£fA-7f-t- -/?- 93 -
Guidelines for
Developing Community
Emergency Exposure
Levels for Hazardous
Substances
COMMITTEE ON TOXICOLOGY
NATIONAL RESEARCH COUNCIL
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Guidelines for Developing
Community Emergency Exposure
Levels for Hazardous Substances
Subcommittee on Guidelines for Developing
Community Emergency Exposure Levels (CEELs)
for Hazardous Substances
Committee on Toxicology
Board on Environmental Studies and Toxicology
Commission on Life Sciences
National Research Council
NATIONAL ACADEMY PRESS
Washington, D.C. 1993
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NATIONAL ACADEMY PRESS 2101 Constitution Aye., N.W., Washington, B.C. 20418
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Copyright 1993 by the National Academy of Sciences.
Printed in the United States of America
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Subcommittee on Guidelines for
Developing Community Emergency
Exposure Levels (CEELs) for
Hazardous Substances
JOHN DOULL (Chair), University of Kansas Medical Center, Kansas
City, Kan.
R. HAYS BELL, Eastman Kodak Company, Rochester, N.Y.
CHARLES E. FEIGLEY, University of South Carolina, Columbia,
S.C.
DONALD E. GARDNER, ManTech Environmental Technology, Inc.,
Research Triangle Park, N.C.
IAN GREAVES, University of Minnesota, Minneapolis, Minn.
ROGENE F. HENDERSON, Lovelace Biomedical and Environmental
Research Institute, Albuquerque, N.M.
DAVID P. KELLY, E.I. DuPont de Nemours & Co., Newark, Del.
RALPH L. KODELL, Food and Drug Administration, National Center
for Toxicological Research, Jefferson, Ark.
TRENT R. LEWIS, Consultant, Cincinnati, Ohio
BERNARD M. WAGNER, New York University Medical Center,
Orangeburg, N.Y.
BAILUS WALKER, JR., University of Oklahoma, Oklahoma City,
Okla.
Staff
RICHARD D. THOMAS, Program Director
KULBIR S. BAKSffl, Project Director
RUTH E. CROSSGROVE, Editor
BEULAH S. BRESLER, Senior Editorial Assistant (until June 1992)
CATHERINE M. KUBEK, Senior Program Assistant
RUTH P. DANOFF, Project Assistant
in
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Committee on Toxicology
ROGENE HENDERSON (Chair), Lovelace Biomedical and
Environmental Research Institute, Albuquerque, N.M.
BERNARD WAGNER (Vice-Chair), Wagner Associates, Inc.,
Millburn, N.J.
R. HAYS BELL, Eastman Kodak Company, Rochester, N.Y.
DEAN E. CARTER, University of Arizona, Tucson, Ariz.
CHARLES E. FEIGLEY, University of South Carolina, Columbia,
S.C.
DONALD E. GARDNER, ManTech Environmental Technology, Inc.,
Research Triangle Park, N.C.
MARY ESTHER GAULDEN, University of Texas Southwestern
Medical School, Dallas, Tex.
WALDERICO GENEROSO, Oak Ridge National Laboratory, Oak
Ridge, Tenn.
IAN GREAVES, University of Minnesota, Minneapolis, Minn.
RONALD A. HTTES, Indiana University, Bloomington, Ind.
CAROLE A. KIMMEL, Environmental Protection Agency,
Washington, D.C.
RALPH L. KODELL, Food and Drug Administration, National Center
for Toxicological Research, Jefferson, Ark.
LOREN D. KOLLER, Oregon State University, Corvallis, Oreg.
ERNEST EUGENE MCCONNELL, Raleigh, N.C.
MICHELE A. MEDINSKY, Chemical industry Institute of Toxicology,
Research Triangle Park, N.C.
ROBERT SNYDER, Rutgers University, Piscataway, NJ.
BAILUS WALKER, JR., University of Oklahoma, Oklahoma City,
Okla.
HANSFETER R. WTTSCfll, University of California, Davis, Calif.
GERALD N. WOGAN, Massachusetts Institute of Technology,
Cambridge, Mass.
GAROLD S. YOST, University of Utah, Salt Lake City, Utah
IV
f, ! :.-:,
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Committee on Toxicology
RICHARD D. THOMAS, Program Director
KULBIR S. BAKSffl, Senior Program Officer
MARVIN A. SCHNEIDERMAN, Senior Staff Scientist
RUTH E. CROSSGROVE, Editor
BEULAH S. BRESLER, Senior Editorial Assistant (until June 1992)
CATHERINE M. KUBEK, Senior Program Assistant
RUTH P. DANOFF, Project Assistant
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Board on Environmental Studies
and Toxicology
PAUL G. RISSER (Chair), University of Miami, Oxford Ohio
FREDERICK R. ANDERSON, Cadwalader, Wickersham & Taft,
JO^C^ABLAR, in, McGill University School of Medicine,
Montreal, Quebec, Canada
GARRY D BREWER, University of Michigan, Ann Arbor, Micti.
JOHN CAIRNS, JR., Virginia Polytechnic Institute and State
University, Blacksburg, Va.
EDWIN H. CLARK, Department of Natural Resources and
Environmental Control, State of Delaware, Dover, Del.
JOHN L. EMMERSON, Lilly Research Laboratories, Greenfield, Ind.
ROBERT C. FORNEY, Unionville, Pa.
ALFRED G. KNUDSON, Fox Chase Cancer Center, Philadelphia, Pa.
KAI LEE, Williams College, Williamstown, Mass
GENE E. LIKENS, The New York Botanical Garden, Millbrook, N.Y.
JANE LUBCHENCO, Oregon State University, Corvalhs, Oreg.
DONALD MATTISON, University of Pittsburgh, Pittsburgh, Pa.
HAROLD A. MOONEY. Stanford University, Stanford, Calif.
GORDON ORIANS, University of Washington, Seattle, Wash.
FRANK L. PARKER, Vanderbilt University, Nashville, Term., and
Clemson University, Anderson, S.C.
GEOFFREY PLACE, Hilton Head, S.C.
MARGARET M. SEMINARIO, AFL/CIO, Washington, D.C.
I GLENN SIPES, University of Arizona, Tucson, Ariz.
BAILUS. WALKER, JR., University of Oklahoma, Oklahoma City,
Okla
WALTER J. WEBER, JR., University of Michigan, Ann Arbor, Mich.
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Board on Environmental Studies and Toxicology Staff
JAMES J. REISA, Director
DAVID J. POLICANSKY, Associate Director and Program Director
for Natural Resources and Applied Ecology
RICHARD D. THOMAS, Associate Director and Program Director for
Human Toxicology and Risk Assessment
LEE R. PAULSON, Program Director for Information Systems and
Statistics
RAYMOND A. WASSEL, Program Director for Environmental
Sciences and Engineering
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Commission on Life Sciences
THOMAS D. POLLARD, Johns Hopkins Medical School, Baltimore,
Md.
BRUCE M. ALBERTS, University of California, San Francisco, Calif.
BRUCE N. AMES, University of California, Berkeley, Calif.
J. MICHAEL BISHOP, Hooper Research Foundation, University of
California Medical Center, San Francisco, Calif.
DAVID BOTSTEIN, Stanford University School of Medicine,
Stanford, Calif.
MICHAEL T. CLEGG, University of California, Riverside, Calif.
GLENN A. CROSBY, Washington State University, Pullman, Wash.
LEROY E. HOOD, University of Washington, Seattle, Wash.
MARIAN E. KOSHLAND, University of California, Berkeley, Calif.
RICHARD E. LENSKI, University of Oxford, Oxford, United
Kingdom
STEVEN P. PAKES, Southwestern Medical Center, Dallas, Tex.
EMIL A. PFTTZER, Hoffmann-La Roche Inc., Nutley, N.J.
MALCOLM C. PIKE, University of Southern California School of
Medicine, Los Angeles, Calif.
PAUL G. RISSER, Miami University, Oxford, Ohio
JOHNATHAN M. SAMET, University of New Mexico School of
Medicine, Albuquerque, N.M.
HAROLD M. SCHMECK, JR., Armonk, N.Y.
CARLA J. SHATZ, University of California, Berkeley, Calif.
SUSAN S. TAYLOR, University of California at San Diego, La Jolla,
Calif.
P. ROY VAGELOS, Merck and Company, Inc., Whitehouse Station,
N.J.
TORSTEN N. WBESEL, Rockefeller University, New York, N.Y.
Staff
ALVIN G. LAZEN, Acting Executive Director
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Other Recent Reports of
The Board on Environmental
Studies and Toxicology
Issues in Risk Assessment (1993)
Setting Priorities for Land Conservation (1993)
Protecting Visibility in National Parks and Wilderness Areas (1993)
Biologic Markers in Immunotoxicology (1992)
Dolphins and the Tuna Industry (1992)
Environmental Neurotoxicology (1992)
Hazardous Materials on the Public Lands (1992)
Science and the National Parks (1992)
Animals as Sentinels of Environmental Health Hazards (1991)
Assessment of the U.S. Outer Continental Shelf Environmental Studies
Program, Volumes I-IV (1991-1993)
Human Exposure Assessment for Airborne Pollutants (1991)
Monitoring Human Tissues for Toxic Substances (1991)
Rethinking the Ozone Problem in Urban and Regional Air Pollution
(1991)
Decline of the Sea Turtles (1990)
Tracking Toxic Substances at Industrial Facilities (1990)
Biologic Markers in Pulmonary Toxicology (1989)
Biologic Markers in Reproductive Toxicology (1989)
Copies of these reports may be ordered from
the National Academy Press
(800) 624-6242
IX
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Acknowledgments
The Subcommittee on Guidelines for Developing Community
Emergency Exposure Levels (CEELs) for Hazardous Substances
gratefully acknowledges the valuable assistance provided by Mr. Ralph
C. Reynolds and Mr. David P. Richardson of Eastman Kodak Company
for preparing sections of this report.
The subcommittee also acknowledges the following scientists for the
valuable contributions to the work of the subcommittee by making oral
presentations and/or providing written material:
Dr. Dean Branson, Dow Chemical Company
Dr. Edward Calabrese, University of Massachusetts
Dr. Roger Garrett, U.S. Environmental Protection Agency
Dr. Daniel Guth, U.S. Environmental Protection Agency
Dr. Annie Jarabek, U.S. Environmental Protection Agency
Dr. Gary Kimmel, U.S. Environmental Protection Agency
Dr. Robert MacPhail, U.S. Environmental Protection Agency
Dr. Alan Susten, Agency for Toxic Substances and Disease
Registry
Dr. Paul Tobin, U.S. Environmental Protection Agency
XI
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Preface
Massive amounts of extremely hazardous substances (EHSs) can be re-
leased accidentally as a result of chemical spills, industrial explosions,
fires, or accidents involving railroad cars and trucks transporting EHSs.
The people in communities surrounding industrial facilities where EHSs
are manufactured, used, or stored, and in communities along the nation's
railways and highways are potentially at risk of being exposed to air-
borne EHSs during accidental releases. The U.S. Environmental Protec-
tion Agency (EPA) has identified 366 EHSs based on acute lethality data
in rodents.
Various governmental agencies and private organizations have recom-
mended inhalation exposure limits for airborne substances for workplace
exposures (8 hr/day, 5 days/week, 50 weeks/year for up to 50 years of
worklife) and for ambient exposures to low levels of common air pollut-
ants (24 hr/day for lifetime of a person). However, inhalation exposure
limits for emergency exposures to EHSs (which might occur once in a
lifetime of a person) have not been recommended for most EHSs. Emer-
gency exposure limits for EHSs would be useful to local emergency
planning committees, as well as state and federal agencies involved in
emergency planning and public health.
At the request of EPA and the Agency for Toxic Substances and Dis-
ease Registry (ATSDR), the National Research Council's Committee on
Toxicology (COT) convened the Subcommittee on Guidelines for Devel-
oping Community Emergency Exposure Levels for Hazardous Substan-
ces, which prepared this report. The members of the subcommittee had
expertise in toxicology, epidemiology, medicine, pharmacology, chemis-
try, industrial hygiene, mathematical modeling, biostatistics, and risk
assessment.
This report addresses risk assessment of short-term exposures to high
levels of chemical toxicants. It describes the criteria and methods for
developing community emergency exposure levels for EHSs for use by
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xiv PREFACE
many organizations, including EPA, the ATSDR, state and local govern-
ments, and chemical industries. . .
The subcommittee gratefully acknowledges the following scientists for
making presentations on issues relevant to setting emergency exposure
limits for hazardous substances: Dr. George Alexeef of California De-
partment of Health Services; Dr. Desai Branson of Dow Chemical Com-
pany Dr Edward Calabrese of the University of Massachusetts; Dr.
Alan'susten of ATSDR; and Drs. Roger Garrett, Daniel Gulh, Annie
Jarabek, Gary Kimmel, Robert MacPhail, and Paul Tobm of EPA.
This report could not have been produced without the untiring efforts
of the National Research Council staff: Richard D. Thomas, program
director; Ruth E. Crossgrove, editor; Beulah Bresler, senior editorial
assistant; Catherine Kubik, senior program assistant; and Ruth Danoff,
project assistant. '. '
Finally the subcommittee gratefully acknowledges the persistence,
patience, and expertise of Kulbir S. Bakshi, project director of the sub-
committee, for preparing the white paper that served as the building
block for the report and for bringing this report to its final form.
John Doull, Chair
Subcommittee on Guidelines for Developing
Community Emergency Exposure Levels (CEELs)
for Hazardous Substances
Rogene F. Henderson, Chair
Committee on Toxicology
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Contents
EXECUTIVE SUMMARY 1
1 INTRODUCTION 9
Acceptable Inhalation Exposure Limits Recommended
by Agencies and Organizations 11
Limitations of the Recommended Exposure
Limits for Use as CEELs 14
2 CRITERIA FOR ESTABLISHING CEELS 19
Populations of Concern 19
Exposure Periods and Patterns 20
Nature and Severity of Toxic Effects 20
Proportion of the Exposed Population Showing
Each Category of Toxic Effect 21
3 SOURCES OF DATA FOR DEVELOPING CEELS 23
Human Data 24
Animal Toxicity Studies 29
Short-Term In Vitro Tests 30
Chemical and Physical Characteristics of Toxicants 33
4 TYPES OF DATA USED IN DEVELOPING CEELS 35
Pharmacokinetics and Metabolism 35
Toxicity End Points: Humans and Animals 44
Other Organ Effects 54
5 RISK ASSESSMENT 57
Overview 57
Noncarcinogenic Effects 61
Carcinogenic Effects 67
xv
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jevi CONTENTS
'
6 DEVELOPMENT OF CEELS
Evaluation of Data
Presentation and Treatment of Data
Translation of Animal Data to Humans
Threshold Toxicity
Nonthreshold Effects
Calculation of CEELs and Rationale
Presentation of CEELs
75
76
84
86
88
93
95
96
REFERENCES
99
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Executive Summary
In the Bhopal disaster of 1984, 2,000 residents living near a chemical.
plant were killed and approximately 20,000 more suffered irreversible
damage to their eyes and lungs following an accidental release of methyl
isocyanate. The toll was particularly high because the community had.
little idea what chemicals were being used at the plant, how dangerous
they might be, and what steps to take in case of emergency. This trag-
edy served to focus international attention on the need for governments
to identify hazardous substances and to assist local communities in plan-
ning how to deal with emergency exposures.
In the United States, the Superfund Amendment and Reauthorization
Act (SARA) of 1986 required the U.S. Environmental Protection Agency
(EPA) to identify extremely hazardous substances (EHSs) and, in cooper-
ation with the Federal Emergency Management Agency, to assist local
emergency planning committees (LEPCs) in developing emergency re-
sponse plans for sites where EHSs are produced, stored, transported, or
used. SARA also required the Agency for Toxic Substances and Disease
Registry (ATSDR), part of the Public Health Service, to determine
whether chemical substances that have been identified at hazardous waste
sites or in the environment present a public health concern.
As a first step in assisting LEPCs, EPA has identified 366 EHSs,
based largely on their "immediately dangerous to life and health" (IDLH)
values in experimental animals. However, few of these substances have
adequate toxicity information from human studies. And while several
public and private groups have established exposure limits for some
substances and some exposures (e.g., workplace or ambient air quality),
these limits are not easily or directly translated into the kind of limits
required for emergency exposures, which typically involve exposure at
high levels but of short duration, usually less than 1 hour, and only once
hi a lifetime.
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2 CEELS FOR HAZARDOUS SUBSTANCES
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Consequently, EPA and ATSDR requested that the Committee on
Toxicology (COT) of the National Research Council (NRC) develop
guidelines that could be used to develop community emergency exposure
levels (CEELs) for EHSs, in order to assist LEPCs in developing plans
for responding to sudden exposures to toxic substances. COT established
the Subcommittee on Guidelines for Developing Community Emergency
Exposure Levels for Hazardous Substances to respond to this request.
The subcommittee's report is intended to be useful to EPA, ATSDR,
state and local governments, LEPCs, and private industry. It provides
step-by-step guidance for EPA, ATSDR, and state and local governments
in setting CEELs for EHSs: what data are needed, what data are avail-
able, how to evaluate them, and how to present the results. This should
also assist LEPCs in developing emergency response plans to deal with
accidental releases—establishing the quantities of EHSs that can be safely
stored, estimating evacuation zones, and taking remedial action to protect
the health of their citizens.
SETTING EMERGENCY EXPOSURE LIMITS
CEELs are ceiling exposure values for the public (i.e., a ceiling is a
concentration of a substance that should never be exceeded), and they are
applicable to emergency exposures of foreseeable magnitude and duration
(usually 1 to 8 hours, although few exposures will exceed 1 hour). The
subcommittee determined that three CEELs are needed for each EHS,
thus providing a series of exposure values that provides more information
than a single value (see Table 1).
• CEEL-1 refers to the concentration of an airborne substance (such
as a gas, vapor, or aerosol) below which direct toxic effects are unlikely
to lead to discomfort in the exposed population, but above which discom-
fort becomes increasing common—for example, eye and nose irritation
or headaches (the description of CEEL values for a chemical must speci-
fy the symptoms to be expected).
• CEEL-2 refers to the concentration below which direct toxic effects
are unlikely to lead to disability, but above which disability becomes
increasingly common—for example, severe eye or respiratory irritation,
disorientation, and organ damage.
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EXECUTIVE SUMMARY 3
• CEEL-3 refers to the concentration below which death or life--
threatening effects are unlikely, but above which death or life-threatening
effects become increasingly likely—for example, pulmonary edema,
cardiac failure, or cancer.
These levels take into consideration the wide range of human respon-
ses to toxic substances. CEELs must be set low enough to protect most
of the population that might be exposed, including those with increased
susceptibilities such as children, pregnant women, and persons with
specific illnesses such as asthma. However, CEELs must also be high
enough to minimize false alarms and over-response. As a result, they
cannot be expected to protect all individuals in a community, particularly
those with unusually high or idiosyncratic susceptibilities.
Most existing exposure limits apply to specific populations, low con-
centrations, and long durations. Hence, workplace standards apply to
healthy workers exposed for 8 hours per day, 5 days per week, for up
to 50 years of working life; ambient air quality standards apply to the
entire community, including susceptible individuals, exposed for 24
hours per day for their entire lives. Because of differences in duration, •
concentration, and population, CEELs cannot be derived from these
standards by simple extrapolation. Two organizations have issued stan-
dards that might be more useful in developing CEELs: the NRC's short-
term public emergency guidance levels (SPEGLs) and the American
Industrial Hygiene Association's 1-hour emergency response planning
guidelines (ERPGs). However, SPEGLs and ERPGs have been devel-
oped for a limited number of hazardous substances—4 and 43, respec-
tively—while CEELs must be developed for 366 EHSs.
CRITERIA FOR DEVELOPING CEELS
In the absence of existing standards, what criteria and methods are to
be used in developing CEELs for hazardous substances? Such standards
must reflect the relationships among four factors:
• Population potentially exposed. In the case of CEELs, this means
the general population, including susceptible individuals but not neces-
sarily those who are seriously debilitated or hypersusceptible.
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4 CEELS FOR HAZARDOUS SUBSTANCES
• Degree and pattern of exposure! Duration and concentration should
reflect realistic scenarios for accidents involving the release of EHSs,
including inventories, location, dispersion, and weather.
