from the Department of Defense stockpile until the environmental implications
of these sales are addressed. In addition, hi past years, DLA sold mercury accumulated and held by
the Department of Energy, which is also considered excess to government needs. DLA suspended
these mercury sales in July 1993 for an indefinite period in order to concentrate on selling material
from its own mercury stockpile.
In general, these data suggest that industrial manufacturers that use mercury are shifting away
from mercury except for uses for which mercury is considered essential. This shift is believed to be
largely the result of Federal bans on mercury additives in paint and pesticides; industry efforts to
reduce mercury in batteries; increasing state regulation of mercury emissions sources and mercury in
products; and state-mandated recycling programs. A number of Federal activities are also underway to
investigate pollution prevention measures and control techniques for a number of sources categories
(see Volume VII of this Report to Congress).
June 1996 3-7 SAB REVIEW DRAFT
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Assessment of Exposure
Exposure Assessment Approach
The exposure assessment draws upon the available scientific information and presents
quantitative modeling analyses which examine the following: (1) the long range transport of mercury
from emission sources through the atmosphere; (2) the transport of mercury from emission sources
through the local atmosphere; (3) the aquatic and terrestrial fate and transport of mercury at
hypothetical sites; and (4) finally, the resulting exposures to hypothetical humans and animals that
inhabit these sites. Exposure to mercury from seafood was estimated using a cross sectional survey
with a three day sampling period and central tendency estimates of mercury concentrations in the
tissues of seafood.
There are no data that conclusively demonstrate a relationship between anthropogenic sources
and increased mercury concentrations in environmental media or biota. Available mercury monitoring
data around sources are extremely limited and no comprehensive database describing environmental
concentrations has been developed. To determine if there is a connection between anthropogenic
emission sources and increased environmental levels, the exposure assessment in this Report utilized
exposure modeling techniques.
Figure 3-2 illustrates the how the various exposure models were integrated to estimate both
human and wildlife exposure.
Long-Range Transport Analysis
The long range transport modeling predicts the regional and national deposition of mercury
across the continental U.S. Details of several studies which demonstrate the long range transport of
mercury are presented in Volume III. In this Report, the long range transport of mercury was modeled
using site-specific, anthropogenic emission source data (presented in Volume II of this Report) to
generate mean, annual atmospheric mercury concentrations and deposition values across the continental
U.S. The Regional Lagrangian Model of Air Pollution (RELMAP) was utilized to model annual
mercury emissions from multiple mercury emission sources.
The chemical form of emitted mercury is a critical factor in its fate, transport and toxicity in
the environment. With the exception of utility boilers, for which there are limited speciated data,
mercury emissions are reported as total measured mercury. The form distributions, or speciation
factors, define the estimated fraction of mercury emitted as elemental mercury (Hg°), divalent mercury
(Hg2"^), or mercury associated with particulates (Hgp) were adopted from Peterson et al. (1995). Since
there is considerable uncertainty about the speciation profiles, an alternate speciation scenario was also
modeled to measure the sensitivity of the RELMAP results to this uncertainty. The speciation factors
for the base case and alternate scenario are discussed in Volume III of this Report. The results of the
modeling using the base case speciation scenario are described below.
From the RELMAP analysis and a review of field measurement studies, it is concluded that
mercury deposition appears to be ubiquitous across the continental U.S., and at, or above, detection
limits when measured with current analytic methods. The southern Great Lakes and Ohio River
Valley, the Northeast, and scattered areas in the South (particularly in the Miami and Tampa areas)
are predicted to have the highest annual rate of deposition of total mercury (above the levels predicted
at the 90th percentile). Figure 3-3 illustrates the pattern of mercury deposition across the U.S. This
figure also illustrates the boundaries of the RELMAP modeling domain. Measured deposition
June 1996 3-8 SAB REVIEW DRAFT
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Figure 3-2
Fate, Transport and Exposure Modeling Conducted in the Combined COMPDEP and RELMAP Local Impact Analysis
Deposition Rate of Mercury =10Mg/m/yr
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Figure 3-3
Total Mercury Wet + Dry Deposition (Base Case)
Units: ug/m2
n 1.0-2.0
D 2.0 -5.0
5.0 -10.0
10.0-20.0
> 20.0
June 1996
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estimates are limited, but are available for certain geographic regions. The data that are available
corroborate the RELMAP modeling predictions for specific areas. These comparisons are discussed in
detail in Volume III.
A wide range of mercury deposition rates is predicted across the continental U.S. The highest
predicted rates (i.e., above 90th percentile) are more than 50 times higher than the lowest predicted
rates (i.e., below the 10th percentile). The three principal factors that contribute to these modeled and
observed deposition patterns are the emission source locations, amount of divalent and particulate
mercury emitted or formed hi the atmosphere; and climate and meteorology. A facility located in a
humid climate is predicted to have a higher annual rate of mercury deposition than a facility located
hi an arid climate. The critical variables are the estimated washout ratios of elemental and divalent
mercury, as well as the annual amount of precipitation. Precipitation is important because it removes
various forms of mercury from the atmosphere and deposits them to the surface of the earth.
Mass Balances of Mercury within the Long-range Model Domain
The general mass balance of elemental mercury gas, divalent mercury gas, and particle-bound
mercury from the RELMAP simulation results using specified speciation profiles are shown in Table
3-2. Using the meteorologic data from the year 1989, the mass balance shows a total of 223.8 metric
tons of mercury emitted to the atmosphere from anthropogenic sources. (This simulated emission total
differs from the national totals indicated in Volume II since the states of Alaska and Hawaii are not
within the model domain and latex paint emissions are not considered.) The simulation indicates that
77.9 metric tons of anthropogenic mercury emissions are deposited within the model domain and 0.6
metric tones remain in the air within the model domain at the end of the simulation. The remainder,
about 145.3 metric tons, is transported outside the model domain and probably diffuses into the global
atmospheric reservoir.
Table 3-2
Modeled Mercury Mass Budget in Metric Tons for 1989
Using the Specified Speciation Profiles
Source/Fate
Total U.S. anthropogenic emissions
Mass advected from model domain
Dry deposited anthropogenic emissions
Wet deposited anthropogenic emissions
Remaining in air at end of simulation
Total deposited anthropogenic emissions
Deposition from background Hg°
Mercury deposited from all sources
Hg°a
92.0
90.4
0.0
1.2
0.4
1.2
33.0
34.2
Hg2+b
92.6
29.9
39.0
23.6
0.1
62.6
0.0
62.6
HgPC
39.1
25.0
0.6
13.4
0.1
14.1
0.0
14.1
Total
Mercury
223.8
145.3
39.6
38.3
0.6
77.9
33.0
111.0
(All figures rounded to the nearest tenth of a metric ton)
4 Hg° = Elemental Mercury
b Kg2"*" = Divalent Vapor-phase Mercury
c = Particle-Bound/Mercury
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The simulation also indicates that 33.0 metric tons of mercury are deposited within the model
domain from this global atmospheric reservoir, suggesting that about four times as much mercury is
being added to the global reservoir from U.S. emissions as is being deposited from it. The total
amount of mercury deposited in the model domain annually from U.S. anthropogenic emissions and
from the global background concentration is estimated to be 111.0 metric tons, or about one-half ef
the total atmospheric emissions from anthropogenic sources in the lower 48 United States.
Of the total anthropogenic mercury mass deposited to the surface in the model domain, 80% is
estimated by the RELMAP to come from Kg2* emissions, 18% from Hgp emissions and 2% from Hg°
emissions when the base-case emission speciation profiles are used. When the deposition of Hg° from
the global background is considered in addition to anthropogenic sources in the lower 48 states, the
species fractions become 56% Kg2"1", 31% Hg° and 13% Hgp. The vast majority of mercury already in
the global atmosphere is in the form of Hg° and, in general, the anthropogenic Hg° emissions do not
greatly increase the existing Hg° concentration. Although Hg° is removed from the atmosphere very
slowly, the global background reservoir is large and extraction of mercury from it is significant in
terms of the total deposition. It should be noted that dry deposition of Hg° is significant only at very
high concentrations and has not been included in the RELMAP simulations. Wet deposition is the
only major pathway for removal of Hg° from the atmosphere. This removal pathway simulated by the
RELMAP involves oxidation of mercury by ozone in an aqueous solution; thus, the Hg° that is
extracted from the atmosphere by the modeled precipitation process would actually be deposited
primarily in the form of Hg2"1".
Of the 92.0 metric tons of anthropogenic Hg° emitted in the lower 48 states, only 1.2 tons
(1.3%) is deposited within the model domain, while of the 92.6 metric tons of Hg2"1" emitted, about
62.6 tons (67.6%) is deposited. Ninety-eight percent of the deposited anthropogenic mercury was
emitted in the form of Hg2"1" or Hgp. Thus, a strong argument can be made that the combined Hg2"1"
and Hgp component of anthropogenic mercury emissions can be used as an indicator of eventual
deposition of those emissions to the lower 48 states and surrounding areas. The emission inventory
and chemical/physical speciations profiles indicate that of all combined Kg2* and Hgp emissions, about
36% is from medical waste incineration, 30% is from municipal waste combustion, 18% is from
electric utility boilers, 11% is from combustion of fossil fuel other than by electric utilities, 1% is
from chlor-alkali plants, 1% is from non-ferrous metal smelting, and 2% is from all other sources.
Limitations of the Long-Range Transport (RELMAP) Analysis
There are a number of uncertainties with the RELMAP analysis. These have to do to a large
degree with the current state-of-the-science concerning atmospheric chemistry and speciation profiles
of mercury emissions. Some of the most important limitations are listed below.
Comprehensive emissions data for a number anthropogenic and natural sources are not
available. This reflects the current developmental nature of emission speciation methods,
resulting in few data on the various species of mercury and proportions of vapor and solid
forms emitted. Both elemental and divalent mercury species as well as gaseous and particulate
forms are known to be emitted from point and area sources.
Atmospheric chemistry data are incomplete. Some atmospheric reactions of mercury, such as
the oxidation of elemental mercury to divalent mercury in cloud water droplets have been
reported. Other chemical reactions in the atmosphere that may reduce divalent species to
elemental mercury have not been reported.
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There is inadequate information on the atmospheric processes that affect wet and dry
deposition of mercury. Atmospheric particulate forms and divalent species of mercury are
thought to wet and dry deposit more rapidly than elemental mercury; however, the relative
rates of deposition are uncertain. There is no validated air pollution model that estimates wet
and dry deposition of vapor-phase compounds close to the emission source. In addition, there
is uncertainty regarding the revolatilization of deposited mercury.
Exposure Assessment of Local Deposition of Mercury
An analysis of the local atmospheric transport of mercury released from anthropogenic
emission sources was undertaken to estimate the impacts of mercury from selected, individual sources.
Model .plants were developed; these are defined as hypothetical facilities that represent actual
emissions from existing industrial processes and combustion sources. The model plants were situated
in hypothetical locations intended to simulate a site in either the Western or Eastern U.S. This
approach was selected because environmental monitoring studies indicate that measured mercury levels
in environmental media and biota may be elevated in areas around stationary industrial and
combustion sources known to emit mercury. These measured data are detailed in Chapter 2 of
Volume III of this Report.
The exposure assessment addressed atmospheric mercury emissions from six combustion and
manufacturing source categories: municipal waste combustors (MWCs), medical waste incinerators
(MWIs), utility boilers, chlor-alkali plants, primary lead smelters and primary copper smelters. It did
not address all anthropogenic emission sources nor did it address emissions from natural sources. In
addition, anthropogenic discharges of. mercury to waterbodies were not addressed.
The following human exposure routes were included: inhalation, consumption of water,
consumption of fish, beef, beef liver, cow's milk, poultry, chicken eggs, pork, lamb, green plants (e.g.,
leafy vegetables, potatoes, fruits, grains and cereals) and ingestion of soil. Dermal exposures that
resulted from contact with soil and water, as well as exposure through inhalation of resuspended dust
particles and exposure through the consumption of human breast milk were not evaluated. The only
exposure route considered for wildlife was the consumption of freshwater fish.
Atmospheric concentrations and deposition rates were used as inputs to a series of terrestrial
and aquatic models described in U.S. EPA's (1990) Methodology for Assessing Health Risks to
Indirect Exposure from Combustor Emissions and a 1994 Addendum. The results of these terrestrial
and aquatic models were used to predict mercury exposure to hypothetical humans through inhalation,
consumption of drinking water and ingestion of soil, farm products (e.g., beef product and vegetables)
and fish. These models were also used to predict mercury exposure in hypothetical piscivorous (i.e.,
fish-eating) birds and mammals through their consumption of fish.
Results of the Exposure Analysis
This exposure analysis, in conjunction with available scientific knowledge, supports a plausible
link between mercury emissions from anthropogenic combustion and industrial sources and mercury
concentrations in air, soil, water and sediments. The critical variables contributing to this linkage are
these:
the species of mercury that are emitted from the sources, with elemental mercury
(Hg°) mostly contributing to concentrations in ambient air and divalent mercury (Hg2+)
mostly contributing to concentrations in soil, water and sediments;
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the overall amount of mercury emitted from a combustion source: and
the climate conditions.
In addition, this study also supports a plausible link between mercury emissions from
anthropogenic combustion and industrial sources and methylmercury concentrations in freshwater fish.
The critical variables contributing to this linkage are the following:
the species of mercury that are emitted, with emitted divalent mercury mostly
depositing into local watershed areas and, to a lesser extent the atmospheric conversion
of elemental mercury to divalent species which are deposited over greater distances;
the overall amount of mercury emitted from a source;
the extent of mercury methylation in the water body; and
the climate conditions.
From the analysis of deposition and on a comparative basis, the deposition of divalent mercury
(Hg^+) close to an emission source is greater for receptors in elevated terrain (i.e., terrain above the
elevation of the stack base) than from receptors located hi flat terrain (i.e., terrain below the elevation
of the stack base). The critical variables are parameters that influence the plume height, primarily the
stack height and stack exit gas velocity.
Based on the local scale atmospheric modeling results in flat terrain, at least 75% of the
emitted mercury from each facility is predicted to be transported more than 50 km from the facility.
The models used in the exposure analysis indicate that, except for utility boilers and intermittent
medical waste incinerators, deposition within 10 Km of a facility is generally dominated by emissions
from the local source rather than from emissions transported from regional mercury emissions sources.
Consumption of fish is the dominant pathway of exposure to methylmercury for fish-
consuming humans and wildlife. There is a great deal of variability among individuals in these
populations with respect to food sources and fish consumption rates. As a result, there is a great deal
of variability in exposure to methylmercury in these populations. The anthropogenic contribution to
the total amount of methylmercury in fish is, in part, the result of anthropogenic mercury releases from
industrial and combustion sources increasing mercury body burdens in fish. As a consequence of
human and wildlife consumption of the affected fish, there is an incremental increase in exposure to
methylmercury. Due to differences in fish consumption rates per body weight and differences in body
weights among species, it is likely that piscivorous birds and mammals have much higher
environmental exposures to methylmercury than humans through the consumption of contaminated
fish. This is true even hi the case of fish consumption by humans who consume above average
amounts of fish. The critical variables contributing to these outcomes are these:
the fish consumption rate;
the body weight of the individual in relation to the fish consumption rate; and
the rate of biomagnification between trophic levels within the aquatic food-chain.
In terms of methylmercury intake on a per body weight basis, the five wildlife species
considered in this analysis can be ranked from high to low as follows:
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Kingfisher
River Otter
Mink, Osprey
Bald eagle
Methylmercury exposures for the most exposed wildlife species (the kingfisher) may be up to two
orders of magnitude higher than human exposures, from contaminated freshwater fish (on a kilogram
fish consumed per body weight basis). This assumes that the fish within different tropic levels of a
given lake are contaminated with the same concentrations of methylmercury.
Some Limitations of the Local Exposure Assessment
Limitations of the local exposure assessment include a lack of information concerning the
movement of mercury from watershed soils to water bodies. There are not conclusive data on the
amount of and rates of mercury methylation in different types of water bodies. In addition, there is a
lack of data on the transfer of mercury between environmental compartments and biologic
compartments; for example, the link between the amount of mercury in the water body and the levels
in fish appears to vary from water body to water body.
On a national scale, an apportionment between sources of mercury and mercury in
environmental media and biota cannot be described in quantitative terms with the current scientific
understanding of the environmental fate and transport of this pollutant There is a lack of adequate
mercury measurement data near the anthropogenic atmospheric mercury sources considered in this
report. To assess how well the modeled data predict actual mercury concentrations in different
environmental media at a variety of geographic locations requires a data base against which to make
these comparisons. The lack of such measured data preclude a comparison of the modeling results
with measured data around these sources. These data include measured mercury deposition rates as
well as measured concentrations in the atmosphere, soils, water bodies and biota. Substantial
additional monitoring data would facilitate such comparison.
Assessment of Fish Consumption
A current assessment of U.S. general population methylmercury exposure through the
consumption of fish is provided in Chapter 3 and in Appendix H of Volume III. This assessment was
conducted to provide an estimate of mercury exposure through the consumption of fish to the general
U.S. population. It is not a site-specific assessment but rather a national assessment. This assessment
utilizes data from the 1989 - 1991 Continuing Surveys of Food Intake by Individuals (CSFII 89-91) to
estimate a range of fish consumption rates among U.S. fish eaters. The CSFII is a cross sectional
survey with a three day sampling period. The survey was conducted over a period of three years; it
included all seasons and both weekend and weekday sampling. Only individuals who reported fish
consumption were considered in the analyses in this Report to Congress. For each fish-eater, CSFII
89-91 study identified the number of fish meals, the quantities and species of fish consumed and the
self-reported body weights of the consumers. The constitution of the survey population was weighted
to reflect the actual U.S. population.
These estimates of fish consumption rates were combined with species-specific mean values
for measured methylmercury concentrations. The marine fish methylmercury concentration data were
obtained from the National Marine Fisheries Service Database. The freshwater fish methylmercury
concentration data were obtained from Bahnick et al., (1994) and Lowe et al., (1985). Through the
application of specific fish preparation factors (USDA, 1995), estimates of the range of methylmercury
June 1996 3-15 SAB REVIEW DRAFT
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exposure from the consumption of fresh water fish were prepared for the fish-consuming segment of
the U.S. population. Per body weight estimates of methylmercury exposure were determined by
dividing the total daily methylmercury exposure from this pathway by the self-reported body weights.
The results of his analysis show that children on a per kilogram body weight basis have higher average
exposure rates to methylmercury through the consumption of fish than adults.
Human Health Effects of Methylmercury
Data in both humans and experimental animals show that all three forms of mercury evaluated
in this Report (elemental, inorganic and methylmercury) can produce adverse health effects. Human
exposure to elemental mercury occurs in some occupations, and exposure to inorganic mercury can
arise from mercury amalgams used in dental restorative materials (U.S. PHS, Environmental Health
Policy Committee, 1995). People, however, are primarily exposed to methylmercury in fish. The
focus of this assessment, therefore, is on methylmercury, which can produce a variety of adverse
effects, depending on the dose and time of exposure.
Individual risk assessors and specific organizations may choose different risk assessment
methodologies. Part of these differences occur when identifying populations or subpopulations of
concern. More than one approach to selection of the population at risk of adverse effect is feasible.
For example, if children are judged to be the subpopulation of greatest concern, specific age-groups
within this subpopulation may be judged to be of greater interest; e.g., birth through 4 years of age.
Alternatively other risk assessors may prefer to consider all children (e.g., birth through 14 years of
age) as a group when evaluating risk to childrea
Neurotoxicity is the effect of greatest concern when exposure occurs to the developing
embryo/fetus during pregnancy as well as when adults and children are exposed to methylmercury.
Two major epidemics of methyhnercury poisoning through fish consumption have occurred. The best
known of these two epidemics occurred among people and wildlife living near Minamata City on the
shores of Minamata Bay, Kyushu, Japan. The source of methylmercury was a chemical factory that
used mercury as a catalyst. A series of chemical analyses identified methylmercury in the factory
waste sludge, which was drained into Minamata Bay. Once present in Minamata Bay, the
methylmercury accumulated in the tissue of shellfish and fish that were subsequently consumed by
wildlife and humans. Fish was a routine part of the diet in these populations. An average fish
consumption was reported to be in excess of 300 g/day (reviewed by Harada et al., 1995); this is a
greater level of fish consumption than is typical for the general U.S. population.
The first poisoning case occurred in 1956 in a 6-year-old girl who came to a hospital
complaining of symptoms characteristic of nervous system damage. Symptoms of Minamata disease
in children and adults included the following:
Impairment of the peripheral vision;
disturbances hi sensations ("phis and needles" feelings, numbness) usually in the hands
and feet and sometimes around the mouth;
incoordination of movements as in writing;
impairment of speech;
impairment of hearing;
impairment of walking; and
mental disturbances.
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It frequently took several years before people were aware that they were developing the signs and
symptoms of methylmercury poisoning.
Over the next 20 years the number of people known to be affected with what became known
as Minamata disease increased to thousands. In time the disease was recognized to result from
methylmercury poisoning, and fish were subsequently identified as the source of methylmercury. As
is often the situation with epidemics, the first cases noted were severe. Deaths occurred among both
adults and children. It also was recognized that the nervous system damage could occur to the fetus if
the mother ate fish contaminated with high concentrations of methylmercury during pregnancy. The
nervous system damage of severe methylmercury poisoning among infants was very similar to
congenital cerebral palsy. In the fishing villages of this region the occurrence of congenital cerebral
palsy due to methylmercury was very high compared to the incidence for Japan in general. After the
source of mercury contamination was identified, efforts were made to reduce the release of mercury
into the bay. After 1969, averagetoercury concentrations hi fish had fallen below 0.5 ppm.
In 1965, an additional methylmercury poisoning outbreak occurred in the area of Niigata,
Japan. As in Minamata, multiple chemical plant sources of the chemical were considered. Scientific
detective work identified the source again to be a chemical factory releasing methylmercury into the
Agano River. The signs and symptoms of disease in Niigata were those of methylmercury poisoning
and strongly similar to the disease in Minamata.
The abnormalities (or pathology) in the human brain that result from methylmercury poisoning
are well described. There is an extremely high level of scientific certainty that methylmercury causes
these changes. Similar pathology has been identified in other countries where methylmercury
poisonings have occurred. Methylmercury contamination of other food products (including grains and
pork products) has resulted hi severe methylmercury poisoning with pathological changes in the
nervous system and clinical disease virtually identical to Minamata disease.
Methylmercury poisoning occurred hi Iraq following consumption of seed grain that had been
treated with a fungicide containing methylmercury. The first outbreak occurred prior to 1960 and
resulted in severe human poisonings. The second outbreak of methylmercury poisoning from grain
consumption occurred in the early 1970s. Imported mercury-treated seed grains arrived after the
planting season and were subsequently used as grain to make into flour that was baked into bread.
Unlike the long-term exposures in Japan, the epidemic of methylmercury poisoning in Iraq was short
in duration, but the magnitude of the exposure was high. Because many of the people exposed to
methylmercury in this way lived in small villages in very rural areas (and some were nomads), the
number of people exposed to these mercury-contaminated seed grains is not known. The number of
people admitted to the hospital with symptoms of poisoning has been estimated to be approximately
6,500, with 459 fatalities reported.
As hi the Japanese poisoning epidemics, the signs and symptoms of disease were
predominantly those of the nervous system: difficulty with peripheral vision or blindness, sensory
disturbances, incoordination, impairment of walking, slurred speech and in some cases, death. Children
were affected, as well as adults. Of great concern was the observation that infants, born of mothers
who had consumed the methylmercury-contaminated grain (particularly during the second trimester of
pregnancy) could show nervous system damage even though the mother was only slightly affected
herself.
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Toxicokinetics of Mercury
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.
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. 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. Elemental mercury is eliminated from the body 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.
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. 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.
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
in the body is considered to be relatively stable and is only slowly demethylated to form mercuric mercury
in rats. It is hypothesized that methytmercury 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.
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.