• Nature, severity, and reversibility of the anticipated effects. Acute
health effects include mortality and adverse clinical signs and symptoms
such as pulmonary effects, effects on eyes and skin, and neurobehavioral
effects. Long-term effects mat also need to be considered include repro-
ductive and developmental toxicity, mutagenicity, and carcinogenicity.
• Proportion of those exposed who are subject to toxic effects. At
each threshold concentration (CEEL-1, CEEL-2, CEEL-3), a small per-
centage of the population might exhibit effects. Because this proportion
cannot be predicted precisely, it is necessary to build adequate safety
factors into the CEELs.
DATA NEEDS ANT' DATA SOURCES
Because there are so few detailed, descriptions of general community
exposures, those developing CEELs will need to conduct a thorough
literature search for other lexicological data that can be used to estimate
CEELs. Human data should be emphasized; they are most likely to
come from case studies of exposed individuals or epidemiological studies
of acutely exposed populations, usually from workplace accidents. These
studies are retrospective, however, so they seldom provide reliable meas-
urements of actual exposure. Controlled-exposure studies using human
volunteers can define thresholds for odor and various toxic effects, but
relatively few such studies have been conducted. The resulting lack of
human data presents a serious problem in developing emergency ex-
posure guidelines.
In the absence of human data, risk assessors must depend on data from
animal studies and other laboratory results. These experimental data
have the advantage that the dose is well known and the conditions can be
closely controlled and monitored. However, animals do not always
respond to a toxin exactly as humans would, and some species are known
to be highly susceptible to specific substances and effects. Nevertheless,
well-designed animal studies can provide a great deal of information
from which CEELs can be developed, including
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EXECUTIVE SUMMARY S
• Organs or systems likely to be affected.
• Specific nature of the toxic effects.
• Dose-response relations.
• Possible recovery or reversibility.
• Cumulative effects (e.g., neurotoxicity and cancer).
• Effects of interaction with other pollutants.
Other useful information can come from so-called "short-term" or in
vitro tests. These techniques are particularly useful for evaluating poten-
tial carcinogens; more than 100 short-term tests have been developed to
detect chemicals that might be carcinogenic in animals and humans.
Additional information can come from the molecular structure of an EHS
(which can determine its chemical activity) and from its pharmacokinet-
ics—how it is absorbed, distributed, metabolized, and eventually excreted
by the body.
RISK ASSESSMENT
Estimation of CEELs requires anticipation of a wide range of adverse
effects, from headaches and eye irritation to pulmonary edema and death.
However, the techniques of quantitative risk assessment are rarely used
for effects other than cancer—for example, estimating the exposure level
corresponding to a lifetime cancer risk of one in 1 million. This expo-
sure also corresponds to a CEEL-3, the concentration at which life-
threatening effects become more likely. Clearly, other approaches will
be needed to estimate CEELs for noncarcinogens, materials that have
toxic effects other than causing cancer.
The use of animal data to establish the toxicity of a chemical is called
hazard evaluation. This process involves four steps:
• Identify the adverse effects produced by acute exposure to the
agent.
• Establish dose-response relationships for each of the effects, ideally
using inhalation exposure route at different rates and durations.
• Determine whether this information is relevant for extrapolation to
humans under actual exposure conditions.
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6 CEELS FOR HAZARDOUS SUBSTANCES
• Determine the no-observed-adverse-effect level (NOAEL) and then
divide the NOAEL by appropriate uncertainty factors to arrive at an
acceptable limit for human exposure to the material.
•.!•:, I ' ' • , ' " •''' •
It is vital to select uncertainty factors that reflect the quality and rele-
vance of the data, differences between test species and humans, and
variation within the human population. Typically, in the past the permis-
sible human exposure has been reduced by a factor of 10 for each ad-
ditional source of variation or uncertainty.
: • i i i . . .
DEVELOPMEJ'ilT OF CEELS
The CEEL team that reviews the available data will be required to
evaluate the quality, adequacy, and relevance of the studies from which
the data were derived, as well as the consistency and plausibility of the
entire data base. Human data should be emphasized, and the most useful
data are (1) the quantitative relationship between dose and response and
(2) the identification of sensitive members of the population. Studies that
show toxic effects of short-term exposure in humans or animals might be
more relevant than long-term studies, but both immediate and delayed
effects of exposure need to be considered.
Animal studies should be evaluated on the basis of methodology, as
well as the appropriateness of the species for extrapolation to humans.
(Rats, mice, hamsters, and rabbits have been identified as species ap-
propriate for extrapolation.) In the absence of a most-relevant species,
data from the most sensitive species should be used. The most useful
studies (human or animal) employ a number of dose levels, determine a
level at which there is no effect, and characterize the toxic responses at
several higher levels.
To expedite assimilation by the LEPCs, data derived from literature
review should be summarized hi a consistent format. Documentation
should include the major assumptions, uncertainties, and rationales for
handling of data. The subcommittee has provided checklists for the
presentation of data from human studies and animal experiments, as well
as guidelines for determining uncertainty factors and procedures for
calculating the CEELs themselves (see Figure 1). Proposed CEELs will
be reviewed by a committee of experts, and where there are significant
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EXECUTIVE SUMMARY 7
data gaps the proposed CEELs should be considered preliminary. All
CEELs should be reviewed and updated every 5 to 10 years, or sooner
if additional data become available.
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J. Introduction
The possibility of sudden contamination of air at sites where hazardous
substances are produced, stored, or used has created the need for guid-
ance regarding potential exposure of people to hazardous chemicals under
emergency conditions. The possibility of acute exposure is considered
when granting an operating permit for a new industrial facility or when
evaluating an existing facility's potential for a catastrophic release. One
of the factors involved in emergency planning for averting or managing
toxic gas releases is consideration of "acceptable" exposure levels for a
"once-in-a-lifetime exposure." As described below, several sources
provide acceptable exposure levels, none of which is ideal.
In 1984, after the Bhopal, India, incident in which methyl isocyanate
was released, killing 2,000 people and causing irreversible health effects
(damage to eyes and respiratory system) in up to 20,000 people, the U.S.
Environmental Protection Agency (EPA) identified 366 extremely hazar-
dous substances (EHSs) on the basis of acute lethality data from the
Registry of Toxic Effects of Chemical Substances (RTECSs). Title III
of the Superfund Amendments and Reauthorization Act of 1986 (SARA)
(also known as the Emergency Planning and Community-Right-to-Know
Act) requires that EPA in collaboration with the Federal Emergency
Management Agency (FEMA) provide information to local emergency
planning committees to help them develop and implement comprehensive
emergency-response plans for sites where EHSs are produced, stored, or
used.
In response to SARA, EPA decided that it needed to develop emergen-
cy exposure levels for all 366 EHSs for the use of local planning com-
mittees. The Agency for Toxic Substances and Disease Registry
(ATSDR), which is responsible for conducting health-hazard evaluations
at hazardous waste sites, also is interested in emergency exposure levels
for EHSs for hazardous-site personnel and the general public living
around those sites. Because the National Research Council's Committee
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10 CEELS FOR HAZARDOUS SUBSTANCES
on Toxicology (COT) has recommended emergency exposure levels for
40 years to the Department of Defense (DOD) and the National Aero-
nautics and Space Administration (NASA), EPA and ATSDR requested
that COT review various guidelines used to define exposure limits, iden-
tify their limitations for use in community exposures to EHSs, and re-
commend its own guidelines for developing emergency exposure levels
for EHSs. In response, COT established the Subcommittee on Guide-
lines for Developing Community Emergency Exposure Levels (CEELs)
for Hazardous Substances.
CEELs are ceiling exposure values for the public and are applicable
to emergency exposures of foreseeable magnitude and duration (usually
1-8 hr) However, few exposures; will exceed 1 hr in duration. The
subcommittee agreed to set three CEEL values for each EHS. The series
Of three exposure values-CEEL-1, CEEL-2, and CEEL-3-provide
much more information than a single value because the series indicates
the slope of the dose-response curve. As explained below, CEEL-2 and
CEEL-3 will not protect against health-related effects for all exposed
persons. ,
CEEL-1 refers to the concentration of an airborne substance (sucn as
a gas vapor, or aerosol) for an exposure lasting 1-8 hr below which
direct toxic effects are unlikely to lead to discomfort in the exposed
population (including susceptible but excluding hypersusceptible in-
dividuals) and above which discomfort becomes increasingly common.
The symptoms for each category will depend on the chemical. For ex-
ample, discomfort due to an irritant gas might cause eye and nose ir-
ritation and an organic vapor might cause headaches. The description of
CEEL values for each chemical must specify the symptoms expected for
each CEEL category.
CEEL-2 refers to the concentration of an airborne substance (such as
a gas vapor, or aerosol) for an exposure lasting 1-8 hr below which
escape is not impaired and direct toxic effects are unlikely to lead to
disability in the exposed population (including susceptible but excluding
hypersusceptible individuals) and above which disability becomes increas-
ingly common.
CEEL-3 refers to the concentration of an airborne substance (such as
a gas, vapor, or aerosbl) for an exposure lasting 1-8 hr below which
death or life-threatening effects are unlikely in the exposed population
(including susceptible but excluding hypersusceptible individuals) and
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INTRODUCTION 11
above which death or life-threatening effects become increasingly com-
mon.
In an emergency situation, personnel responsible for protecting the
public health need to know the dose-response characteristics of the hazar-
dous substances that have been released. For example, emergency per-
sonnel need to know if the concentration of a compound that causes eye
irritation is close to the concentration that causes death or if the lethal
concentration is several thousandfold higher than the irritant level.
The characteristics of the three CEEL categories are shown in Table
1.
ACCEPTABLE INHALATION
EXPOSURE LIMITS RECOMMENDED BY
AGENCIES AND ORGANIZATIONS
Many organizations worldwide are engaged in developing and issuing
inhalation exposure guidelines or promulgating inhalation health stan-
dards. Some are private organizations; others are governmental agen-
cies. Although the approaches for determining inhalation exposure limits
have generic similarities, the philosophies, organizational structures, and
chartered mandates of each group active in such endeavors have finite
differences. However, all the inhalation exposure limits have been
deemed adequate within the framework of the various organizations to
protect the health of an exposed individual throughout his or her lifetime
under defined exposure conditions.
The greatest number of inhalation exposure limits and their issuing
organizations relate to occupational exposures—i.e., exposure for 40 hr
per week over a working lifetime. Among the private organizations in
the United States active in recommending permissible occupational ex-
posure guidelines are the Threshold Limit Values (TLV) Committee of
the American Conference of Governmental Industrial Hygienists
(ACGIH), the American Industrial Hygiene Association (AIHA), the
American Society for Testing Materials (ASTM), and the American
National Standards Institute (ANSI). Of these, the TLV Committee of
ACGIH has had the greatest national and international impact. Recom-
mendations by these private organizations are not legally binding but are
jften voluntarily utilized by industry. Industrial corporations and trade
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12 CEELS FOR HAZARDOUS SUBSTANCES
! •
TABLE 1 Characteristics of CEELs 1-3
Category
Characteristics
Death or life-threatening Death or life-threatening effects immediately or
effects soon after exposure
CEEL-3
Disability External assistance is needed because
• persons; are disabled by exposure and cannot
take actions necessary to protect themselves or
escape;
j • exposed persons acquire an illness or condition
with permanent or long-lasting residual ef-
fects, including effects on the outcome of an
existing or subsequent pregnancy.
CEEL-2
Discomfort Exposed persons might request assistance, but
their condition, though unpleasant and possibly
amenable to symptomatic relief, does not
• impair escape;
i • produce disablement;
• result fin permanent or long-lasting effects.
CEEL-1
•'."'! ' ! • ' i "'
Detectability Exposed persons might complain, inquire, or
express aoxiety, but exposure, if perceived at all,
will be perceived only by smell, taste, or sight or
by sensations (mild sensory irritation) that do not
persist after exposure ceases. There are no direct
effects of exposure on health.
.:'.': I i i .
Source: Adapted from ECETOC, 1991.
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INTRODUCTION 13
associations also develop occupational exposure limits to apply to their
particular facilities and customers. These limits are particularly impor-
tant when an industry is the sole manufacturer or user of the substance,
the number of potentially exposed workers is small and limited to a
single company, and other organizations are committed to developing
limits for the vast number of other chemicals used in commerce.
In the United States, the authority to promulgate federal health stan-
dards is delegated by Congress to the executive branch of the govern-
ment (Administrative Conference of the United States, 1983). The pro-
cedure of administrative rule-making encompasses the processes of notice
and comment. The public is given notice of an agency's proposed rule
that would affect the conduct of activities in the workplace or have an
impact on the ambient environment. The public, which includes indus-
try, labor, academia, and government, participates in the rule-making
process by presenting written or oral comments and scientific data on the
various issues addressed in the proposed rule. Subsequently, the agency
publishes a final rule that is the product of the original proposed rule and
the comments and data received from the public. The final rule has legal
status and is binding on the regulated industry or party. Violations may
be punishable by fines or imprisonment or both.
Legal health standards are the responsibility of a number of federal
agencies. The Occupational Safety and Health Administration (OSHA),
of the U.S. Department of Labor (DOL) is responsible for promulgating
and enforcing health standards in the majority of work environments (the
Occupational Safety and Health Act of 1970, U.S. Public Law 91-596).
The Mine Safety and Health Administration (MSHA) of DOL is respon-
sible for promulgating and enforcing metal and nonmetal mining air-
quality standards (the Federal Mine Safety and Health Amendment Act
of 1977, U.S. Public Law 95-164). The National Institute for Occupa-
tional Safety and Health (NIOSH) of the U.S. Department of Health and
Human Services (DHHS) is responsible for developing health-related
criteria and making recommendations to OSHA and MSHA for the prom-
ulgation of legal standards. NIOSH also provides written comments and
oral testimony during the OSHA or MSHA process. A fourth federal
agency, EPA, is charged with the establishment of community exposure
limits for air and water pollutants (the Air Quality Act of 1967 as amend-
ed by the Clean Air Act Amendments of 1970, U.S. Public Law 91-604,
and the Safe Drinking Water Act of 1974, U.S. Public Law 93-523). In
sases in which the substance to be regulated is a known or suspected
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14 CEELS FOR HAZARDOUS SUBSTANCES
human carcinogen (animal carcinogen), other federal agencies, such as
the National Cancer Institute and the National Toxicology Program of
DHHS, may be active in the federal rule-making process.
On the international level, exposure limits have been established by a
number of countries or multinational groups. Occupational exposure
limits have been developed by Germany, Russia, The Netherlands the
Nordic countries, the International Labor Organization and the World
Health Organization. Other countries, such as the United Kingdom and
Australia, which previously used ACGIH's TLV list as a mandatory
standard, have recently introduced guidelines of their own for establish-
ing occupational exposure standards. The newly founded European
Economic Community is also active in establishing initiatives regarding
toxic chemicals and protecting potentially exposed individuals The
International Agency for Research on Cancer (IARQ of the World
Health Organization through its monographs on carcinogenic risks to nu-
mans provides valuable assistance to nations and international groups
active in establishing permissible exposure guidelines or standards.
LIMITATIONS OF THE RECOMMENDED EXPOSURE
LIMITS FOR USE AS CEELS
There are several key issues that influence how previously established
exposure limits are used for the development of CEELs. The issues are
(1) the duration of exposure and the concentration of the toxic agents,
determinants of the total dose, or the threshold of response in the ex-
posed individual; (2) the margin of safety or uncertainty applied to the
levels that induce the principal health risks; and (3) whether the limit is
intended to protect all exposed persons, including the young, aged,
chronically ill, and those with genetic or acquired hypersusceptibihties
to chemical insult.
With the exception of carcinogens, inhalation exposure limits are
based on the premise that the amount of damage to a person's health is
dependent on the total dose of the toxic agent and that, with a continuous
decrease of dose, a quantity will be reached at which damage, or devia-
tion from the norm, will not occur or will escape detection by the most
sophisticated procedures. It is imperative to recognize that this no-ob-
served-adverse-effect level (NOAEL) will vary among individuals be-
cause of the variability in health status and genetic composition of hu-
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INTRODUCTION 15
mans. The variation in human susceptibility to a toxic substance, cou-
pled with the imprecision of laboratory, clinical, and epidemiological
studies and the difficulties of extrapolating animal data to humans, neces-
sitate the application of "safety" or "uncertainty" factors to the NOAEL
hi establishing all exposure guidelines and standards.
Occupational and community air exposure limits differ regarding the
fundamental bases inherent to their establishment; likewise CEELs will
differ from other exposure limits for the same reasons. Occupational
exposure limits are established to protect against adverse health effects
in nearly all workers who are repeatedly exposed over a 40-hr workweek
for a working lifetime. The population at risk in occupational settings
is composed of adults whose health is usually better than that of the
general population.
Evidence for the healthier state of worker populations has been found
repeatedly in epidemiological studies in which morbidity and mortality
rates are lower in working populations than in the general population
except when diseases or deaths are caused by specific toxic agents pres-
ent at the worksite. This phenomenon of apparent selection of persons
at lower risk at the time of employment for many causes of disease or
death has been termed "the healthy worker effect" by epidemiologists and
is a form of selection bias that inadvertently occurs when the mortality
or morbidity experience of workers is compared with that of the general
population. Conversely, community air exposure limits must be estab-
lished for all elements of the general population over a chronological
lifetime and must reflect exposures that can be tolerated by the general
populace 24 hr per day for a lifetime. In contrast, CEELs are designed
to protect community populations, including potentially sensitive popula-
tions, that might be exposed to a chemical on a rare occasion, most
likely not more than once in a lifetime. Thus, even though differing
safety or uncertainty factors must be used to derive CEELs that protect
the young, the old, the ill, and the idiosyncratic, they might not protect
all hypersensitive persons all the time.
Because occupational exposure limits and community air exposure
limits have differing exposure durations and populations at risk for ex-
posure to hazardous substances, CEELs cannot be derived by simple
jxtrapolation from other existing exposure limits. Discussion of CEELs
rbr each chemical substance should represent committee judgments,
Allowing evaluation of known mechanisms of injury, experimental ani-
nal and human clinical and epidemiological data, and conditions in
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16 CEELS FOR HAZARDOUS SUBSTANCES
which emergency exposures might occur. Effects must be reversible and
not incapacitate an exposed individual either mentally or physically; he
or she must be able to recover or return to normal without significant
medical treatment.
Several types of exposure guidelines have been developed for use in
emergency situations involving a single high-level exposure to substances
that might produce adverse health effects.
COT has recommended emergency exposure guidance levels (EEGLs)
for exposures of 1-24 hr for about 70 substances. An EEGL is defined
as a concentration of an airborne substance (such as a gas vapor or
aerosol) that may be judged by DOD to be acceptable for the perfor-
mance of specific tasks during rare emergency conditions lasting 1-24 hr.
Such an exposure could result from a fire, a spill, a line break, or any
event mat is unanticipated but nevertheless has a rate of occurrence that
is predictable. Exposure at an EEGL might produce reversible effects
that do not impair judgment and do not interfere with proper responses
to the emergency. It must be noted, however, that EEGLs are not hy-
gienic or safe. The EEGL is a suggested peak level of exposure and is
not to be regarded as a standard in any form or use. EEGLs were devel-
oped for military use and are intended for healthy young military person-
nel Therefore, EEGLs are not applicable to general populations consis-
ting of the elderly, the very young, pregnant women, and ill persons
COT also recommended short-term public emergency guidance levels
(SPEGLs) for civilians (comprising healthy adults, infants, children,
elderly, and ill persons) residing in and near military installations A
SPEGL is defined as a concentration of an airborne substance (such as
a gas, vapor, or aerosol) for unpredicted single short-term emergency
exposure of the general public. In contrast to the EEGL, the SPEGL
takes into account the wide range of susceptibility of the general public
SPEGLs include sensitive subgroups, such as children, the elderly, and
Ul persons. Effects of exposure on the fetus and on the reproductive
capacity of both men and women also would be considered. However,
SPEGLs were recommended for only four substances by NRC. Further-
more EEGLs and SPEGLs do not exist for many commercially impor-
tant chemicals. Thus, most of the NRC values are not directly applicable
to the general public. .
In the 1970s, NIOSH developed a series of exposure values-immedi-
ately dangerous to life and health (IDLHs). The IDLH values were
developed for respirator-selection purposes, not for setting permissible
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INTRODUCTION 17
exposure limits. The EDLH value represents a maximum concentration
from which, in the event of respirator failure, a person could escape
within 30 min without experiencing any escape-impairing effects or
irreversible health effects.