Some Limitations of the Assessment
In both the Iraqi and Japanese epidemics, the levels of methylmercury consumed were much
higher than the levels currently reported in the U.S. food supply. While there are no data to indicate
that methylmercury absorption is affected by food type, it must be noted that one of the severe
poisoning episodes was through a means not expected to be prevalent in the U.S.; that is, the
consumption of contaminated grain.
Health endpoints other than neurotoxicity were evaluated by U.S. EPA using established risk
assessment Guidelines. Data for other endpoints than developmental neurotoxicity were limited.
Methylmercury has been shown to cause tumors in mice at high doses that produce severe non-cancer
toxicity. Low-dose exposures to methylmercury are not likely to cause cancer in humans. Data on
effects related to mutation formation (changes in DNA) indicate that methylmercury could increase
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What Is A Reference Dose?
A reference dose or RfD is defined in the following way by U.S. EPA: 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. RfDs are reviewed by Agency scientists for accuracy, appropriate use of risk assessment
methodology, appropriate use of data and other scientific issues. When consensus has been reached by the
workgroup, information on the RfD is made available to the public through a U.S. EPA database; namely,
the Integrated Risk Information System (IRIS).
The RfD is based.on the best available data that indicate a "critical effect"; this is generally the
first indicator or most subtle indicator of an adverse effect in the species under study. In calculating RfDs
U.S. EPA generally uses a no observed adverse effect level (NOAEL). This is found from either
inspection or modeling of dose-response data on the critical effect. It is a means of estimating the
threshold for effect in the reported study. Tfie NOAEL is most useful when it is from a study in whi6h a
determination of the lowest observed adverse effect level (LOAEL) can also be done. The LOAEL is the
lowest tested dose at which the critical effect was seen in the species under study.
In calculating the RfD the U.S. EPA divides the NOAEL or LOAEL by a series of uncertainty
and modifying factors in order to extrapolate to the general human population. The uncertainty factors
(which may be as much as 10 each) are for the following areas: extrapolation of data to sensitive human
subpopulations; extrapolation from animal data to conclusions for humans; lack of chronic data; lack of
certain other critical data; and use of a LOAEL in the absence of a NOAEL.
The RfD is used for risk assessment judgments dealing with evaluations of general systemic
toxicity. It is intended to account for sensitive (but not hypersensitive) members of the human population;
the rationale is that if exposure to the RfD is likely to be without appreciable risk for sensitive members of
the population, then it is without appreciable risk for all members of the population. The RfD is generally
applicable to men and women and to adults, to children and to the aged, unless data support the
calculation of separate RfDs for these groups.
The RfD is a quantitative estimate of levels expected to be without effect. Exceedance of the
RfD does not mean that risk will be present Acceptability of uncertain risks is a risk management
decision. Risk management decisions may consider the RfD but will take into account exposures, other
risk factors and non-risk factors as well.
frequencies of mutation in human eggs and sperm. These data were not sufficient, however, to permit
estimation of the amount of methylmercury that would cause a measurable mutagenic effect in a
human population.
How Much Methylmercury is Harmful to Humans?
Information on the amount of methylmercury exposure producing particular combinations of
signs and symptoms in people has been analyzed to yield what are called quantitative dose-response
assessments. Both the Japanese and Iraqi epidemics are important to understanding how methyl-
mercury from food produces neurological disease in humans. In the epidemics in Minamata and
Niigata, the exposures were long-term, and the tissues of fish and shellfish were the sources of
methylmercury exposure. This establishes with highest scientific confidence that methylmercury in
fish can produce human disease. A limitation to these data is that many patients were severely
June 1996 3-19 SAB REVIEW DRAFT
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affected. The extent of methylmercury poisoning was so severe that finding subtle indications of
disease is difficult. Subtle indicators of poisoning are important in identifying levels of exposure
which will not cause any adverse effects. The U.S. EPA calculates one such measure, called a
reference dose or RfD (see the box above).
U.S. EPA has on two occasions published RfDs for methylmercury which have represented the
Agency consensus for that time. These are discussed at length in Volume IV, and the uncertainties
and limitations are described in Volume VI. At the time of the generation of the Mercury Study
Report to Congress, it became apparent that considerable new data on the health effect of
methylmercury in 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 PBPK models to published work.
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 interagency process, with external involvement, will be undertaken for the
purpose of review these new data, their evaluations and evaluations of existing data. An outcome of
this process will be assessment by U.S.EPA of its RfD for methylmercury to determine if change is
warranted.
The current U.S. EPA RfD for methylmercury was based on data on neurologic changes in 81
Iraqi children who had been exposed in utero: their mothers had eaten methylmercury-contaminated
bread during pregnancy. The data were collected by interviewing the mothers of the children and by
clinical examination by pediatric neurologists approximately 30 months after the poisoning episode.
The incidence of several endpoints (including late walking, late talking, seizures or delayed mental
development,, and scores on clinical tests of nervous system function) were mathematically modeled to
determine a mercury level in hair (measured hi all the mothers hi the study) which was associated with
no adverse effects. These effects were delays in motor and language development defined by the
following:
Inability to walk two steps without support by 2 years of age;
t,
inability to respond to simple verbal communication by age 2 years among children
with good hearing;
scores on physical examination by a neurologist that assessed cranial nerve signs,
speech, involuntary movements, limb tone, strength, deep tendon reflexes, plantar
responses, coordination, dexterity, primitive reflexes, sensation, posture, and ability to
sit, stand, walk, and run; and
assessment of mental development or the presence of seizures based on interviews with
the child's mother.
In calculating the mercury level in hair which was associated with no adverse effects; the U.S.
EPA chose a benchmark dose (in this instance the lower bound for 10 percent risk of neurological
changes) based on modeling of all effects in children. This lower bound was 11 ppm hair
concentration for methylmercury. A dose-conversion equation was used to estimate a daily intake of
1.1 ug methylmercury/kg body weight/day that when ingested by a 60 kg individual will maintain a
blood concentration of approximately 44 ug/L of blood or a hair concentration of 11 ug mercury/gram
hair (11 ppm).
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Data on the behavior of mercury in the human body were used to estimate the amount of
mercury ingested per day at this no adverse effect level. Due to variability in the way individuals
process methylmercury in the body and the lack of data on observed adult male and female
reproductive effects, an uncertainty factor of 10 was used to derive the RfD from the benchmark dose.
The RfD for methylmercury was determined to be 1x10~4 mg/kg-day; that is a person could consume
0.1 jag methylmercury for every kg of his/her body weight every day for a lifetime without anticipation
of risk of adverse effect. The RfD is a daily ingestion level anticipated to be without adverse effect to
persons, including sensitive subpopulations, over a lifetime. The RfD may be considered the midpoint
in an estimated range of about an order of magnitude. This range reflects variability and uncertainty
in the estimate.
The RfD is a risk assessment tool, not a risk management decision. Judgments as to a "safe"
dose and exposure represent decisions that involve risk management components.
Limitations and Uncertainties in the Assessment
The range of uncertainty in the RfD and the factors contributing to this range were evaluated
in qualitative and quantitative uncertainty analyses. The uncertainty analyses indicated that paresthesia
(numbness or tingling) in the hands and feet, and occasionally around the mouth, in adults is not the
most reliable endpoint for dose-response assessment because it is subject .to the patient's recognition
of the effect Paresthesia in adults is no longer the basis for U.S. EPA's methylmercury RfD as it was
in the mid-1980s. There are, however, uncertainties remaining on the current RfD based on
developmental effects from methylmercury in children exposed -in utero. There are difficulties with
reliability in recording and classifying events like late walking in children, especially as the data were
collected approximately 30 months after the child's birth. It should be noted, however, that the
endpoints used represented substantial developmental delays; for example, a child's inability to walk
two steps without support at two years of age, inability to talk based on use of two or three
meaningful words by 24 months, or presence of generalized convulsive seizures. There is uncertainty
in the physiologic factors which were used in estimating the ingested mercury dose. There is also a
degree of uncertainty introduced by the size of the study population (81 mother-child pairs).
Nevertheless, the RfD for methylmercury is a reasonable estimate based on currently available data.
The RfD is supported by additional studies in children exposed in utero. These include
investigations among Cree Indians in Canada and New Zealanders. consuming large amounts of fish.
In these studies the hair concentration of mercury is used to monitor mercury exposure over time.
Conclusions by the investigators in their official reports cite developmental delays among the children
born of mothers whose hair mercury concentrations during pregnancy were 6 to 18 ppm, consistent
with the benchmark dose of 11 ppm.
Currently a number of research studies are underway that further address the question of what
exposures to methylmercury in fish are associated with neurological disease. These studies include
more subjects than did the Iraqi study, are prospective in design, and utilize endpoints that are
anticipated to be more sensitive than the clinical signs and symptoms of methylmercury poisoning
observed in Iraq. These studies of fish consumption, rather than poisoning, are conducted in the
Seychelles Islands in the Indian Ocean, the Faroe Islands in the North Atlantic Ocean (sponsored by
the Department of Health and Human Services), and in the United States; this last study is sponsored
by the Agency for Toxic Substances and Disease Registry (ATSDR). Data from these studies, when
available, should be useful in decreasing the uncertainty surrounding both the benchmark dose and the
RfD. The U.S. FDA has determined that revisions of its action level for mercury concentrations of
fish in interstate commerce should wait until the new studies have reduced the level of uncertainty.
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The availability of results from the above studies will likewise enable U.S. EPA to re-examine and
adjust its RfD as needed.
Levels of Methylmercury Exposure Addressed by the U.S. Food and Drug Administration,
World Health Organization and State Recommendations
The U.S. EPA RfD is a daily intake level and is a risk assessment tool; the use of the RfD is
not limited to fish. The discussion that follows covers risk assessment and risk management activities
concerning fish. These consider fish consumption patterns and risk management policy factors. The
existing advice and action level of the U.S. FDA is compatible with the U.S. EPA RfD and the
assessment information presented in this Mercury Study.
There are numerous local and state warnings in the U.S. to limit intake of fish because Of
chemical contamination. Warnings are issued because of a number of contaminants. Methylmercury
is most often included as one of the contaminants that form the basis for the warning. Often these
warnings are issued based on local conditions.
Recommended limits on methylmercury exposure have been expressed in these units: ng/kg
body weight/day; concentrations of mercury hi tissues such as blood, hair, feathers, liver, kidney,
brain, etc.; grams of fish per day; number of fish meals per time interval (e.g., per week). Reference
values for mercury concentrations (expressed as total mercury) in biological materials commonly used
to indicate human exposures to .mercury were published by the WHO/IPCS (1990). The mean
concentration of mercury in whole blood is approximately 8 ug/L, in hair about 2 ug/g, and in urine
approximately 4 ug/L. Wide variation occurs about these values (WHO/IPCS, 1990).
A number of different estimates exist for hair mercury levels that are associated with low risks
of neurological endpoints such as paresthesia. These estimates are sensitive to variables such as the
half-life of mercury in the body (time to eliminate half the dose of mercury). Half life is usually
estimated as an average of 70 days, with extremes of about 35 to just over 200 days reported for
different individuals. The half-life of mercury in pregnant women has not been directly measured.
The half-life of mercury in women during lactation is shorter, possibly due to excretion of mercury
into milk produced during lactation.
Cross-comparison of
World Health Organization
(WHO) recommendations
regarding risk associated
with hair mercury concentra-
tions is facilitated by data
reported by the WHO on
mercury concentrations in
559 samples of human head
hair from 32 locations in 13
countries. The WHO report
found that mercury concen-
trations in hair increased
with increasing frequency of
fish consumption (see Table
3-3).
Table 3-3
WHO Data on Mercury in Hair
Fish Consumption Frequency
No unusual mercury exposure
Less than one fish meal per month
Fish meals twice a month
One fish meal a week
One fish meal each day
Average Mercury Concentration in
Hair (ng mercury per g of hair)
2
1.4 (range 0.1 to 6.2)
1.9 (range 0.2 to 9.2)
2.5 (range 0.2 to 16.2)
11.6 (range 3.6 to 24.0)
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The World Health Organization's International Programme for Chemical Safety (WHO/IPCS)
concluded that the general population of adults (males and non-pregnant females) does not face a
significant health risk from methylmercury when hair mercury concentrations are under 50 ug
mercury/gram hair. In recent evaluations of the Niigata epidemic of Minamata disease, study authors
reponed lower thresholds with mean values in the range of 25 to approximately 50 ug mercury/gram
hair.
Clinical observations in Iraq suggest that women during pregnancy are more sensitive to the
effects of methylmercury with fetuses at particularly increased risk. The WHO/IPCS (1990) analyzed
the Iraqi data and identified a 30 percent risk to the infant of abnormal neurological signs when
maternal hair mercury concentrations were over 70 ug/g. Using an additional statistical analysis,
WHO/IPCS estimated a 5 percent risk of neurological disorder in the infant when the maternal hair
concentration was 10 to 20 ug mercury/gram of hair. The U.S. EPA RfD is within an order of
magnitude of the dose described by WHO.
WHO/IPCS recommended that as a preventive measure, in a subpopulation that consumes
large amounts of fish (for example, one serving or 100 grams per day), hair levels for women of
child-bearing age should be monitored for methylmercury.
The WHO/IPCS estimated (1990) that a daily methylmercury intake of 0.48 ug mercury/kg
body weight will not cause any adverse effects to adults and that a methylmercury intake of 3 to 7
ug/kg body weight/day would result in a <5 percent increase in the incidence of paresthesia in adults.
Risk to this extent would be associated with hair mercury concentration of approximately 50 to 125 ug
mercury per gram hair. By comparison, the U.S. EPA's reference dose, or the amount of
methylmercury any person (including children and pregnant women) can ingest every day without
harm is 0.1 ug/kg body weight per day. This was based on a benchmark dose equal to 11 ppm (ug/g)
hair. Children are expected to have a higher exposure to methylmercury (on a per kg body weight
basis) than do adults.
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
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 ug, respectively, per kilogram of body weight Using the
values for 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 part 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.
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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, 1979; 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 hi 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.
Characterization of Risk to Human Populations
The characterization of risk to U.S. human populations focuses on exposure to methylmercury.
Although methylmercury is found in other media and biota, it accumulates to the highest concentra-
tions in the muscle tissue of fish, particularly fish at the top of the aquatic food chain. As a result,
fish ingestion is the dominant'exposure pathway. The dominance of this pathway reflects both
bioaccumulation of methylmercury in the fish and the efficiency with which methylmercury passes
through intestinal walls. The critical elements in estimation of methylmercury exposure from fish are
these: the species of fish consumed; the concentration of methylmercury in the fish; the quantity
consumed and the frequency of consumption.
There are three ways to assess the risk to populations from methylmercury exposure. The first
way used in this analysis was based on predicted increases in methylmercury concentrations in fish
due to anthropogenic emissions coupled with predicted exposure to human (and wildlife) populations.
This type of analysis has the advantage of predicting the direct impact of anthropogenic emissions on
fish concentrations. The second way risk was assessed was by using dietary surveys to identify the
amount and type of fish consumed by populations in the U.S. The advantage of this methodology is
that a total exposure from fish can be evaluated, even though the contamination may have come from
sources other than anthropogenic emissions. The third way to determine whether members of the
population are at risk was to consider hair mercury levels as methylmercury exposures for the general
populations are reflected by these levels. This type of assessment would be the best measurement of
actual mercury exposure because biological samples are utilized. These three methodologies and
conclusions regarding the risk characterization are presented below.
Modeled Anthropogenic Emissions and Predicted Fish Methylmercury Levels
The key issue addressed in the risk characterization was whether anthropogenic mercury
emissions from U.S. sources have the potential to increase mercury concentrations hi freshwater fish
such that subsequent consumption of these fish would result hi increased risk to the consumer. Due to
limitations in the science regarding methylation and bioaccumulation of mercury in marine environ-
ments, the analysis did not address the potential impact of these emissions on marine species of fish.
As described hi previous sections, this approach used models to evaluate exposures that result
from atmospheric mercury emissions from U.S. sources. Exposure to mercury consumption from fish
depends on both the mercury concentration in the fish and the amounts of fish consumed. The
modeling analysis predicted that some of the mercury emitted from local emission sources deposits on
local watersheds and water bodies where a fraction of it is methylated and incorporated into the
aquatic food chain. Since mercury emissions are also transported across great distances, the deposition
of mercury from distant sources were also considered to contribute to the environmental loading of
June 1996 3-24 SAB REVIEW DRAFT
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mercury around a single source. As noted in the discussion of the exposure analysis above, the U.S.
EPA concludes that there is a plausible link between anthropogenic emissions and increases in
methylmercury concentrations in freshwater fish.
The highest levels of methylmercury in fish (e.g., greater than 1 ppm) were predicted in the
trophic level 4 fish; that is, those predator species at the top of the food web. These high predictions
generally result from using relatively conservative assumptions. However, the highest predicted fish
concentration was 4.9 ppm which is at the nigh end of the range of measured values in the U.S.
Measured values range from less than 0.1 ppm to 8.94 ppm; typical values are between 0.11 and 0.26
ppm.
Local water bodies in proximity (e.g., within 2.5 km) to industrial and combustion sources that
emit substantial amounts of divalent mercury from low stacks or at a slow rate appear to be more
highly impacted by atmospheric mercury releases. For water bodies located in remote areas, the
predicted concentrations in fish are influenced by the overall proximity to anthropogenic sources,
increased soot and ozone concentrations and elevated rainfall.
Given these potential methylmercury concentrations, the issue becomes the fish consumption
rate of populations eating fish from these waters. Consumption of fish from these waters was assumed
for three types of human populations: an adult with a high fish consumption rate ("high-end
consumer"), a child of a high-end consumer and a recreational angler. The consumption and body
weights used in the analysis are shown below in Table 3-4.
Table 3-4
Body Weights and Fish Consumption Values Used in Exposure Modeling
Subpopulation
Adult High-End Consumer
Child High-End Consumer
Adult Recreational Angler
Assumed Body Weight
(kg)
. 70
17
70
Assumed Local Fish
Consumption Rate
(g/day)
60
20
30
Results of the modeling analysis show that humans consuming fish with predicted mercury
concentrations above 1 ppm at the above consumption rates would be ingesting mercury at levels
approaching or exceeding the product of 10 times the U.S. EPA's RfD. It is not possible to predict the
degree of risk from exposures above the RfD for methylmercury as the RfD is not a probability
estimate. A level of 10 times the RfD is equal to the NOAEL. The NOAEL was the lower bound on
a 10 percent effect level; it is not possible to estimate likelihood of risk in a large population exposed
to the NOAEL.
A limitation of this modeling analysis is that the size of the population potentially at increased
risk cannot be estimated because hypothetical water body locations and exposure scenarios are
employed. It is known, however, that there are locations in the U.S. where fish concentrations exceed
1 ppm. For example, the U.S. EPA has found mercury residues in fish at 92 percent of more than 370
surface water bodies tested in the U.S. Mercury levels above 1 ppm were found at 2 percent of the
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sites surveyed, and above 0.5 ppm at 15 percent of the sites. Figure 3-4 illustrates the geographic
location of these sites. In addition, as mentioned previously, a relationship exists between the
methylmercury content in fish and lake pH, with higher methylmercury concentrations in fish tissue
typically found in more acidic lakes. Although pH is only one parameter that can influence mercury
methylation and subsequent bioaccumulation, it is informative to note geographic areas that are both
predicted to have elevated mercury deposition rates and include surface waters already impacted by
acid deposition. Figure 3-5 displays the regions of the U.S. that meet these criteria.
The potential for a consumer to be at increased risk from fish consumption is modified by at
least three important factors. First, many States have issued advisories regarding the consumption of
certain species of fish from certain water bodies on account of mercury contamination. These
advisories are meant to prevent the public from consuming fish with harmful levels of mercury in
them. Thus, exposures to high concentrations are hopefully avoided. (It is known however, that not
all anglers heed this advice.)
Figure 3-4
Distribution of Mercury Concentrations in EPA-Sampled
Fish Tissue Throughout the U.S.
Pwcaflt at MM in category cumulative
Total Site: 374
Fatal Only Situ: 128
Maximum was Whoto Body:
15
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Figure 3-5
Surface Water with pH < 5.5 and Anthropogenic Mercury Deposition ,
NORTHEAST
WEST
MID-ATLANTIC
COASTAL PLAIN
6-20% pH < = 6.6, Deposition 1-6 ug/m2
6-20% pH < = 6.6, Deposition 6-10 U8/m2
6-20% pH < = 6.6. Depoiltion > 10 ug/m2
> 20% pH < = 6.6, Deposition 1-6 ug/m2
> 20% pH < = 6.6. Depoiltion 6-10 ug/m2
> 20% pH < = 6.5, Deposition > 10 ug/m2
FLORIDA
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Second, most anglers fish from a variety of water bodies. Several studies indicate that many
anglers may travel extended distances to fish. These individuals who consume fish from a variety of
locations decrease their chance of exposure to methylmercury at lexicologically significant doses
because the extent of mercury contamination can differ significantly between water bodies. Although
some areas of the U.S. are known to have fish contaminated with levels above 1 ppm, the national
average for freshwater fish (based on the Lowe et al.s 1984 data set) is 0.11 ppm, or 0.26 ppm based
on data from Bahnick et al., (1985).
Third, some members of the population, even though they consume large quantities of fish, are
likely to obtain their fish from both local water bodies and from commercial sources. By eating a
variety of fish in the diet, including fish obtained commercially, it is likely that fish with a range of
mercury levels are being consumed. A consumer buying fish may be purchasing fish with lower
mercury levels than those locally caught Thus, -overall exposure would be reduced. For example, the
top ten seafood species that make up 80 percent of the seafood market all have methylmercury levels
less than 0.2 ppm. These species are listed in Table 3-5. Note however, that there are some saltwater
species, notably shark and swordfish, that do have elevated levels of mercury. These are not
frequently consumed species, but their mercury levels are sufficiently high to have potential for
increased risk if consumed regularly. Consequently, the FDA advises pregnant women, and women of
childbearing age intending to become pregnant, to limit their consumption of shark and swordfish to
no more than once a month.
The FDA advises persons other than pregnant women and women of child-bearing age to limit
their consumption of fish species with methylmercury levels around 1 ppm to about 7 ounces per week
(about 1 serving). For fish with levels averaging 0.5 ppm, regular consumption should be limited to
about 14 ounces per week ( about two servings). Consumption advice is unnecessary for the top 10
seafood species listed in Table 3-5 as mercury levels are low and few people eat more than the
suggested weekly limit of fish (2.2 pounds) for this level of contamination. These consumption
advisories are consistent with the findings of the U.S. EPA's modeling analyses described above.
In summary, conclusions that can be drawn from the above discussion are these.
Emission sources can plausibly linked with incremental increases in fish
methylmercury levels in surface waters in the U.S.
In some regions of the U.S., fish levels approach or exceed 1 ppm. Increased or
continued mercury deposition from anthropogenic emission sources in the U.S. have
the potential to increase the mercury concentration in fish above current levels.
The populations with the highest potential of increased risk are those who routinely
and exclusively eat freshwater fish from a single location or region that is known to be
impacted by mercury contamination. Consumers are thus urged to heed the advice of
state and local health departments concerning local conditions.
The typical consumer eating fish in moderation from a variety of sources and eating a
variety of fish species are not believed to be at increased risk. These consumers are
not being advised to limit fish consumption.
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Table 3-5
Mercury Concentrations in the Top Ten Types of Fish
Consumed by U.S. Residents
Fish"
Mercury
Concentration
(»»g/g, wet
weight)*
Comments
Tuna
0.206
The mercury content for tuna is the average of the mean
concentrations measured in 3 types of tuna: albacore tuna (0.264
ug/g), skipjack tuna (0.136 ug/g) and yellowfin tuna (0.218 ug/g).
The U.S. FDA measured the methylmercury concentration in 220
samples of canned tuna in 1991; the average amount of
methylmercury measured in these samples was 0.17 ug/g and the
measured range was <0.1 - 0.75 ug/g (Yess, 1993).
Shrimp
0.047
The mercury content for shrimp is the average of the mean
concentrations measured in seven types of shrimp: royal red
shrimp (0.074 ug/g), white shrimp (0.054 ug/g), brown shrimp
(0.048 ug/g), ocean shrimp (0.053 ng/g), pink shrimp (0.031
ug/g), pink northern shrimp (0.024 ug/g) and Alaska (sidestripe)
shrimp (0.042 ug/g).
Pollack
0.15
The Pesticide and Chemical Contaminant Data Base for U.S.
FDA (1991/1992) reports the methylmercury concentration in
pollack in commerce as 0.04 ug/g.
Salmon
0.035
The mercury content for salmon is the average of the mean
concentrations measured in five types of Salmon: pink (0.019
ug/g), chum (0.030 ug/g), coho (0.038 ug/g), sockeye (0.027
ug/g), and Chinook (0.063 ug/g).
Cod
0.121
The mercury content for cod is the average of the mean
concentrations in Atlantic Cod (0.114 ug/g) and the Pacific Cod
(0.127 ug/g).
Catfish
0.088
0.16
The sources of mercury content in catfish are Bahnick et al.,
1994 and Lowe et al., 1985. Both data sets were collected from
U.S. freshwater sources. The Bahnick data (mean = 0.088)
include channel, largemouth, rock, striped and white catfish, and
the Lowe data (mean = 0.16) include channel and flathead
catfish. It should be noted that neither survey included farm-
raised catfish, which is the type of catfish predominantly
consumed in the U.S. The mercury content of farm-raised catfish
may be significantly different than freshwater sources. The
Pesticide and Chemical Contaminant Data Base for U.S. FDA
(1991/1992) reports the methylmercury concentration in catfish as
0.02 ug/g.
Clam
0.023
The mercury content for clam is the average of the mean
concentrations measured in four types of clam: hard (or quahog)
clam (0.034 ug/g), Pacific littleneck clam (0 ug/g), soft clam
(0.027 ng/g), and geoduck clam (0.032 ug/g).
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Table 3-5 (continued)
Mercury Concentrations in the Top Ten Types of Fish
Consumed by U.S. Residents
Fish3
Mercury
Concentration
(Hg/g, wet
weight)1*
Comments
Flatfish (Flounder)
0.092
The mercury content for flounder is the average of the mean
concentrations measured in nine types of flounder: Gulf (0.147
ug/g), summer (0.127 ug/g), southern (0.078 ug/g), four-spot
(0.090 ug/g), windowpane (0.151 ug/g), arrowtooth (0.020 ug/g),
witch (0.083 ug/g), yellowtail (0.067 ug/g), and winter (0.066
Crab
0.117
The mercury content for crab is the average of the mean
concentrations measured in five types of crab: blue crab (0.140
ug/g), dungeness crab (0.183 Mg/gX king crab (0.070 ng/g), tanner
crab (C. opilio) (0.088 ug/g), and tanner crab (C. bairdl) (0.102
Scallop
0.042
The mercury content for scallop is the average of the mean
concentrations measured in four types of scallop: sea (smooth)
scallop (0.101 ug/g), Atlantic Bay scallop (0.038 ug/g), calico
scallop (0.026 ug/g), and pink scallop (0.004 ug/g).
a List of fish types from U.S. FDA (1995).
b Mercury concentrations sources are described in the comments, refer to Volume III for complete citations.
Human Exposure to Methylmercury Based on Dietary Surveys
The discussion above focused on potential risk to human populations due to consumption of
fish having relatively high concentrations of mercury. The analysis of mercury exposure using dietary
surveys is aimed at identifying populations that eat much greater amounts of fish than the average
consumer. Their potential for increased risk is not necessarily due to elevated concentrations in fish, it
is more a function of the amount of fish consumed on a regular, usually daily, basis.
The analysis of the at-risk population eating above average amounts of fish focuses on that
part of the population which consumes on average 100 grams or more of fish or shellfish per day
(approximately 3.5 ounces). The basis for this focus on persons eating 100 grams or more is a
recommendation made by the World Health Organization's International Programme for Chemical
Safely (WHO). The WHO'S recommendation is that as a preventive measure, in a subpopulation that
consumes large amounts of fish (for example, 100 grams per day), hair levels for women of child-
bearing age should be monitored for methylmercury.
General U.S. Population. Three groups are potentially at increased risk from methylmercury:
pregnant women, women of child-bearing age (i.e., between the ages of 15 and 44) and children ages
14 and younger. Pregnant women are of concern because of the adverse effects of methylmercury on
the fetal nervous system. Women of child-bearing age rather than only pregnant women are of
concern for two reasons. The first is that methylmercury persists in tissues. Measured half-lives for
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methylmercury in adults range from about 1 month to 9 months, although half-lives of just over 2
months are usually observed. Thus, dietary intakes just prior to pregnancy are of concern rather than
only methylmercury intakes during pregnancy. The second reason is that women usually do not know
they are pregnant until the pregnancy is past many of the critical stages of fetal development.
Children may be at a higher risk of methylmercury exposure than are adults because they
appear to have higher exposures on a per kilogram body weight basis, and they may be inherently
more sensitive than adults given the developmental state of the nervous system. In the methylmercury
poisoning epidemics in Japan and Iraq, children were affected, as well as adults. These effects were
not seen only in children exposed to methylmercury in caero, but included children exposed through
ingesting methylmercury from food. Whether or not children differ from adults in sensitivity to
methylmercury neurotoxicity is not known.
The 100 gram per day ^commendation by the WHO can be used as a screening analysis to
identify populations potentially at increased risk. The significance of the risk is, as mentioned above,
is also a function of the methylmercury concentrations of the fish consumed. The U.S. EPA used two
types of dietary surveys to identify these populations.
Dietary surveys can be classified into longitudinal or cross-sectional surveys. Cross-sectional
data are used to give a "snap shot" in time and are typically used to provide information on the
distribution of intakes for groups within the population of interest. Cross-sectional data typically are
for 24-hour or 3-day sampling periods and may rely on recall of foods consumed following
questioning by a trained interviewer, or may rely on written records of foods consumed. The cross-
section al survey used in the Report was the Continuing Surveys of Individual Food Consumption for
the period 1989 to 1991. Typically long-term or longitudinal estimates of intake can be used to reflect
patterns for individuals (e.g., dietary histories); or longitudinal estimates of moderate duration (e.g.,
month-long periods) for individuals or groups. A longitudinal survey used for comparative purposes
in this Report is the National Purchase Diary, Inc survey. Additional discussion of these issues are
found on Appendix H to Volume HI.
The U.S. EPA first evaluated data on fish consumption for a general population in the United
States based on the United States Department of Agriculture's Continuing Surveys of Individual Food
Consumption for the period 1989 to 1991 (CSFII 89/91). (The survey and analyses are described in
detail Appendix H to Volume HI and discussed in Volume VI.) The CSFII 89/91 was a cross-
sectional survey conducted over a three-year period. Participants kept three-day food consumption
diaries which recorded the amount of various foods consumed as well as self-reported body weight.
Data were collected over all months of the year and included week days as well as weekends.
During the past decade, reviewers of dietary survey methodology (for example, the Food and
Nutrition Board of the National Research Council/National Academy of Sciences; the Life Sciences
Research Office of the Federation of American Societies of Experimental Biology) have evaluated
various dietary survey techniques with regard to their suitability for estimating exposure to
contaminants and intake of nutrients. Having evaluated a number of data sets, the Academy's
Subcommittee concluded that 3 days of observation may be more than is required for the derivation of
the distribution of usual intakes.
For comparison, the U.S. EPA also used results of a longitudinal food consumption survey;
namely, the National Purchase Diary, Inc. data on fish consumption which was conducted between
1973 and 1974 (NPD).
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Based on the reported quantities of fish consumed in CSFII 89/91, it was estimated that 2 to 5
percent of the population of women who consume fish eat, on average, 100 grams of fish per day.
The CSFII 89/91 does not include the States of Alaska and Hawaii. Because of their substantial
coastal areas, fish consumption in these states is likely to be higher than in the continental U.S. Thus,
the population calculated to consume fish in excess of 100 grams per day may be underestimated
using this survey. In the one-month sampling period surveys conducted by NPD about 1 percent of
the study population consumed over 100 grams of fish per day. Thus, the results of the two surveys
are in substantial agreement. Using 1990 census data, the number of women of child-bearing age and
of children were calculated with the result multiplied by the percentages above to provide- the size of
the various populations consuming greater than 100 grams of fish per day. The number of women of
child-bearing age who are also pregnant was calculated using the public health statistic that in any
given year approximately 9.5 percent of women are pregnant The results of these analyses are
summarized below in Table 3-6. The range represents the results of both dietary surveys.
Table 3-6
Size of the Populations of Concern Consuming 100 Grams or More of Fish Per Day
Population of Concern
Women of Child-Bearing Age
Pregnant Women
Children
Size of the Population
(Represents 1* to 5" percent of total population)
547,000* - 887,000"
51,900' -84,300"
503,000* -665,000"
" Based on data from CSFII/89-91
been reported to increase, on average, 26% between 1970 and 1990. Because it is uncertain whether
this increase applies to the extremes of the distribution, no attempt was made to adjust the estimate
derived from the. 1973/74 NPD data set.
Populations Consuming Greater than 100 Grams of Fish Per Day. Populations that consume
fish at 100 grams per day or higher are likely to include several segments of the U.S. population. The
term "subsistence fishers" has been used to describe various persons who rely on fish as a major
source of protein. "Subsistence fishers" are not defined by whether the fish/shellfish are self-caught or
purchased. Groups with high fish intake are typically determined by social, economic, ethnic, and
geographic characteristics. An additional group of people consumes high levels of fish in response to
numerous health-based messages that have promoted the consumption of fish to reduce the likelihood
of disease, particularly of the cardiovascular system. Furthermore, there are large numbers of people
who simply prefer fish and shellfish as a source of protein. These consumers are represented by these
groups: recreational anglers, members of some Native American Tribes, members of ethnic groups
who consume higher than typical intakes of fish, persons who preferentially select fish for health-
promotion purposes, individuals who prefer the taste of fish, and persons who rely on self-caught fish
from local sources because of limited money to buy food. All of these groups may be more reliant on
local sources of self-caught fish than is the general population.
Whether or not the dietary surveys of the general population described above adequately
represent subpopulations, such as recreational anglers, subsistence fishers, or Native Americans
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remains a concern. Some groups, for example native populations in Alaska, consume on average,
quantities of fish and marine mammals far higher than the overall U.S. population. A review of the
published literature on quantities of fish eaten by groups that include anglers, subsistence fishers, and
some Native Americans shows that average fish intake is higher for these groups than for the general
population. Figure 3-6 illustrates the various consumption rates of these groups.
Estimates of mercury exposure for these consumers can be made based on the following
scenario. The average mercury concentration of the mixture of fish/shellfish reported in CSFII 89/91
to be consumed by persons surveyed is 0.134 ppm. As a result, a 100 gram serving would contain
13.4 ug mercury per 100 grams. The quantities of fish and shellfish consumed by women of child-
bearing age at the 95th, 97.5th and 99th percentile of respondents in CSFII 89/91 are 111, 133 and
175 grams per day, respectively. Analysis of the CSFII 89/91 data also indicated that 33 percent of
fish and shellfish come from freshwater and estuarine species. Data for Wisconsin anglers and for
members of "two tribes of Native Americans living near Puget Sound support the observation that
persons who consume freshwater or estuarine fish, also purchase a portion of their fish/shellfish in the
marketplace.
Based on the scenario that both locally caught and commercial fish are consumed,
consumption of fish from local sources would increase exposure above that which would be expected
if only commercial fish were consumed, if the fish from local sources contained more than 0.134 ppm
mercury. If local sources are lower in mercury than 0.134 ppm, exposure to local sources could
reduce overall mercury exposure. However, if fish high in the aquatic food web are preferentially
consumed, mercury exposure would increase. Data from several states indicate that mercury
concentrations of locally caught fish are likely to be higher than 0.134 ppm.
Determining actual methylmercury intake for groups that rely on locally caught fish or fish
obtained from a limited geographic region or from only a few species of fish requires individual
assessment. These particular groups of women and children would benefit from a more specific
evaluation. An example of such evaluation is a biological monitoring program based on analyses of
hair or blood for mercury.
In summary, the U.S. EPA analysis indicates that the commercial U.S. fish supply is safe for
the U.S. population who consume less than 100 grams/day of fish and shellfish, and a wide variety of
fish types. Those consumers who eat large quantities of predatory marine species may be at some
level of risk from exposure to methylmercury. Consumers of freshwater fish are also, in general, not
expected to be at an elevated risk level, unless their sources of fish are contaminated with more than
average levels of mercury.
Hair Mercury Measurements
Actual measurements of hair mercury levels would be the best way to assess mercury exposure
and risk because mercury exposure is reflected by hair mercury levels. Because fish are the primary
exposure pathway for methylmercury there is a broad-based scientific literature describing increases in
hair mercury concentrations with increases in fish consumption. Maternal hair mercury concentrations
predict mercury concentrations in fetal brain, fetal blood, umbilical cord blood and newborn hair.
The WHO has concluded that the general population of adults (males and non-pregnant
females) does not face a significant health risk from methylmercury when hair mercury concentrations
are under 50 |jg mercury/gram hair. However, in recent evaluations of the Niigata epidemic of
June 1996 3-33 SAB REVIEW DRAFT
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Figure 3-6
Distribution of Fish Consumption Rates of Various Populations
4S2-,
Wolfe & Walker '87 Highest Response Group Mean in AK
350 -
156'
109'
-I CRITFC'94 99th %ile Adult
LEGEND - POPULATIONS
GENERAL U.S. POPULATION
NPD 73/74 §
CSFII
RECREATIONAL ANGLERS
PUFFER
FIORE
CONNELY
SUBSISTENCE FISHERS
WOLFE & WALKER A
NATIVE AMERICANS
CRITFC A
TOY. TULALIP 9
NOBMAN
EPA '92 Wl TRIBES
| Toy '95 Tulalip Tribe 90th %ile
Fiore '89 95th %ile
Wl Anglers
NPD 73/74 Adult 99th %ile j
Nobmann '92 AK Tribes Mean |
| CRITFC '94 Adult Mean
Toy '95 Tulalip Tribe Median
Puffer '81 Median1+
Fiore '89 75th %ile Wl Anglers
NPD 73/74 Adult 90th %ile
Anglers 10^|Connoly '90 NY Anglers Mean]+
| CSFII Age 15-44 Mean ? andc?
NPD 73/74 Adult 50th %ile
3-34
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Minamata disease, study authors reported lower thresholds with mean values in the range of 25 to
approximately 50 ug mercury/gram hair.
In addition, clinical observations in Iraq suggest that women during pregnancy are more
sensitive to the effects of methylmercury with fetuses at particularly increased risk. The WHO
analyzed the Iraqi data and identified a 30 percent risk to the infant of abnormal neurological signs
when maternal hair mercury concentrations were over 70 ug/g. Using an additional statistical analysis,
WHO estimated a 5 percent risk of neurological disorder in the infant when the maternal hair
concentration was 10 to 20 ug mercury/gram of hair.
Although data on hair mercury concentrations from a sample representative of the United
States population with adequate documentation of quality assurance/quality control do not exist, data
from individual studies conducted within the United States are available and are discussed in Volume
VI (See Table 5-8 of that volume). These surveys were conducted in widely diverse geographic areas
within the United States. The mean hair mercury concentrations identified for subjects in these studies
are typically under 1 ug/g or 1 ppm. For a number of the surveys the detection limit was greater than
1 ppm indicating that a substantial number of zero or trace values were included in the mean
concentration. The maximum values reported in these individual surveys range from 2.1 to 15.6 ppm.
The highest maximum value (15.6 ppm) was reported from a study that specifically focused on
persons from the Florida Everglades who consumed wildlife from this area.
Unpublished data (submitted to U.S. EPA by the U.S. Food and Drug Administration) from
the early 1980s on a group of United States women of child-bearing age indicate a mean hair mercury
concentration of 0.48 ppm for the overall sample of 1,431 women and a mean hair mercury
concentration of 0.52 ppm for the 1,009 women who reported consuming some seafood (statistical
analyses of variability in these data [e.g., error estimates] were not included in the data provided to
U.S. EPA). These mercury concentrations correspond to hair mercury concentrations associated with
fish consumption at the level of less than one meal per month to one meal per week based on data
shown in Table 3-3. The magnitude of the increase in hair mercury concentration shown in these
unpublished data for those women reporting seafood consumption differs from the patterns observed in
most surveys identified in the literature on hair mercury concentrations. This difference between these
unpublished data and most literature underscores the need for survey data on hair or blood mercury
concentrations from a representative sample of the United States population to estimate body burden of
mercury in the general United States population. Until appropriate survey data for the general United
States population exist, the overall pattern of hair mercury concentrations for the United States remains
unclear. For adequate prediction of methylmercury exposure for the general United States population,
the data should be obtained from subjects who are chosen based on a sampling strategy that can be
extrapolated to the United States population, and must include appropriate quality assurance/quality
control procedures.
Summary of the Risk Characterization
Individuals who consume more than 100 grams (about 3.5 ounces) of fish per day on average
may ingest methylmercury in quantities which could pose health effects. Groups such as some Native
Americans or subsistence fishermen do consume fish in these large quantities for cultural or economic
reasons. The U.S. EPA estimates that between 1 and 5 percent of the U.S. population eats fish in
these quantities. The actual risk of exposure depends on the methylmercury concentration of the fish
being consumed. These individuals should avoid frequent consumption of fish species which have
relatively high methyl mercury concentrations. Pregnant women, women of child-bearing age and
June 1996 3-35 SAB REVIEW DRAFT
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children consuming above 100 grams (about 3.5 ounces) of fish per day are the populations of most
concern.
Fish which have relatively high concentrations of methylmercury have been identified by the
FDA to include shark and swordfish. Fish consumers should also follow advice of States and local
authorities regarding fish caught in fresh water bodies for which mercury consumption advisories have
been issued.
Limitations of the Assessment
The primary purpose of the Mercury Study Report to Congress was to assess the impact of
U.S. anthropogenic emissions on mercury exposure to humans and wildlife. The size of some,
populations of concern have been estimated; namely women of child-bearing age and children who eat
fish. In the general population, people typically obtain their fish from many sources. The question on
whether or not the impact of mercury from anthropogenic ambient emissions can be proportioned to
the overall impact of methylmercury on wildlife is a much more difficult issue.
As with environmental monitoring data, information on body burden of mercury in populations
of concern (blood and/or hair mercury concentrations) are not available for the general U.S.
population. Data oh higher-risk groups are currently too limited to discern a pattern more predictive
of methylmercury exposure than information on quantities of fish consumed. The selenium content of
certain foods has been suggestive as a basis for modifying estimates of the quantities of
methylmercury that produce adverse effects. Currently, data on this mercury/selenium association
form an inadequate basis to modify quantitative estimates of human response to a particular exposure
to mercury.
Available data for human health risk assessment have limitations as described in the Report
and in this summary. Studies of human fish-consuming populations in the Seychelles and Faroes
Islands address some of these limitations; they are expected to be published within a year of release of
this Report. Additional studies on U.S. populations who consume fish from the Great Lakes are in
progress. Public health agencies of the U.S. government as well as the U.S. EPA will evaluate these
new data when they are available.
The benchmark dose methodology used in estimating the RiD required that data be clustered
into dose groups. Most data on neurologically based developmental endpoints are continuous; that is,
not assigned to dose groups. For example, scoring on scales of IQ involves points rather than a
"yes/no" type of categorization. Measurements on the degree of constriction of the visual field involve
a scaling rather than a "constricteoVunconstricted" type of variable. Although arbitrary scales can be
constructed, these groupings have generally not been done in current systems. Use of alternative dose
groupings (as described in Volume IV) had no significant effect on calculated benchmark doses. An
additional difficulty occurs in estimation of benchmark dose for multiple endpoints that have been
measured. Further research on appropriate methods for mathematical modeling is needed. For some
situations such information is known, but for methylmercury exposure and multiple endpoints
assessing the same system (i.e., developmentally sensitive neurological, neuromotor and
neuropsychological effects) the time-course/dose-response of such changes have not been clearly
established. Development of the mathematical models needs to be accompanied by understanding the
physiological/pathological processes of methylmercury intoxication.
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How Much Methylmercury Exposure is Harmful to Wildlife and What Are the Effects?
Massive poisonings of birds and wildlife from methylmercury-treated seed grains were
identified during the decades preceding the 1970s. These findings resulted in substantial limitation on
use of methylmercury-treated seed grains. However, methylmercury contamination of the aquatic
foodchain from many sources continues to adversely affect wildlife and domestic mammals and wild
birds. In Minamata, Japan from about 1950-1952 (prior to recognition of human poisonings) severe
difficulties with flying and other grossly abnormal behavior was observed among birds. Signs of
neurological disease including convulsions, fits, highly erratic movements (mad running, sudden
jumping, bumping into objects) were observed among domestic animals, especially cats that consumed
seafood.
Generally the place of wildlife in the aquatic foodchain of the ecosystem and their feeding
habits determine the degree to which the species is exposed to methylmercury. Fish-eating
(piscivorous) animals and those which prey on other fish-eaters accumulate more mercury than if they
consumed food from terrestrial food chains. In a study of fur-bearing animals in Wisconsin, the
species witfi the highest tissue levels of mercury were otter and mink, which are top mammalian
predators hi the aquatic food chain. Top avian predators of aquatic food chains include raptors such as
the osprey and bald eagle. Smaller birds feeding at lower levels in the aquatic food chains also may
be exposed to substantial amounts of mercury because of their high food consumption rate
(consumption/day/gram of body weight) relative to larger birds.
Laboratory studies under controlled conditions can be used to assess the effects of
methylmercury from fish on mink, otter and several avian species. Effects can occur at a dose of 0.25
ug/g bw/day or 1.1 ug/g methylmercury hi diet. Death may occur in species at 0.1-0.5 ug/g body
weight/day or 1.0-5.0 ug/g hi the diet. Smaller animals (for example, minks, monkeys) are generally
more susceptible to mercury poisoning than are larger animals (for example, mule deer, harp seals).
Smaller mammals eat more per unit body weight than larger mammals. Thus, smaller mammals may
be exposed to larger amounts of methylmercury on a body weight basis.
Whole body residues of mercury in acutely poisoned birds usually exceed 20 ug/g fresh
weight. Although sublethal effects include a number of different organ systems, reproductive effects
are the primary concern. These occur at concentrations far lower than those that cause overt toxicity.
The broad ecosystem effects of mercury are not completely understood. No applicable studies
of the effects of mercury on intact ecosystems were found. Consequently, characterization of risk for
non-human species did not attempt to quantify effects of mercury on ecosystems, communities, or
species diversity. The characterization focused on quantities of mercury that adversely affects the
health of sensitive subpopulations of wildlife species and on the co-location of these populations with
areas of elevated mercury exposure secondary to ambient, anthropogenic emissions of methylmercury.
To this end wildlife criteria (WC) were calculated for three piscivorous birds and two mammals (see
Table 3-7). The WC is a mercury level in water which is expected to be without harm for the species.
The WC considers the bioaccumulation of mercury in the large and small fish eaten by the mammals
or birds. A bioaccumulation factor (BAF) was used in the WC calculation; the BAF was based on
data on mercury in fish and the water from which they were taken. The effects data for mammals
were from a short-term study of neuro-toxicity in mink. The data for fish-eating birds were from a
3-generation study in mallard ducks.