EDLH values are based primarily on secondary-literature review. The
basis for each EDLH value has not been published formally, nor were the
values subject to peer review. No formal mechanism or procedure exists
to update the values.
Other potentially useful inhalation exposure limits are short-term
exposure limits (TLV-STEL) and ceiling limits recommended by ACGIH
for workplace exposures. Although developed for daily occupational
exposures, the values have been designed for healthy workers and are not
relevant for the general public.
Other organizations, such as the World Bank and the European Eco-
nomic Community, provide guidance in terms of acutely hazardous quan-
tities rather than concentrations. Their recommendations are based on
LC50 values. LC50 values are less appropriate for emergency planning
since lethality is the only health effect they consider. Human-experience
data, escape-impairing effects, and serious but nonlethal effects, such as
lung or liver injury, are not considered.
Emergency response planning guidelines (ERPGs) have been devel-
oped under the auspices of AIHA. The AIHA committee decided to set
three planning guidelines: ERPG-3, ERPG-2, and ERPG-1.
• ERPG-3 is the maximum airborne concentration below which it is
believed that nearly all individuals could be exposed for up to 1 hr with-
out experiencing or developing life-threatening health effects.
The ERPG-3 level is a worst-case planning level above which there is
the possibility that some members of the community will develop life-
threatening health effects.
• ERPG-2 is the maximum airborne concentration below which it is
believed that nearly all individuals could be exposed for up to 1 hr with-
out experiencing or developing irreversible or other serious health effects
or symptoms that would impair an individual's ability to take protective
action.
Above ERPG-2, for some members of the community, there might be
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18 CEELS FOR HAZARDOUS SUBSTANCES
significant adverse health effects or symptoms that could impair an in-
dividual's ability to take protective action. Those symptoms include
severe eye or respiratory irritation oir central nervous system effects.
• ERPG-1 is the maximum airborne concentration below which it is
believed that nearly all individuals could be exposed for up to 1 hr with-
out experiencing any symptoms other than mild transient adverse health
effects or without perceiving a clearly defined objectionable odor.
The ERPG-1 identifies a level thatt does not pose a health risk to the
community but might be noticeable because of slight odor or mild ir-
ritation. In the event of a small nonthreatening release, the community
may be notified that odor or slight irritation might be noticeable but that
concentrations are below those that could cause adverse health effects.
Although ERPGs have been widely used by states, local communities,
departments of transportation and energy, and industry, there are ERPGs
for only 43 substances. Information from these ERPG reports might be
useful for developing CEELs on those 43 substances, but CEELs are
needed for about 350 chemicals.
This report describes the criteria and methods for developing CEELs
for hazardous substances, which will be useful to EPA, ATSDR, in-
dustry, and state and local governments in responding to chemical emer-
gencies.
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2 Criteria for Establishing CEELs
Any numerical index of toxic effect or any standard that is set to protect
against toxic effects must reflect the relationships among the following
factors:
1. The population potentially exposed.
2. The degree and pattern of exposure.
3. The nature, severity, and reversibility of the anticipated toxic
effects.
4. The proportion of those exposed who are subject to toxic effects.
Thus, for example, LC50 values refer to a specific animal species (1),
an exposure concentration for a specified time (2), a specific toxic effect
(death) (3), and a specific proportion of animals affected (50%) (4).
TLV-STEL values refer to workers (1), represent the exposure for 15
min by inhalation (2), refer to a level not expected to have significant
adverse health effects (3), and refer to nearly all workers (4).
Those four factors are necessary to establish a definition for CEELs.
POPULATIONS OF CONCERN
The population concerned in any industrial accident involving the
release of hazardous substances is the work force and others within the
premises and the general public who live or work in the vicinity. The
general public might include people who are more susceptible to chemi-
cal exposure than the average person (e.g., the elderly, the young, preg-
nant women, and those with acute or chronic illnesses). CEELs should
take into account such susceptible individuals, but not necessarily more
seriously debilitated persons or those with enhanced susceptibilities (e.g.,
those with pneumonia or myocardial infarction) current or historical.
19
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20 CEELS FOR HAZARDOUS SUBSTANCES
EXPOSURE PERIODS AND PATTERNS
I Js
After an accidental release of a quantity of a chemical, the concentra-
tion at any point where the chemical disperses will be influenced by
meteorological conditions and topography. As the gas cloud disperses,
the concentration at a particular point generally will rise to a maximum,
which will be sustained for a period and men fall toward zero. It was
considered that the most practical descriptors of exposure would be the
exposure duration and the concentration that was not exceeded during the
exposure. CEELs would be most useful if the exposure period or peri-
ods to which they refer were of a duration pertinent to realistic scenarios
for accidents involving the release of hazardous substances. The descrip-
tion of the scenarios should incorporate the evolution of release over time
as determined by product inventory, isolation, and other mitigating de-
vices and dispersion factors as determined by meteorological and topo-
graphical conditions. In some cases, the exposure period will be fairly
clear and similar for a wide range of concentrations—for example, when
release is uniform and ceases abruptly and wind velocity is high. In
other cases, exposure duration will vary considerably according to the
concentration of interest; in those cases, an iterative process must be
used to establish the appropriate duration for each CEEL beginning with
an arbitrary period.
CEELs should be established for each toxic effect for a number ot
exposure periods, up to 1-8 hr, when the data allow it, so that values
most appropriate for predicted exposure periods can be chosen. How-
ever, few exposures will exceed 1 hr in duration.
NATURE AND SEVERITY OF TOXIC EFFECTS
Toxic effects are of many types! and each can occur with varying
degrees of severity. The important effects following accidental industrial
releases of toxic materials are those that occur in humans following an
acute exposure to an airborne subsitance. Systemic effects will result
chiefly from absorption of materials through the lungs; local effects will
involve the eyes, respiratory tract, and skin.
The possibilities are limitless for describing the grades of severity of
the immediate toxic effects of acute exposures. It is essential in setting
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CRITERIA FOR ESTABLISHING CEELS 21
CEELs that each indicates a type of toxic effect that clearly requires a
specific emergency response. The subcommittee recognized the need for
three indexes (i.e., CEELs 1-3) to represent the transitions between four
graded groupings (see Table 1).
Death or life-threatening effects are the most severe effects for which
an exposure index can be provided; they are easily defined and are used.
by society to judge the severity of accidents. Two other grades of ef-
fect—disability and discomfort—though less well defined, place distinct
demands on emergency and health-care services. The term disability is
used here to indicate the situation where persons will require assistance
or where the effects of exposure will be more severe or prolonged with-
out assistance. Persons suffering discomfort, though distressed and
possibly requesting assistance, will not be dependent on assistance for
minimizing the severity or duration of the effect of exposure. Exposure
insufficient to cause discomfort or adverse health effects might be per-
ceived nevertheless by means of smell, taste, or sensations (mild sensory
irritation) that are not uncomfortable. The awareness of exposure might
lead to anxiety and complaints and constitutes what is termed here detec-
tability. Except for death, these graded categories of effect are not
sharply demarcated, but each merges into adjacent categories.
PROPORTION OF THE EXPOSED POPULATION
SHOWING EACH CATEGORY OF TOXIC EFFECT
CEELs should indicate exposures that would be thresholds for the
occurrence of (1) death or life-threatening effects, (2) disability, or (3)
discomfort in the population. At such a threshold concentration, a small
proportion of the population might exhibit effects.
Precision in defining "a small proportion" is impossible and unneces-
sary for the following reasons:
• The categories of effect are not precisely demarcated.
• The data available for setting CEELs are imprecise and derived
from various sources, including experiments and clinical observations
usually not designed for setting CEELs.
• The incidence of effects at CEELs will depend on the proportion of
susceptible people in the population, which is variable and uncertain.
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22 CEELS FOR HAZARDOUS SUBSTANCES
• The response will depend on the actual pattern of exposure rather
than the predicted exposure category! '..'''.
• Models used to predict the pattern of exposure provide information
that is inherently imprecise.
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Sources of Data for
Developing CEELs
Ideally, the data from which CEELs are developed would be well-docu-
mented descriptions of the clinical effects seen in a representative sample
of the general population exposed to accurately measured concentrations
of the substance under consideration for relevant exposure periods. In
practice, such data never exist. Reliance has to be placed on toxicologi-
cal data from animal studies and human data, usually from workplace
accidents. Thus, it is essential that a thorough literature search be con-
ducted for data relating to accidental and experimental human exposures
and to lexicological studies in animals. Although the most immediately
or directly applicable data would come from reports of acute inhalation
studies in animals and humans, the search should not be confined to
those studies but should be widened to include all routes of exposure and
all exposure periods. When the reports of inhalation studies are scant or
inadequate, the subsidiary information might be helpful and, in all cases,
would provide confirmatory information on the toxicity of the substance.
In some circumstances, for example, when the data available on the
chemical itself are inadequate, data relating to a close chemical analogue
might contribute significantly to the information to be considered.
All data sources should be consulted, including the published scientific
literature and any unpublished information from industry and public
bodies. Every attempt should be made to obtain the full text of original
articles to extract the details of the experimental conditions and results.
Secondary sources should be used only when original articles cannot be
obtained. Unpublished reports and individual expertise may be used to
supplement published reports provided they are scientifically credible.
Various types of evidence should be assessed in establishing CEELs.
Information should be collected from (1) studies and case reports of
human exposures, (2) animal toxicity studies, (3) in vitro toxicity studies,
and (4) chemical and physical characterizations of the potential toxicant.
23
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24 CEELS FOR HAZARDOUS SUBSTANCES
HUMAN DATA
Human data relating to the acute effects of hazardous chemicals have
been collected in relation to unintentional and intentional exposures to
toxic agents.
Unintentional man-made disasters that are life-threatening often are
sudden and devastating (e.g., Bhopal and Chernobyl) but also might be
chronic and insidious (e.g., Minamata). In both circumstances, however,
the health consequences are most often a combination of acute and long-
term effects.
Emergency exposures to chemicals usually arise from various sources:
• Accidental releases at industrial sites.
• Spills during transportation.
• Explosions, fires, or other mishaps.
• Escape from toxic waste sites.
The timing and location of toxic releases are difficult to predict and
have often been unanticipated. Thus, the at-risk populations and the
possible health effects among them have seldom been determined before
a toxic release. As a result, health studies among populations exposed
to toxic releases almost always have been retrospective in nature.
Two main types of health data have been collected following acute
exposures to toxic agents: (1) case studies of individuals affected to
various degrees by acute exposures; and (2) epidemiological studies of
acutely exposed populations.
Epidemiological and clinical data on toxic health effects also come
from workers exposed to toxic agents during the course of their work.
Those data include some of the most detailed studies of the relationship
of human exposure and response to toxic agents. Health data on com-
mon industrial chemicals, such as sulfur dioxide, ammonia, chlorine,
hydrogen sulfide, and phosgene, agents that have killed workers, are
readily available. Exposures to workers, particularly following industrial
spills or explosions, generally are higher than those encountered by the
public during acute emergency releases. Workers usually are at greater
risk of life-threatening exposures because they are closer to the source
of toxic release. When large quantities of highly toxic agents are stored
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SOURCES OF DATA FOR DEVELOPING CEELS 25
close to populations (as occurred in Bhopal), however, there is the poten-
tial for severe, life-threatening toxic exposures to large numbers of the
general public. Under such circumstances, an environmental release
leads to health effects among the population similar to those seen in
industrial workers.
Another important source of human data comes from volunteers who
•are exposed intentionally to toxic agents under controlled clinical con-
ditions, again at levels less than those usually found during acute emer-
gencies.
Case Studies
Important clinical information has been obtained from survivors of
acute massive exposures to toxic agents. By their nature, case studies
describe individuals who develop clinical problems caused by such ex-
posures. When affected subjects are followed over time, information is
obtained about the natural history of health effects caused by particular
agents, as well as the clinical patterns of recovery or progression to
chronic disease.
The initial description of reactive airways dysfunction syndrome
(RADS) exemplifies how a series of individual case histories can demon-
strate an important effect following a single acute toxic exposure (Brooks
et al., 1985). Some individuals, even though they have neither asthma
nor atopy beforehand, might inhale a high concentration of a respiratory
irritant and quickly develop highly reactive airways with a clinical pic-
ture indistinguishable from asthma (Brooks et al., 1985; Boulet, 1988).
Asthmatic symptoms typically appear within 24 hr of the acute exposure
and persist for many months. Further case reports have confirmed that
inhalation of high concentrations of many different irritant gases, fumes,
vapors, and aerosols might lead to RADS.
Recent findings among 56 workers exposed to an acute release of
acetic acid showed that the prevalence of RADS was 21.4% among those
most highly exposed and that the risk of RADS in the highest exposure
group was 10 times greater than in less exposed individuals (Kern,
1991). Increasing recognition of RADS and the extent to which it might
occur following a single acute exposure to a respiratory irritant make it
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26 CEELS FOR HAZARDOUS SUBSTANCES
an important health effect to be considered by the risk assessor. Criteria
for the diagnosis of RADS have been discussed (Brooks et al., 1985;
Kern, 1991).
'' I .' ' '
Epidemiological Studies
Virtually all attempts to study the effects of acute releases of toxic
agents on exposed populations have been retrospective. Longitudinal
data on exposed groups (in contrast to case studies of affected individuals
over time) are relatively uncommon and often subject to methodological
problems. , . ,
A major problem is defining the exposed or at-nsk populations and a
similar unexposed population that can serve as a valid comparison group.
In the immediate response to an acute emergency, records are seldom
obtained of everyone who might have been exposed. To reconstruct that
information after the fact is difficult, and incomplete definition of the
exposed group is a source of potential bias.
Reliable exposure measurements are seldom obtained at the time ot a
toxic release, so most exposures a*e estimated retrospectively. For ex-
ample, when a toxic agent is released into the atmosphere from a source,
exposure can be estimated by mathematical modeling based on the
amount and rate of the release, the prevailing atmospheric conditions
and knowledge of where the subject was located relative to the point of
release. That approach contains several assumptions (uniformity ot
release-exposure and exposure-dose relationships based solely on large-
scale physical factors) that cannot account for differences in the immedi-
ate environment of individuals, differences that might affect exposures
greatly or might affect the substantial individual variation in exposure-
dose relationships. .
Misclassification of an individual's exposure is inevitable whenever
exposure is estimated. Such misclassification might occur randomly or
nonrahdomly and can influence the data adversely. Random misclas-
sification of exposure causes bias toward the null hypothesis: noisy data
lead to a decrease in statistical power for any exposure-related effect and
therefore the possible dismissal of an exposure effect because of a lack
of statistical significance, when a real effect actually might be present.
If exposure misclassification is nonrandom, it becomes a source of bias
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SOURCES OF DATA FOR DEVELOPING CEELS 27
that can cause a spurious association or lack of association between the
exposure and health effects.
A different approach to exposure assessment has been to measure the
concentration of toxic agents and their metabolites in body fluids and
tissues. That method of determining exposure is best suited for agents
that are absorbed systemically and are excreted slowly. The association
between such exposure measurements and related health effects will be
closest and most meaningful when the fluid or tissue sample is taken
from the target organ or a related tissue. The most frequently sampled
fluids and tissue are blood, urine, and fat cells. Exhaled air also can
provide useful information. Blood and urine are used widely for such
biological monitoring, and the relevance of exposure measurements
derived from each fluid depends greatly on the pharmacokinetics of the
agent and the timing of the sample in relation to exposure. Samples of
fat tissue have been used particularly to determine exposure to organo-
chlorine compounds. Many organochlorines accumulate and persist for
years in fatty tissue. The persistence of those compounds has been use-
ful, for example, in determining past exposures to phenoxyacetic acid
derivatives and polychlorinated biphenyls. Recent exposures to volatile
organic compounds can be detected by the presence of the compound in
exhaled breath. Workers are exposed frequently to toxic agents in the
workplace. Epidemiological studies of working populations have provid-
ed evidence of the nature of adverse health effects caused by many toxic
agents and their exposure-response relationships. It is important to note,
however, that workers usually are exposed to low to moderate levels for
8 hr per day and that standards for workplace exposures are often aver-
aged over the work period.
Extrapolation of exposure limits developed for workers might not be
relevant to the large fraction of nonworking people in the general popula-
tion. Currently, workers usually are healthier than nonworkers. By
their nature, working groups exclude the very young and the elderly,
who are regarded as more susceptible to many of the adverse effects of
toxic hazards. Workers are thus a biased sample of the general popula-
tion in terms of their health status and their susceptibility to hazardous
agents.
In addition, workers who experience adverse health effects from toxic
agents in the workplace often migrate to jobs where they are not exposed
to such hazardous agents; that self-selection process is a form of the
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28 CEELS FOR HAZARDOUS SUBSTANCES
healthy-worker effect and has been noted in many working populations.
The healthy-worker effect causes bias in epidemiological studies and is
seen most commonly in studies of cross-sectional design; it is less likely
in longitudinal cohort studies in which self-selection effects can be moni-
tored by the investigator.
Controlled Exposures of
Human Subjects
' I „ .' !• , • ,; II , "
Important and unique data have been obtained from studies of volun-
teers subjected to controlled exposures to toxic agents at low concentra-
tions, that approach complements case studies and epidemiological data
by defining exposure-response relationships among healthy subjects and
susceptible subgroups of the general population (such as asthmatics).
Such data also are useful in validating animal studies, conducted at high-
er concentrations, and extrapolating to human effects.
Low-level controlled exposures are designed to investigate single
agents or simple mixtures. They have been used to define odor thresh-
olds exposures at which respiratory, skin, and eye irritation occur, and
neurobehavioral effects. Although such studies are not usually designed
to assess nonspecific systemic effects, exposure of volunteers at low
levels also might help identify exposures that are associated with mild
subjective symptoms such as nausea, headache, dizziness, or fever.
Controlled exposures to airborne agents have been either brief individ-
ual exposures via a nasal mask or mouthpiece or exposures to one or
more people in an environmental chamber. The latter procedure has
greater flexibility in terms of the exposure route, the number of subjects
exposed, their activities, the duration of exposure, and the ease of con-
comitant measurements of physiological function and other indexes of
health effects.
Much of the data available from controlled clinical exposures to air-
borne hazards relate to the criteria for air pollutants defined by EPA
(such as ozone, nitrogen dioxide, sulfur dioxide, and respirable parti-
cles); less information is available on common industrial pollutants (such
as ammonia and chlorine).
For many agents, the lack of human data on acute health effects at low
exposure levels presents a serious problem for the risk assessor who is
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SOURCES OF DATA FOR DEVELOPING CEELS 29
required to evaluate the human toxicity of a given chemical and to devel-
op emergency exposure guidelines. In the absence of relevant human
data, the risk assessor must resort to less direct information, including
acute toxicity studies in animals.
When the Bhopal disaster occurred in 1984, limited human data exist-
ed on the health effects of methyl isocyanate, and very limited data exist-
ed that applied directly to that chemical's exposure-response relationship
in humans. Thus, at the time of the release there was little quantitative
information on the toxicity of methyl isocyanate in people.
ANIMAL TOXICITY STUDIES
The data necessary to evaluate the relationship between exposure to a
pollutant and its effects on a population are frequently not available from
human experience. For many air pollutants, studies in animals have
provided the only useful data. Ideally, the data should be derived from
at least two species and by the inhalation route.
Inhalation experiments with animals provide a basis for estimating
possible effects in humans and the concentrations at which the effects
occur. They are useful in the identification of adaptations that might
occur following repeated exposure. They permit the testing of hypothe-
ses about the mechanisms of the toxic actions of pollutants. They offer
a good opportunity to explore interactions between pollutants and other
factors that might affect toxicity.
Data from skin absorption, ingestion, and parenteral studies also are
potentially useful. The usefulness of animal data depends in part on the
species used. Relevance to humans might be limited in the absence of
information on target organs and pharmacokinetics in animals and hu-
mans.
Well-conducted animal studies will report the following:
• The most sensitive target organs or body systems affected by ex-
posure to the contaminant in question.
• The nature of the effect on the target organs.
• Data to establish dose-response relationships for the target organs—
from no observed effect to severe effects. (The distinction between
exposure and dose needs to be made.)
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30 CEELS FOR HAZARDOUS SUBSTANCES
• The rate of recovery from reversible effects, if any.
• The nature and severity of injury for effects that are not reversible.
• Cumulative effects, if any (e.g., neurotbxicity and cancer)^
• Pharmacokinetic data for comparison with data obtained from hu-
U13HS. • i_ u •
• The effects of interaction, if my, of the toxicant with other air
pollutants and the minimum concentrations at which the interaction ap-
pears to occur.
• Techniques used to assure quality and avoid bias.
SHORT-TERM IN VITRO TESTS
The major effort to date in the development of short-term tests has
been in genetic toxicity testing for predicting carcinogenicity after obser-
vation of genotoxicity. More than 100 short-term in vitro tests have been
developed or proposed to detect rodent carcinogens and, by implication,
chemicals that might be carcinogenic in humans. Test combinations (or
batteries) and sequential testing schemes have been proposed to improve
the predictability. In general, the batteries and combinations have not
proved to be better predictors than the best of the single tests. Too
often batteries of tests have increased sensitivity (finding more positives)
at the cost of decreased specificity (labeling materials positive that were
really negative). . .
Short-term tests have been developed for other lexicological end
points such as reproductive and developmental toxicity; however, the
level o'f investigation for the majority of those end points has not been as
extensive as that for carcinogenicity. Therefore, short-term tests for
carcinogenicity can be used as the paradigm for short-term tests in gener-
al and the mistakes made and lessons learned in this field can be used
to' guide development of comparable tests for other pathological end
P°Snort-tenn tests often are defined with respect to the long-term effect
that they are to identify or measure. There are three reasons to perform
short-term tests:
1 For screening purposes to replace costly long-term testing or
provide information that could be used to help design subsequent tests.
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SOURCES OF DATA FOR DEVELOPING CEELS 31
2. As an adjunct to other tests to aid in making regulatory or in-
dustrial decisions or to aid in elucidating the mechanisms of toxicity.
3. To replace existing tests with easier tests.
Short-term tests need to be validated. As in any predictive test, short-
term tests will give false positives and false negatives. The tests are
useful in elucidating the mechanism of action.
In Vitro Reproductive and
Developmental Effects
A number of in vitro approaches have been used for evaluating repro-
ductive and developmental toxic effects. Whole-embryo cultures, in vitro
fertilization procedures, organ-tissue cultures, and cell-culture systems
have been developed; some of those systems have been tested for
replicability and reliability with both positive and negative test agents.
In vitro test systems can provide significant information on target-site
interaction and mechanisms of action, but none of the test systems alone
is considered adequate for risk-assessment purposes or useful in setting
CEELs, because most in vitro systems do not account adequately for
effects that might occur in the whole animal. For example, the maternal
influence on dose to the developing organism via piacental transfer dur-
ing pregnancy or via milk transfer during the preweaning period cannot
be modeled in in vitro systems. In the case of in vitro fertilization as-
says, several steps in the interaction between the sperm and the female
reproductive tract are bypassed. Thus, those test systems may be used
in conjunction with whole-animal tests of developmental toxicity, fer-
tility, or reproductive organ function, but they may not be used as the
only indicator of an adverse effect.
In Vitro Genotoxicity Assays
Many of the in vitro carcinogenicity tests are in fact tests for detecting
agents causing damage to DNA. The existence of an extremely high
positive correlation for inducing the two end points (tumors and muta-
tions) for a compound indicates a relationship that is more than coin-
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32 CEELS FOR HAZARDOUS SUBSTANCES
cidental, and much of current cancer research is devoted to elucidating
the mechanisms related to the steps of tumor initiation. One feature of
chemicals that appears to be significant for inducing both mutations and
tumors is the ability to form reactive electrophilic molecular species that
will interact with nucleic acids. Most direct-acting carcinogens are high-
ly reactive electrophilic agents per se and most procarcinogens typically
form electrophilic metabolites.
A significant body of other indirect evidence also associates one or
more critical steps in cancer induction with change in cellular genetics.
For example, humans with hereditary defects in their ability to repair
DNA damage have a statistically higher susceptibility to cancer induc-
tion The active forms of most initiating chemical carcinogens are also
the active mutagens. Most tumors appear to be of single-cell origin, and
that origin would be a requirement if mutation were an etiological com-
ponent in the process of initiation.
In vitro mutagenicity tests include those using microorganisms, such
as the Ames Salmonella strains, as indicators of genetic activity and those
using mammalian tissue extracts as sources of metabolic activation to
identify enzyme-formed mutagens. Yeasts can detect the kinds of genetic
damage that are identifiable only in diploid eukaryotic cells. Other in
vitro test systems include those using mammalian cells (e.g., Chinese
hamster ovary cells) for detecting point mutations and chromosomal
aberrations and damage.
Mutagenicity assays are designed to identify compounds that are con-
verted to reactive electrophilic compounds (or are converted to these
compounds) hypothesized to be involved in tumor initiation. According
to this postulate, only carcinogens that have a reactive electrophilic state
are expected to be detected by mutagenicity assays. Chemicals that
promote initiated cells or induce tumors by indirect means, such as hor-
mones, physical carcinogens, heavy metals, and certain other chemicals
are unlikely to induce genetic alterations. One important function or
short-term in vitro genotoxicity tests is to distinguish between direct-
acting carcinogens (chemicals that produce neoplastic transformation) and
indirect-acting carcinogens (chemicals that promote tumor development).
It might be possible to develop acceptable exposure levels for indirect
carcinogens. The results of carcinogenicity tests appear to support that
model because some apparent indirect carcinogens (e.g., saccharin, some
chlorinated hydrocarbons, and nitrilotriacetic acid) require high-dose
levels to produce effects, whereas most direct carcinogens are active at
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SOURCES OF DATA FOR DEVELOPING CEELS 33
low doses. Thus, negligible risk can be expected from very low environ-
mental levels of many indirect carcinogens but not necessarily from
direct carcinogens.
Structure-Activity Relationships
Structure-activity relationships (SARs) are commonly used in phar-
macology and toxicology to predict chemical structures that have the
ability to cause certain responses. Because carcinogenesis is a multistage
process, SARs tend to be more complicated than SARs associated with
simpler processes—for example, the inhibition of specific enzymes, such
as acetylcholinesterase, by certain pesticides. Certain chemical struc-
tures, however, are often but not always associated with carcinogenic
activity. Those chemicals include such compound classes as polycyclic
aromatic hydrocarbons, aromatic amines, Af-nitroso compounds, and
biological alkylating and arylating agents.
SARs are of great value in the pharmaceutical industry in deriving new
products effective against very specific targets. A carcinogen might
induce cancer by different mechanisms in different tissues of the same
species. SARs should be used with care and are most useful if the can-
didate chemical contains one or more of the key groups associated with
the induction of cancer. In such cases, the known carcinogenicity or
noncarcinogenicity of structurally related compounds might be sufficient
to indicate whether the chemical is likely or unlikely to be a carcinogen.
For further information on this topic, the reader is referred to the work
of Klopman (1985) and Klopman and Rosenkranz (1991).
Besides predicting carcinogenicity, SAR is useful in predicting other
toxic effects, such as reproductive and developmental toxicity. When
there are no data for the compound under consideration, its CEELs can
be estimated from toxicity of surrogate chemicals that are closely related
to it in its chemical class.
CHEMICAL AND PHYSICAL
CHARACTERISTICS OF TOXICANTS
The chemical and physical characteristics of a chemical provide valu-
able information on both the dosimetry of the compound within the body
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34 CEELS FOR HAZARDOUS SUBSTANCES
1 • ! '
and on the likely toxic effects. A highly lipophilic substance can be
expected to accumulate in fatty depots in the body and might be retained
and available for metabolism to toxic metabolites for some time after an
exposure. Water-soluble substances can be expected to clear the body
more rapidly except for metal ions that bind to specific proteins. Much
is known about the substrate specificity of xenobiotic metabolizing en-
zymes and metabolic pathways and potential metabolites for many chem-
icals can be predicted by the structure of the chemical. For aerosols, the
physical size, shape, and density of the aerosols can be used to determine
the probable sites of deposition in the respiratory tract and the degree to
which the aerosols are respirable. The water solubility of inhaled gases
is known to strongly influence where the gases are deposited and retained
in the respiratory tract. Thus, the chemical and physical characteristics
of a substance can be used to predict the potential toxicity of that sub-
stance and to set CEELs, especially for substances for which few, if any,
toxicity studies have been conducted.
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Types of Data Used in
4 Developing CEELs
The preceding chapter summarized the sources of data for use in devel-
oping CEELs. In this section, the two types of data obtained from those
sources and their importance in developing CEELs are described. The
types are (1) dosimetry and (2) toxicity end points.
PHARMACOKINETICS AND METABOLISM
The evaluation of the health effects of any chemical in a given envi-
ronment is greatly improved if there is an understanding of its physiolog-
ical disposition in the body (pharmacokinetics and metabolism). Metabo-
lism of a chemical might lead to detoxification or might result in meta-
bolic activation leading to toxic effects. Because metabolic events usual-
ly are enzyme-mediated, they usually are driven by the concentration of
substrate available for the reaction. The concentration of substrate in the
target site is a result of the level of exposure and the pharmacokinetics
of the chemical. Modern pharmacokinetic studies are aimed increasingly
at developing dispositional models, which compare input of chemical
with outflow for the whole system as well as for individual organs.
The toxic metabolites of each chemical and factors that control the rate
at which they form are important for developing CEELs. It is important,
however, to distinguish between the production of toxic metabolites in
organs where toxicity is observed and metabolism in the liver where the
bulk of most chemicals are metabolized. The relationship between me-
tabolism in liver and toxic effects in either liver or other organs is an
important factor because metabolism of chemicals in a liver already
damaged by other agents or conditions (diet, microbial disease, toxic
chemicals, etc.) is likely to differ from metabolism in a normal liver.
Furthermore, if toxic metabolites are exported from the liver to other
organs, damaged livers may alter the production of toxic responses.
55
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36 CEELS FOR HAZARDOUS SUBSTANCES
Toxic metabolites have been observed to be highly reactive chemically
and have been characterized as "biologically reactive intermediates."
The reactive metabolites can covalently bind to nucleic acids, proteins,
or lipids, which in turn can alter DNA replication or the process of
transcription. Attempts to repair damage to DNA might involve
misrepair, leading to erroneous DNA replication or function, inhibition
of protein synthesis if RNA is the target, or inhibition of an enzyme or
other activity if proteins are the target. In addition to the formation of
reactive intermediates that are metabolites of the chemicals, metabolic
activity can give rise to species of active oxygen, which can damage
nucleic acids or proteins or yield lipid peroxidation. The results might
range from target-organ toxicity to carcinogenesis.
Biological Markers
Recently, the utility of biological markers as indicators of exposure to
polluted air has been investigated (NRC, 1989). Biological markers of
various types within an exposed individual can indicate the degree of
exposure to a pollutant, the initial structural, functional, or biochemical
changes induced by exposure, and, eventually, the changes associated
with adverse health effects. Thus, biological markers are indicators of
change within an organism that link an exposure to polluted air to subse-
quent development of an adverse health effect. It is convenient to divide
biological markers into three classes: (1) biological markers of exposure,
(2) biological markers (early predictors) of effects of exposure, and (3)
biological markers of susceptibility to effects of exposure.
Biological markers of exposure can be thought of as "footprints" that
the chemical leaves behind upon interacting with the body. Such mark-
ers contain the chemical itself 01 a metabolic fragment of the chemical
and thus are chemical specific. Examples of such markers are the chemi-
cal adducts formed with macromolecules such as nucleic acids or pro-
terns, particularly blood proteins. Another example is the presence of
volatile chemicals in exhaled air. That type of marker has been used by
Wallace (1987) to assess exposure to benzene during filling of gasoline
tanks in passenger cars, exposure to tetrachloroethylene in dry cleaning
shops, exposure to chloroform from contaminated hot shower water in
homes, and exposure to volatile aromatic compounds in tobacco smoke.
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TYPES OF DATA USED IN DEVELOPING CEELS 37
Physiologically based pharmacokinetic models (Ramsey and Andersen,
1984) have been used to relate biological markers of exposure to prior
exposure conditions.
Biological markers of the effects of inhalation exposure are the total
list of clinical signs of chemically induced disease states. The biological
markers of greatest interest are those that are early predictors of late--
occurring effects. Such markers would be invaluable hi assessing what
levels of pollutants can be tolerated without causing irreversible deleteri-
ous health effects. For example, cell proliferation can result in clonal
expansion of initiated cells (Swenberg, 1989). Thus, persistent cell
proliferation could be a biological marker predictive of an increased
incidence of late-developing neoplastic lesions. Few markers, however,
have been validated as predictive of late-stage diseases.
The third type of biological marker, markers of susceptibility to the
effects of exposure to airborne chemical pollutants, is a potentially im-
portant, useful tool. For example, polymorphisms related to acetylation
and DNA repair can be related to susceptibility to chemically induced
tumors. One must be cautious, however, in the use of such indicators
of susceptibility, as they pose numerous moral and ethical problems.
The appearance of metabolites in excreta have been useful in the past
and will be even more useful in the future as biological markers of expo-
sure, effect, and susceptibility. When the metabolites are specific to the
chemical, they will be most useful. When similar compounds related to
normal diet are found, the interpretation of the results might be more
complex. Other types of biological markers that are undergoing evalua-
tion include chromosome damage and covalent binding of reactive metab-
olites to DNA or proteins in circulating blood cells.
Route Considerations
Inhalation Route
The primary function of the respiratory tract, to provide the organism
with oxygen and remove carbon dioxide, presents a particular dilemma
with respect to exposure to airborne toxicants because, unlike ingestion
or dermal exposure, we cannot choose not to breath. Extreme measures
are often required to remove a toxic pollutant from the breathing zone of
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38 CEELS FOR HAZARDOUS SUBSTANCES
the individual. Understanding the key features of the respiratory tract as
a portal of entry will permit better predictability of exposure scenarios
resulting in inhalation hazards. The key features are the respiratory
tract's unique anatomy, the mechanisms by which inhaled particles and
gases deposit in the respiratory tract/and the mechanisms for removal or
clearance of deposited materials. More detailed information can be
found in reviews by McClellan and Henderson (1989), Gardner et al. (in
press), Bond (1990), Snipes (1990), and Schlesinger (1990).
The respiratory tract is grossly divided into three compartments: the
nasopharyngeal region, consisting of the nasal cavity, pharynx, and
larynx; the tracheobronchial region, consisting of the trachea and bron-
chial tree down to and including the terminal bronchioles; and the pulmo-
nary region, composed of respiratory bronchioles, alveolar ducts, alveoli,
and alveolar sacs. The last compartment is the functional area of the
lungs, the site for gas exchange.
Because inhaled pollutants move through the respiratory tract in an
airstream, the principles of fluid dynamics govern their transport and
eventual interaction with the respiratory-tract surface, a phenomenon
termed deposition. One of the most important factors in determining if
and where a pollutant will deposit in the respiratory tract is the aerody-
namic diameter of the pollutant particle. The aerodynamic diameter of
a particle is defined as the diameter of a unit-density sphere with the
same settling velocity as the particle in question. Thus, a small, very
dense particle will act like a larger, less dense particle on the basis of
where it deposits in the respiratory tract.
Fluid-dynamic mechanisms that govern the deposition of uncharged
aerosol particles include sedimentation, inertia! impaction, interception,
and diffusion. Diffusion, or movement due to Brownian motion, is most
important for particles with physical diameters of < 0.2 /mi. Sedimenta-
tion arid impaction are the most important mechanisms for deposition of
particles > 0.5 /mi diameter. One generalization that can be made is that
particles >5 /mi will deposit primarily in the nasopharyngeal region
because of inertial impaction; particles 1-5 /mi will deposit in the tra-
cheobronchial and pulmonary regions because of sedimentation; and
smaller particles will deposit in the pulmonary region because of diffu-
sion. The smallest particles (<0.01 /mi), such as radon progeny, will
deposit in the nasopharyngeal region because of diffusion.
The site for deposition of fibers in the respiratory tract is also depen-
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TYPES OF DATA USED IN DEVELOPING CEELS 39
dent on aerodynamic diameter. The aerodynamic diameter of fibers such
as asbestos depends primarily on the diameter of the fiber and is indepen-
dent to a large extent of the length of the fiber. Inhaled fibers align
parallel to the airways and act as spheres of approximately equivalent
diameter. An important mechanism for deposition of fibers is intercep-
tion or contact of the fiber with the respiratory-tract walls at the carina.
The structure of the respiratory tract affects particle deposition. For
example, airway diameter determines the distance that a particle must
traverse before contacting a surface, and the cross-sectional area deter-
mines the velocity at which the airstream travels through the particular
airway segment. Variations in pathway lengths in the bronchial tree
influence regional deposition of pollutants. Pathways with the shortest
lengths generally will have the highest concentrations of deposited parti-
cles in the alveolar region at the end of the pathway. Finally, the pattern
of breathing and the mode of breathing, such as nasal, oral, or a combi-
nation, will alter the deposition patterns of inhaled pollutants.
Critical determinants in the deposition and uptake of inhaled gases and
vapors include the solubility of the gas in body fluids and its reactivity
with tissue components or fluid lining the respiratory tract. Gases or
vapors that are very water soluble (e.g., methanol, acetone, hydrogen
chloride, and ammonia) deposit predominantly in the nasopharyngeal and
tracheobronchial regions (Buckley et al., 1984; Gerde and Dahl, 1991;
Johanson, 1991). For those pollutants, respiratory airflow patterns play
a major role in determining sites of deposition on the airway lining. Not
all water-soluble gases are toxic to respiratory-tract tissues. However,
if the gases react with respiratory-tract tissues, deposition patterns influ-
ence the distribution and severity of toxic responses and might account
for interindividual differences in susceptibility to associated respiratory-
tract disease or injury. Formaldehyde is an example of a reactive, highly
water-soluble vapor. Studies in rats have shown that the nasal mucosa
is the site for deposition of inhaled formaldehyde. Toxicity studies in
rats have shown that the anterior nasal cavity is the site for development
of tumors (Heck et al., 1990).
Reactive gases with low water solubility, such as ozone, will penetrate
farther into the respiratory tract. Exposure to low concentrations of
ozone causes injury predominantly in the small airways and the proximal
region, which is the alveolar region closest to the terminal bronchioles.
The proximal alveolar portions of the airways have a small total volume
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40 CEELS FOR HAZARDOUS SUBSTANCES
and are protected by a thick coat of mucus. As the inhaled ozone moves
into the large volume comprising the distal alveoh, the concentration is
diluted. Tlie dilution results in a localized injury to a specific portion of
the respiratory tract (Crapo et al., 1990). r^wi;-
Gases and vapors that are not water soluble but are more lipophihc
(such as the volatile organic compounds, benzene, styrene methylene
chloride, and trichloroethylene) are not completely removed by interac-
tion with the surfaces of the upper respiratory tract. As a result, those
gases can continue down into the pulmonary region where they might
diffuse across the alveolar epithelium into the blood (Gerde and Dahl,
1991) Once in the systemic circulation, their disposition will be similar
to the disposition after other routes of administration. The theoretical
equilibrium that can be attained between the volatile pollutant m fee
blood and that in the alveolar air is generally termed the blood-air parti-
tion coefficient. The extent of pulmonary exhalation of volatile organic
compounds is determined by the blood-air partition coefficient (Pb). The
proportion of circulating volatile compound that will be exhaled in a
single pass through the lungs when inhaled air contains none of the
Seal is approximately 1/[1 + PJ (NRC. 1986a). That situation
often occurs if the volatile pollutant is administered by another route
such as oral, dermal, or injection. Many of the highly toxic, unregulated
pollutants identified by the 1990 Clean Air Act Amendment, such as
benzene and 1,3-butadiene, fall into the category of volatile organic
opa or gases that are inhaled will be deposited on the
surfaces of the respiratory tract or absorbed into the blood. A large
fraction of inhaled pollutants are exhaled. Fractional deposition of parti-
cles is dependent upon particle sire. For large (>5/™) or s™
«0 01 urn) particles, deposition efficiencies approach 80-90%. Mini-
mum deposition occurs for particles in the 0.3- to 0.5-Mm range. Parti-
cles in that range are minimally influenced by impaction, sedimentation,
and diffusion. As a result, they undergo minimal deposition after inhala-
tion and most are carried out of the respiratory tract in the exhaled air.
Average deposition efficiencies for those particles range from approxi-
mately 10% to 30%. A.^,11.