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Table 3-7
Wildlife Criteria for Mercury
Organism
Mink
River otter
Kingfisher
Osprey
Bald eagle
Wildlife Criterion
(pg/L)
415
278
193
483
538
The evaluation of data and calculation of WC hi this Report was done in accordance with the
methods and assessments published in the Final Water Quality Guidance for the Great Lakes System:
Final Rule. Availability of additional data led to differences in calculated values of the WC in this
Report and those published in the final rule. Differences were the result of three factors. The Report
uses more recent data to derive BAF. Second, the final rule appropriately used some region-specific
assumptions that were not used in the nationwide assessment in the Report; for example, consumption
of herring gulls by eagles. Finally different endpoints were used for the evaluation of mammals
because the purposes of the assessments in the Report and final rule were different. In the final rule, a
risk-management decision was made to base the wildlife criterion on endpoints likely to influence
whole populations (mortality, growth). In this Report a more sensitive endpoint was selected with the
goal of assessing the full range of effects of mercury. The difference in the results reflects the amount
of discretion allowed under Agency Risk Assessment Guidelines.
In restricted wildlife populations, effects of mercury originating from point sources have been
conclusively demonstrated and provide a residue basis for evaluation of mercury levels in animal tissue
as an indicator of risk to other populations. Although clear causal links have not been established,
mercury originating from airborne deposition may be a contributing factor to population effects on
bald eagles, river otters and mink. There is evidence to support the possibility of toxic effects on the
common loon and the Florida panther.
Derivation of a WC to protect the Florida panther is complicated by the possibility that prey
items (e.g., raccoon) accumulate mercury to an even greater extent than the fish represented by trophic
level 4. Other prey (e.g., deer) probably contain relatively lower levels of mercury. Calculation of a
WC protective of the panther, therefore, requires collection of additional information on the diet of this
species and mercury residues in that diet Existing data are insufficient to support such an analysis. A
chronic NOAEL for domestic cats was reported to be 20 ug/kg/d. This is close to that of 5.5 ug/kg/d
estimated for mink (that is, the subchronic NOAEL of 55 ug/kg/d divided by a UFS of 10). Cats (and
presumably larger felines) do not, therefore, appear to be uniquely sensitive or insensitive to the toxic
effects of mercury.
Methylmercury (as described in Volumes IV and V of this Report) has deleterious effects on
the chordate nervous system. The human health endpoint of concern is developmental neurotoxicity.
The health endpoints of concern for the avian wildlife species are reproductive and behavioral deficits
and for the mammalian quadrupeds are neurological effects. Assuming that the effects are of similar
June 1996
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concern for the well-being of individuals within a species, the NOAELs, LOAELs and the human and
wildlife WCs for these health endpoints can then be compared across species.
The human benchmark dose of 11 ppm mercury in hair was considered operationally
equivalent to a NOAEL in the derivation of the methylmercury RfD. A LOAEL of 52.5 ppm mercury
in hair was estimated for purposes of this risk characterization from inspection of data in Table 5-4 of
Volume VI. The NOAEL of 11 ppm mercury in hair and the LOAEL of 52.5 ppm mercury in hair
correspond to ingestion levels of 1 ug/kg-day and 5.3 ug/kg-day, respectively; these dose conversions
were made by applying the methods for converting hair mercury concentrations to ingestion levels
used in the derivation of the RfD in Volume IV of this Report.
The avian RfD was based on the data from a series of studies by Heinz and collaborators on
mallard ducks. A NOAEL could not be identified The estimated LOAEL, based on reproductive and
behavioral effects, was 64 ug/kg bw/day. The mammalian RfD was based on the data from a series of
\ studies by Wobeser and collaborators done on ranch mink. A NOAEL of 55 ug/kg bw/day was
estimated frojn these studies. The estimated LOAEL, based on damage to the nervous system and
liver, was 180 ug/kg bw/day.
Based on the data developed for the health assessment, the human LOAEL and RfD are orders
of magnitude lower than the corresponding LOAELs and RfD.of the other animals (Table 5-5). There
is a great deal of uncertainty in this comparison. It must be noted that the effects in humans are based
on the RfD definition of a critical effect; that is the most sensitive reported adverse effect or indicator
of adverse effect The effects reported for mammals (i.e., neurologic damage in the mink) and birds
(i.e., reproductive effects in mallards) would be considered frank effects in the human RfD
methodology. The observations in laboratory animals indicate that it would be reasonable to expect
more subtle and less damaging effects of methylmercury to occur at lower doses than the wildlife
LOAEL and NOAEL.
The information assessed hi this Report suggests that ecosystems most at risk from airborne
releases of mercury exhibit one or more of the following characteristics:
They are located in areas where atmospheric deposition of mercury is high;
they include surface waters already impacted by acid deposition;
they possess characteristics other than low pH that result in high levels of
bioaccumulation; and/or
they include sensitive species.
Areas which meet the first two criteria are delineated in Figure 3-5 The adverse effects of
methylmercury on wildlife have been described and quantified. For wildlife the importance of
site-specific effects of mercury exposure are anticipated to be greater than for humans in the general
population because wildlife obtain their fish from a much more limited geographic area than do
people.
Limitations of the Assessment
There is uncertainty and variability associated with each WC. These include lack of long-term
studies for mammals, lack of a no adverse effects level for birds, and extrapolation from one species to
June 1996 3-39 . SAB REVIEW DRAFT
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another. It is not known if the species selected for WC development are the most sensitive or
appropriate species; also, it is hot known if protecting individual animals or species will guarantee
protection of their ecosystem from harmful effects of mercury. There are uncertainties and expected
variability in the BAF; it was the subject of a quantitative uncertainty analysis.
For wildlife risk assessment, as for humans, mercury toxicity among wildlife involves
neurological effects. Available toxicology data from laboratory-based studies of wildlife exposed to
methylmercury have measured only gross clinical signs and symptoms of disease and death or
pathological changes accompanying these clinically evident changes. Physiologically based evaluation
of wildlife has not been done. The importance of more subtle endpoints of neurological function is
anticipated to be relevant to such practical questions as the ability of visual hunters such as the loon to
find food.
The risk assessment for wildlife made the assumption that the primary source of mercury
exposure to the selected species was contaminated fish. Since mercury bioaccumulation is largely
through aquatic ecosystems, it is reasonable to focus attention on wildlife species whose feeding habits
are tied to these systems. Existing data permit a general treatment of mercury exposure and effects on
such populations. For some species, such as the kingfisher and river otter, it can be reasonably
assumed that fish always comprise a high percentage of the diet. For others, such as the eagle and
mink, considerable variations in diet are likely to exist Still others, such as the Florida panther,
consume prey (such as the raccoon) which consume variable amounts of aquatic biota, but which in
South Florida are closely linked to the aquatic food chain. A more accurate characterization of the
risk posed by mercury to a specific group of animals occupying a given location will depend on the
collection of necessary supporting information: food habits, migratory behavior, breeding biology, and
mercury residues in preferred prey items.
To improve the characterization of risk, research needs highlighted in the preceding sections
should be addressed. Additional work to decrease uncertainty should be directed toward the exposure
assessment. Validated local and regional atmospheric fate and transport models are needed. This
should utilize long-term national monitoring networks. Data to improve understanding of movement
of mercury through environmental media are also needed. The bioaccumulation factors are major
sources of uncertainty. Tin's uncertainty will be decreased by improved data to use in the parameters
of the bioaccumulation factor equations and by increased understanding of mercury biogeochemistry in
water bodies.
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4. MANAGEMENT ALTERNATIVES
Possible Control Strategies
Effective control of mercury emissions may require a mix of strategies including pollution
prevention, materials separation and conventional regulatory strategies to control mercury emissions at
the stack. Pollution prevention would be suitable for those processes or industries where a mercury
substitute is demonstrated and available (e.g., mercury cell chlor-alkali plants). Material separation is
an appropriate approach for processes where mercury-containing products are disposed of by
incineration, or where mercury can be reduced in the fuel prior to the fuel being combusted (e.g.,
medical waste incineration). The third approach, conventional regulatory strategies, may be applicable
when mercury is emitted to the environment as a result of trace contamination in fossil fuel or other
essential feedstock in an industrial process (e.g., smelting). Other non-traditional approaches such as
emissions trading or application of a use tax, or other market-based approaches may also prove
feasible for mercury control. In addition, emissions control is only one possible means for risk
control; reduced human exposure, for example through the use of fish advisories, is another alternative
that would need to be explored when selecting among strategies for reducing risks to human health
(though not to ecosystems).
The analyses of control technologies and costs presented in this Report are not intended to
replace a thorough regulatory analysis, as would be performed for a rulemaking. The information
presented is intended to present the range of available options and provide a relative sense of the
extent of mercury reductions achievable and the general magnitude of the cost of such reductions.
The three major types of control techniques reviewed are:
Pollution prevention measures, including product substitution and process modification;
Materials separation; and
Flue gas treatment technologies.
Table 4-1 summarizes mercury control techniques for selected source categories.
a
Pollution Prevention Measures
One possible means of achieving reductions in mercury emissions is through the use of
pollution prevention or source reduction. Such approaches to achieving reductions involve changes in
processes or inputs to reduce or eliminate emissions of mercury from a particular product or process.
They could include, for example, the replacement of mercury with an appropriate substitute or the use
of low-mercury content inputs.
In considering opportunities for pollution prevention or source reduction it is important to
consider both the potential reductions achievable and the costs of these options. Any consideration of
the potential reductions should examine whether (and the extent to which) emission reductions from
the particular sources in question will yield reductions in risk to public health and the environment. It
is also essential to understand the costs associated with implementing a pollution prevention measure,
including any changes in the quality of the end product. ,,
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Table 4-1
Summary of Mercury Control Techniques for Selected Source Types
Mercury Control
Technique
Applicable Source
Type
Estimated Mercury
Removal Efficiency
Cross-Media
Impacts?"
Other Pollutants
Controlled
Comments
Product substitution (e.g.,
batteries, fluorescent lights)
MWCs, MWIs
Variable, depending on the
extent of substitution
Yes
Could include other
components of mercury-
containing batteries,
fluorescent lights and
other products
Product substitution has reduced the use of
mercury in household batteries
Use of mercury-containing fluorescent lights lias
increased because of their energy efficiency, but
lower mercury content is being achieved
The impact of product substitution to other areas
depends on specific circumstances, including
technical and economic feasibility
Process modification
Mercury cell chlor-
alkali plants
100%
Yes
None directly
In 1994, about one-half of the chlor-alkali plants
used mercury-free processes
Because the membrane cell process has lower
electricity demands than the mercury cell process,
plant conversion results in an energy savings
Additional savings presumably also result by
avoiding costs of recycling or disposing of
mercuric wastes
Materials separation
MWCs and MWIs
Variable, depending on the
extent of separation
Yes
Could include other
components of mercury-
containing wastes
burned in MWCs or
MWIs
Separation of low-volume materials containing
high mercury concentrations (e.g., butteries,
fluorescent lights, thermostats and other electrical
items) can reduce mercury input to a combustor
without removing energy content of the waste
stream
Household battery separation has been
implemented by several communities; program
efficiency ranges from 3 to 25 percent
Pilot studies conducted at hospitals have been
successful
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Table 4-1 (continued)
Summary of Mercury Control Techniques for Selected Source Types
Mercury Control
Technique
Carbon filter beds
Wet scrubbing (single-
staged polishing scrubbers)
Depleted brine scrubbing
Treated activated carbon
adsorption
Applicable Source
Type
MWCs, utility
boilers, industrial
boilers
MWCs, MWls,
boilers
Chlor-alkali plants
Chlor-alkali plants
Estimated Mercury
Removal Efficiency
99%
Can be >90% for water-
soluble species; limited for
elemental mercury
98% '
90%
Cross-Media
Impacts?"
Yes
Yes
Yes
Yes
Other Pollutants
Controlled
Residual organic
compounds, other heavy
metals, S02, acid gases
Acid gases, metals,
particulate matter,
dioxins, furans
None
Residual organic
compounds, other heavy
metals, SO2, acid gases
Comments
Currently applied to five full-scale power plants in
Germany, and planned to be installed on five
hazardous waste incinerators in Europe
Technically feasible to other sources, such as
MWls or smelters, but has not been applied
Potential negative effects associated with the
disposal of spent carbon and the potential lor fires
in the bed
Applied to one MWI in the U.S.
Have not been applied to MWCs or boilers in the
U.S., although they have been used at MWCs in
Europe
Requires treatment of wastewater prior to disposal
« May form more toxic, lesser-chlorinated dioxin
and furan congeners
Very little information is available on this
technique
Very little information is available on this
technique
In 1984, carbon bed systems were in use at 8 of
the 20 chlor-alkali plants in operation in (lie U.S.
at that time
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Table 4-1 (continued)
Summary of Mercury Control Techniques for Selected Source Types
Mercury Control
Technique
Applicable Source
Type
Estimated Mercury
Removal Efficiency
Cross-Media
Impacts?"
Other Pollutants
Controlled
Comments
Selenium filters
Primary copper
smelters, primary
lead smelters, and
(more limited)
MWCs, crematories,
power plants
90%
Yes
Particulate matter, acid
gases
Factors that influence performance include inlet
mercury concentrations, flue gas temperature and
flue gas dust content
Four known applications at smelters as well as a
MWC and a crematory in Sweden; known
installation at a German power plan; potentially
applicable to MWIs
Spent filter containing selenium and mercury must
be landfilled after use
More information needed on the possibility of
selenium being emitted from the filter itself
Activated carbon injection
MWCs, MWIs,
utility boilers
50-90+%
Yes
Chlorinated dioxins and
furans, potentially other
semi-volatile organics
Activated carbon injection efficiencies reported for
utility boilers are based on pilot-scale data and as
such have a high degree of uncertainty
Factors that influence performance include Hue
gas temperature, amount of activated carbon
injected, type of particulale matter collector,
concentration and species of mercury in Hue gas
and type of carbon used
Addition of carbon could have significant impact
on amount of particulate matter requiring disposal
from utility boilers, but not from MWCs or MWIs
* For the purpose of this table, cross-media impacts refer to the potential to transfer and release mercury to media other than air, such as soil, ground water, and surface water. For example,
carbon filter beds and wet scrubbers remove mercury from air emissions but result in the generation and disposal of mercury-containing solid and liquid wastes, respectively.
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Materials Separation
Removing mercury-containing products such as batteries, fluorescent lights and thermostats
from the waste stream can reduce the mercury input to waste combustqrs without lowering the energy
content of the waste stream. The mercury removal efficiency would vary, however, depending on the
extent of the separation. Many materials in wastes contain mercury. Materials that comprise a large
portion of the waste stream, such as paper, plastic, dirt and grit and yard waste, contain very low
concentrations of mercury. Therefore, obtaining appreciable mercury reduction from separation of
these types of materials would require separating a large fraction of the total waste stream. Separating
these materials would counter the intended purpose of the combustion process, which is to disinfect
and reduce the volume of waste materials.
Other materials contain higher concentrations of mercury, but make up only a very small
portion (less than 1 percent) of the waste stream. These materials include mercuric oxide batteries,
fluorescent lights, thermostats and other electrical items.. Separation of such materials can reduce
. mercury input to a combustor without removing any of the energy content of the waste stream. To
evaluate a materials separation program, the feasibility and costs of separating a particular material
should be compared with the mercury emission reduction achieved. Furthermore, the current and
future mercury reduction achieved by separating a certain material should be considered since the
mercury contribution of some materials such as household batteries has already declined considerably.
Coal cleaning is another option for removing mercury from the fuel prior to combustion. In
some states, certain kinds of coal are commonly cleaned to increase its quality and heating value.
Approximately 77 percent of the eastern and midwestern bituminous coal shipments are cleaned in
order to meet customer specifications for heating value, ash content and sulfur content.
There are many types of cleaning processes, all based on the principle that coal is lighter than
the pyritic sulfur, rock, clay, or other ash-producing impurities that are mixed or embedded in it.
Mechanical devices using pulsating water or air currents can physically stratify and remove impurities.
Centrifugal force is sometimes combined with water and air currents to aid in further separation of
coal from impurities. Another method is dense media washing, which uses heavy liquid solutions
usually consisting of magnetite (finely ground particles of iron oxide) to separate coal from impurities.
Smaller sized coal is sometimes cleaned using froth flotation. This technique differs from the others
because it focuses less on gravity and more on chemical separation.
Some of the mercury contained in coal may be removed by coal cleaning processes. Volume
II of this report (Inventory of Anthropogenic Mercury Emissions in the United States) presents
available data on the mercury concentrations in raw coal, cleaned coal and the percent reduction
achieved by cleaning. These data, which cover a number of different coal seams in four states
(Illinois, Pennsylvania, Kentucky and Alabama), indicate that mercury reductions range from 0 to 64
percent, with an overall average reduction of 21 percent. This variation may be explained by several
factors, including different cleaning techniques, different mercury concentrations in the raw coal and
different mercury analytical techniques. It is expected that significantly higher mercury reductions can
be achieved with the application of emerging coal preparation processes. These include selective
agglomeration and advanced column flotation. Bench-scale testing is also being carried out to
investigate the use of naturally-occurring microbes to reduce the mercury and other trace elements
from coal.
Any reduction in mercury content achieved by coal cleaning results in a direct decrease in
mercury emissions from boilers firing cleaned coals. The mercury removed by cleaning processes is
June 1996 4-5 SAB REVIEW DRAFT
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transferred to coal-cleaning wastes, which are commonly in the form of slurries. No data are available
to assess the emissions of mercury from coal-cleaning slurries.
Flue Gas Treatment Technologies
With the exception of mercury, most metals have sufficiently low vapor pressures at typical
air pollution control device operating temperatures that condensation onto particulate matter is
possible. Mercury, on the other hand, has a high vapor pressure at typical control device operating
temperatures, and collection by particulate matter control devices is highly variable. Factors that
enhance mercury control are low temperature in the control device system (less than 150 °Celsius [°C]
[300 to 400 "Fahrenheit (°F)]), the presence of an effective mercury sorbent and a method to collect
the sorbent In general, high levels of carbon in the fly ash enhance mercury sorption onto particulate
matter which is subsequently removed by the particulate matter control device. Additionally, the
presence of hydrogen chloride (HC1) In the flue gas stream can result in the formation of mercuric
chloride (HgCl2), which is readily adsorbed onto carbon-containing particulate matter. Conversely,
sulfur dioxide (SC>2) in flue gas can act as a reducing agent to convert oxidized mercury to elemental
mercury, which is more difficult to collect
Add-on controls to reduce mercury emissions are described in detail in Volume VII of this
report, including information on commercial status, performance, applicability to the specified mercury
emission sources, and secondary impacts and benefits. The controls described are:
Carbon filter beds;
Wet scrubbing;
Depleted brine scrubbing;
Treated activated carbon adsorption;
Selenium filters; and
Activated carbon injection.
The most important conclusions from the assessment of flue gas treatment technologies include:
Conversion of mercury cell chlor-alkali plants to a mercury-free process is technically
feasible and has been previously demonstrated.
Control technologies designed for control of pollutants other than mercury (e.g., acid
gases and particulate matter) vary in their mercury-removal capability, but in general
achieve reductions no greater than 50 percent
Selenium filters are a demonstrated technology in Sweden for control of mercury
emissions from lead smelters. Carbon filter beds have been used successfully in
Germany for mercury control on utility boilers and MWCs. These technologies have
not been demonstrated in the U.S for any of these source types.
Injection of activated carbon into the flue gas of MWCs and MWIs can achieve
mercury reductions of at least 85 percent The addition of activated carbon to the flue
gas of these source types would not have a significant impact on the amount of
particulate matter requiring disposal.
No full-scale demonstrations of mercury controls have been conducted in the U.S. for
utility boilers. Based on limited pilot-scale testing, activated carbon injection provides
June 1996 4-6 SAB REVIEW DRAFT
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variable control of mercury for utility boilers (e.g., the same technology might capture
20 percent of the mercury at one plant and 80 percent at another). The most important
factors affecting mercury control on utility boilers include the flue gas volume, flue
gas temperature and chloride content, the mercury concentration and chemical form of
mercury being emitted.
The chemical species of mercury emitted from utility boilers vary significantly from
one plant to another. Removal effectiveness depends on the species of mercury
present To date, no single control technology has been identified that removes all
forms of mercury.
The addition of activated carbon to utility flue gas for mercury control would
significantly increase the amount of paniculate matter requiring disposal.
A number of research needs were also identified in the area of control technologies. These
are included in Section 5 of this volume.
Cost of Controls
The overall approach for assessing the cost of flue gas treatment technologies was to select a
subset of source categories on the basis of either their source category emissions in the aggregate or
their potential to be significant point sources of emissions. Consideration was also given to whether a
particular source category was a feasible candidate for application of a control technology-based
standard under section 112 of the CAA. This narrowed the analyses to six source categories: MWCs,
MWIs chlor-alkali plants, utility boilers, and primary lead and copper smelters.
In addition to determining the cost effectiveness of applying mercury control technology, a
financial analysis was performed to evaluate the affordability of mercury control (in terms of potential
price increases or impacts on financial impact) for the selected source categories. This analysis is
presented in Volume VII of this report
Table 4-2 presents the six source categories for which a control technology and cost analysis
was performed. The table presents the number of facilities in each category and the percent
contribution of each to the national inventory. Potential national mercury reductions, potential national
control costs and cost-effectiveness estimates are also presented. These estimates are based on the
assumption that all plants within a source category will achieve the same reductions and incur the
same costs as the model plants used in the analysis. Because this assumption would not be applicable
in all circumstances, the estimates of potential reductions and costs should be used only for relative
comparisons among the source categories to give an initial indication as to where mercury controls
could provide the most emission reduction for the least cost.
The cost of mercury control incurred by any specific facility may be underestimated by the
cost analysis presented in this Report because of variability inherent in the assumptions that were made
in the analyses. These include the efficiency of the various control techniques for reducing mercury,
the amount of mercury in the flue gas stream and other site-specific factors such as down-time and
labor costs. In addition, costs for monitoring and record keeping were not included in the cost
analyses. On the other hand, the costs represent retrofit application of controls. Installation of
controls at new facilities can be significantly less expensive than retrofitting an existing facility.
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Table 4-2
Potential Mercury Emission Reductions and Costs for Selected Source Categories
Mercury Source Category
Municipal waste combustors
Medical waste incinerators
Coal-fired utility boilers
Chlor-alkuli plants using the
mercury cell process
Primary copper smelters
Primary lead smelters
Total
Number of
Facilities
149
-3,700
426
(1,043
boilers)
14
8
3
-4,900
% of U.S.
Mercury
Emission
Inventory
23
27
21
2.7
0.3a
3.7
78
Mercury Control Techniques
Material separation
Product substitution
Activated carbon injection
Carbon filter beds
Polishing wet scrubber
Material separation
Activated carbon injection
Polishing wet scrubber
Fuel switching
Advanced coal cleaning
Carbon filter beds
Activated carbon injection
Process modification
Depleted brine scrubbing
Treated activated carbon adsorption
Selenium filters
Selenium filters
Potential National
Reductions'1
50 tons
(90% reduction)
60 tons
(90% reduction)
24-44 tons
(50-90% reduction)*
6.5 tons
(100% reduction)
>0.7 tons
(90% reduction)
8 tons
(90% reduction)
..;:;:;:.;.:.:..
Potential
National Annual
Costs0
$56 million
$24 million
$2.9 billion
$70 million
$7.7 million
$0.8 million
~$3 billion
Cost-
Effectiveness
($/lb of mercury
removed)11
$211-870
$228-955
$5,240-28,000
$4,590
$497
$1,061
NOTE: The underlined mercury control techniques are the techniques on which potential national reductions and potential national annual costs are based.
" Reflects one smelter only; a national estimate would be higher.
b Estimated reductions assuming every facility could achieve the reduction listed.
c Potential national costs are estimates only and assume all facilities would incur the same costs as the model plants used in the analysis.
d Where cost-effectiveness values are presented as a range, the values reflect the range across facilities of different sizes. t
e The range in potential national reductions reflects the'variable efficiency of activated carbon injection to control mercury emissions from coal-fired utility boilers. Activated carbon injection
has not been demonstrated for a full-scale utility boiler application. Control costs are based on the installation of spray cooler, fabric filter and carbon injection systems.
June 1996
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The estimates of cost for mercury reduction also do not illustrate two important
considerations. One is that all of the cost of control is attributed to mercury removal. As described
previously in this Report, many of these controls achieve reductions of other pollutants as well. The
benefits of these additional reductions should also be considered. Second, the technologies available
for mercury control represent relatively new applications of these technologies. Thus, it is possible
that new or emerging control technologies will improve the cost-effectiveness.
Clean Air Act Provisions Applicable to Mercury Control
Mercury is a priority pollutant across numerous U.S. EPA programs including air, water,
hazardous waste and pollution prevention. The focus of this chapter is the statutory authority under
the CAA that could be used to control mercury emission sources. A brief summary of these
authorities is provided below.
Section H2(a) Lesser Quantity Emission Rates
The U.S. EPA Administrator has the discretion to redefine major sources by setting an
emissions cutoff lower than the 10 tons per year emission rate level for a single pollutant or 25 tons
per year emission rate for a mixture of pollutants. This is referred to as a lesser quantity emission rate
(LQER). The CAA states that LQERs are pollutant-specific and should be based on public health or
environmental effects.
The major implications of setting an LQER are that all the requirements for a major source,
including setting maximum achievable control technology (MACT) standards, mandatory residual risk
analyses, calculation of the MACT floor, modification provisions and Title V permitting requirements
become applicable to what was previously defined as an area source category.
Section 112(c)(6) List of Specific Pollutants
Section 112(c)(6) requires that by 1995, sources accounting for not less than 90 percent of the
aggregate emissions of each of seven specific pollutants must be listed on the source category list, and
be subject to standards under 112(d)(2) or (4) no later than 2005. The pollutants are: alkylated lead
compounds; polycyclic organic matter; hexachlorobenzene; mercury; polychlorinated biphenyls;
2,3,7,8- tetrachlorodibenzo-p-dioxin; and 2,3,7,8-tetrachlorodibenzofuran. This provision also
includes a specific reference to utility boilers. It reads: "This paragraph shall not be construed to
require the Administrator to promulgate standards for such pollutants emitted by electric steam
generating units."
Section 112(d) Emission Standards
Section 112(d) requires that emission standards be established for each source category listed
on the source category list. The emission standards are applicable to both new and existing sources
and are based on the application of MACT. MACT is defined differently for new and existing sources
as explained by 112(d)(2) and (3). Under 112(d)(4), if the pollutant is a threshold pollutant (e.g.,
noncarcinogen), the emission standard can be based on a health threshold with an ample margin of
safety. A health threshold is a level where the risk of an adverse effect from exposure to the pollutant
is negligible. Section 112(d)(5) allows the Administrator the discretion to apply generally available
control technology (GACT) to area sources rather than MACT (or any other technologies that may be
required of the source category on account of residual risk analyses under 112(f)).
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Section 112(f) Residual Risk Program
Section 112(f) requires the U.S. EPA to report to Congress on the methods that will be used
to calculate the risk remaining after the promulgation of MACT emission standards under Section
112(d). This report should address the public health significance of the risk and the actual health
effects experienced by persons living in the vicinity of emitting sources, and make recommendations
on legislation regarding the risk. This report is due to Congress on November 15, 1996. If Congress
does not accept any of the recommendations provided for reducing the residual risk, the Administrator
has the authority to promulgate any additional standards required in order to protect public health with
an ample margin of safety. The report is currently under development.
Section 112(k) Urban Area Source Program
By 1995, a national strategy to control emissions of hazardous air pollutants (HAPs) from area
sources in urban areas is required to be transmitted to Congress. The strategy must identify not less
than 30 HAPs which present the greatest threat to public health in the largest number of urban areas.
Source categories accounting for at least 90 percent of the aggregate emissions of each HAP must be
listed on the source category list and be subject to 112(d) standards. The strategy, when implemented,
is to achieve a 75 percent reduction in cancer incidence attributable to these sources.
Mercury is a likely candidate for the urban area source program.
112(m) Atmospheric Deposition to Great Lakes and Coastal Waters (Great Waters)
The Great Waters study is an ongoing study with periodic reports to Congress required. This
program must identify and assess the extent of atmospheric deposition of HAPs to the Great Waters, .
the environmental and public health effects attributable to atmospheric deposition and the contributing
sources. The first report was submitted in May 1994 and is to be submitted biennially hereafter.
Mercury was identified as a priority pollutant under the Great Waters program. The Administrator
must determine if other provisions under Section 112 will adequately control these sources. If not, by
1995, further emission standards to control these sources must be promulgated.
The recommendations of the first Great Waters Report to Congress included (1) the U.S. EPA
should strive to reduce emissions of the identified pollutants of concern, including mercury, through
implementation of the CAA; (2) a comprehensive approach should be taken both within the U.S. EPA
and between the U.S. EPA and other Federal agencies to reduce and preferably prevent pollution in the
air, water and soil; and (3) the U.S. EPA should continue to support research for emissions inventories,
risk assessment and regulatory benefits assessment
J12(n)(l)(A) Study of Hazardous Air Pollutants for Electric Utility Steam Generating Units
The Utility Study is required to address the hazards to public health that are reasonably
anticipated to occur as a result of emissions by electric utility steam generating units of ... [hazardous
air pollutants] ... after imposition of the requirements of the Act. The list of 189 HAPs is presented in
section 112(b) of the CAA. In the study, the U.S. EPA must develop and describe alternative control
strategies for HAPs that may require regulation under section 112, and, if appropriate and necessary,
the U.S. EPA is to proceed with rulemaking to control HAP emissions from utility boilers. Mercury is
one of the pollutants of concern for utilities.
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Section 129 Solid Waste Combustion
Under this section, the Administrator must establish emission guidelines and standards for
solid waste incineration units, including municipal waste combustors, medical waste incinerators and
commercial and industrial waste incinerators. The performance standards must specify numerical
emission limits for mercury as well as a number of other pollutants. The U.S. EPA has already issued
final rules for municipal waste combustors (59 FR 48198) and proposed rules for medical waste
incinerators (60 FR 10654). Emission limits for hazardous waste combustors will be forthcoming
under the Agency's Combustion Strategy.
Ongoing Activities
The U.S. EPA already has efforts underway to reduce mercury emissions from industrial
sources. Specific actions being taken under the Clean Air Act include the following:
The U.S. EPA has promulgated final emission limits for municipal waste combustors
and is pursuing promulgation of proposed emission limits for medical waste
incinerators under the authority of section 129 of the CAA.
The U.S. EPA is evaluating the impacts of mercury reductions for the following
source categories under the authority of section 112(c)(6): commercial/ industrial
boilers, primary lead smelters, primary copper smelters, chlor-alkali plants using the
mercury cell process and portland cement kilns.
The U.S. EPA plans to evaluate whether secondary mercury production should be
added to the source category list under section 112(c) of the CAA and subsequently
evaluated for regulation under the authority of section 112(c)(6).
Numerous CAA requirements involve utilities either directly or indirectly. Section
112(n)(l)(B) which required this Mercury Study Report to Congress specified utility
boilers for analysis as did section 112(n)(l)(A) which is referred to as the Utility Air
Toxics Report to Congress (Utility Study). The Utility Study is charged with
evaluating the hazards to public health reasonably anticipated to occur as a result of
emissions by electric utility steam generating units of pollutants listed under Section
112(b), including mercury, and to evaluate the impact of other provisions of the CAA
on these emissions. The other provisions of the CAA would include the Acid Rain
program as well as provisions pertaining to National Ambient Air Quality Standards.
The Utility Study is also required to offer a regulatory recommendation with respect to
regulation of utility boilers under section 112 of the CAA.
To address cross-media issues, additional pollution prevention options and regulatory
authorities, the U.S. EPA has established a Mercury Task Force to consider strategies for coordinating
various programs for use, management and disposal of mercury. The Task Force has recommended to
the Department of Defense that the Defense Logistics Agency suspend sales of mercury from federal
stockpiles through the fiscal year 1996 sales cycle, pending development of an EPA strategy for
mercury. The U.S. EPA will make a final recommendation on the stockpile sales as part of this
overall strategy.
During 1995 and beyond, the Mercury Task Force will consider several approaches for
reducing mercury releases and environmental and human health risks associated with mercury
June 1996 4-11 SAB REVIEW DRAFT
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exposure. The U.S. EPA will examine a wide range of options, within a multi-media framework,
advocating common-sense pollution prevention programs. Some areas which the Task Force will
explore include evaluation and information transfer of ongoing prevention and control efforts at local,
national and international levels; consideration of pollution prevention ideas including product
substitution and innovation; recycling and disposal options; research and science needs; and
coordination within U.S. EPA for consistent mercury regulatory programs, as well as coordination with
Federal agencies managing mercury.
The findings of the Mercury Study Report to Congress will be considered by the Mercury
Task Force as it develops a strategy.
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5. RESEARCH NEEDS
The following sections summarize the major research needs identified for each of the study
areas addressed in this Report.
Anthropogenic Mercury Emissions in the United States
An effort has been made to characterize the uncertainties (at least qualitatively) in the
emissions estimates for the various source categories described. There are inherent uncertainties in
estimating emissions using emission factors. To reduce these uncertainties, a number of research
needs remain, including the following.
1. Source test data are needed from a number of source categories that have been
identified as having insufficient data to estimate emissions. Notable among these are
mobile sources, hmdfills, agricultural burning, sludge application, coke ovens,
petroleum refining, residential -woodstoves, mercury compounds production and zinc
mining. A number of manufacturing sources were also identified as having highly
uncertain emissions estimates. Notable among this category are secondary mercury
production, commercial and industrial boilers, electric lamp breakage, primary metal
smelting operations and iron and steel manufacturing. The possibility of using
emissions data from other countries could be further investigated.
2. Development and validation of a stack test protocol for speciated mercury emissions is
needed.
3. More data are needed on the efficacy of coal cleaning and the potential for slurries
from the.cleaning process to be a mercury emission source.
4. More data are needed on the mercury content of various coals and petroleum and the
trends in the mercury content of coal burned at utilities and petroleum refined in the
U.S.
5. Additional research is needed to address the potential for methylmercury to be emitted
(or formed) in the flue gas of combustion sources.
6. The importance (quantitatively) of re-emission of mercury from previously deposited
anthropogenic emissions and mercury-bearing mining waste needs to be investigated.
This would include both terrestrial and water environments. Measuring the flux of
mercury from various environments would allow a determination to be made of the
relative importance of re-emitted mercury to the overall emissions of current
anthropogenic sources.
7. Determination of the mercury flux from natural sources would help determine the
impact of U.S. anthropogenic sources on the global mercury cycle as well as the
impact of all mercury emissions in the United States.
8. The use of more sophisticated fate and transport models for mercury will require more
detailed emissions data, particularly more information on the chemical species of
mercury being emitted (including whether these species are particle-bound) and the
temporal variability of the emissions.
«
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Exposures from Anthropogenic Mercury Emissions in the United States
During the development of the mercury exposure assessment, many areas of uncertainty and
significant data gaps were identified. Many of these have been identified in the document, and several
are presented in the following list.
1. Improved analytical techniques for measuring speciated mercury air emissions are
needed as well as total mercury emissions from point sources. Laboratory evidence
suggests that divalent mercury gas -emissions will wet and dry deposit much more
readily than elemental mercury gas. Particle-bound mercury is also likely to deposit
relatively quickly. Current stack sampling methods do not provide sound information
about the fraction of mercury emissions that are in oxidized form. While filters are
used to determine paniculate mercury fractions, high temperature stack samples may
not be indicative of the fraction of mercury that is bound to particles after dilution and
cfeoling in the first few seconds after emission to the atmosphere. Methods for
determination of the chemical and physical forms of mercury air emissions after
dilution and cooling need to be developed and used to characterize significant point
sources.
2. Evaluated local and regional atmospheric fate and transport models are needed. These
models should treat all important chemical and physical transformations which take
place in the atmosphere. The development of these models will require comprehensive
field investigations to determine the important atmospheric transformation pathways
(e.g., aqueous cloud chemistry, gas-phase chemistry, particle attachment, photolytic
reduction) for various climatic regions.
3. The evaluation of these models will require long-term national (possibly international)
monitoring networks to quantify the actual air concentrations and surface deposition
rates for the various chemical and physical forms of mercury.
4. Better understanding of mercury transport from watershed to water body including.the
soil chemistry of mercury, the temporal aspects of the soil equilibrium and the impact
of low levels of volatile mercury species in surface soils and water bodies on total
mercury concentrations and equilibrium.
5. Better understanding of foliar uptake of mercury and plant/mercury chemistry. (The
most important questions: do plants convert elemental or divalent mercury into forms
of mercury that are more readily bioaccumulated? Do plants then emit these different
forms to the air?) A better understanding of the condensation point for mercury is
needed.
6. Better understanding of mercury movement from plant into soil (detritus). May need
to refine the models used to account for movement of mercury in leaf litter to soil.
7. The impact of anthropogenic mercury on the "natural," existing mercury levels and
species formed in soil, water, and sediments needs better understanding. How does
the addition of anthropogenic mercury affect "natural" soil and water mercury cycles?
Natural emission sources need to be studied better and their impacts better evaluated.
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8. Improved understanding of mercury flux in water bodies and impact of plant and
animal biomass are needed. Unlike many other pollutants, most of the methylmercury
in a water body appears to be in the biological compartment. The sedimentation rate
as well as benthic sediment:water partition coefficient require field evaluation.
Important to consider rivers and other larger water bodies in these flux analyses.
9. The bioaccumulation factors (BAF) contains a substantial level of uncertainty. A more
appropriate BAF can probably be developed when the data base upon which the
estimate is based is enlarged; i.e., need data from more than four studies. The
availability of more data would enable the possible development of lake-type
adjustment factors for the mercury BAF possibly based on color, acidification
susceptibility, etc., or species-specific BAF adjustment factors for freshwater species
most commonly consumed. Also need a time analysis of fish mercury uptake which
could lead to the development of a dynamic fish model. A mercury BAF for marine
fish is needed.
10. Better estimates of fish consumption rates for high-end consumers (subsistence) as
well as recreational anglers are needed. Fish species-specific consumption rates are
also needed. Improving these estimates would require additional dietary surveys.
11. Need to improve the biotransfer factors for mercury from soil and plants to beef.
12. Long-term studies using ultra-clean sampling techniques and state-of-the-art analytical
methods are needed to help resolve questions of mercury concentration trends over
time, particularly in soils, sediments and biota.
13. A research need for this area is for biological monitoring (for exposure and effect) of
populations with either greater than U.S. average fish consumption (such as one
serving of 100 grams per day) or consumption of fish predicted or measured to have
higher than average amounts of methylmercury.
Health Effects of Mercury and Mercury Compounds
1. In addition to the ongoing studies identified in the health effects review, further
research is necessary for refinement of the U.S. EPA's risk assessments for mercury
and mercury compounds. In order to reduce 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. Some studies should be targeted to populations
identified in this Report as likely to experience methylmercury exposure in fish (e.g.,
subsistence fishers).
2. 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
June 1996 5-3 SAB REVIEW DRAFT
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information from developmental toxicity studies would allow determination of RfDs
for developmental toxicity (RfDdt) for elemental and inorganic mercury.
3. Research needs include studies which will delineate the most appropriate indicators of
neurotoxic effects for exposed adults, children and individuals exposed to
methylmercury in utero. Well conducted studies are also needed to clarify critical
levels at which other toxic effects could occur in humans.
4. 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).
5. 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.
6. 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 in
producing tumors will be of particular use in defining the nature of the dose response
relationship.
7. 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.
8. 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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Ecological Assessment for Anthropogenic Mercury Emissions in the United States
1. Process-based Research. Mechanistic information is needed to understand the
variability that presently typifies the mercury literature. This research includes
laboratory and field studies to identify the determinants of mercury accumulation in
aquatic food chains and kinetic information that would allow researchers to describe
the dynamics of these systems. Areas of uncertainty include these: (1) factors that
determine net rates of methylation and demethylation; (2) dietary absorption efficiency
from natural food sources; (3) effect of dietary choice; and (4) bioavailability of
methylmercury in the presence of dissolved organic material and other potential
ligands.
In time it is anticipated that this information can be used to develop process-based
models for mercury bioaccumulation in fish and other aquatic biota. ^Significant
progress in mis direction is represented by the Mercury Cycling Model (MCM),
presently being developed and evaluated by the Electric Power Research Institute
(Hudson et al., 1994).
2. Wildlife Toxicity Data. There is a need to reduce the present reliance on a relatively
few toxicity studies for WC development. Additional data are needed for wildlife that
constitute the most exposed organisms in various parts of the country (e.g., the Florida-
panther). There is also a critical requirement for toxicity data that can be related to
effects on populations (see Table 2-1), including effects on organisms that comprise
the lower trophic levels.
3. Improved Analytical Methods. Efforts to develop and standardize methods for analysis
of total mercury and methylmercury in environmental samples should be continued.
Such methods must recognize the importance of contamination, both during the
collection of such samples and during their analysis. It is particularly important that
mercury measurements which are at present operationally defined (e.g., "soluble",
"adsorbed to organic material") be made in such a way that mercury residues in fish
can be correlated with the bioavailable mercury pool.
As validated methods become available, it is important to analyze for both total and
methylmercury whenever possible so that differences between aquatic systems can be
definitively linked to differences in methylmercury levels. Analyzing the two mercury
species together will contribute to an understanding of existing data, most of which is
reported as total mercury. It is also anticipated that developing BAFs in terms of
methylmercury will reduce the variability that currently exists around BAF estimates
based on total mercury.
4. Complexity of Aquatic Food Webs. Present efforts to develop WC values for mercury
are based on linear, four-tiered food chain models. Research is needed to determine
the appropriateness of this simple paradigm and to develop alternatives if field data
suggest otherwise. Of particular interest is whether zooplankton and phytoplankton
should be modeled as two different trophic levels. Current information for detritivores
and benthic invertebrates is extremely limited, even though their importance in
mobilizing hydrophobic organic contaminants has been demonstrated.
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5. Accumulation in Trophic Levels 1 and 2. Ongoing efforts to understand mercury
bioaccumulation in aquatic systems continue to be focused on trophic levels 3 and 4,
despite the fact that uncertainties in predator/prey factors (PPFs) are relatively small.
Additional emphasis should be placed on research at the lower trophic levels. In
particular, there is a need to understand the determinants of mercury accumulation in
phytoplankton and zooplankton, and how rapid changes in plankton bio mass impact
these values.
6. Field Residue Data. High qualityfield data are needed to support process-based
research efforts and to determine residue concentrations in the fish and other aquatic
biota that wildlife eat Whenever possible, it is desirable to collect residue data at all
trophic levels and to analyze mercury levels in the abiotic compartments of a system
(e.g., water and sediments). It is particularly important that such measurements be
made in a broader array of aquatic ecosystem types (including both lakes and rivers)
so that a better understanding of mercury cycling and accumulation can be obtained.
Residue data from wildlife are also needed to identify populations that are being
adversely impacted or are potentially at risk. Feathers and fur hold considerable
promise in this regard because of the potential for "non-invasive" determination of
mercury residues. Laboratory research is required, however, to allow interpretation of
these data. Factors such as age, sex, and time to last moult are likely to' result in
variability among individuals of a single population, and need to be understood.
Sampling efforts should be targeted on areas receiving high levels of mercury
deposition and/or regions containing large numbers of poorly buffered surface waters,
as discussed throughout this report.
7. Natural History Data. The development of WC requires knowledge of what wildlife
eat. Fish sampling efforts are frequently focused on species that are relevant to human
consumers but that may be of little significance to wildlife. There is an additional
need to collect information for macroinvertebrates and amphibians. Seasonal and
spatial effects on predation should be explored and methods developed to describe this
information adequately. Additional life history data is needed to characterize fully the
nature and extent of exposure to mercury. Complicating factors must be considered,
including migratory behaviors and sex-specific differences in distribution and resource
allocation. It is particularly important that information be collected to support the
development of predictive population models for sensitive species.
Risk Characterization
1. A monitoring program is needed to assess either blood mercury or feather/hair
mercury of piscivorous wildlife; particularly those hi highly impacted areas. This
program should include assessment of health endpoints including neurotoxicity and
reproductive effects.
2. There is a need to collect additional monitoring data on hair or blood mercury and
assess health endpoints among women of child-bearing age and children. This study
should focus on high-end fish consumers and on consumption of fish from
contaminated water bodies.
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3. There is a need for improved data on effects that influence survival of the wildlife
species as well as on individual members of the species.
4. There is a need for controlled studies on mercury effects in intact ecosystems.
5. Monitoring data sufficient to validate or improve the local impact exposure models are
needed.
Mercury Control Technologies
1. Data are needed from full-scale testing of activated carbon injection at a coal-fired
utility boiler.
2. Additional data are needed on the efficiency of activated carbon injection, and various
impregnated carbons, in reducing the different chemical species of mercury present in
flue gas.
3. Additional information is needed on the efficiency and cost of other technologies for
mercury control that are currently in the research stage. These include impregnated
activated carbon, sodium sulfide injection and activated carbon fluidized bed.
4. More data are needed on the ability of conventional or advanced coal cleaning
techniques to remove mercury from raw coal and advanced coal-cleaning techniques
such as selective agglomeration and advanced column floatation. The potential for
mercury emissions from coal-cleaning slurries need to be characterized.
4. Additional analyses are required on the feasibility and cost effectiveness of other
mercury emission prevention measures such as emissions trading, emissions averaging,
energy conservation, renewable energy, and fuel switching.
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APPENDIX A
SUMMARY OF THE SCIENTIFIC PEER REVIEW
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1. OVERVIEW OF THE REVIEW PROCESS
Ensuring the quality of scientific underpinning actions by U.S. EPA has been a major thrust
of Agency policy. In order to ensure top quality science, U.S. EPA has been advised by review bodies
including its Science Advisory Board (SAB) to consider peer review of its programs, methods and
products to be a top priority. To this end Administrator Carol Browner issued a memorandum
requiring expert review wherever appropriate and requiring U.S. EPA Programs and the Office of
Research and Development (ORD) to develop specific guidelines for scientific review. The ORD final
guidelines for review of scientific products were issued in November 1994. These were used as the
basis for the peer review plan for this Report The Mercury Study Report to Congress was considered
by ORD and the Office of Air Quality Planning and Standards, Office of Air and Radiation
(OAQPS/OAR) to be one of U.S. EPA's major and most visible outputs. As such the Report was
considered to fall into category 1, requiring the highest level of scientific peer review. The components
for category 1 review include the following: approval of the peer review plan by the Assistant
Administrator of ORD; review of the product by appropriate U.S. EPA scientists; review of the
product by appropriate scientists external to the Agency; convening a peer review meeting; and
stringent recordkeeping on all phases of the review process.
Because of the wide scope of the Report and the interest in mercury by many stakeholders, it
was felt that the process of generating the Report should be open to external input. Meetings with
U.S. EPA Report authors were held with members of the public at their request; for example, during «
early stages of Report generation, U.S. EPA staff met on a quarterly basis with scientists and engineers
representing the Electric Power Research Institute (EPRI). Meetings were also held with the Portland
Cement Association and with other requestors. The Agency accepted and reviewed submissions of
data and mercury assessment material throughout the study period; these were used as was considered
appropriate by U.S. EPA scientists.
In order to gather input and critiques on preliminary assessments, several of these were
presented at conferences and scientific meetings. Early results of the emissions inventory (found in
Volume II) were presented at both regional and national meetings. Draft health assessments were also
shown for purposes of discussion at scientific meetings on mercury. In January of 1994 a review draft
of the emissions inventory was made publicly available.
Internal scientific review of a draft of the entire Report (minus Volume I, Executive
Summary) was begun in November of 1994. Following procedures for review and clearance
established for the Office of Health and Environmental Assessment (now the National Center for
Environmental Assessment, NCEA) within ORD, the draft was reviewed by four scientists from that
Office. In addition, the draft was reviewed by a U.S. EPA Mercury Study Work Group consisting of
staff from the following U.S. EPA offices: Office of Science, Planning and Regulatory Evaluation
(OSPRE/ORD); Office of Health Research (OHR/ORD); Office of Policy Analysis and Review
(OPAR/OAR); Office of Water (OW); Office of Solid Waste and Emergency Response (OSWER);
Office of Prevention, Pesticides and Toxic Substances (OPPTS); Office of Policy, Planning and
Evaluation (OPPE); and Region V. Scientists representing the State of New York and the State of
Michigan also participated in the Work Group and in this phase of review.