Likewise, not all inhaled gases and vapors are deposited on the walls
of the respiratory tract or absorbed into the blood. For very reactive
volatile chemicals, deposition efficiencies are high, and because these
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TYPES OF DATA USED IN DEVELOPING CEELS 41
reactive gases are not likely to re-entrain into the airstream, uptake by
the respiratory-tract surface can approach 100%. For nonreactive va-
pors, the rate of uptake depends on the Pb. As the Pb increases, the rate
of uptake increases. The rate of uptake of the chemical will also vary as
equilibrium with blood is achieved. For vapors with a low Pb, total
uptake might amount to only a few percent of the amount inhaled. The
majority of the inhaled vapor will be exhaled. Even for vapors with a
high Pb, the upper limit for uptake appears to be about 60%. That is due
to the cyclic nature of respiration. On inspiration, vapors with a high Pb
are temporarily absorbed by the respiratory-tract surfaces. On
exhalation, a portion of those vapors will re-entrain into the airstream
and be exhaled. The remainder will be absorbed into the blood (Gargas
et al., hi press).
For pollutants that have deposited on the respiratory-tract lining,
mechanisms for clearance or removal include mechanical clearance via
the mucociliary escalator and transfer to the gastrointestinal tract, phago-
cytosis by pulmonary macrophages followed by migration of the
macrophages to the lymph nodes or to the upper airways, and dissolution
of the particle in the fluid lining the respiratory tract and diffusion into
the blood. Mucociliary clearance is a rapid process beginning in a mat-
ter of minutes after the particle has deposited on the ciliated cells lining
large portions of the upper respiratory tract. Transfer to the gastrointes-
tinal tract, by swallowing of mucus containing particles, means that all
inhalation exposures to particulate material can result in a significant oral
exposure component (ICRP Task Group on Lung Dynamics, 1966).
The surface of the bronchial tree is nonhomogeneous. There are areas
that are not covered by ciliated cells, usually at the bifurcations. Mucus
flow is interrupted over those regions and clearance is decreased. Addi-
tionally, the bifurcations may be sites for increased deposition of parti-
cles due to inertia! impaction as the airstream changes direction. Com-
bined slower clearance and increased deposition indicates that the bifur-
:ations may receive an increased dose of pollutants compared with other
ureas of the bronchial tree.
Particles also can be cleared from the respiratory tract by dissolution
uto the blood. In general, particles that are water soluble will dissolve
within minutes or days. Chemicals that are not water soluble but have
i moderate degree of lipophilicity (log octanol-water partition coefficient
< 106), such as benzo[a]pyrene? nitropyrene, and anthracene, will also
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42 CEELS FOR HAZARDOUS SUBSTANCES
clear rapidly into blood (Bond et al., 1985). Those that are not water
soluble or are highly lipophilic (octanol-water partition coefficient >
106) will not dissolve and will be retained in the lungs for long periods.
Dioxin and other highly chlorinated hydrocarbons are examples of highly
lipophilic pollutants (Henderson et al., 1988). Inorganic particles pro-
duced during high-temperature combustion processes, such as metal
oxides: oxides of transuranic elements, including plutonium oxides; and
carbon particles, including diesel soot, are examples of water-insoluble
particles. Those kinds of particles might be retained in the lungs for
years They will be removed slowly through phagocytosis by pulmonary
macrophages and through dissolution. In addition to the solubility char-
acteristics of the particle, dissolution rate is also a function of the surface
area of the particle. Large specific surface area (square centimeter per
gram) facilitates dissolution.
The presence of xenobiotic metabolizing enzymes in respiratory-tract
tissues raises the possibility of both a first-pass metabolism of inhaled
pollutants and a bioactivation of these materials to compounds that can
be directly toxic to respiratory-tract tissue (Bond, 1990). All segments
of the respiratory tract, from the nasal cavity to the periphery of the
pulmonary compartment, contain enzymes that metabolize xenobiotic
compounds. However, the distribution of those enzymes throughout the
respiratory tract is not uniform. Additionally, within a region of the
respiratory tract, enzymatic activity varies with different cell types. A
number of studies have demonstrated that the nonciliated Clara cells in
broncniolar tissue contain the highest concentrations of xenobiotic metab-
olizing enzymes. Those cells are also a target for many toxicants, in-
cluding carbon tetrachloride, chlorinated ethylenes, and naphthalenes.
Skin—Percutaneous Absorption
The skin, as the outermost organ of the body («1.7 m2), provides an
important protective shield for underlying organs. Xenobiotic substances
that make contact with the skin must circumvent this barrier tissue to
gain entrance into the body. The protective function of skin is imperfect,
quite variable, and influenced by a host of factors (Marzulli, 1962).
Percutaneous absorption (skin penetration) provides a more indirect
route to the bloodstream than inhalation; nevertheless, both routes share
certain similarities. In both cases, the structure, function, and metabo-
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TYPES OF DATA USED IN DEVELOPING CEELS 43
lism at the point of entry modulate the dose of the xenobiotic and its
metabolites that reach the circulatory system before distribution within
the body. Both local and systemic effects are possible, therefore, when
xenobiotic substances make contact with the skin (EPA, 1992).
When a xenobiotic substance is inhaled, some is retained in various
parts of the respiratory tract and some is exhaled. Similarly, losses oc-
cur when xenobiotics make contact with skin. The first loss is the evap-
orative loss that occurs at the skin surface. Additional losses occur if the
substance binds to the outermost layers of dead tissue (stratum corneum).
Metabolism in the underlying living epidermis might bring about chemi-
cal changes in the parent molecule (Bronaughs and Collier, 1991).
The amount of xenobiotic that reaches the systemic circulation is
determined by certain chemical and physical factors (concentration,
molecular size, dissociation state, lipid and aqueous solubilities, and
reactivity) and exposure factors (skin contact area, skin site, duration of
xmtact, occlusion, and decontamination efforts). Intact skin is important
in functioning as a barrier to foreign substances in contact with the skin
Flynn, 1990).
In percutaneous absorption, the xenobiotic compound bypasses the
iver, so that first-pass liver metabolism is avoided; nevertheless, some
netabolism, albeit with less activity than the liver, may take place in the
;kin itself because it contains metabolic enzymes.
Metabolism may occur during percutaneous absorption, preceding the
;enobiotic's entry into the bloodstream. At present, it is not known to
vhat extent metabolic activity in animal skin duplicates that taking place
n human skin. Some exploratory information suggests that human skin
s not as metabolically active as animal skin (Kao and Carver, 1991).
Skin penetration is quantitatively estimated by one of two techniques.
"he first technique involves measuring the steady-state rate at which the
«netrant traverses the skin's barrier tissues in an in vitro diffusion-cell
ystem. The second technique involves measuring the fraction of the
pplied amount that penetrates skin in vivo (usually with radiolabeled
laterials) (EPA, 1992). With the first technique, results are reported in
;rms of amount penetrating per unit of skin per unit of time. In the
econd method, results are reported as percent of applied amount that is
bsorbed.
To normalize effects of concentration, when using the steady-state
pproach, results can be reported in terms of a permeability constant.
hus, with the simple form of Pick's law, r = pC, where r = rate in
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44 CEELS FOR HAZARDOUS SUBSTANCES
' ' " ' ' -i •' • . ". • ..•;.,•. . , " •„.). ' ....<• . •
micrograms per square centimeter per hour, p = permeability constant
in microcentimeters per hour, and C = concentration in grams per milh-
liter (Marzulli et al., 1969).
TOXICITYEND POINTS: HUMANS AND ANIMALS
Mortality
The most commonly reported end point for acute animal toxicity
testing is lethality. The information is expressed as either median lethal
concentration (LC50), when the exposure is by inhalation, or as median
lethal dose (LDSO), when the test substance is administered by some other
route The data should be expressed in statistical terms necessary to
describe the lethal response of a substance in a particular population
under a discrete set of experimental conditions. The exact numerical
value of the median lethal dose should not be used as an absolute num-
ber because many extraneous factors, such as animal species, strain,
sex', age, and duration of exposure, can influence or alter the precision
of the data Lethality data also might not reflect accurately the tull
spectrum of toxicity associated with such exposures, because some chem-
icals (e.g., some carcinogens and teratogens) might have a low acute
toxicity but produce serious long-term health effects.
In the absence of median lethality data, alternative data might be
useful in determining acute toxicity, such as identifying the lowest dose
or concentration (LDLO or LCLO) mat results in the test animal s death
following exposure. Such information should be expected to be quite
variable, but it can be used to compare toxicity of various chemical types
when LDSO data are not available. _
Lethality is not the only significant end point obtainable from acute
toxicity testing. Much information can be derived from the slope of the
dose-response curve, time of death, and signs of toxicity, all of which
are critical responses that are useful in characterizing acute toxicity.
Clinical Signs and Symptoms
Physical examination, paying attention to signs and symptoms, can
contribute to the biomedical and behavioral assessment of exposure. Eye
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TYPES OF DATA USED IN DEVELOPING CEELS 45
irritation and tearing, for example, are among the most sensitive indica-
tors of excessive exposure to oxidants. Complaints by exposed individu-
als provide an important criterion on acceptability of pollutants. Al-
though they might not be related to any direct health effects, complaints
need to be considered as an indication of unacceptable exposure.
Effects on Respiratory Tract
The majority of exposures in a community emergency situation most
likely will be through the inhalation route, which can result in immediate
danger to life and health. In addition to causing pulmonary effects, the
inhaled toxicants can be rapidly transported from the initial target organ
and cause potentially serious lexicological effects on other biological
systems.
The earliest indicator of pulmonary toxicity to high concentrations of
airborne contaminants might be respiratory irritation. Chemicals that are
sensory irritants usually act relatively rapidly, and the irritation tends to
:ease quickly when the chemical exposure is removed. Such substances
iave a characteristic odor and a tendency to evoke a burning sensation
jf the nasal turbinates, to cause tearing, coughing, and headaches, and
:o induce bronchoconstriction of the airway passages. The irritant effects
:end to be more concentration-dependent than time-dependent and, as
such, are generally noncumulative in nature. Such responses can provide
i certain warning of potential danger, but they cannot be considered a
•eliable indicator of pending hazards.
Inhalation experiments with animals can provide a useful basis for
>redicting possible immediate and delayed health effects in humans and
:an indicate the concentration at which such effects might occur. Vari-
>us pulmonary-function parameters have been measured in animals and
lumans. Many of the techniques are noninvasive and are useful for com-
laring effects in animals with similar measured responses in humans.
[Tie data can be useful in detecting the presence of impairment, the type
•f impairment, and its severity. The most common measurements in-
lude lung volumes (tidal volume, vital capacity, residual volume, and
Dtal lung capacity), mechanical behavior of the lung (airway resistance
nd compliance), gas-exchange proficiency (diffusion capacity and distri-
>ution), and ventilatory exchange (minute ventilation and respiratory
ate).
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46 CEELS FOR HAZARDOUS SUBSTANCES
Whenever possible, multiple end points should be considered in evalu-
ating -the toxicity of hazardous materials because the lung can respond to
airborne pollutants in different ways. Multiple end points can be used
to assess toxic injury in an emergency situation and are valuable in estab-
lishing safe exposure guidelines. Table 2 lists toxic effects that result
from short-term inhalation exposures.
Neurotoxicity
When administered in large doses, many chemical substances produce
marked changes in human behavior and neurological function, such as
depression of the central nervous system, that disappear rapidly and with
no apparent sequelae after exposure has ceased. The same compounds
might produce other potentially irreversible neurotoxic effects in persons
chronically exposed to lower doses for long periods. The mechanisms
underlying those two (or more) neurotoxic effects might be entirely
different. Although some compounds, such as n-hexane and toluene,
cause various types of neurodegenerative diseases after prolonged expo-
sure, others, such as methyl ethyl ketone and acetone, do not appear to
caiise such effects. .
Evidence that environmental agents produce adverse effects in tne
human nervous system can be provided through clinical evaluations of
exposed humans and studies in experimental animals. Definitive demon-
stration that a substance is neurotoxic comes from complementary inves-
tigations showing that the suspect agent produces the same type of disor-
der in humans and in one or more appropriate test species. In the ab-
sence of data on humans, convincing demonstration that a substance is
neurotoxic in an appropriate animal species can be taken as evidence that
the agent is probably neurotoxic in humans. For example, certain well-
studied classes of chemicals, such as the organophosphates, reliably
produce human-like disorders in fowl.
Chronic neurotoxicity might develop many years after exposure to
certain chemical substances (e.g., phenothiazines). Negative results ob-
tained from studies conducted over shorter periods should be interpreted
carefully, because lifetime studies to evaluate agents for neurotoxic prop-
erties rarely have been undertaken.
Short-term in vitro methods for assessing neurotoxic activity are being
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TYPES OF DATA USED IN DEVELOPING CEELS
47
TABLE 2 Data for Identifying Effects Associated with Short-Term
Inhalation Exposures3
Immediate
Effect Effect
Respiratory Effects:
Pulmonary edema +
Pulmonary +
hemorrhage
Spasm-suffocation +
Chronic obstructive —
lung disease
Restrictive lung —
disease
Emphysema —
Cancer —
Hypersensitivity +
Hyperreactivity —
Predisposition to +
infection
Nauseating odor +
Eye Effects:
Corneal clouding +
Burning +
Lachrymators •*•
Availability of
Delayed Procedures and
Effect Protocols
•*• Yes
+ Yes
Yes
+ Yes
+ Yes
+ Yes
+ Yes
+ Yes
+ Yes
+ Yes
- Yes
+ Yes
+ Yes
Yes
Quality
of Data
•H- +
+ + +
•H- +
++
+
+
—
±
±
+ +
±
•f -H-
+•(-+
+ H- +
aData from EPA, 1990a.
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48 CEELS FOR HAZARDOUS SUBSTANCES
developed. Tissue-culture systems, especially those that exploit a com-
plex cellular structure and function comparable to that foundm parts of
the human nervous system, ultimately might provide surrogates for in
vivo animal bioassays of potential neurotoxicants. Organotypic tissue
cultures composed of structurally and functionally coupled spinal cord,
dorsal root ganglia, peripheral nerve, and muscle develop specific types
of pathological change (e.g., sensory neuronopathy, axonopathy, and
myelinopathy) that also are seen in humans and animals challenged with
the same substances. Tissue-culture systems also have been used to
study alterations of neurotoxic response to one agent («-hexane) by con-
current exposure to a second agent (e.g., methyl ethyl ketone, ethanol,
and toluene). .
In some cases, structure-activity considerations might be usetut in.
assessing whether a substance poses a hazard to the human nervous
system. Examples of compounds with structural similarities that consti-
tute neurotoxicants include some organophosphorus esters (phosphates
and phosphonates), pyrethrins, and 1,4-dicarbonyl aliphatic hydrocar-
bons.
Reproductive and Developmental Toxicity
Single or short-term exposures to chemical agents have been shown to
result in reproductive and developmental toxicity in a number of cases
(Schardein, 1985). A variety of outcomes are possible, including alter-
ations in sexual behavior, onset of puberty, fertility, gestation, parturi-
tion, lactation, pregnancy outcomes; (including prenatal and early postna-
tal death, structural abnormalities, growth retardation, and postnatal
functional deficits), premature reproductive senescence, and modifica-
tions in other functions that are dependent on the integrity of the repro-
ductive system (Kimmel, 1991).
A number of stages in the reproductive cycle and during development
are known to be sensitive to exposure to toxic agents, and the types of
effects seen may depend on the timing of exposure (Wilson, 1973;
Dixon, 1986). Available data are often from studies employing expo-
sures to pregnant animals during major organogenesis, and some extend
the study into the preweaning period to examine the products of concep-
-------
TYPES OF DATA USED IN DEVELOPING CEELS 49
tion and the growth and development of the liveborn offspring. Addi-
tional information on reproductive performance in exposed adults and on
pregnancy outcomes is sometimes available from two-generation studies.
Data from those studies usually form the primary data base for evaluating
reproductive and developmental hazards (Manson and Kang, 1989;
Zenick and Clegg, 1989). When human data are available, they should
be considered with the animal studies to make judgments about hazard
and risk (EPA, 1991).
Inhalation exposure, but more often oral exposure, is used in conduct-
ing reproductive and developmental toxicity studies in experimental
animals. Data from oral exposure studies should be considered in deter-
mining CEELs when evaluating the potential for an agent to produce
reproductive or developmental effects from inhalation exposure.
Pharmacokinetic data from appropriate studies also should be considered
for route-to-route extrapolation of reproductive and developmental toxici-
ty data.
Acute or short-term exposure levels that produce adult systemic toxici-
ty are likely in some, but not all, cases to cause reproductive or develop-
mental effects. With some agents, reproductive or developmental chang-
es may be produced at doses that do not cause other forms of overt
toxicity, and these agents are of greatest concern in assessing risk for
humans (Zenick and Clegg, 1989; EPA, 1991).
The adjustment of doses to account for duration of exposure in repro-
ductive and developmental toxicology has not been explored in great
ietail. EPA's "Guidelines for Developmental Toxicity Risk Assessment"
[1991) gives the most recent guidance on that issue. As indicated in the
EPA guidelines, a single exposure is assumed to be sufficient to produce
idverse reproductive and developmental effects. Therefore, exposure
jstimates and doses usually are based on a daily dose that is not adjusted
ibr duration or pattern of exposure unless pharmacokinetic data are avail-
ible that indicate bioaccumulation with repeated exposure. In developing
DEELs, data from studies that are most similar to the scenario (i.e.,
ihort-term exposures from one to several hours per day) of exposure by
nhalation are most appropriate for use. When such data are not avail-
ible, data from other types of studies should be considered, and the
guidance discussed above should be employed, incorporating scientific
udgment as necessary. Further development of short-term exposure
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50 CEELS FOR HAZARDOUS SUBSTANCES
guidelines is under discussion by several groups, including scientists at
EPA, and work is in progress to develop more appropriate methods tor
risk assessment in this area.
Immunotoxicity
Immunotoxicology is a scientific discipline that explores the effectsof
physical, biological, and chemical agents on the immune system. The
immune system is complex and highly sophisticated and is self-regulated
as well as influenced by other systems, namely the endocrine and central
nervous systems. Immunotoxicology has experienced considerable
growth since its inception nearly two decades ago. Today, several exper-
imental procedures are available that have been validated to assess
xenobiotic-induced immune dysfunction in animals. Fewer assays are
available to evaluate immune dysfunction in humans. Nevertheless, pro-
cedures are available and others are being developed to assess immune
function in animals and humans. Care must be taken in using immune
data for risk analysis to ensure that the data are a correlate of immune
function (i.e., that it represents some type of response that correlates
with innate or naturally acquired resistance to disease, neoplasia, autoim-
munity, and hypersensitivity).
Procedures that can be used in animals and humans to assess immune
function are measurement of humoral (B-cell) antibody responses to
challengeby T-dependent antigens; delayed-type hypersensitivity respons-
es1 cytotoxicity of T lymphocytes, macrophages, natural killer cells and
lymphokine-activated cells; and cytokine production and activity. Tests
developed specifically for humans include assessing antibody levels to
ubiquitous antigens (blood groups); responses to protein and polysaccha-
ride antigens (such as diphtheria, tetanus, poliomyelitis, pneumococcus,
and meningococcus); primary (keyhole limpet hemocyanin) and second-
ary (Candida, diphtheria, and tetanus) delayed-type hypersensitive reac-
tions; use of multiple-antigen skin-test kits; natural-killer-cell cytotoxici-
ty and serum levels of cytokines (such as interleukins 1 and 2). Al-
though tests for humoral, cellular, and nonspecific immunity have great
value in studies of profound hereditary immunodeficiency states, they
have not been specifically evaluated for their ability to detect more mod-
est immunodeficiencies that might occur in individuals exposed to
immunotoxic agents. Therefore, although these procedures have been
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TYPES OF DATA USED IN DEVELOPING CEELS 51
validated in animal models, their use as biomarkers in humans must be
validated prospectively in populations exposed to putative immunotoxi-
cants and in control groups to establish their ability to predict develop-
ment of disease associated with xenobiotic-induced immunodeficiency.
Several other immune procedures might be applied eventually in as-
sessing xenobiotic-induced immune dysfunction in humans. However,
those procedures will require considerable testing before they can be
accepted as positive correlates of immune function for risk-assessment
purposes.