Included as part of the Report are summaries of human health risk assessments which
comprise parts of the Agency's Integrated Risk Information System (IRIS). IRIS is a publicly
available computerized data base which provides U.S. EPA consensus health risk assessment
information. IRIS files must undergo specific forms of internal and external review before they are
made available on the system. The IRIS documents on mercury were reviewed as part of the Mercury
June 1996 A-l SAB REVIEW DRAFT
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Study Report to Congress. The following six IRIS summaries were distributed to reviewers as
Appendix B of Volume IV of the Report: reference concentration for elemental mercury; cancer
assessment for elemental mercury; reference dose for inorganic mercury; cancer assessment for
inorganic mercury; reference dose for methylmercury; and cancer assessment for methylmercury.
Internal review for the IRIS documents consisted of the appropriate Agency Work Group discussion
and closure (referred to as "verification"). The Work Groups charged with reviewing IRIS information
and achieving consensus on its validity are comprised of U.S EPA scientists from a variety of
disciplines relevant to human health risk assessment and who represent ORD, the Regions and the
Program Offices. The two Work Groups have been organized around either the assessment of
carcinogenic effects (the Carcinogen Risk Assessment Verification Endeavor (or CRAVE) or the
assessment of general systemic toxicity (RfD/RfC) Work Group). To enhance the mercury expertise
of the Work Groups and to allow for discussion of alternate risk assessment approaches, scientists
from FDA, ATSDR and the State of New Jersey were invited to participate in the RfC/RfD Work
Group discussions; they were not part of the consensus process, however, and did not participate in
Agency decisions on the assessments. After consensus on the assessment was achieved, IRIS
documents were revised and received external review (see below) as part of the external review draft
of the Report. Following external review and revision the IRIS documents were either reviewed and
cleared by the Work Group chair (RfDs and RfC) or given a pass-around review by the whole Work
Group (CRAVE).
External review of the Mercury Report to Congress and the appended IRIS documents was
done in two steps: a Federal interagency review and a non-Federal external review. A meeting of
Federal reviewers was held at the U.S. EPA, Washington DC on January 9, 1995 to discuss scientific
issues in the Report. Representatives of the following Agencies were invited to attend and to submit
written comments at the time of the meeting: ATSDR, NIEHS, NOAA, USD A, DOE, FDA and the
National Biological Service. The names and addresses of the reviewers can be found at the beginning
of each volume of the Report. Written comments were received from all Agencies participating in the
review. A summary of reviewer comments, consensus opinion of reviewers and U.S. EPA's response
to comments can be found hi this Appendix.
The second phase of external review included comments from non-Federal experts. Reviewers
were chosen based on scientific expertise and availability. An attempt was made to include
representatives of a spectrum of groups with interest in mercury: academe, research groups, State
agencies, industrial concerns and environmental groups. The names and affiliations of reviewers can
be found at the beginning of each volume. All reviewers were required to submit written comments
on the report including the IRIS documents. A public review meeting was held January 25-26, 1995
at U.S. EPA in Cincinnati, OH. A notice of the meeting was published in the Federal Register, and
there was time set aside each meeting day for members of the public to comment. The pre- and post-
meeting reviewer comments, synopses of meeting discussions and conclusions of the meeting
comprised the external review report. Examples of external review comments are in Appendix A to
this Volume. In response to reviewer comments, specific changes were made in the External Review
Draft. As the Risk Characterization (Volume VI) was revised* a second review of that volume was
done. ^ £,-/tj ft rtf£iJ
One revised component was reviewed in advance of the remainder of Volume VI; this was the
estimate of population size, amount of fish consumed and measured amount of mercury in marine and
freshwater fish. This assessment (now included in Volume III, Appendix H and summarized in
Volume VI) was sent to two external reviewers expert in statistics and demographics. These reviewers
were selected by a U.S. EPA contractor who was provided with criteria for reviewers and a list of
potential candidates. The entire revised Risk Characterization was subjected to internal and external
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review. Scientists in ORD, OAQPs and Office of Water were sent the volume and requested to submit
comments. Four external reviewers were selected by a U.S. EPA contractor based on criteria provided
by U.S. EPA. Among these criteria were that two reviewers be included who had commented on the
External Review draft. Written comments on the risk characterization were provided by these four
external scientific reviewers. Copies of all review comments and external input are archived at the
National Center for Environmental Assessment in Cincinnati, Ohio.
This appendix summarizes the major comments provided by external and Federal interagency
reviewers, along with U.S. EPA's responses. Section 2 presents an overview of the charge to the
external and interagency reviewers. Section 3 provides a summary of the external review process, the
non-Federal, external reviewers comments and U.S. EPA's disposition. Finally, Section 4 summarizes
U.S. EPA's notes and responses to comments from Federal interagency reviewers.
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2. CHARGE TO REVIEWERS
Reviewers were asked to focus on that portion of the report that matched their area of
expertise. The following are issues or questions considered by the reviewers, including the
development of premeeting comments.
All Volumes
Are additional data or analyses available that would have a major impact on the
conclusions presented in any volume of the report?
Are arguments and conclusions presented clearly and in a logical manner?
Do the Research Needs chapters of particular volumes present a program of research
projects that will address uncertainties in the evaluation of mercury impacts?
Volume I: Executive Summary
Does the summary adequately reflect the conclusions of the other volumes?
Is additional information presented in the report that should be added to the summary
for clarity or completeness?
Is the summary sufficiently clear and informative to function as a stand-alone volume,
or does it rely too heavily on familiarity with the report as a whole?
Volume II: An Inventory of Anthropogenic Mercury Emissions in the United States
Please critique the emission factors approach used in the inventory.
Are you aware of information for source categories identified as having insufficient
data for evaluation?
Volume III: An Assessment of Exposure From Anthropogenic Mercury Emissions in
the United States
Please critique the conclusions of the exposure modeling. Are the conclusions well
supported by the analyses presented in the text of Volume III?
Is there material in the text of Volume III that would be more appropriately presented
in an appendix?
Please critique methods used and assumptions made for the local impact analysis.
Do the appendices provide necessary supporting information concerning methods
described in the text?
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Volume IV: Health Effects of Mercury and Mercury Compounds
Is the information provided on pharmacokinetics sufficient for evaluating human health
effects associated with mercury?
Please critique the weight-of-evidence categorizations for carcinogenicity,
developmental toxicity, and germ cell mutagenicity. Is the level of detail in the report
descriptions in Volume IV sufficient to permit evaluation of these endpoints?
' No quantitative dose-response assessment was conducted on carcinogenicity for
inorganic or methyl mercury. Are the arguments against conducting a quantitative
assessment presented cogently and are they supported by the information given in this
volume?
* Are the reference doses (RfDs) and reference concentrations (RfCs) properly
calculated? Were the appropriate critical study and endpoint(s) ckosen? Were the
proper uncertainty factors and modifying factors used?
Are there any factors modifying mercury toxicity in humans that have not been
addressed in the volume?
Volume V: An Ecological Assessment for Anthropogenic Mercury Emissions in the
United States
Please critique the methods used for generating a trophic level three BAF and a
trophic level four BAF.
Please critiques the methods used for generating an uncertainty analysis.
Were appropriate endpoints and studies selected for generating wildlife RfDs?
Were appropriate assumptions used in developing wildlife water criteria?
Are there other species of concern that should be considered in this volume?
Are there other geographic areas of concern that should be included in this volume?
Volume VI: Characterization of Human Health and Wildlife Risks From Anthropogenic
Mercury Emissions in the United States
Are the summaries of human and wildlife risk assessment sufficient for a scientific
critique?
Are there major areas of uncertainty, defaults, or assumptions that were not discussed?
Please critique the uncertainty analyses.
Please critique both the methods and results of the comparative discussion of risk
presented in this volume.
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Volume VII: An Evaluation of Mercury Control Technologies and Costs
Are you aware of any quantified benefits of mercury control? Please specify.
Are you aware of data on the efficacy of materials separation programs or other
pollution prevention measures other than that presented in this volume? Please specify.
Please critique the cost analysis presented in this volume.
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3. SUMMARY OF NON-FEDERAL EXTERNAL REVIEWERS
COMMENTS AND U.S. EPA DISPOSITION
On January 25-26, 1995, a I'/i-day workshop was held at the U.S. EPA's Andrew W.
Breidenbach Environmental Research Center in Cincinnati, Ohio, to provide external review of the
draft Mercury Report to Congress. A draft report was prepared by U.S. EPA's Office of Air Quality
Planning and Standards and Office of Research and Development in response to Section 112(n)(l)(B)
of the Clean Air Act Amendments of 1990, which requires U.S. EPA to submit a report to Congress
on mercury emissions. The draft report consisted of six volumes at the time it was distributed for
review:
Volume II: An Inventory of Anthropogenic Mercury Emissions in the United States.
Volume HI: An Assessment of Exposure from Anthropogenic Mercury Emissions' in
the United States.
Volume IV: Health Effects of Mercury and Mercury Compounds.
Volume V: An Ecological Assessment for Anthropogenic Mercury Emissions in the
United States.
Volume VI: Characterization of Human Health and Wildlife Risks from
Anthropogenic Mercury Emissions in the United States.
Volume VII: An Evaluation of Mercury Control Technologies and Costs.
Volume I, the Executive Summary, was not yet complete and ready for review along with the
other volumes. In preparation for the workshop, Eastern Research Group, Inc., providing contractor
support to U.S. EPA, identified 15 independent external scientists to review the document The
reviewers' expertise covered a variety of subject areas relevant to the report, including mercury
emissions and sources of mercury emissions; the transport to and fate of mercury in the environment;
the physicochemical and biotic transformation among mercury forms in environmental compartments,
particularly of inorganic to methylmercury; exposure of human and ecological populations to
methylmercury and other mercurials; human and ecological toxicology; quantitative risk assessment;
and risk management. Each reviewer was asked to focus on that portion of the report that matched his
or her area of expertise. Reviewers prepared and submitted premeeting comments on the report prior
to the workshop.
Fourteen reviewers,1 10 U.S. EPA representatives involved in writing and/or revising the
mercury report, and 39 observers attended the workshop. The agenda included plenary sessions and
breakout groups. The first day of the workshop began with a presentation, by the two breakout group
chairs, of summaries of the reviewers' premeeting comments for Volumes II, III, IV, and V. The
participants then broke into two groups one to discuss Volumes II and III, and the other to discuss
Volumes IV and V. During a plenary session at the end of the first day, the breakout group chairs
presented a summary of their groups' discussions and observers commented on the report.
lOne of the 15 original reviewers was unable to attend.
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The second day of the workshop consisted of a half-day plenary session. Two reviewers
presented a summary of the premeeting comments on Volumes VI and VII, and all the reviewers then
discussed these two volumes. Following this discussion, additional observers presented their
comments on the mercury report.
A summary of the major comments provided by external reviewers along with U.S. EPA's
responses is presented in the following sections: a report of the workshop chair (Section 3.1);
summary of premeeting comments (Section 3.2); summary of breakout group discussions (Section 3.3);
overview of reviewer discussion in the plenary session (Section 3.4); and the summary of revisions
made in response to reviewers comments (Section 3.5).
3.1 Report of the Workshop Chair - Paul Mushak, Ph.D.
Overview
The draft report and reviewers' comments clearly show that, while we do not know nearly as
much as we would like to about environmental mercury, we know a lot. In fact, we know more about
environmental mercury than about most contaminant metals or metalloids of concern.
The principal challenge for both the authors and external reviewers of the draft report was to
critically evaluate the problems associated with integrating what we do and do not know into a
scientifically credible synopsis. One of these problems appears to be that the extensive database for
mercury is mainly available as discrete blocks of information within various scientific disciplines,
while the congressional mandate requires U.S. EPA to establish and quantify linkages between these
blocks of data. For example:
Information in one block tells us that the forms of mercury addressed in the draft
report particularly methylmercury are intrinsically toxic, with a relatively high
degree of toxicological potency to humans and various other biological receptors. The
types of toxic responses known or anticipated in both ecological and human
populations are qualitatively recognized.
Information in a second block* tells us that mercury is emitted to the environment from
a variety of sources, and that one can generally determine the relative contribution of
different anthropogenic mercury source categories.
Information in a third block tells us that some fraction of the mercury emitted to the
atmosphere from a point source will eventually be deposited by precipitation processes
onto land and water bodies. Direct or indirect post-depositional processes not only
will impart mobility to the contaminant but also will transform mercurial species.
Information in a fourth block tells us that inorganic ionic mercury entering certain
environmental compartments will undergo biomethylation to methylmercury, and that
methylmercury will accumulate and biomagnify in the human food web, particularly in
high-trophic-level predator fish. Data in this block also show that mercurial forms can
contaminate several environmental media, depending on the exposure particulars.
These examples of what we know clearly indicate that the difficulties in synthesizing all this
information into a coherent statement about the potential health and ecological risks posed by mercury
June 1996 A-8 SAB REVIEW DRAFT
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in the United States are rooted in uncertainties about how to quantitatively link these blocks together.
Areas of uncertainty include, for example:
How much of current anthropogenic atmospheric emissions is deposited in various
environmental compartments?
What is the link between natural and anthropogenic mercury in terms of proportional
contamination and subsequent impact?
How much of this post-deposition mercury is converted to highly toxic
methylmercury?
*
How much of any increased toxicity risk associated with consumption of
methylmercury-contaminated fish can be traced back to anthropogenic atmospheric
emissions of mercury?
The draft report was variably successful in dealing with the numerous complexities,
uncertainties, and data gaps connected with quantifying linkages. The essence of the reviewers'
comments concerned whether the report under- or overstated these uncertainties, particularly with
reference to risk characterization.
General Review Panel Assessments of the Report
In their comments before and during the workshop, the peer reviewers recommended revisions
to strengthen the report's scientific credibility. Reviewers generally agreed that the report would serve
a useful purpose once it had been revised and improved in the various ways they had suggested. Few,
if any, reviewers felt the report should not be submitted at all, and no reviewer thought the report
should be transmitted without revision.
The review panel generally agreed that some portions of the report underestimated the
uncertainty associated with modeled estimates or pathway analyses. The panel suggested that one way
to better acknowledge this higher uncertainty was to use a range of values rather than point estimates
in the estimating exercise; some panel members also argued that a more refined point estimate could
be presented in certain casesfor example, in deriving the reference dose (RfD) for methylmercury.
On the other hand, the panel also generally agreed that the draft report conveyed too much
uncertainty by failing to include important peer-reviewed data available in the recent scientific
literature. For example, the exposure breakout group generally agreed that data do exist to indicate a
relationship between point-source mercury emissions and gradients in mercury deposition consistent
with a point-source contribution.
The review panel was similarly concerned about including or excluding available information
on other topics in the report. The panelists felt that the authors should revisit the most recent
scientific information to close any gaps that affect quantification of the linkages noted above.
Reviewers also were concerned about the role of modeling in the report. However, they had
different opinions about how much data from the recent literature could be used to complement the
model estimates. Panelists generally agreed that the report volumes should be more consistent and
integrated, particularly concerning information relevant to risk characterization.
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3.2 Summary of Premeeting Comments
This section presents the summary of the premeeting comments for the following Volumes of
the draft Mercury Study Report to Congress:
Volume II (Emissions) and Volume III (Exposure) Gerald Keeler, Ph.D., and Paul
Mushak, Ph.D;
Volume IV (Health Effects) Steven Bartell, Ph.D., and Paul Mushak, Ph.D;
Volume V (Ecological Effects) Steven Bartell, Ph.D., and Paul Mushak, Ph.D; and
»
Volume VI (Risk Characterization) Pamela Shubat, Ph.D., and Paul Mushak, Ph.D.
\
3.2.1 Volume n (Emissions) and Volume HI (Exposure) Gerald Keeler. Ph.D.. and Paul Mushak,
Ph.D.
Reviewers felt that Volume II probably was the best of the four volumes reviewed. The
approach used to characterize emissions was reasonable. However, the volume provides no estimates
of natural and baseline emissions and ignores several potentially important sources. Specific
comments on the various sections are provided below.
Natural Emissions
The inadequate coverage of natural sources of mercury detracts from the entire report.
Chapter 2, Natural Sources of Mercury Emissions, which consists of only a single page in Volume II,
is incomplete and misleading. The topic of natural sources of mercury is controversial and qualitative
at best If the authors want to include this topic in the report, they should provide a more complete
and defensible assessment of natural emissions. Reviewer William Fitzgerald, Ph.D., recommended
that natural emissions could be roughly calculated using an approach similar to that of Mason et al.
(1994).2 This approach suggests that natural emissions in the United States are approximately 20
percent of anthropogenic emissions. A recent estimate of natural emissions in Europe gave a similar
result of 25 percent of the total emissions (Axenfeld et al., 1992).3 However, the quantitative data
concerning natural emissions are very limited, and there are numerous problems with the estimates in
the literature.
Anthropogenic Sources
The report's list of source categories for mercury emissions is complete with respect to the
major source categories. Many of the source categories discussed have relatively low annual mercury
emissions. For a few source categories for which insufficient information was found, the report
provides no emission estimates. Emission factors and data are missing for several potentially
important sources, including hazardous waste incinerators, primary mercury production, mercury
compounds production, by-product coke production, refineries, and mobile sources. In addition,
2Mason, R.P., W.F. Fitzgerald, and F.M.M. Morel. 1994. Aquatic biogeochemical cycling of elemental mercury:
anthropogenic influence. Geochim. Cosmochim. Acta 58:3191-3198.
3Axenfeld, F., J. Munch, and J.M. Pacyna. 1991. Europaische Test-Emissionsdatenbasis von Quecksilber-Komponenten
fur Modellrechnungen. Dornier Report Friedrichshafen, Germany.
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Volume II provides no information or discussion on emissions from iron-steel production and primary
zinc productioa Emission factors and data are available for European sources and could be used to
estimate the U.S. emissions to determine their potential importance.
The report to Congress provides only very limited information on emissions of various
physical and chemical forms of mercury. Better information is needed on mercury speciation in both
emissions and environmental samples. These could be identified as research needs.
The report could be strengthened by adding maps showing the actual location of point sources
for categories like utilities (by fuel type), incinerators (sludge, municipal), iron-steel production, coke
ovens, and cement production. The spatial distribution of the gridded emissions presented at the
workshop by report author Martha Keating should also be included.
Lastly, the report lacks information regarding seasonal or temporal variations in emissions by
source category. While utilities may have fairly constant emissions both diumally and seasonally,
other sources do not. Operations involving multiple steps over different time periods will probably
have time-varying emissions.
Exposure to Mercury
A comprehensive quantitative assessment of the relationship between anthropogenic mercury
releases to the atmosphere and the potential exposure of people, wildlife, and terrestrial and aqueous
systems to these releases may not be possible due to the apparently limited state of knowledge of the
mercury cycle in nature and the environmental consequences from anthropogenic emissions of
mercury. The report states that the exposure assessment is a "qualitative study based partly on
quantitative analyses." As noted by reviewer William Fitzgerald in his premeeting comments:
...this important exposure assessment provides a valuable guide for research.
Although the results and conclusions are qualitative, this extensive and essential
modeling effort provides a credible means for evaluating the present sparse data base,
and for identifying major gaps, inconsistencies and weaknesses associated with major
aspects of the biogeochemical cycle of Hg at the Earth's surface....
As the report confirms, human exposure to methylmercury is almost exclusively from
consumption of fish and fish products. Intake of methylmercury through consumption of nonlocal fish
and seafood should be evaluated. Such intake should not be considered "background," as the mercury
found in coastal environments and in saltwater fish may be of anthropogenic origin. The report lacks
an assessment of the exposure of the marine environmentespecially the coastal zoneto
anthropogenic mercury emissions and of the effects of such exposure.
The report suffers from a general lack of recent information and actual measurement data in
the recent peer-reviewed literature. References will be provided by the reviewers, and the Monterey
Mercury Meeting Book will be provided by Donald Porcella (Electric Power Research Institute).
Inclusion of more recent information will address such comments as "There is a general recognition of
uncertainty," "So much is said about uncertainty that it appears as if we do not know much about
mercury," and "Little of the most recent knowledge has found its way into this report."
The modeling results should be "ground-truthed" where possible. The report's estimates of
deposition and water concentrations often are more than an order of magnitude greater than any
actually measured in the United States.
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The meaning of some key terms used in the report, such as "total emissions" and
"background," was confusing. The peer reviewers strongly recommended that the authors add to the
report the definitions provided in the Atmospheric Mercury Expert Panel Report and that they use the
various terms consistently throughout the report based on these definitions.
3.2.2 Volume IV (Health Effects) Steven Bartell. Ph.D.. and Paul Mushak. Ph.D.
Several of the reviewers' key premeeting comments concerned the major sources of intake and
exposure in human populations. Some reviewers suggested that the report should address the
contributions to human mercury exposure of dental amalgams containing mercury. Similarly,
reviewers recommended that the drinking water pathway be further examined, including the potential
human health risks associated with drinking water at locations known to have elevated mercury
concentrations in ground water. The report should clearly explain why particular papers concerning
human health endpoints are cited while others were omitted.
Reviewers also commented on the subject of mercury disposition among biological indicators
of mercury exposure, particularly exposure to methylmercury. The derivation and use of a constant
ratio of mercury in hair to mercury in blood for estimating blood levels of mercury may require
additional attention. Reviewers expressed reservations about the time-scale differences implicit in
comparing blood mercury with hair mercurynamely, that mercury concentrations in hair reflect
exposure over a longer time scale, while mercury concentrations in blood may correspond to a shorter
time frame. The reported variability may reflect interindividual variability rather than just
measurement error as Volume IV suggests. Reviewers identified an error in the equation used to
calculate the methylmercury concentration in blood; an additional term defining blood volume is
needed to make the units in this equation work out to those stated.
The quantitative linkage of mercury intake by exposed populations and the expression of some
toxic endpoint is mediated through the toxicokineticsi.e., the uptake, distribution, and
retention/excretionof the particular mercurial. The modeling of the systemic behavior of
methylmercury is particularly critical in this regard. The reviewers felt that the derivation of the
parameters used in the pharmacokinetic modeling needed additional explanation and justification. For
example, the elimination rate or half-life used to describe methylmercury conversion to inorganic
mercury and its subsequent removal from the body in feces is an important model parameter;
reviewers disagreed about the most appropriate value. Differences in this parameter can result in
appreciable variability in the modeled mercury concentrations for the human populations of interest.
Chapter 4 on toxic effects of various mercurials, particularly methylmercury, was the subject
of several comments. The organization and presentation of toxic endpoints in the chapter could
benefit by progressing from lethal through acute effects to subchronic and chronic effects. Distinct
subsections organized along this framework would improve the presentation. The rationale for
selecting the set of core studies of toxic responses should be clarified.
Not surprisingly, many comments involved the chapter on dose-response relationships.
Several reviewers were concerned that the current RfD for methylmercury might not be protective,
particularly for more subtle neurotoxic endpoints such as neurobehavioral and neurodevelopmental
endpoints. One reviewer pointed out some confusion regarding the interpretation and presentation of
the apparent association between maternal methylmercury exposure and abnormalities in deep tendon
reflexes in their male children. Two reviewers recorded their disagreement regarding the adjustment
of No Observed Adverse Effect Levels (NOAELs) and Lowest Observed Adverse Effect Levels
(LOAELs) to lifetime exposures for different exposure pathways (e.g., inhalation, ingestion) in the
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derivation of RfDs and reference concentrations (RfCs). Exposure resulting from these pathways
would be more realistically described as intermittent, shorter-term events. There was apparent
confusion regarding the derivation and use of uncertainty factors (UFs) and modification factors
(MFs). The values were not carried through the analysis according to the usual protocols. Reviewers
pointed out some confusion and inconsistency regarding the relative sensitivity of adult and fetal
developmental toxicity used to derive overall human health assessment endpoints.