Carcinogenicity
Current theories based on experimental research in animals indicate
that at least three distinct stages exist in the development of carcinogene-
sis: initiation, promotion, and progression. Each of the stages appears
to be influenced by such factors as age, heredity, diet, metabolic activity,
and previous and current exposures to xenobiotic compounds, which can
interact and potentiate, antagonize, or synergistically influence the devel-
opment of tumors. Mouse skin serves as a good model for chemical
carcinogenesis; the model's additional use is for studying the mechanisms
of action of initiators and promoters. Rat and mouse liver models also
have led to useful discoveries about stages of tumor development, and
promotion has been demonstrated in other tissues, such as the lung and
breast. Some chemicals, such as asbestos, estrogens, and some constitu-
ents of cigarette smoke, appear to be mainly tissue-specific promoters,
whereas others, such as 2-acetylaminofluorene, can act as promoters in
the mouse bladder but as initiators (and promoters) in the liver of the
same animal. The effect of a promoter can vary depending on the se-
quence in which it is administered with an initiator. Therefore, it might
be desirable to test potential promoters and inhibitors of promotion in
vivo in several species and strains, to examine several organs for the
response, and to test different sequences of administration.
Recent investigations into the role of oncogenes seem to support the
multistage theory of carcinogenesis. Of the oncogenes thus far discov-
ered, most have been either of two types, immortalizing genes or trans-
forming genes, which may be regarded as the biological counterparts of
initiation and progression in tumor induction. A third type of oncogene
recently identified might cause a cancer cell to metastasize. Ideally,
-------
, '"i!
i
52 CEELS FOR HAZARDOUS SUBSTANCES
then, the risk modeler should consider at least three stages of tumor
induction to be consistent with human and experimental evidence. Risk
models also must take into account the diversity of promoting agents and
their multiplicity of action at the cellular level.
The most widely used tests for carcinogen evaluation and regulation
are long-term animal bioassays, which are considered predictors of re-
sponse in humans. However, animal bioassays not only are time con-
suming but also are difficult to conduct, are extremely expensive, do not
necessarily predict the target organ in humans, and cannot duplicate the
wide range of susceptibilities, simultaneous exposures, and genetic back-
grounds of the human experience.
Short-term in vitro assays of cultured cells, requiring only a fraction
of the time, can test for toxic end points manifested during the initiation
phase of carcinogenesis. Other tests, such as cell transformation assays,
examine toxic end points for later events in carcinogenesis. Short-term
tests provide information on the mechanism of carcinogenicity and can
contribute to the weight of evidence.
There is a need to develop short-term tests for promotion. A battery
of tests is necessary for screening chemicals for genetic toxicity. False-
negative and false-positive results might occur because the tests do not
reflect the complexity of interactions in the whole animal, and although
knowledge of the chemical's principal mechanism of action is crucial to
the selection of short-term tests with meaningful predictive value, that
knowledge is rarely available. Despite the fact that in vitro tests are
technically difficult to design and interpret, they have considerable poten-
tial and should be developed further.
Mutagenicity
Genetic toxicology is" concerned with health effects resulting from the
mutagenic action of physical and chemical agents on DNA. Mutagenesis
involves gene (molecular) and chromosomal (gross structural and numeri-
cal aberrations) changes in germ and somatic cells. Mutations in both
types of cells do not result in immediate adverse health effects but might
result in late effects. Germ-cell mutations affect the subsequent and
future generations of the exposed person, whereas somatic-cell mutations
can affect the individual in whom they are produced, primarily by initiat-
ing carcinogenesis and, in the embryo-fetus, teratogenesis. Low-dose
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TYPES OF DA TA USED IN DEVELOPING CEELS 53
exposure to some chemicals is capable of inducing mutations, but acute
high-dose exposure that permits survival requires special attention be-
cause not only is the probability of mutation induction higher but also
more mutations can be induced by high-dose exposure than by low-dose
exposure. The increase in mutations gains significance in view of recent
evidence suggesting that additional gene mutation is involved in the later
stages of carcinogenesis (Stowers et al., 1987; Kennedy, 1991).
Germ-cell mutations, either gene or chromosomal, usually are deleteri-
ous to some degree in the individual who inherits them, the mode of
expression—recessive, dominant, or polygenic—varying with the gene
affected. The involvement of somatic-cell mutation in carcinogenesis is
now widely accepted. Carcinogenesis is a complex multistage phenome-
non, three stages of which have been identified: initiation, promotion,
and progression. The best understood is initiation (i.e., a change in a
cell that leads eventually, through a chain of subsequent events, to a
clinical tumor). The finding that most human tumors have arisen from
a single cell (Nowell, 1991) and the discovery that single mutated
growth-regulating genes or growth-suppressing genes (oncogenes) can
cause cancer (cf. Bishop, 1987) leave little doubt that mutation is the
main initiating event. Progression is probably controlled by different
genes that permit transformed cells to invade and metastasLze (Hill,
1990).
Over the last two decades, many data have been accumulated on the
mutagenic effects of many chemicals, primarily with non-human cell
systems. In recent years, data from human cells have been emphasized.
Research on in vivo gene mutation has centered on red blood cells
(RBCs) and T lymphocytes (cf. Albertini et al., 1990). In RBCs, the
end points are mainly mutations in the hemoglobin and the GPA
(glycophorin-A) genes, and in T lymphocytes, the end points are mainly
mutations in the HGPRT (hypoxanthine-guanine phosphoribosyltrans-
ferase) and the HLA (human leukocyte antigens) genes. Chromosomal
changes have been and are being extensively examined in lymphocytes
hi vivo and in vitro including structural and numerical aberrations, sister
chromatid exchanges, micronuclei, unscheduled DNA synthesis, and
single-strand breaks in DNA.
The rapid growth of molecular biology has resulted in a new approach
to in vivo genetic toxicity in humans, namely, molecular epidemiology
(Weinstein and Perera, 1982; Perera et al., 1985, 1991; Perera, 1990;
Marx, 1991). This interdisciplinary field ties the highly sensitive labora-
-------
III I ! I t!»
54 CEELS FOR HAZARDOUS SUBSTANCES
tory techniques for detecting initial chemical interactions with cell targets
to the analytical methods of epidemiology, one goal of which is early
detection of individuals at high risk for cancer, thereby permitting inter-
vention or prevention of the disease.
Molecular epidemiology focuses on detecting molecular markers (pri-
marily carcinogen-DNA or carcinogen-protein adducts) of damage in
populations of people who are exposed to chemicals known to be or
suspected of being carcinogenic. Perera (1990) provided a summary of
the laboratory techniques. One of the advantages of molecular epidemi-
ology is that adducts are formed during or very quickly after exposure
but their early formation can be a disadvantage when late sampling of
subjects is necessary because many adducts are relatively short-lived
Among other advantages are the ability to detect minute amounts of
molecular markers (one adduct in 109-1010 nucleotides in DNA); protein
adducts in body fluids (blood, urine, breast milk, and semen), exhaled
air, and adipose tissue; and new proteins, such as those produced by
activated oncogenes.
With one exception (cf. Perera et al., 1991), results do not yet allow
quantitative risk assessment, but they do show that molecular epidemiolo-
gy is promising (e.g., Hemminki et al., 1990; Mayer et al., 1991) and
probably will be one of the prominent methods for identifying and as-
sessing cancer risks in humans. As Perera (1990) has pointed out mo-
lecular epidemiology is not a panacea but supplements existing methods
for environmental monitoring, experimental testing, and epidemiology.
' ' • ".!.•„'• .'••'•' • t ,,i. • • . ...... , I
• • ••" ' ' • ' ' '
OTHER ORGAN EFECTS
Ocular Effects
Acute eye irritation (including inflammation or opacity of the cornea
and irritation of the bulbar and palpebral conjunctiva and the iris) is a
prominent feature of ocular effects that must be considered in relation to
emergency exposures to hazardous chemicals. The cornea and ins are
important in vision and thus those effects warrant special attention.
A measurement of the pH and buffering capacity of an acid or alkali
provides a first look at the potential' of the xenobiotic to irritate the eye
and produce possible serious deep injury. The intimacy and duration of
contact between the offending substance and the eye as well as the pene-
-------
TYPES OF DATA USED IN DEVELOPING CEELS 55
tration capacity and chemical reactivity of the xenobiotic with tissue
components of the eye are important predictors of ocular damage. Use-
fulness and availability of water as an eye decontaminant also might be
critical determinants of ocular damage.
The most capable test of disclosing ocular irritation potential is the
Draize rabbit eye irritation test; however, if serious ocular damage is
expected, it can be avoided by first applying the test material to skin for
a short time. As the skin and eye are anatomically similar, skin, which
is tougher and more easily repaired, can serve as a surrogate for the eye.
Organic solvents, alkylating agents, surfactants, and detergents are
known to produce serious deep ocular damage or delayed injury on
contact with the eye; less toxic chemicals might produce only stinging,
lacrimation, and discomfort (Grant, 1974).
Dermal Effects
As the outermost organ of the body, skin might be the first tissue to
come in contact with xenobiotic chemicals during emergency exposures.
Systemic or local effects are possible following skin contact with
xenobiotics. Organophosphate pesticides and other toxic chemicals might
penetrate skin without apparent gross skin damage, producing systemic
effects. Some skin penetrants might be more toxic by the percutaneous
route than by the oral route if metabolic activity is high by the oral
route. Herbicides, fungicides, and rodenticides are typical farm chemi-
cals that might produce systemic toxicity when they contact skin and are
absorbed into the bloodstream following percutaneous penetration.
Skin irritation, skin sensitization, and phototoxicity are local effects
that might occur when xenobiotic substances come in contact with the
skin during emergency exposures. Skin irritation is a local inflammatory
response, skin sensitization is an immune response involving skin and the
immune system, and phototoxicity is an exaggerated sunburn-like skin
response produced by chemicals that are activated by light.
Hepatic Effects
The liver is the single most important organ concerned with protecting
the body against invasion by potentially toxic xenobiotic chemicals and
-------
!"'ii!!i!i: I, •'": I ! .I'lE1 •' \\S' ih iiilllJ^l' "'I!1'?"' '' 'il "'•!"" "!'ii, ill ' lii'll1 'It'1
T'li1' !N.' '"• '"S,"!1 !' ,,i.I'.
56 CEELS FOR HAZARDOUS SUBSTANCES
1 " i1 • '" ' ' ' ' ' ' !j ''
therefore is among the organs most vulnerable to chemical insult. The
liver avidly accumulates many chemicals, often after oral administration,
leading to the so-called first-pass effect. However, accumulation of
chemicals by the liver and toxic responses of the liver also can occur
after exposures via inhalation or other routes.
Some chemicals, such as phosphorus, can be directly toxic to the
liver. Chemicals passing through the hepatic circulation can be acted
upon to yield detoxification products or can be activated metabohcally to
become more toxic chemical entities. Metabolic activation of many
chemicals leads to liver damage. Mechanisms by which liver injury
might occur include the accumulation of lipids, lipid peroxidation, cova-
lent binding of reactive metabolites to critical cellular macromolecules,
depletion of antioxidants such as glutathione leading to oxidative stress,
interactive lexicological effects originating in Kupffer cells that lead to
damage in hepatocytes, disruption of calcium compartmentation, and
other aberrations that can lead to cell death. The effects of chemicals
include local or generalized lesions leading to cell death and necrosis,
inflammation, fatty infiltration, cholestasis, cirrhosis, or carcinogenesis.
Underlying these injuries are factors such as excessive alcohol intake,
diabetes mellitus, or starvation that induce increases in the levels of cyto-
chrome P-450HE1,an enzyme involved in the metabolic activation of
many halogenated hydrocarbons and other chemicals to lexicologically
active or carcinogenic species. Some halogenated hydrocarbons, such as
halothane, can cause immunological responses in liver following repeated
exposure. Nutrilional deficiencies rnighl exacerbate potenlial liver injury
caused by chemicals such as elhanol.
Liver damage can be delected by the appearance of jaundice, measure-
ment of hepatic enzyme levels in circulating blood, finding of radioiso-
lope accumulation in the liver, use of various imaging techniques, or,
more invasively, evaluation of liver samples obtained by biopsy.
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Risk Assessment
OVERVIEW
Recommendations for permissible exposure limits to chemicals usually
have been determined by the nature of the adverse effect, the exposure
scenario, and the anticipated susceptibility of the population for which
the permissible exposure limit is being set. The quality of such recom-
mendations for materials that have been in use for a long time is highest
when the recommendations are based on epidemiology or clinical toxicol-
ogy studies carried out under conditions that are identical to those for
which the recommendation is intended. Even in that ideal situation,
however, the recommendations are for the average or typical population
and might not be predictive for a specific individual within the popula-
tion. When the information on humans is not ideal for setting permissi-
ble exposures for a chemical or exposure situation, the first approach is
to extrapolate from the best available human data, recognizing that differ-
ences in dose, exposure route, rate, or duration; population differences
in sex, age, occupation, or dietary habits; and other differences might
bias or invalidate the permissible limits set. If human data on the desired
chemical is lacking or inadequate, several other approaches suggest them-
selves. For new materials, laboratory data usually from animal studies
combined with data on surrogate chemicals might be helpful. The chem-
ical and biological properties of the target chemical should closely resem-
ble those of the surrogate chemical, for which human data are adequate.
The use of animal data to predict the effects of human exposure to a
chemical is most useful when the test species and humans respond in the
same manner to the chemical. Unfortunately, data on corresponding
responses are rarely available. Studies can be carried out in animals that
are neither ethical nor practical in humans. Animal studies can be de-
signed, however, to provide information that can contribute to the appro-
priate setting of guidelines for human exposure to chemicals.
57
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5S CEELS FOR HAZARDOUS SUBSTANCES
The use of animal data to establish the toxicity of a chemical is called
hazard evaluation. Full hazard evaluation involves developing informa-
tion concerning the chemical, the exposure conditions, and the adverse
effects in the test species. The first step in the hazard evaluation process
is to identify the adverse effects produced by (acute or chronic) exposure
to the agent. The second step is to establish dose-response relationships
for each of the adverse effects. Ideally, the studies would provide infor-
mation on the effects of administration by various routes at different rates
and durations of exposure arid information on other test species. That
information and data on the chemical and physical-chemical properties
of the agent, kinetic data in various species, genotoxicity studies, devel-
opmental toxicity and reproductive studies, and other types of target-
organ and mechanistic studies constitute the toxicity data base for the
chemical. Of course, if the material turns out to be so toxic that the
manufacturer does not wish to put it on the market or if the likely per-
missible exposure is too low; much of the additional work need not be
done The third step is to determine, whether the information in the data
base is appropriate and relevant for extrapolating to humans under actual
exposure conditions. In situations where the information is adequate and
relevant and where there is a threshold or no-effect level for a specific
adverse effect on which to base the extrapolation, the final step in the
process is to divide the experimental threshold or no-observed-adverse-
effect level (NOAEL) by appropriate uncertainty factors to develop an
upper limit for exposure to the material. A modification of that approach
has been proposed by Crump (1984a), in which the fitted dose-response
curve is used to calculate benchmark doses or concentrations for low-
incidence responses (1%, 0.1%, etc.), and then appropriate uncertainty
factors are used to develop the reference dose or concentration (RrD or
RfC) The benchmark-dose approach is more responsive to the number
of animals used at each dosage level and avoids the use of an arbitrary
NOA^L value but is basically an extension of the traditional RfD-RfC
approach. One of the advantages of the RfD-RfC approach is that it is
simple and easy to understand. However, in practice it involves a great
deal of judgment in selecting test data and uncertainty factors. The
selection of an uncertainty factor should reflect not only the confidence
of the ©valuator in the quality and relevance of the data for the prediction
but also the differences in susceptibility between the test and target spe-
cies and the differences among individual members of the exposed popu-
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RISK ASSESSMENT 59
lation. The goal of the benchmark approach is to predict a reference
dose for exposure to an agent below which a significant increase in toxic
effects is not expected above background levels. This approach for
protecting the health of specific groups, such as workers, is the most
well-established and most widely used approach against adverse effects
from exposure to chemicals, but it is not applicable for materials that
produce nonthreshold toxicity, such as cancer.
Although it has been argued that there should be a threshold for every
adverse effect, the threshold might be virtually impossible to distinguish
from the zero- or no-exposure level. In addition, there are theoretical
reasons why some toxicities are no-threshold phenomena. Therefore, in
those situations where a threshold or no-effect level cannot be demon-
strated or theoretically should not exist, an alternative approach is need-
ed. For example, a so-called quantitative risk-assessment (QRA) ap-
proach is currently used for carcinogens, which assumes zero response
only at zero dose.
Several models have been proposed for use in extrapolating low-dose
(lower than the lowest experimental dose) responses to carcinogenic
materials within the test species. The earliest models were so-called
susceptibility models—the probit and logit models. Those models were
rapidly replaced by the multistage model of Armitage and Doll (1961),
which was consistent with the age-related incidence patterns of human
cancer. More recently, other biologically driven models have been
proposed and are being studied extensively, including the Moolgavkar
two-stage model (Moolgavkar and Venzon, 1979) and several pharmaco-
dynamic models. At low doses, the Armitage-Doll model predicts a
linear dose-response curve. Regulatory agencies are using the linearized
multistage model most frequently.
For between-species extrapolations, there are concerns about the ap-
propriateness of extending the results from one species to another. Cur-
rently, extension of results from one species to another hinges on wheth-
er milligrams per kilogram of body weight, milligrams per square meter
of surface area, or some other allometric approach is used. The surface-
area approach (milligrams per square meter) leads to the most risk-aver-
sive "acceptable" doses and is favored by EPA. In addition, findings
from pharmacokinetic studies might assist in extending the results from
experimental species to humans. There are arguments that some carcino-
gens operate through indirect or secondary mechanisms, which are relat-
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-,!! " 'I!!' •• < ?
I ('I:' If
60 CEELS FOR HAZARDOUS SUBSTANCES
ed at times to excess cell proliferation (thus increasing the probability of
mutations leading to cancer) or perhaps to hormone-like activity. Under
those circumstances, it is argued that secondary carcinogens would exhib-
it thresaold-like behavior and for them the uncertainty factor and refer-
ence dose approach would be appropriate.
Exposure limits frequently are established for different purposes, some
of which are not related directly to health. At times, exposure limits
have been set on best-available technology (BAT), analytical chemistry
detection limits, and the flammability and explosive properties of the
chemical. There are exposure limits established for different popula-
tions, such as workers (ACGIH's TLVs, OSHA's PELs,a NIOSH^s
RELs,b AIHA's WEELs,c and the International Labor Organization s
EOLs^), and for military and space applications (COT's EEGLs e
SPEGLs f and SMACs&). The purpose of most exposure limits is self-
explanatory. For those limits that are not health-based, it would be
helpful if the basis for establishing the values were provided. Although
the general procedure used to establish most of the values is documented,
it is not always clear from the documentation on individual chemicals
how the decision was reached and whether the recommendation was
based on human or animal data or on threshold or QRA analysis. Ex-
posed persons need to be better informed about the level of protection
that is provided by the exposure limits, although current right-to-know
laws have improved communication. Those using exposure limits also
should be concerned about the level of protection provided by the expo-
sure limit for different segments of the population, such as pregnant or
older workers. Setting a range of limits might provide a good indication
of the protective value by reflecting the biological variation associated
with responses to exposures. Using a range of values rather than a
single number could also be advantageous in dealing with different expo-
permissible exposure limits.
bRecommended exposure limits.
''Workplace emergency exposure limits.
dEmergency occupational exposure limits.
Emergency exposure guidance levels.
fShort-term public emergency guidance levels.
^Spacecraft maximum allowable concentrations.
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RISK ASSESSMENT 61
sure situations. The range concept needs to be evaluated by all the
groups involved in establishing exposure limits.
Setting exposure limits for workers, military personnel, and other
population groups exposed to chemicals is the major practical application
of the risk-assessment process. Groups such as the ACGIH's TLV com-
mittee and the NRC's COT have been engaged in this effort for over 40
years; the U.S. Food and Drug Administration has been using essentially
the same process to regulate drugs, cosmetics, and food additives for an
even longer period. The basic elements of the process are case-by-case
evaluation, priority of human over animal data, use of the threshold
approach to establish safety, and reliance on good science and judgment
rather than rigid protocols to achieve the optimal balance between bene-
fits and risks.
NONCARCINOGENIC EFFECTS
Traditional Threshold-Based Approaches
Risk assessment for health effects other than cancer traditionally has
been based on the concept that an adverse health effect will not occur
below a certain level of exposure, even if exposure continues over a
lifetime. The existence of a so-called threshold dose is supported by the
fact that the toxicity of many agents is manifest only after the depletion
of a known physiological reserve. In addition, the biological repair-
capacity of many organisms can accommodate a certain degree of dam-
age by reversing toxic processes (Klaassen, 1986; Aldridge, 1986).
Above the threshold dose, however, the homeostatic physiological pro-
cesses that allow compensatory mechanisms to maintain normal biologi-
cal function might be overwhelmed, leading to organ dysfunction. Thus,
the objective of traditional lexicological risk assessment is to establish a
threshold dose below which adverse health effects are not expected to
occur or are extremely unlikely to occur.
The concept of a no-effect level was introduced by Lehman and
Fitzhugh (1954), who proposed that an acceptable daily intake (ADI)
could be calculated for contaminants in human food. That concept was
endorsed by the Joint FAO-WHO (Food and Agricultural Organization
and World Health Organization) Expert Committee on Food Additives
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62 CEELS FOR HAZARDOUS SUBSTANCES
in 1961 and subsequently adopted by the Joint FAO-WHO Meeting of
Experts on Pesticide Residues in 1962 (cf. McColl, 1990). Formally,
the ADl was defined by
ADI = NOEL/SF,
where NOEL stands for the noK)bserved-effect level in toxicological
studies (the highest exposure level at which there are no statistical y _ or
biologically significant increases in frequency or seventy of effec ^s be-
tween the exposed population and its appropnate conttol) and SF repre-
sents a safety factor to allow for variations in sensitivity to fce tes agent
in humans as compared with experimental animals and for variations
within tie human population. Those two sources of vana ^f*™
been accommodated through the use of a 10 x 10 = 100-fold SF as
reviewed by the NRC's Food Protection Committee (NRC, 1970).
In 1977 the NRC's Safe Drinking Water Committee reviewed the
methods that had evolved for establishing ADIs and made several signifi-
cant recomiileiidations. First; the committee proposed that the NOEL be
expressed in milligrams per kilogram of body weight rather than ^milli-
grams per kilogram of diet to adjust for dietary consumption ipatt m.
Second, the committee suggested reducing the trad.tional 100-fold SF to
only 10-fold in the presence of adequate dose-response data derived from
human studies. Third, the committee proposed augmenting the tradition-
al 100-fold SF to 1,000-fold in the absence of adequate toxicity data
uthe use of SFs is now accepted practice in toxicological risk
assessment for health effects other than cancer, the NOEL-SF approach
is subject to certain limitations (Munro and Krewski, 1981) Because toe
ADI is only an estimate of the assumed population threshold dose or the
true no-effect level, it does not provide absolute assurance of safety
(Crump, 1984a). Smaller experiments tend to yield larger NOELs and
hence larger ADIs, than do larger, more sensitive experiments (Mantel
and Schneiderman, 1975). SFs of 10-fold that are used to account for
both inter- and intraspecies variation in sensitivity might not aftord ade-
quate protection in all cases. Therefore, the ADI should not be viewed
as possessing a high degree of mathematical precision but rather as a
guide to exposure limits that are not expected to present serious health
risks to humans.
.-: 11
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RISK ASSESSMENT 63
EPA has recommended using the term uncertainty factor (UF) rather
than safety factor in recognition of the uncertainty associated with the
ADI, and the ADI has been relabeled as a reference dose (EPA, 1988;
Barnes and Dourson, 1988). EPA also introduced an additional modify-
ing factor (MF) to account for specific scientific uncertainties in the
experimental data used to establish the RfD. With the NOAEL defined
as the highest experimental dose at which there are no statistically signifi-
cant adverse effects in the test animals beyond that exhibited by the
control group, the RfD is defined by the equation
RfD = NOAEL/(UF x MF).
In the present context, an adverse effect is defined as any effect that
contributes to the functional impairment of an organism or that reduces
the ability of the organism to respond to additional challenges (Dourson,
1986). When the data do not demonstrate a NOAEL, a LOAEL (lowest-
observed-adverse-effect level) may be used. A LOAEL is defined as the
lowest experimental dose at which a statistically significant adverse effect
occurs.
Five factors may contribute to the composite UF x MF. The factors
are (1) the need to accommodate human-response variability, including
sensitive subgroups; (2) the need to extrapolate from animal data to
humans when human exposure response data are unavailable or inade-
quate; (3) the need to extrapolate from subchronic to chronic exposure
data when the latter are unavailable; (4) the need to account for using a
LOAEL when a NOAEL is unavailable; and (5) the need to extrapolate
from a data base that is inadequate or incomplete. The first four factors
are incorporated into the UF, whereas the fifth, which addresses the
quality of the data base, is represented by the MF. Factors between 1-
and 10-fold commonly are used to account for each of those sources of
uncertainty.
EPA has adapted the oral RfD method to estimate inhalation reference
concentrations (RfCs) to be consistent in setting exposure levels for
health effects other than cancer (EPA, 1990b). The inhalation RfC
method departs from the oral RfD paradigm by using dosimetric adjust-
ments to scale the exposure concentration for animals to a human equiva-
lent concentration.
For health effects other than cancer, the RfD and RfC methods would
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ll1
64 CEELS FOR HAZARDOUS SUBSTANCES
be useful in setting CEELs. In that approach, UFs between 1 "and 10
might be used for each source of uncertainty listed above, depending on
the nature and severity of the adverse effects. For setting CEELs; the
duration extrapolation, if any, would be in the opposite direction of that
indicated in factor 3 above (i.e., it would be from long-term to acute
exposure). An exposure-extrapolation approach such as Haber s law
might be accompanied by the highest UF, 10, if there were considerable
uncertainty as to the validity of Haber's law.
' '''" '
Mathematical-Model-Based Methods
Recently, efforts have been devoted "to mathematical and statistical
modeling of adverse health effects other than cancer, the objective being
to exploit the shape of the dose-response curve and to improve the preci-
sion of the estimates of acceptable exposure levels of a chemical.
Benchmark Dose
The concept of a benchmark dose (BD) has been proposed "as a re-
placement for the NOAEL (Crump, 1984a; see also Kimmel and Gaylor,
1988) The BD is defined as a statistical lower confidence limit on the
dose that produces a predetermined relatively small increase in response
rate (risk) of 1-10%. It is calculated with the aid of a matoematical
dose-response model that is fitted to the experimental data. The BD is
promoted as representing a toxicologically relevant quantity because it is
in the experimental dose range, although the BD does not usually coin-
cide with an actual experimental dose level. The BD makes appropriate
use of the sample size, as reflected in the magnitude of the confidence
limit It exploits the shape (steepness) of the dose-response curve in the
experimental range but does not depend strongly on the particular mathe-
matical model used, because the model is not followed below the 1%
response level.
Crump (1984a) recommended redefinition of the ADI as
"",': , "'. ADI = BD/SF. '; '""",; . "' ' '"
Gaylor (1983) proposed that the BD could be used to determine the size
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RISK ASSESSMENT 65
of the UF needed to achieve a desired low level of risk. He argued that
the relationship ADI = BD/SF can be viewed as equivalent to linear
low-dose extrapolation. EPA's recent publication "Guidelines for Devel-
opmental Toxicity Risk Assessment" (EPA, 1991) recommends that a BD
be calculated to supplement the NOAEL or LOAEL used in determining
an RfD for developmental toxicity (RfDDT). The BD is considered
useful as a comparison and as a possible option to the LOAEL or
NOAEL.
Gaylor (1992) examined 120 bioassay experiments on developmental
toxicity. He showed that the estimated risk of embryo lethality or mal-
formations at the NOAEL ranged from 0 to 4.5% and exceeded 1% in
one-fourth of the cases. Gaylor (1992) suggested that the BD often
would be closer to a true no-effect level than the NOAEL.
Various dose-response models (e.g, probit and logit) can be used to
facilitate calculation of the BD for most binary quantal-response data.
Provided the model gives an adequate description of the data, it need not
have strong biological motivation because it is used almost entirely in the
observed experimental range. Thus, popular mathematical models for
carcinogenesis (e.g., one-hit and multistage) frequently suffice. There
have been, however, special efforts directed at dose-response modeling
for developmental effects (Rai and Van Ryzin, 1985; Faustman et al.,
1988; Chen and Kodell, 1989; Ryan, 1989). Perhaps more important
than the model itself is the overdispersion (greater than binomial varia-
tion) exhibited by the data due to litter effects (Kupper et al., 1986).
Modeling on a probability scale (to characterize risk) is not well devel-
oped for continuous quantitative-response data, such as changes in organ
and body weights, clinical chemistry end points, hematology measure-
ments, and neurotoxic effects. Crump (1984a) suggested modeling such
effects on their untransformed scale without a specific risk connotation.
Application of that method to behavioral-response data was demonstrated
by Glowa (1991). A fundamental approach to dose-response modeling
of quantitative health effects other than cancer on a probability scale was
illustrated by the NRC's Safe Drinking Water Committee (NRC, 1980)
and further developed by Gaylor and Slikker (1990). The process in-
volves four steps. First, a dose-response relationship for the expected
value of a given quantitative end point must be postulated. Second, a
statistical distribution of individual measurements about the dose-response
curve (e.g., normal) must be assumed. Third, an abnormal or adverse
range of the distribution for the given end point must be defined (e.g, a
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.!" Mi:
66 CEELS FOR
SUBSTANCES
, , „, .....
low percentage point, say 0. i% , of the distribution in control subjects).
Finally the previous three steps are combined to calculate the probability
(risk) of an adverse effect as a function of dose. Methods for calculating
statistical confidence limits for this methodology to define BD appropri-
ately remain to be worked out. As with the quantal-response models, the
dose-response model for quantitative responses is intended to improve the
estimation of a BD at the lower end of the data range and is not for
extrapolation below that point.
New Approaches
r t ,„ ,„ i ri|| „ f i I' • • " • 1111 i1 i' •'• » i i
Recent research within EPA has focused on improving risk-assessment
methodology for noncarcinogenic effects. Jarabek and Hasselblad (1991)
have proposed a confidence profile method (CPM) to improve on the
RfC-RfD approach. With the CPM, a posterior probability distribution
is estimated for a concentration of chemical that either (1) produces a
defined response level of a specified quanta! effect, or (2) produces a
defined level of change in a specified quantitative effect. Thus, the CPM
enlarges on the BD concept in providing more information on the uncer-
tainty in the data; it emphasizes total distribution as opposed to single
percentage points (or level of confidence), whereas the BD is associated
with a particular level of confidence (e.g., 95%). Jarabek and
Hasselblad (1991) illustrated that the CPM could be used to combine
effects from different experiments, even if those effects were defined
according to the two approaches expressed in points 1 and 2 above.
Another approach that has been developed by EPA and continues to
be refilled is categorical regression analysis (Hertzberg and Miller, 1985;
Hertzberg, 19^89; Guth et al., 1991). With that approach, health effects
other than cancer are assigned to severity categories based on evaluation
of the reported information and consideration of biological and statistical
significance. The aggregate group of animals at any particular dose level
and exposure duration is classified as giving evidence of a specific sever-
ity of response. The actual response level, if available, is used only to
make the severity determination; it is not used in the regression analysis.
The logistic regression model is applied with the severity code as the
dependent variable and the exposure concentration and duration as the
independent variables. Advantages of the method are that it allows
incorporation of both quanta! and quantitative data and it enables the
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RISK ASSESSMENT 67
simultaneous analysis of many studies. Also, it provides a concentration-
by-duration profile for any desired probability level either as a point
estimate (Guth et al., 1991) or as a confidence limit (Clement Interna-
tional Corp., 1991). That capability is especially useful for acute-expo-
sure durations, since extrapolation according to an approach such as
Haber's law is not required. Finally, if sufficient human data are avail-
able to include in the regression analysis, then interspecies uncertainty
is reduced. One criticism of the approach has been that it sacrifices
information on good studies by reducing the data to severity categories.
Another criticism is that the severity rankings themselves are subjective.
Because the method does not use individual animal data, it cannot be
used to analyze single experiments. Although disaggregation of the data.
from multiple experiments according to different target sites and mecha-
nisms of action might be desirable, it can result in apparently poorer fits
and possibly even failure to converge (Guth et al., 1991). However, a
recent refinement in the method (Clement International Corp., 1991) to
allow the use of individual animal data holds promise for alleviating the
problem.
As. indicated above, the RfD-RfC approach should constitute a first
approach in setting CEELs for noncarcinogenic effects. However, the
more refined mathematical-model-based methods should be given consid-
eration as secondary approaches. In particular, BDs should be calculated
whenever possible for comparison with NOAELs and, optionally, ,used
as substitutes for calculating RfDs and RfCs. Finally, the mathematical
approaches should be reviewed by a committee of experts (such as
COT), which would take them into consideration in deriving CEEL
values.
CARCINOGENIC EFFECTS
Genotoxic Carcinogens
Extrapolation from High to Low Doses
For carcinogenic effects that are known or assumed to be due to direct
genetic events (e.g., mutations), thresholds are assumed not to exist.
Thus, beginning with the pioneering work of Mantel and Bryan (1961),
attempts have been made to estimate carcinogenic risks on a precise
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68 CEELS FOR HAZARDOUS SUBSTANCES
quantitative basis all the way down to zero dose rather than to seek a
positive experimental dose level associated with presumed zero risk, such
as a NOAEL. Because clinical or epidemiqlogical data rarely have been
sufficient to assess the carcinogenic risk to humans at low levels of
chemical exposure, cancer risk assessment largely has involved fitting
mathematical models to animal bioassay data and extrapolating those
models downward to predict risks at doses well below the experimental
liin
The probit-log-dose model used by Mantel and Bryan (1961) was a
carryover from the tolerance distribution models used for analyzing
dose-response mortality data from bioassays. After initial acceptance,
the probit-log-dose model was replaced by mathematical models derived
on the basis of presumed mechanisms of carcinogenesis. The mechanis-
tic model most frequently used as the underlying basis for low-dose
extrapolation is the multistage model of Armitage and Doll (1961). A
historical perspective on the evolution of the multistage model has been
provided by Whittemdre and Keller (1978). According to the multistage
theory, a malignant cancer cell develops from a single stem cell as a
result of a number of biological events (e.g., mutations) that must occur
in a specific order. The model predicts that the age-specific cancer-
incidence rate should increase in proportion to age raised to a power
related to the number of stages in the model, and it provides a good
description of many forms of human cancer, if two to six stages are
assumed.
When the transition between stages in the multistage model are ex-
pressed as linear functions of dose, the model is characterized by
low^dose linearity, a property generally assumed for chemical carcino-
gens mat interact directly with genetic material. Low-dose linearity is
supported by theoretical considerations in carcinogenesis (Krewski et al.,
1989) and by the linearity of DNA binding observed at very low doses
with a number of chemical carcinogens (Lutz, 1990). Although low-dose
linearity might not be applicable in cases in which carcinogenesis occurs
by certain nongenotoxic mechanisms (e.g., as a result of tissue toxicity),
its assumption is widely made in regulatory applications of low-dose risk
assessment in the absence of clear information to the contrary (OSTP,
1985). Indeed, if the mechanism of the carcinogen whose dose response
Is being modeled is assumed to be the same as the background mecha-
nism of tumor induction (i.e., the background is dose-additive), then
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RISK ASSESSMENT 69
even for nonlinear dose-response models that do not exhibit low-dose
linearity from mechanistic considerations, the dose response will ap-
proach linearity at low doses (Crump, et al., 1976; Hoel, 1980; Krewski
and Van Ryzin, 1981).
It has become common policy among regulatory agencies in the United
States to extrapolate downward in some linear fashion from an upper
confidence limit on excess risk (FDA, 1985; EPA, 1986). That proce-
dure generally is followed by COT for setting emergency and short-term
emergency guidance levels (EEGLs and SPEGLs) for chemicals of inter-
est to the U.S. Department of Defense (NRC, 1986b). Perhaps the best
known procedure is what EPA calls the linearized multistage (LMS)
model (EPA, 1986). With this procedure, an upper confidence limit,
#!*, is obtained on the coefficient of the linear term that appears in the
exponent of a generalized version of the multistage model (Crump,
1984b). The value
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I
70 CEELS FOR HAZA&DOUS WJ3STANCES
: .„ 'I
and PO represents a specified "acceptable" excess risk (e.g., 10"4 to KT6),
then the corresponding dose, d^, is found by solving
Thus, PO is an upper bound on excess risk corresponding to dose d0, and
dQ represents a lower bound on dose for risk level P0.
Some chemical carcinogens are known to require metabolic activation
to exert their effects. If metabolic activation could be characterized
adequately in terms of a suitable pfaarmacokinetic model, then the dose
delivered to the target should be used in place of the administered dose
for puiposes of dose-response modeling and low-dose extrapolation (Hoel
et al., 1983; Starr, 1990). In general, the use of delivered rather than
administered doses might be expected to lead to more accurate predic-
tions of carcinogenic risk (NRC, 1987). Greater accuracy might not
occur, however, if complicated pharmacokinetic models with many pa-
rameters, each subject to uncertainty, are used for tissue dosimetry
(Portier and Kaplan, 1989).
Extrapolation from Long-Term
To Shprt-Term Exposures
" •-''; .' :, : •:' :". »,:'. v ",.fc,: . . ' ! ! ^ I,,",,; ;, >;,(:• j ^ £: ' ••
Most bioassay data suitable for low-dose extrapolation reflect continu-
ous exposure to a carcinogen, necessitating the translation of risks calcu-
lated for continuous exposures to risks associated with short-term expo-
sures. Crump and Howe (1984) developed methods for applying the
multistage model to carcinogenic risk assessment when exposure to a
carcinogen is of short duration. Their results have been adapted by COT
for setting EEGLs and SPEGLs (NRC, 1986b). Kodell et al. (1987) and
Murdoch and Krewski (1988) have studied the multistage model with
respect to accommodating age at the time of exposure. The results indi-
cate that early exposures will be of greater concern than late exposures
when an early stage iii the carcinogenic process is dose-dependent.
Conversely, late exposures have higher risk than early exposures when
the late stage is dose-dependent. In the context of the classical multistage
model, the potential increased excess risk from exposure for a fraction,
/, of a lifetime at a given dose rate, d, will never be more than k times
11
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RISK ASSESSMENT 71
the excess risk from full lifetime exposure at dose rate^f, where k is the
number of stages in the model (Kodell et al., 1987).
The determination of CEELs for genotoxic carcinogens generally will
require extrapolating risks estimated for long-term exposure conditions
to short-term situations. The methods proposed by Kodell et al. (1987)
on the basis of the multistage model can be used to set those CEELs.
Suppose that data from a long-term animal bioassay or human epidemiol-
ogy study have been used to calculate an average daily lifetime exposure
level, d, of a particular chemical, corresponding to an "acceptable"
lifetime excess carcinogenic risk (e.g., lO"6). Assume that d has been
estimated using the linearized multistage model along with any available
pharmacokinetic information on target-tissue dose level (and interspecies
conversion, if necessary).
Suppose that the carcinogen of interest affects only a single stage of
a multistage process and that there are k stages in total. Let / (i — 1, 2,
..., K) denote the index of the stage that is dose-related. From the results
of Kodell et al. (1987), it can be shown that an individual exposed to
constant daily dose D from t0 to ra will have the same excess risk as an
individual exposed at constant daily dose d from birth to age t, where
and t denotes the time when excess risk is evaluated (usually the average
human lifetime).
In the case of near-instantaneous exposure (e.g., single-day duration
or less), the total dose D at time tQ that will give equivalent risk to that
of exposure at constant daily dose d from birth to age t is
As an example of how large an adjustment might be made, consider a
three-stage process with only the first stage dose-related, and assume that
-------
r i :,:4'
-I i
72 CEELS FOR HAZARDOUS SUBSTANCES
a 70-year lifetime exposure at dose d is collapsed to an instantaneous
(1-hr) exposure D at age 20. Then, D = 400,624 d. For an 8-hr expo-
sure, D = 50,078 d. ,_. * -r-
The latter expression and others like it derived on the basis of specific
multistage models can be useful for setting 1-hr and 8-hr CEELs, but
generally information will be insufficient to postulate the total number of
stages in the cancer process and the stages that are dose-related. In such
situations, the limiting case of the time-weighted-average dose can be
used whereby instantaneous dose D at time f0 having equivalent lifetime
excels carcinogenic risk as daily dose d up to time t is approximated by
. • • • • ...... -.. . ,. - • *, .