Reviewers disagreed with the presentation regarding the possible interactions between mercury
and selenium, particularly the implication that interaction with selenium may mitigate the human toxic
effects of mercury.
3.2.3 Volume V (Ecological Effects) Steven Bartell. Ph.D.. and Paul Mushak. Ph.D.
Reviewers were concerned with the efficacy of the overall approach to the report's ecological
assessment, which involves defining overlapping areas of potentially high mercury exposures with the
distribution of sensitive piscivorous birds and mammals. For example, the life history and distribution
of the Florida panther differ considerably from those of the mink or kingfisher. Failure to address life
history and migration patterns in developing this overall approach might lead to inaccurate assessments
of risk.
Reviewers also .pointed out the report lacked a consideration of mercury effects on organisms
at lower tropic levels (e.g., plankton, invertebrates). Additional reservations were expressed over the
absence of wading birds, particularly species of declining abundance that are known piscivores.
Effects of mercury on fish and reptiles should also be explored, or their omission should be further
justified.
Reviewers were concerned about the report's dependence on assessment approaches and data
that emphasized the Great Lakes and upper midwestern lakes, for example, in developing the
bioaccumulation factors (BAFs). Concern also was expressed regarding the removal of surface waters
with pH > 5.5 from regions of concern. This approach would exclude the circumneutral waters of the
Florida Everglades that are suspected of posing mercury-related risks to resident populations of birds
and mammals.
A major review issue focused on the use of NOAELs as endpoints for developing the wildlife
criteria for the ecological assessment. This approach removes any consideration of a dose-response
relationship from the assessment. If measured or modeled mercury exposures exceed the wildlife
criteria values, we would not know the nature or magnitude of the expected response. Also, this
approach implies different time scales between the shorter-term toxicity data used to develop the
wildlife criteria and the longer-term exposure values. The fact that limited data were used to develop
NOAELs for the selected wildlife species also calls into question the efficacy of the report's overall
approach for estimating ecological risks.
In developing the BAF values, the report essentially ignored the complex chemistry of
mercury in surface waters. Instead, these factors were developed using constant ratios of
methylmercury to total water column mercury. Reviewers expressed serious concerns with this
assumption, which ignores the complex environmental chemistry of mercury. Also, in developing the
BAF values, the assumption was made that the selected piscivores restrain their feeding to specific
"trophic level" fish. This assumption is certainly open to question; it remains unclear what the impacts
of this assumption are on the resulting estimates of BAFs and wildlife criteria used as endpoints for
the assessment. The assumption of a simple linear food chain implied by this approach was similarly
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of concern; the draft does not address spatial and temporal variations in diet and feeding behavior that
might increase or decrease exposures for the selected piscivores.
It was not clear what the exposure models (RELMAP, COMPMERC) really provide to the
assessment. The different spatial scales of these exposure models were not related to the spatial scale
of the distributions of the selected species.
Finally, the reviewers noted that the sensitivity/uncertainty analyses did not comprehensively
address all the components of the equations used to develop the BAFs or the final wildlife criteria
values. The reported analyses addressed some of the models' structural uncertainties (e.g.,
correlations), but did not adequately address parameter uncertainty. The results of the sensitivity
analyses do not lend themselves to defining future research needs in relation to reducing uncertainty on
the endpoints of the assessment
\
3.2.4 Volume VI (Risk Characterization) Pamela Shubat Ph.D.. and Paul Mushak, Ph.D.
Reviewers agreed that Volume VI fell short of expectations for a risk characterization of
health and ecological effects from mercury emissions. One reviewer felt that the necessary data to
conduct a risk assessment are lacking, considering that a risk characterization should estimate the
probability of health effects.
Reviewers noted that the volume should have compared the measurements of fish mercury
levels and the incidence of health effects in populations to the volume's assumptions and results. The
volume assumed a body weight and a fish consumption rate for each species; it also assumed a
NOAEL and LOAEL for the selected species and derived a fish concentration that would permit
consumption without exceeding the NOAEL or LOAEL. Reviewers felt that more data were needed
to support this approach, and they expressed particular concern about the NOAEL and LOAEL
selected for each species.
Reviewers felt that the assumptions, in the relative exposure ranking, that a given lake has
only a single mercury concentration and a single trophic level were not accurate. The exposure
rankings for the eagle, kingfisher, otter, and other species should be compared to measured values in
tissue samples from these species.
3.3 Summary of Breakout Group Discussions
This section presents the summary of breakout group discussions on the following volumes of
the Mercury Study Report to Congress:
Exposure Breakout Group (Volumes II and DI) Gerald Keeler, Ph.D., and Paul
Mushak, Ph.D; and
Effects Breakout Group (Volumes IV, V, and VI)Steven Bartell, Ph.D., and Paul
Mushak, Ph.D.
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3.3.1 Exposure Breakout Group (Volumes II and III) Gerald Keeler, Ph.D., and Paul Mushak.
Ph.D.
Volume II (Emissions)
Panelists suggested that the "minor sources"i.e., those not included in the quantitative
assessmentmay contribute as much as an additional 20 percent to the total amount of mercury
emitted annually. European emission factors should be used to improve the accuracy of this
assessment of the minor sources.
Reviewers stressed that, to provide a complete picture of the atmospheric flux of mercury and
to properly assess anthropogenic contributions to environmental mercury, the report should assess
natural sources of atmospheric mercury as well as the reemission of mercury previously deposited on
both aquatic and terrestrial environments by anthropogenic emissions.
Reviewers suggested that a national network of atmospheric mercury monitoring be
established to validate emission data and to provide necessary information on trends in mercury
deposition.
The panel felt that the division of sources into point and area source categories should be
unproved. For example, mercury emissions from residential heating furnaces should be defined as
area sources, while crematories and medical waste incinerators should be categorized as point sources.
The panel agreed with the appropriateness of the emission factor approach. Many of the
emission factors are based on actual test data and measurements, which contributes to the accuracy of
the inventory. The emission estimates, when compared on a per capita basis, are quite similar to those
in selected industrialized countries in Europe. In addition, the total U.S. anthropogenic mercury
emissions are similar in magnitude to those of other industrialized nations in the world.
Volume III (Exposure Assessment)
a
The exposure volume utilized state-of-the-art methods in investigating the relationships
between mercury emissions and exposures. Nevertheless, only plausible relationships between
" anthropogenic emissions and exposure could be defined.
The draft report does not assess the impact of anthropogenic mercury emissions in coastal
environments. However, since fish consumption is the dominant exposure pathway, seafood or
saltwater fish should be included in the total exposure estimates.
The analysis presented in the report supports the conclusion that current levels of emissions
from major combustion/industrial sources result in incremental exposures above background to both
humans and wildlife through the consumption of contaminated freshwater fish.
The group discussed the use of exposure estimates derived from the RELMAP and
COMPMERC models. The discussants felt that the report should better describe how the model
estimates were added. After questioning the modelers directly during the breakout group, the
reviewers suggested that the authors consider alternative strategies for the risk assessment. For
example, decoupling the regional impact provided by RELMAP from the local-scale exposure
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scenarios may improve the site-specific risk analysis and provide a clearer definition of the
uncertainties in the exposure estimates utilized in the risk assessment.
Reviewers recommended that actual observations (i.e., measured mercury concentrations)
could be used to "ground-truth" the model estimates or could themselves be used in the local-scale risk
assessments. Although a wealth of high-quality atmospheric mercury data or mercury deposition data
is not available, enough data are available from the Great Lakes programs to perform a risk assessment
at a similar or better level of accuracy than the models provided. The only drawback to this approach
would be the lack of assignment of risk to specific source categories.
Additional suggestions for improving the assessments include:
Evaluate the existing exposure to methylmercury via seafood consumption. Base this
evaluation on existing data and not the model results.
Perform the risk assessment and exposure to methylmercury from existing freshwater
fish data. (This could be time-consuming because so many data are available.)
Utilize existing wet and dry deposition data as input to the Indirect Exposure Model
(IBM) to see what is predicted. This approach would remove two of the greatest
uncertainties from the modeling and could be used to estimate the risk in the risk
characterization.
Attempt to identify a better indicator of the central tendency (perhaps the median)
from the exposure assessment uncertainty analysis, which used the distributions rather
than the high-end (maximum) estimates.
In conclusion, the panel members felt that the accuracy of the estimates decreases as the
report moves from the initial emissions inventory through the exposure modeling using RELMAP and
COMPMERC to the risk assessment phases. This results in a risk assessment that may have relatively
large uncertainties and, therefore, may not provide a sound basis for decision- or policy-making.
The report would be improved by providing linkage between the risk management and the
emissions inventory. The type and cost of mercury control technologies depend largely on the form of
mercury in an emission and, thus, on the source category being considered for emission reduction.
3.3.2 Effects Breakout Group (Volumes IV. V. and VI) Steven Bartell. Ph.D.. and Paul Mushak.
Ph.D.
Volume IV (Health Effects)
After some discussion, all or most group members generally agreed with the views and
recommendations reported below. Dissenting views on key issues, where they occurred, are noted.
The group expressed several concerns about the organization and accuracy of Volume IV.
Chapter 4 is difficult to follow, but group members generally agreed that its goal was to provide
toxicity data for a human health risk assessment.
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The description and discussion of lipophilicity of mercury compounds was not entirely
accurate. The term is simplistic and does not account for current knowledge of binding and ligand-
transfer interactions of methylmercury and other mercurials.
With respect to toxicity endpoints, -the group noted that developmental impacts in the neonatal
period should not be dismissed, since neonatal effects of elemental mercury have been reported in
mice.
Differential sensitivity to mercurials among human populations is well established, and the
fetus is now assumed to be the most sensitive to effects of methylmercury. The basis of such
sensitivity includes physiological vulnerability, population variability concerning biotransformations
(e.g., demethylation of methylmercury by gut flora), and variable patterns of exposure. Overall,
sufficient data are not available to generate a highly resolved summary of differential sensitivity.
Of concern to the reviewers was treatment of the time course of exposure-effect
relationshipsi.e., are we dealing with latency or a masking phenomenon with long-term exposures?
Some reviewers were critical of the RfD calculation for inorganic ionic mercury (i.e., back-
calculating from the drinking water equivalent level [DWEL]). Some also questioned how good a
surrogate the Brown Norway rat is for humans sensitive for renal effects in the form of an
autoimmune glomerulonephritis. One reviewer thought that the Integrated Risk Information System
(IRIS) document is not convincing in this regard, and recommended that the mercury report at least
reproduce the DWEL.
How UF factors were used in the analysis was not clear; the RfDs and RfCs need a closer
look. Authors should reexamine the original data to see if they can justify how they used the
numbers, and they should better explain their rationales.
The report should indicate that additional studies are under way (other than the Iraqi data set),
although it is not known when the data will be available. Basically, the message here was to proceed
with caution, but proceed.
Either Chapter 2 of Volume IV should be expanded to provide a concise summary of the
integrated exposures to mercury, or an integrating final section should be added in Volume III. The
authors should include more information on mercury exposure from dental amalgams and from ground
waters that are or will be drinking water sourcesparticularly when mercury concentrations in these
waters approach or exceed the RfC or RfD. Information should be added on how dietary components
(other than methylmercury in fish) contribute to human exposure. This should include, information,
however qualitative, on any linkages of nonfish dietary mercury to atmospheric emissions.
Several comments concerned the mechanisms of mercury toxicology in humans and test
animals. Although mechanisms of toxicity are critical to understanding the plausibility of
epidemiological relationships reported for different populations and to understanding where thresholds
for toxic effects may lie, the report gave them short shrift The report should expand the discussion of
this topic and should address how mercury forms move in and out of cells. However, reviewers
recognized that a complete mechanisms sections might require an effort beyond the scope of the
report.
Reviewers generally agreed that the health endpoints selected for the assessment and the dose-
response relationship for each of the three forms of mercury were appropriate for the risk assessment.
June 1996 A-17 SAB REVIEW DRAFT
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However, they thought the authors should strengthen the discussion of the validity of the endpoints
and epidemiological data selected. Also, the group recommended that authors scrutinize the numbers
employed from modeling, such as the fraction that goes into blood, the half-life, and the elimination
parameter. The hairblood ratio of 250 seems to be a middle-of-the road number and is probably
acceptable. Reviewers questioned why the report did not use distributional analysis rather than
selecting point values that might result in an unknown bias.
The group's comments on Appendix C of Volume IV mainly concerned model uncertainty
and not variability in data-based parameters.
Reviewers considered the issue of selenium-mercury interactions. They felt this issue was
complicated because the data sets are isolated and have no mechanistic underpinning. The critical
question is how selenium in diet affects long-term exposures and associated chronic toxic endpoints.
Was the Iraqi population at risk because of dietary habits (i.e., because they were grain eaters)? On
the other hand, the reported selenium content of cereal grains is not vastly different than the selenium
content measured in certain fishes. Although the selenium issue may have a bearing on which
population exposed to methylmercury is valid for risk characterization, reviewers felt it premature to
use selenium intake as a criterion for selection. One problem concerning the selenium-mercury
connection is that the clearest associations are seen in gross endpoints, such as high-dose teratogenesis.
Regarding which dose-response data to use in risk characterization, reviewers expressed some
sentiment for using at least two RfDs: one for the general adult population and one for pregnant
women. Reviewers emphasized that the methylmercury RfD used in the assessment should be
reported as an interim value, and that the assessment should be formulated to facilitate near-term (i.e.,
within the next several months) modifications to the RfD.
Some comments expressed in the effects breakout group also concerned the risk
characterization volume. For example, the values of the NOAELs or LOAELs should be carried forth
into the risk assessment instead of transforming them into permissible fish tissue concentrations.
Volume V (Ecological Assessment)
The group generally agreed that the goal was to provide data for a risk assessment and that
the appropriate species were identified except for lower trophic levels and wading birds.
There was consensus that methylmercury was the compound of interest in addressing the toxic
effects of mercury on piscivores. The consensus was further evidenced by the reported mortality of
panthers, which was diagnosed as mercury toxicosis. The group also discussed the fact that the
population of wading birds in the Everglades has significantly decreased in the last 5 years. Loss of
habitat and exposure to mercury were listed as the suspected causes of these declines. One reviewer
reported that loons in Minnesota also were suffering increased mortality from mercury exposure.
Analyses showed elevated mercury concentrations in the feathers of juvenile loons. Approximately
2,500 loons died in coastal waters off Florida, in part from mercury exposure.
One reviewer pointed out that ethylmercury was measured in the Everglades, but this
compound was not expected to be environmentally or toxicologically important in the overall
assessment. Ethylmercury has not been identified in fish, for example. Dimethylmercury also exists
in nature, but is quite volatile and, based on known information and the compound's fundamental
chemistry, is not expected to represent any significant ecological threat.
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Reviewers generally agreed that the report's treatment of methylmercury as a constant fraction
of total mercury in the water column was an oversimplification. Additional work might be undertaken
to determine the impacts of this assumption on the final estimates of the BAF and wildlife criteria
values developed as assessment endpoints.
The group discussed the fact that chronic toxicity tests for methylmercury are extremely
limited and that such effects are difficult to demonstrate under field conditions. For example, eggs can
be collected from the nests of mercury-contaminated birds; however, it is not easy to detect toxic
effects of mercury (e.g., hatching success, survivorship, growth). Different histories of exposure for
adult birds may also make it difficult to establish effects in the field. As a result the reviewers
suggested that the use of toxic effects measured in the laboratory is justified, particularly
developmental effects. In other words, laboratory-to-field extrapolations should be conserved. The
group expressed concern about whether frank toxicity is the most appropriate endpoint, but
acknowledged that frank effects are-the best known.
A couple of reviewers thought that the dose-response relationships were adequately treated,
the choice of a NOAEL and LOAEL was acceptable, and the limited toxicity data were used in an
appropriate manner to develop the NOAELs and LOAELs used in the assessment. Some discussion
ensued concerning the utility of toxicity data from laboratory studies on other animals (e.g., domestic
animals and birds); these data might be used to at least help define the range of toxic exposure
concentrations. The assessment needs to clarify the use of the wildlife criteria values developed in an
approach paralleling human health risks (i.e., protection of individuals) for protecting populations of
the selected wildlife species.
There was considerable discussion and concern regarding the validity of the overall conceptual
model for the ecological assessment This relates in part to the consideration of the complex
chemistry of mercury in surface waters, where different physicochemical factors might determine
exposure. Reviewers noted that lakes located side by side might show very different concentrations of
mercury in fish. This multifactor complexity calls into question the linearity implied in the current
approach for developing the BAF and wildlife criteria values. The concern is particularly important
given the national scope of the intended assessment.
The reviewers noted the need to better articulate the uncertainty regarding the BAFs and the
selection of the mean value. They also felt the report needed better discussions of distributions and of
the nature of the uncertainty analysis.
Volume VI (Risk Characterization)
The effects breakout group's primary concern regarding Volume VI was its lack of emphasis
on risk integration. Volume VI mainly reiterates and summarizes the material presented in the first
five volumes. The reviewers were disappointed to find that the wildlife criteria values developed in
Volume V were not carried directly through to the risk characterization. Substituting fish tissue
mercury concentrations that are consistent with the wildlife criteria values is acceptable as long as the
authors can clearly explain in the report why this was done. Nevertheless, the tissue concentrations
(or, preferably, the wildlife criteria), should be developed as distributions, not single values. These
distributions should be compared with distributions of expected mercury exposures on a regional basis
for each of the selected piscivores. Such comparisons, which are more consistent with a probabilistic
framework for ecological risk, will quickly identify species and regions of concern. They also will
highlight where current information on exposure or toxic endpoints is insufficient to develop
distributions that are precise enough for an assessment. Methods such as sensitivity and uncertainty
June 1996 A-19 SAB REVIEW DRAFT
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analysis can then be used to examine the variance underlying such imprecision to pinpoint the major
factors (e.g., model structure, model parameters) contributing to the overall uncertainty. Identifying
the sources of uncertainty is important to promote efficient and effective allocation of limited resources
and to improve precision, reduce bias, and refine the overall ecological assessment.
Reviewers felt the risk characterization might also address the risks posed by mercury to
production dynamics at lower trophic levels. Clearly, such impacts have a profound effect on fish
production that is independent of the direct accumulation and toxic effects on fish. These indirect
effects are also relevant for assessing human and piscivore exposure to contaminated fishfewer,
smaller fish translates into reduced exposure, or at least a greater effort to obtain fish and, thus,
significant mercury exposure if a larger number of smaller fish are consumed.
The group also expressed concern regarding the report's nearly total reliance on unverified
models to produce the risk assessment Where possible, the models that provided estimates of regional
deposition and exposure should be evaluated in relation to known mercury concentrations. Any efforts
at "ground-truthing" either the exposure or the toxicity models should be pursued within the resource
and time constraints imposed by the overall schedule for delivering the report.
3.4 Overview of Reviewer Discussion in the Plenary Session Paul Mushak, Ph.D.
Volume VI (Risk Characterization)
Panelists noted that a considerable portion of Volume VI consisted of summaries of Volumes
II, III, IV, and V. These summaries covered human and wildlife health effects, overlay maps of
sensitive wildlife populations with predicted high mercury depositions, and the uncertainties and
assumptions in modeling emissions. Volume VI then provided a relative exposure ranking, a relative
dose-response ranking, and levels of methylmercury hi fish tissue that would be of concern for fish
eaters.
The panel found the summaries to be confused and lacking; they failed to provide a
comprehensive or quantitative discussion of the uncertainties and assumptions, and they did not discuss
the extent and magnitude of the harmful exposures. Insufficient attention was given to linkages
between anthropogenic emissions and background mercury data with the risk characterization.
One reviewer suggested that an ecological risk assessment be performed by using distributions
of the parameters used to develop Tables 4-3 and 4-4 of Volume VI. Reviewers were impressed with
the uncertainty analysis for the human RfD value found in Volume IV, Appendix C, and were
interested in a discussion of propagated uncertainties.
The methodology and results in the comparative risk chapter of Volume VI were major areas
of concern. Reviewers pointed out that the NOAELs and LOAELs are not based on the same set of
endpoints and, therefore, are not directly comparable; hi fact, the NOAELs and LOAELs may reflect a
wide range of adverse responses. Another important concern was that the human NOAEL did not
account for uncertainty areas such as different sensitivities. This indicates that use of the RfD would
be more appropriate.
Regarding the wildlife criteria, reviewers felt that use of the published rat and monkey dose-
response data would potentially capture more subtle effects in the rat. Notwithstanding the problems,
information is available to enhance the accuracy of the criteria.
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Reviewers offered several caveats regarding the strength of the linkages between point source
emissions of mercury and increased levels of methylmercury in fish. Reviewers agreed there is no
doubt that fish in certain areas exceed advisory limits. One reviewer claimed that all the conclusions
in Volume VI are based on models rather than actual data. The volume would benefit from a
discussion of the pathways for which there are claimed to be no data. Reviewers discussed the extent
to which the report went beyond actual data, but did not come to a clear consensus.
In terms of fish consumption rates, reviewers felt the estimates of the distribution of such
intakes should be improved.
Reviewers agreed that there is a significant need for systematic collection of data on increased
levels of methylmercury in exposed wildlife populations.
In the aggregate, the discussion clearly indicated a need to better integrate the exposure and
health effects datafor example, by comparing distributions of fish mercury levels with distributions
of wildlife criteria. Some reviewers argued that background (baseline) determinations were needed to
better determine increases over time. The panel also suggested that the RfD be clearly defined as
"interim" and that it be revisited periodically as new data become available. Panelists also questioned
the validity of comparing a human NOAEL to overt toxicity-based guidelines in wildlife, and why an
RfD was not used.
Several comments concerned specific chapters in Volume VI. Deposition rates drive the
overall analysis, and field verification is desirable. With reference to this, the exposure breakout group
chair reemphasized that very recent data document the linkage between anthropogenic mercury
emissions and deposition (e.g., the existence of a gradient with distance). Also, reviewers agreed that
the report should better characterize seafood consumption, since it elevates the baseline for mercury
exposure to which freshwater mercury intakes are added for the overall risk characterization. In
addition, the panel recommended that seafood levels not be called "background" because some fraction
of mercury in seafood is likely to come from anthropogenic sources.
Volume VII (Risk Management)
Reviewers agreed that Volume VII was generally good, but felt that it emphasized controls
and did not adequately examine pollution prevention options. Pollution prevention could include
banning products containing mercury (e.g., Minnesota's ban on mercury batteries). Reviewers also
expressed concern about the volume's cost estimates for mercury control. For example, could the
aggregate cost of reducing mercury emissions by half be calculated?
Reviewers thought it economically inaccurate to allocate all the costs of mercury reduction
strictly to mercury, since typical reduction technologies also remove other contaminants. They
suggested that the authors lower the cost estimate for mercury reduction by distributing reduction costs
over all contaminants controlled by the technologies.
The panel felt that the absence in Volume VII of recommended actions and research needs is
a major gap that should be filled. Recommendations could include, for example, market-based
approaches, product reformulations, product bans, and recycling. The European experience was
suggested as a valuable source for information on market-based approaches.
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3.5 Summary of Major Revisions Made in Response to Reviewers Comments
All volumes:
Executive summaries re-written to be more informative
Executive summaries written to include conclusions categorized by degree of
confidence in the findings, summaries of uncertainties and research to improve the
assessment.
Volume II: An Inventory of Anthropogenic Mercury Emissions in the United States
Revised natural emission inventory information to be consistent with Expert Panel
Report.
Added industrial use trends and historical trends
Updated municipal waste combustor (MWC) inventory to include 50 closures; this
resulted in a decrease in the emissions estimate of 10 metric tons/yr to 55 metric tons.
Added impacts of proposed medial waste incinerator (MWI) and MWC rules.
*
Revised inventory to use 1993 instead of 1992 Bureau of Mines data.
Incorporated maps showing locations of sources.
Incorporated industry-specific comments.
Volume HI: An Assessment of Exposure from Anthropogenic Mercury Emissions in the
United States
Numerous recent peer-reviewed studies were incorporated.
Sections added on exposure from anthropogenic , non-ambient sources including
dental amalgam, occupational exposure and consumption of marine fish.
Section added on measured mercury concentrations near multiple local sources
Additional mercury measurement data from various media added and compared to
modeled estimates. These measurements included air concentrations, deposition rates
and soil concentrations.
An assessment of the mercury exposures that result from the input of measured
mercury air concentration, deposition rate and soil concentration data to the indirect
exposure models was added.
Modeling assumptions were modified to accommodate new data.
Increased percentage of divalent mercury assumed to be particulate bound.
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Flat terrain only was modeled and effects of complex terrain addressed
separately in an uncertainty analysis.
The configuration of the watershed was changed and area-averaged deposition
rates were utilized.
The aquatic trophic levels, which wildlife were assumed to consume, were
modified.
The assumed quantity of background atmospheric mercury was modified.
Deposition velocities for vapor-phase divalent mercury were modified to
account for lower dry deposition rates at night
The assumption related to the bioconcentration of atmospheric mercury into
green plants was modified to account for lower measured concentrations in
edible portions of grains and legumes.
Exposure models were re-run to accommodate the above assumptions and the revised
emissions inventory.
A section (Appendix H) was added to estimate the size of the fish consuming human
population in the U.S., the amounts of fish consumed by the general U.S. population
and several high-end-fish-consuming populations, and the amount of mercury
measured in surveys of marine and fresh-water fish. These data were used to generate
estimates of mercury exposure from fish consumptioa These mercury exposure
estimates were not attributed to individual sources or source categories.
Volume FV: Health Effects of Mercury and Mercury Compounds
Section added on pharmacokinetic models. No pharmacokinetic model was chosen for
use in the health assessment.
Germ cell mutagenicity assessment was re-written to remove the numbered
classification.
Additional studies on developmental toxicity of elemental mercury were added raising
the overall weight of evidence judgement to "Sufficient Animal Data" for
developmental toxicity
The newly verified RfD for methylmercury was described. A section on input data
and derivation of the benchmark dose was added as was discussion of plausible
alternatives to the U.S.EPA RfD.
Section on interactions of other materials with mercury and section on selenium were
re-written.
A section on mechanism of action of mercury was eliminated.
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Volume V: An Ecological Assessment for Anthropogenic Mercury Emissions in the United
States
Discussion of several new studies supplied by the reviewers was added.
Enhanced discussion of non-mammalian, non-avian life forms
Obtained original doctoral dissertation describing effects in mink and used as basis for
reevaluation of mammalian wildlife criteria.
Used revised no observed adverse effect level from dissertation
Described uncertainty factor of 10 for subchronic to chronic extrapolation
Re-evaluated criteria for avian species
described available data on loons, but did not calculate a wildlife criterion for
this species
Described studies form the National Biological Service on levels of mercury in
eagle feathers.
Describe uncertainty in LOAEL to NOAEL extrapolation and species
extrapolation
Analyzed data from laboratory animal studies to bound uncertainty on wildlife criteria
Clarified assumptions, uncertainties and methods in development of wildlife criteria.
Described variability and uncertainty in wildlife feeding habits.
Volume VI: Characterization of Human Health and Wildlife Risks from Anthropogenic
Mercury Emissions in the United States
Volume was completely re-organized to meet specifications of new U.S.EPA guidance
on risk characterization
Discussion of plausible alternatives to the U.S. EPA RfD on methylmercury included.
Revised and expanded discussion of uncertainty and variability
Included estimates of size of "at risk " human and wildlife populations
Human estimate based on data from National Center for health Statistics
(CDC), U.S. census data, and the Continuous Survey of Food Intake by
Individuals. This was combined with measured levels of mercury in marine
and fresh-water fish.
Wildlife estimates made from literature.
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Highlighted exposure as the major source of variability vs. Species -specific
differences in susceptibility to toxic effects.
Added comparison of mercury exposure estimates with methylmercury RfD or
equivalents for humans and wildlife.
Volume VII: An Evaluation of Mercury Control Technologies and Costs
Enhanced discussion of pollution prevention opportunities. These were discussed in
qualitative terms and quantified when data were sufficient
Integration of control costs with benefits was done, as well as final section on
management alternatives and statutory authorities.
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4. SUMMARY OF INTERAGENCY REVIEWERS COMMENTS
AND DISPOSITION
Reviews of the External Review Draft of the Mercury Study Report to Congress were
obtained from the following U.S. government agencies:
Agency for Toxic Substances and Disease Registry (ATSDR), Public Health Service,
U.S.Department of Health and Human Services.
National Institute of Environmental Health Sciences (NIEHS), National Institutes of
Health
National Oceanic and Atmospheric Administration (NOAA), Department of Commerce
U.S. Department of Agriculture (USDA)
U.S. Department of Energy (DOE)
U.S. Food and Drug Administration (FDA)
National Biological Service
A meeting of reviewers was held at the U.S. EPA, Washington DC on 1/9/95 to discuss
scientific issues concerning the report. Representatives of the above Agencies attended the meeting
with the exception of NIEHS; comments from NIEHS were submitted in writing after the meeting.
Written comments were requested of all reviewers; responses were received from ATSDR, DOE,
NIEHS, USDA and the National Biological Service. At the meeting and in written reviews point of
congruency among Federal risk estimates and methodologies were identified; points of divergent
opinion were also noted.
Major critiques are described below as well as U.S. EPA's response (in italics). It is the
Agency's intent to describe in the final Report alternate points of view or risk estimates in those
instances wherein U.S. EPA disagrees with another federal agency.
General Comments on the Report.
Reviewers noted that some references were incomplete or missing.
These were completed. To the extent possible within deadlines, papers submitted by
the reviewers were cited in the Report.
Reviewers felt that the Report would be greatly enhanced in its usefulness if general
conclusions on the extent of mercury contamination or degree of hazard could be
articulated in plain language.
This was done for inclusion in Vol. I Executive Summary, which was prepared after
the interagency review was completed. In addition, each volume was revised to
include a general conclusions summary in its own executive summary.
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Reviewers discussed section 112(n)(l) of the Clean Air Act Amendments of 1990.
This specifies the following.
The Administrator shall conduct, and transmit to the Congress not later than 4 years
after the date of enactment of the Clean Air Act Amendments of 1990, a study of
mercury emissions from electric utility steam generating units, municipal waste
combustion units, and other sources, including area sources. Such study shall consider
the rate and mass of such emissions, the health and environmental effects of such
emissions, technologies which are available to control such emissions, and the cost of
such technologies.
FDA proposed that U.S. EPA was not required to determine or comment upon a
threshold for adverse effects of mercury in humans and that it was inappropriate for
U.S. EPA to make such a determination in this Report.
U.S. EPA is obliged to follow consistent methodologies and published Guidelines for
Human Health Risk assessment in it evaluation of potential human health impacts of
environmental agents. For general systemic non-cancer health endpoints this includes
consideration/calculation of reference doses (RfD) or reference concentrations (RfC).
The methods used for derivation ofRfDs and RfCs are based on the assumption of a
population threshold for response in the absence of data which indicates that no
threshold exists. It was agreed by both scientific staff and U.S. EPA management that
application of state-of-the art methodologies for calculation ofRfDs and RfCs was an
appropriate part of the Mercury Study. A statement of the FBA critique is included in
the section of the Report summarizing reviewer comments.
Reviewers noted that the Report did not deal with the impacts of global mercury use
or emissions or of "natural" mercury.
U.S. EPA was directed in the CAAA to deal with emissions from various specified
sources and "other sources, including area sources". When data were sought and
models constructed, it became obvious that contemporary, reliable emissions data on
mercury were not sufficient to support a national survey. Neither the extant data nor
modeling technology permitted accurate modeling of emissions from countries other -
than the U.S. The acknowledged global cycling of mercury was accounted for in the
incorporation of a 1.6 ng/m3 "background level" into the long-range transport
modeling (RELMAP). The Report describes the impossibility of determining whether
mercury is of "natural" or anthropogenic origin; there is, for example a discussion of
hypotheses that mercury soil levels in sites distant from emissions sources can be the
consequence of deposition over time of mercury released as a result of human
activities. The Executive Summary and Exposure volumes indicate that any local
evaluation of mercury hazard must use heal determinations of mercury in media.
Volume II: Inventory of Anthropogenic Mercury Emissions in the United States.
The emissions inventory was thought generally to be comprehensive and well
described. The was general agreement with the conclusions on relative source
contributions.
June 1996 A-27 SAB REVIEW DRAFT
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An explanation should be given in this volume of the use of emissions data in the
exposure modeling.
This was done.
A description of derivation of emission modifying factors (EMF) was requested;
specifically, were these numbers means, representative values, etc.
This information was added.
USDA proposed the lack of an estimate of mercury emissions from landfills was a
serious deficit
Dataware not available which permit any sort of generalization about the magnitude of
emissions from this source type. There is only one study of mercury emitted from a
landfill area; this was done in Minnesota, and there is no indication that this site was
representative of other waste sites. Studies on landfills as a potential mercury source
have emphasized pathways leading to groundwater contamination rather than release
to the air.
USDA also remarked that mercury from application of sewage sludge to farm land
was not considered as a source.
The Report does include some information on'sewage sludge incineration and its
potential for mercury release to the atmosphere. Data on consequences of land
application of sludge, to the extent provided by the USDA, were included in Vol II or
III as appropriate.
The National Biological Service recommended adding more information on the re-
emission of deposited mercury of anthropogenic origin.
This is discussed in the Report as a source for which data are not available and as a
contributor to possible underestimation of over all emissions.
Volume III: An Assessment of Exposure from Anthropogenic Mercury Emissions in the
United States.
There was general agreement that mercury deposition model results were reasonable
predictions given available data. There was discussion of impacts of using emission
factors from washed coal and from seams most recently used by coal fired utility
boilers on the relative ranking of source contributions. There was discussion of the
likelihood that methylmercury is released from utility stacks; consensus opinions of
U.S. EPA and reviewers were that there were no conclusive data on methylmercury
release.
USDA raised concern that parameters (e.g., amounts of foodstuffs consumed by human
populations) used in Vol III modeling were inconsistent with those used in the sludge
evaluations.
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The parameters were compared, and any departures from the sludge methodologies
are described; justifications for departure are provided.
USDA identified consumption of wild mushrooms as a source of mercury.
Description of this source based on material if provided by USDA was included in Vol
III.
ATSDR noted that use of the term "subsistence fisher" in the assessment was
inaccurate because the consumption rate used was not sufficient to constitute dietary
subsistence.
Use of "high end fish consumer" or some other more descriptive term was substituted.
DOE and others cautioned against using "significant" outside a statistical context.
Another term was used when statistical significance is not being described.
There was discussion of the availability and usefulness of mercury total body burden
data.
It was agreed to incorporate such data as were available from ATSDR and on recent
reports from Sweden and on a group of Chippewa Native Americans. The Report
describes the limitations on comparison of the modeled predictions with body burden
data. Body burden data include cumulative exposure to non-anthropogenic mercury
and mercury in marine fish.
There was discussion on variability in estimates of percent of mercury in food sources
as methylmercury.
The Report describes this variability. Sources of the estimates were checked to ensure
that attribution is clear.
There was a brief discussion of the impact of dental amalgams on total mercury body
burden.
Discussions in the report on amalgam mercury release were reiterated in the
beginning of Vol III in the section outlining those sources which were modeled, how
background is considered, etc.
Several reviewers pointed to the lack of information on marine fish. It was noted by
FDA that one cannot generalize as to whether marine fish or freshwater fish are likely
to have higher concentrations of mercury.
The modeling of mercury deposition employed by U.S. EPA of necessity dealt only
with mercury in U.S continental, fresh water lakes and streams. The Report contains
one table on measured mercury levels in commercial marine fish. This was enhanced
with data supplied by the reviewers, and the accompanying discussion was expanded
and moved to the beginning of the report. Conclusion statements of the Report
June 1996 A-29 SAB REVIEW DRAFT
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acknowledge that the majority offish consumed in the U.S. is marine fish. Marine
seafood consumption estimates are included in discussions in Volume VI.
The NOAA and the National Biological Service reviewers said that the mercury
species found in fish flesh varies with the type and trophic level of fish.
Variation reported in the literature is described in Volume HI.
The reviewer from the National Biological Service took issue with some assumptions
used in the deposition modeling; specifically, he asserted that most precipitated
mercury is paniculate bound and that methylmercury can be introduced into systems
by wet deposition.
The recommended papers (Benoit, Fitzgerald and Damman, 1994., Holtberg et al.,
Monterey Conference Proceedings) were evaluated as to inclusion in the Report. Note
that Benoit et al. is in press and was not available for evaluation in the time frame of
U.S. EPA's Report deadlines.
The National Biological Service registered a strong objection to the use of
bioaccumulation factors or other means to make generalized statements as to
relationship between mercury in water and predicted concentrations of mercury in fish
inhabiting the water. The reviewer stated that local biogeochemistry is highly variable
with the result that fish taken from water bodies with the same mercury concentration
can have very different mercury concentrations in the tissue. Discussion focussed on
factors governing this variability; it was acknowledged that there are no data to allow
modeling of any one factor or combination of factors. There was discussion of use of
the EPRI Mercury Cycling Model (MCM). There was agreement that this is not
appropriate as a basis for local or general conclusions as to relationship between water
and fish mercury concentrations. The objection to the MCM stems in large part from
its basis on data from a water body not considered to be representative of other U.S.
freshwater lakes.
U.S. EPA maintains that some form of estimate offish tissue mercury level is needed
to evaluate the potential impact of anthropogenic mercury emissions on human and
wildlife health. The Report was amended to include local biogeochemistry as a source
of substantial variability in fish mercury predictions. Ranges of mercury fish levels
provided by the National Biological Service were used to describe the extent of
variability. This was added to the discussion of limitations of use of the modeled
estimates for any site-specific evaluation.
USDA described the potential for sheep to consume beet greens which may be a
source of mercury.
Volume III was reviewed to ensure that sources of mercury contamination not modeled
are mentioned.
0
Volume IV: Health Effects of Mercury and Mercury Compounds.
There was agreement with the hazard identification categories for carcinogenicity,
developmental toxicity and germ cell mutagenicity for elemental, inorganic and
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methylmercury with the exception noted below. It was agreed that no low dose
extrapolation for potential carcinogenicity of inorganic mercury or methyl mercury is
supported by existing data. It was agreed that immune-mediated glomerulonephritis is
the critical effect for a reference dose for inorganic mercury. It was agreed that a
reference concentration for elemental mercury of 3xlO~4 mg/m3 is reasonable. After
much discussion (excerpted below) it was agreed that a reference dose for
methylmercury is within an order of magnitude of 10~4 mg/kg-day.
It was recommended that "methylmercury" be substituted for "organic mercury".
This change has been made in the Report.
U.S. EPA discussed the pending revisions to the Guidelines for Risk Assessment of
Carcinogens. *
While the alphanumeric classification was maintained in the text, discussion of these
classifications was enhanced to conform to the narrative classifications which U.S.
EPA will likely use in the near future. In addition the number classification in the
discussion of germ cell mutagenicity was dropped.
ATSDR indicated that there are new data on developmental effects resulting from
inhalation exposure to elemental mercury.
These studies were evaluated and the classification of "insufficient evidence for
developmental toxicity" re-examined.
In its derivation of an intermediate MRL for inorganic mercury (2xlO~3 mg/kg-day)
ATSDR used a NOAEL 0.23 for F344 rats gavaged for six months as part of the
subchronic range-finding component of a cancer bioassay (NTP, 1993).
This study was not available at the time that U.S. EPA convened an expert panel to
derive its RfD for inorganic mercury. That panel recommended use of data from short
term studies in Brown Norway rats as an animal model appropriate to estimation of
potential toxicity in sensitive human subpopulations. The NOAEL and LOAEL from
the NTP bioassay are within the range observed in three studies in the Brown Norway
rat. U.S. EPA scientists have concluded that the existing RfD, described in the 1988
Drinking Water Criteria Document for Inorganic Mercury is.not impacted by the more
recent data from NTP. The ATSDR evaluation is described and compared to U.S.
EPA's in the risk assessment chapter of Vol IV.
The FDA reviewer stated that in deriving an RfD for methylmercury (and other
agents) U.S. EPA does not estimate or predict the degree of risk but rather estimates a
measure of a "safe" level of exposure. The reviewer felt that the "bright line"
approach dose not constitute a risk assessment.
There was some agreement with the reviewer's position, particularly in the utility of
predicting risk above a hypothetical threshold. U.S. EPA, however, has not completed
analyses which would support such an estimate of risk. The question of whether the
data used (neurologic deficits in children of Iraqi mothers who ingested contaminated
grain during gestation, Marsh et al 1987) are suitable to this type of analysis is an
June 1996 A-31 SAB REVIEW DRAFT
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open one. At this time U.S. EPA does not include any estimate of risk above the RfD
in the Report. Discussion will be continued by U.S. EPA and FDA scientists with the
goal of deriving an estimate of methylmercury risk for ingestion levels.
There was discussion of the impact of current studies of developmental effects in
populations which consume high end levels of marine fish and/or mammals (the Faroe
Islands and Seychelles Islands studies). Some results these epidemiologic
investigations have been presented at recent meetings and have been published in
abstract in proceedings. It was the opinion of the FDA reviewer that these studies
show no (or little) neurologic impairment in children exposed in utero to mercury
levels associated with observed effects in the Iraqi population on which U.S. EPA
based its RfD. U.S. EPA was encouraged to use these data in their quantitative
assessment of non-cancer effects.
%
£7.5. EPA can only use data which are available to the scientific community and have
undergone a process of peer review. The deadlines specified in the CAAA do not
permit delay until the studies have-been published in the peer reviewed press or the
data submitted to U.S. EPA for a process of expert review. (It was noted that U.S.
EPA has missed the submission date (11/94) specified in the CAAA.) The Faroe and
Seychelles Islands studies as reported in abstract are described in Volume IV. In
response to the critique that there has been no influence of these results in U.S. EPA's
risk evaluation, the Report was amended in the following ways. In both Vol IV and
Vol VI (Risk Characterization), the potential for the Faroes and Seychelles results to
decrease uncertainty in the RfD is described. Alternative approaches are described;
specifically, decreasing the uncertainty factor or using the upper bound on the 10%
risk level for the benchmark dose (vs the lower bound which U.S. EPA employed).
These alternatives are used to describe the range around the U.S. EPA RfD of IxW4
mg/kg-day.
Volume V: An Ecological Assessment for Anthropogenic Mercury Emissions in the United
States.
There was agreement that data are insufficient for evaluation of mercury impacts on
any ecosystem. There was agreement that data were insufficient to calculate a wildlife
criterion for Florida panthers. There was no objection to development of wildlife
criteria for methylmercury only. It was agreed that lack of data on sensitive indicators
of toxic effect in wildlife species is a major contributor to uncertainty in the estimates.
The National Biological Service reiterated its concern with use of any method (such as
a BAF) which relates water concentrations of mercury to fish concentrations.
Volume V repeats discussions of variability in fish concentrations due to local
biogeochemistry.
USDA felt that in derivation of the trophic level 3 and 4 B AFs that a geometric mean
was more appropriate than the simple mean used.
All calculations were performed on the logs of values; arithmetic values presented in
tables were converted from logs after derivation of means and percentiles. Geometric
June 1996 A-32 SAB REVIEW DRAFT
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water bodies. The reviewer felt that the maps (with the Florida panther as the example)
were misleading and gave a false impression that no problem exists for some species.
The maps -were designed to show only predicted high mercury deposition and do not
rule out the likelihood of mercury contamination in areas (particularly wetlands)
contiguous to high deposition areas. The purpose of the overlay procedure was to
highlight areas and species of concern, not to eliminate areas as of no interest. The
extent to which overlap can be quantified is being examined; results will be included
in the Report as feasible. The purpose and limitations of the overlay maps will be
explicated more completely in Vol V.
ATSDR indicated in the derivation of wildlife RfDs and criteria that interspecies
extrapolation not based on pharmacokinetic data will have an unacceptable degree of
uncertainty.
Thus far, no useful data on pharmacokinetics in the species of interest have been
available. Additional literature searches are being conducted in that area. U.S. EPA
scientists feel that an adjustment of the NOAEL reported for mink is not need for
application to otters. The adjustment of the NOAEL derived in mallards for
application to three fish-eating birds will be re-evaluated if data permit.
Several reviewers queried whether the wildlife criteria were conservative. Questions
were raised about the likelihood that wildlife have evolved protective adaptations to
mercury toxicity.
Data are insufficient to answer either question. The endpoints tested in the wildlife
species are neither as sensitive nor as subtle as those detected in humans exposed to
methylmercury. There is no indication whether individual species or ecosystems are
being impacted by mercury such that viability or reproduction is reduced. Discussion
of this uncertainty will be expanded and reiterated in Volumes V and VI. Information
from the National Biological Survey on correlation between eagle feather mercury
levels and reproductive rates will be included.
Volume VI: Characterization of Human Health and Wildlife Risks from Anthropogenic
Mercury Emissions in the United States.
After much discussion there was agreement that data (limited as they are) for wildlife
and humans do not show special sensitivity of one species over the others. The range
of (adjusted) NOAELs is in within an order of magnitude.
There was much discussion on the comparisons made at the end of Volume VI:
NOAELs and LOAELs for human and wildlife populations, levels of mercury in fish
which would result in exposure to NOAELs or LOAELs given assumptions of fish
consumption. The utility of this approach was questioned by some reviewers; the
soundness of the data and extrapolations were questioned by others.
U.S. EPA is reconsidering the comparisons made. Our preference at this time is for
some form of interspecies comparison; an holistic approach to assessment "o^fisk for
human and non-human species is the direction which ORD is taking, based on recent
mandates and advice to U.S. EPA. The method of comparison used in the Report is
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untried. It may be advisable to limit the scientific uncertainty by backing up a few
steps; that is to limit comparisons to LOAELs and NOAELs without the additional step
of including exposure assumptions to calculate reference levels of mercury in fish. The
advantage of the last step is that it makes clear the relationship between measured
adverse endpoint in species of concern and guidance levels such as fish advisories.
Several reviewers found they could not follow the process of wildlife NOAEL
estimation from the text or tables in Volume VI.
The estimation of all NOAELs and LOAEL will be explained more fully in Volume VI.
The use of uncertainty adjustments as proposed in the Great Lakes Initiative will be
explained.
Reality checks as to measured levels of mercury in wildlife were requested by
reviewers.
Information from Vol HI, Vol V and new information supplied by the National
Biological Survey (eg., levels of mercury in feathers) will be carried over to Vol VI.
There was agreement among all parties that the Report and Volume VI in particular
should present conclusions as emphatically and clearly as the science permits.
Conclusions for all volumes will be articulated and presented in each Executive
Summary chapter. These conclusions will be re-stated in Volume VI (for risk
assessment) and in Volume I (for all conclusions).
Volume VII: An evaluation of Mercury Control Technologies. Costs and Regulatory Issues.
There was agreement that the description of control technologies and the costs of
controls was comprehensive and as accurate as extant data permit.
Reviewers discussed the "societal cost" chapter of Vol VII. DOE asked whether a
cost/benefit analysis was done. Reviewers asked if impacts on international trade (eg
GATT) were considered. FDA inquired specifically if benefits of fish consumption
(health and societal) were weighed against costs.
The CAAA mandate did not specify a cost/benefit analysis for this report. The study
included only material which could be used for cost/cost comparisons (e.g., cost of
mercury control vs. loss of revenue from recreational fishing). It was agreed after
discussion that (unlike the situation for lead exposure) there are insufficient population
data or economic impact data for subtle health effects to permit a suitable cost/benefit
analysis.
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