As shown by Kodell etal. (1987), the actual risk at dose D will not
exceed k times the presumed risk. In the above example, the approxima-
tion gives D = 613,200 d for a 1-hr CEEL and D = 76,650 d for an
8-hr CEEL. . . '
If prudence dictates conservatism, then the above approximation to L>
may be reduced by a factor of 2 to 6 (the number of assumed stages in
the multistage model), according to the result of Kodell et al. (1987). In
determining cancer-based CEELs, extrapolation from a daily lifetime
exposure level to near-instantaneous exposure levels should follow this
approach on the basis of the multistage theory of carcinogenesis, unless
there is evidence to the contrary.
Nongenotoxic Carcinogens
Extrapolation from High
To Low Doses
• i ; • ,",/:„ ii , :t f • '"'. •('',' ;: rj;»;. > 1;:
Dissatisfaction with the linearized' multistage procedure for (perceived)
nongenotoxic chemical carcinogens has prompted the search for an alter-
native method for establishing exposure levels. Much attention has
focused on a stochastic model that includes only two genetic events but
allows for tissue growth and for proliferation of cells that have experi-
enced the first of these events (Moolgavkar and Venzon, 1979;
Moolgavkar and Knudsori, 1981; Greenfield et al., 1984). This model
-------
RISK ASSESSMENT 73
has various descriptions: a two-stage clonal expansion model, a stochas-
tic birth-death-mutation model, and an initiation-promotion-progression
model. Initiation refers to the occurrence of the first genetic event that
predisposes a cell to malignancy, and progression to the occurrence of
the second genetic event that transforms the initiated cell to a malignant
one. Promotion generally refers to the expansion of the pool of initiated
cells. Carcinogenic agents can be classified as initiators, promoters, or
progressors according to the components of the model that the agents are
presumed to affect (Moolgavkar, 1986; EPA, 1987). Without the pro-
motional component, this model reduces to the ordinary Armitage-Doll
two-stage model (Armitage and Doll, 1961). Recently, Bogen (1989) has
investigated the two-stage clonal expansion model in the context of a
general class of cell-kinetic multistage models.
Application of the two-stage clonal expansion model in risk assess-
ment for a subclass of nongenotoxic carcinogens (i.e, putative tumor
promoting agents) has been investigated on a limited basis (Thorslund
and Charnley, 1988). When the background tumor incidence arises from
a mechanism that is independent of the promoting agent, then the
dose-response model can be nonlinear at low doses and can have a zero
slope at zero dose; but if additive background is assumed, models for
both pure initiators and pure promoters show low-dose linearity (Portier,
1987). The ability to distinguish between initiation and promotion effects
by using only standard bioassay data is poor, even for independent back-
ground mechanisms; with zero background risk, the ability to distinguish
improves (Portier, 1987). Thus, the use of the two-stage clonal expan-
sion model requires a good understanding of the biological processes.
Before the model can be recommended for routine application, its statisti-
cal properties require further study, particularly with respect to predic-
tions of risk at low doses.
For nongenotoxic carcinogens that act to induce cancer by secondary
mechanisms other than tumor promotion (e.g., hormonal mechanisms),
no general class of mathematical models has been proposed. It has been
suggested that thresholds do exist for certain secondary carcinogens. In
fact, even for certain tumor promoters (i.e., those that cause cell prolif-
eration as a result of toxic tissue injury), there might be a threshold dose
below which cellular proliferation, and hence cancer, will not be induced
(EPA, 1987). For nongenotoxic carcinogens, a procedure other than
linear low-dose extrapolation can be used to establish cancer-based
-------
• ' ISIill1,)'"!1!!!"'
.', !,. ' !J'l'l|:li, '; :flv :,ll|lli. >!,"',,"Hi ,11, i;!»||' i|
it'
IK
74 CEELS FOR HAZARDOUS'SUBSTANCES
CEELs, if known mechanisms of action so indicate. In some cases, risk-
assessment techniques that are used to set acceptable levels for
honcafcinogens may be applied (e.g., BD/SF), provided knowledge of
the mechanism of action is sufficient to provide a high degree of confi-
dence with respect to the existence of a threshold. For a putative tumor
promoter (with or without presumed threshold), the pure-promoter model
may be used from the class of initiation-promotion-progression models;
however, the importance"of knowing the underlying biological processes
when using the pure-promoter model cannot be overemphasized.
Like the multistage model, the two-stage model has been studied with
respect to accommodating age at time of exposure (Chen et al., 1988;
Murdoch and Krewski, 1988). Recall that for a fc-stage multistage mod-
el, the potential increased excess risk from exposure for a fraction,/, of
a lifetime at a given dose rate, d, will never be more than k times the
excess risk from full lifetime exposure at dose rate fd. With the
two-stage clonal expansion model, however, the increased risk can be
substantial when the agent of interest greatly increases the proliferation
rate of the initiated cell population, thus, for a tumor promoter whose
mechanism of action is adequately described by the two-stage clonal
expansion model, care must be taken riot to underestimate risk when
extrapolating from long-term to short-term exposures.
When nongenotoxic carcinogens act through secondary mechanisms
having presumed thresholds, acceptable exposure levels are calculated for
long-term exposures, but those levels might not be useful for estimating
acceptable exposure levels for short-term exposures.
-------
U Development of CEELi
CEELs were defined in Chapter 1 as a series of concentrations that
indicate certain toxic effects that might be observed in some members of
the general population exposed to an agent over a specified time. The
exposure is assumed to result from the accidental release of the agent.
CEELs, which are defined in Table 1, delineate concentrations above
which exposure may lead to death or life-threatening effects (CEEL-3),
disability requiring rescue or treatment (CEEL-2), or discomfort that
does not cause incapacity to take appropriate actions (CEEL-1). Below
CEEL-1, persons may detect the presence of the agent, which may lead
to complaints, inquiries, or anxiety, but for the most part they will not
suffer any direct adverse effects on their health. CEELs for 1-8 hr are
likely to apply to most emergency situations, but for most situations,
only the 1-hr CEEL will be needed because few exposures will exceed
1 hr.
CEELs are designed to protect almost all people in the general popula-
tion from the toxic effects of an agent; thus, extrapolation factors (i.e.,
safety or uncertainty factors) will be incorporated into setting CEELs.
Although CEELs are designed to protect "sensitive" individuals, some
hypersusceptible individuals might not be protected, even at CEEL-1.
Because emergency exposures might occur once in a person's lifetime,
the acute toxic effects of the agent are of primary importance.
The process of setting CEELs involves issues of data collection, evalu-
ation of the adequacy of data bases, presentation and treatment of data,
use of other standards and guidelines in setting CEELs, and calculation
of CEELs on the basis of key studies chosen during the process.
The sources and types of data useful for setting CEELs are discussed
in Chapters 3 and 4. It is important to emphasize that those who set
CEELs should use primary sources of data; secondary sources should be
used only if an original report cannot be obtained. Documentation used
by other organizations to set standards and guidelines for the agent (e.g.,
75
-------
IMS*
ii I,!),
76
CEELS FOR HAZARDOUS SUBSTANCES
Kit En
TLVs PELS, IDLHs, EEGLs, SPEGLs, and STELs) also should be
available for use. Although those values might not be extrapolated di-
rectly to set CEELs, the documentation provides a useful checklist ot
studies to ensure that all pertinent data have been identified.
Before starting the process for setting CEELs for a particular com-
pound, the practicality of setting levels and the need for them should be
considered. Some very toxic materials do not warrant CEELs because
they do not pose the types of hazards that are addressed by CEELs.
Those materials might never be used or stored in significant quantities,
or they might not be liable to sudden, massive airborne generation be-
cause of their physical properties. Alkali hydroxides, for example, are
highly corrosive in aqueous solution, but the possibility that they or their
solutions could become airborne in large amounts should be considered
carefully before setting CEELs for them. Similar considerations might
be appropriate for organic liquids with very high boiling points or low
vapor pressures.
EVALUATION OF DATA
t'.lllll ii,
Qualitative Review
The aim of the CEEL methodology is to establish relationships be-
tween airborne concentrations of an agent and specific human health
effects Data will be evaluated from many sources and disciplines of
varying adequacy. The CEEL team that evaluates data should include
reports from scientific specialties encompassing the range of disciplines.
The reviewers will be required to judge the adequacy of the experimental
procedures with respect to the following:
• Description of the methods.
• Statistical analysis .
• Atmospheric analysis: estimated, nominal, or actual.
• identification of NOELs, NOAELs, LOELs, or threshold levels.
• Dose-response characterization with quantitative grading of respons-
•M- ' ....... . ......... ......... - •' ....... - - ' . ..... + ......... I ......... ' ............... ..... ............ I ' ...........................
es.
• Clearly defined elements of exposure.
• Controls comparable with test subjects in all respects except treat-
ment Variable.
II I
!, 'Jliilill1, i
-------
DEVELOPMENT OF CEELS 77
• Relevant toxicity end points measured.
• Conclusions justified by data in report and consistent with current
scientific understanding.
• Study conducted according to sound scientific practices.
The key studies for setting CEELs will be those studies with quantita-
tive data that can be extrapolated to provide numerical CEELs that are
protective of human health. Inherent in this process is evaluating the
weight of evidence of an agent's hazard to humans. CEELs cannot be
set without sufficient data from in vivo studies that provide quantitative
dose-response information. Thus, most in vitro studies and nonmam-
malian in vivo screening studies are of limited use in setting CEELs
except for hazard identification. The two types of studies that will pro-
vide useful information for setting CEELs are epidemiological, clinical,
or case studies in humans and experimental studies in mammalian ani-
mals. Information on the structure-activity relationship of closely related
chemicals can be used to support CEELs; however, that information
alone is seldom sufficient for setting CEELs.
It is obvious that human data should be emphasized in the setting of
CEELs. Studies that show toxic effects of acute or short-term exposure
in humans or animals might be more relevant than long-term studies that
show only subtle effects. Animal data derived from well-conducted
studies might be preferred over human studies that are not appropriately
conducted. If no acute or short-term data are available, then data from
subchronic or chronic exposures must be used, if available. Although
short-term exposures are more likely to give pertinent information, both
immediate and delayed effects of exposure should be considered. Except
for reproductive or developmental effects, however, delayed effects of
short-term exposures rarely are determined in animal studies or available
from human experience.
Studies of human experience and studies in animals have specific
qualities that should be considered in assessing their relevance to the
setting of CEELs.
Human Studies
The use of data from human experience removes one of the major
uncertainties involved in setting CEELs—i.e., the need for cross-species
-------
M'f li id ........... •" ,- }'!,.! ; ..... '!-
Siiii'ist;11;;1 ........ :;•
j. : i si
78 CEELS FOR HAZARDOUS SUBSTANCES
extrapolation of data. Nevertheless, adequate data from short-term expo-
sures of humans rarely are available. Although acute effects in humans
are sometimes observed'as a result of accidental exposure, quantitative
exposure data are seldom available: The most useful data in setting
CEELs are (1) the quantitative relationships between dose and response
for certain cohorts, and (2) the identification of sensitive members of the
population. The specific requirements germane to studies of human ex-
perience have been reviewed by EPA (1990b).
Human data also might include clinical data and case reports. Clinical
studies might contain exposure-response information that can be used in
estimating health-related effects. Individual case reports of adverse ef-
;^~ duetto, a specific agent can provide some help in evaluating the
potential risk from exposure.
Si1 K'1'11,1 , ii
Criteria for Identifying
Sensitive Individuals
it I!!'*
As CEELs are designed to protect most people in the general popula-
tion, information from human studies, including epidemiological and
clinical studies, can help identify susceptible people so that appropriate
extrapolation factors can be incorporated into the calculation of CEELs.
The criteria recommended by EPA should be considered when examining
studies to identify sensitive or liypersusceptible individuals (EPA,
1990b).
Establishing Causality
Statistical methods used in epidemiological studies cannot establish
proof of a causal relationship but can define an association with a certain
probability. Determination of the causal significance of an association
is a matter of judgment that goes beyond any statement of statistical
probability. To assess the causal significance of an air contaminant with
respect to a health effect, a number of criteria, none of which is patho-
gnomonic, must be used. The criteria have been provided by EPA
(EPA, 1990b).
-------
DEVELOPMENT OF CEELS 79
Animal Studies
When, as is frequently the case, the data base lacks appropriate and
adequate information on the effects of an agent in humans, CEELs must
be based on experiments conducted in animals. Even in cases in which
apparently adequate human data are available, the animal data base is
useful as supportive evidence to confirm, if possible, that the critical
toxicities have been identified for each CEEL level and that the numbers
generated from the human data are not widely disparate compared with
the animal data. The experimental animals most often used for toxicity
studies include rats, mice, guinea pigs, hamsters, dogs, and non-human
primates. Although studies in animals are usually conducted under con-
trolled exposure conditions on relatively homogeneous populations, they
present the risk assessor with uncertainties when extrapolating dose and
exposure regimens to the human situation. Some of the factors that
influence the interpretation of animal studies and the extrapolation of
quantitative data to humans are discussed here.
Quality of Animal Studies
Evaluation of the quality of toxicity studies requires consideration of
factors associated with each study's hypothesis, design, execution, analy-
sis, interpretation, and reporting. Guidelines for assessing animal studies
have been provided (Society of Toxicology, 1982; Muller et al., 1984;
NRC, 1984; James, 1985; Lu, 1985).
An ideal animal study addresses a clearly defined hypothesis, follows
a carefully prescribed protocol, adheres to good laboratory practices, and
includes appropriate analysis to support its conclusions. EPA's Good
Laboratory Practice Standards (EPA, 1983a,b) are designed to ensure the
integrity of data used in hazard evaluation. Studies that do not precisely
follow those guidelines might still be judged adequate if it is determined
that the deviations are not important. The types of deviation, their mag-
nitude, and their potential interaction with other variables must be as-
sessed (NRC, 1984). For example, a study might be judged adequate
despite an insufficient number of test animals as specified by the appro-
priate reference protocol guidelines if the results are so definitive that the
-------
80 CEELS FOR HAZARDOUS SUBSTANCES
addition of more test animals almost certainly would not affect the con-
( 'I .ili'l!' „ » • ' n.v.Vt
elusion.
U&iUlJt. . T J U TTT) A
Ideally, an acceptable study should meet the criteria outlined by EPA,
which fundamentally represent good scientific practice (EPA,
Appropriateness of Species
As a Model for Humans
Selection of an animal model and key study for each CEEL level
depends on the depth of understanding of the human disease syndrome,
adverse effect, or indicator of toxicity selected as the criterion for evalua-
tion Even though a particular animal species shares a number of simi-
larities with humans in respiratory-tract physiology, dissimilarity in
crucial factors make it inadequate as a model. This subject area has been
reviewed (Hakkinen and Witschi, 1985) and various mammalian species
(rat hamster, and rabbit) have been identified as appropriate species tor
extrapolation from several perspectives. Other reviews that discuss the
current limitations and need for the development of animal models as
surrogates for humans inclule thBse of Reid (1980), Slauson and Hahn
1980), ^ Calabrbse (1983)_ .
For agents with toxicological outcomes that depend on degree to which
they are metabolized, selection of the most appropriate animal species is
contingent on proper evaluation of the numerous interspecies differences
with respect to metabolism. The studies of Plopper et al. (1980) suggest
that animal species differ widely in the metabolizing potential of the
respiratory tract. The metabolizing potential is much greater in hamsters
and rabbits than in monkeys and rats. Interspecies differences in meta-
bolic pathway might serve as a basis for selecting one study for CEEL
derivation and rejecting another. .
Appropriate animal model selection might be contingent on pathologi-
cal Identification of early changes in a species that are consistent with the
human syndrome; for example, a clear choice of an appropriate animal
species has not been established for emphysema (Snider et al., 1986).
People with deficient antitrypsin blood concentrations because of a genet-
ic defect are characterized as a high-risk subgroup for emphysema.
Hamsters which have low antiprotease concentrations, might represent
a good choice, but primates, which have antitrypsin profiles comparable
to humans, might offer a better model.
'..' "I!*
-------
DEVELOPMENT OF CEELS 81
Species-dependent variables in mucous production and secretion also
are factors to be considered in selecting an appropriate animal model for
inhalation assays.
Establishing Relevance
Of Animal Studies
Identification of the most appropriate study (key study) for each toxici-
ty end point that will be used to set a CEEL is the end result of an inter-
pretive process that examines all aspects of each study in the data base
from study design, performance, and interpretation to relevance to the
human situation. Although selecting the most sensitive species might
have the advantage of affording the greatest degree of protection, it could
lead to undue alarm and unnecessary evacuation of a population. The
species most sensitive to an agent might not be as toxicologically relevant
as other species for extrapolation to humans; therefore, all available
information on the agent must be considered in choosing the key study
for each toxic end point.
Determination of validity and relevance involves evaluating a number
of factors, including all elements of exposure definition (dose, duration,
administration route, and physical and chemical characterization of the
chemical), relevance of the dose level tested to the anticipated human
exposure level, nature of the effect, and similarities and differences
between the test species and humans (e.g., deposition, absorption, reten-
tion, clearance, and metabolism).
Animal studies are conducted with a variety of exposure scenarios, in
which the magnitude, frequency, and duration of exposure might vary
considerably. Studies might use different durations (acute, subchronic,
and chronic) and schedules (single, intermittent, and continuous) of
dosing. All the studies contribute to the hazard identification portion of
the risk assessment. Special consideration should be given to those
studies in which exposure duration is appropriate for the CEEL to be
determined.
Exposure concerns (dose and duration) are compounded when the risk
assessor is presented with data from several animal studies. An attempt
to identify the animal model most relevant to humans should be made on
the most defensible biological rational (e.g., morphology, comparable
metabolism, and pharmacokinetic profiles). In the absence of such a
-------
""il!"1!" " ,:!!;!
ill11 I»"! '" * • »'"
82 CEELS FOR HAZARDOUS SUBSTANCES
model the most sensitive species (i.e., the species showing a toxic effect
at the lowest administered dose) should be used. The selection process
is more difficult when the animal data are for various exposure routes,
esfieciaily when the routes are different from that in the human situation
bfconcern. For example, if data are insufficient for the inhalation route,
CEEL reviewers might have to use toxicity information obtained from a
less relevant exposure (unless convincing contrary evidence exists).
However, consideration must be given to the differences in pharmacoki-
netics resulting from different exposure routes of the chemical.
Bioavailability of the chemical administered is another important factor
for consideration in the evaluation of dose. Extrapolation from the
noninhalation route might be inappropriate when acute high-level expo-
sure primarily affects the pulmonary system.
'" di*:"^',;<•;> if-.V ,':- '•' .••• .•••.:•{" •'. ,'jy
Appropriateness of Data
• " '" ' \;''* ;i:;'"'i": !fpV' Extrapolation
Several factors will contribute to the evaluation of the complete data
base as supporting the hazard assessment and the choice of the key stud-
ies for quantitative extrapolation. Those factors include the consistency
of findings among studies in humans or animals and the biological plausi-
bility of the associations.
Consistency of Findings Among
Studies in Animals and Humans
The strength of an association between exposure and response is in-
creased if the specific response to an exposure is seen as follows:
• In a single species having different dosing regimens.
• In a number of tests in a single species.
• In a single species having different routes of exposure.
• In different species of animals.
• In studies of animals and humans.
• In different epidemiblogical studies of humans.
."111"..
I!,!;, •!„
-------
DEVELOPMENT OF CEELS 83
In general, the strength of the association increases as one goes down
the list.
Biological Plausibility
Biological plausibility is derived from an examination of the available
data. The following increase confidence in the association between the
data and the effect:
• Dose-response gradient is observed in epidemiological or animal
tests.
• Short-term in vitro or in vivo tests indicate plausible mechanism of
action.
• Biotransformation and pharmacokinetic studies indicate constancy
of mechanism across species or show that mechanisms are of types gen-
erally found among all relevant species.
• Pharmacodynamic studies or organ pathology indicates constancy
of target organs and toxic mechanisms across species.
• Structure-activity relationships suggest constancy of action for simi-
lar structures.
Consideration of Dose-Response Relationship
The primary purpose of the evaluation of experimental and epidemio-
logical evidence is to provide quantitative data that can be extrapolated
to humans exposed to the agent under the hypothetical conditions of an
accidental release of the agent. The data needed to define the three
CEEL levels ideally would show a gradient of effects with increasing
dose. The primary objective in the case of threshold toxicity is to identi-
fy NOELs, NOAELs, LOELs, and frank-effect levels (FELs) for the
most sensitive end points and to set CEELs on the basis of those values.
The most useful studies are experimental animal or human studies that
have employed a number of dose levels, have determined a level at
which there is no effect, and have characterized the toxic responses at
several doses above NOEL. For epidemiological studies, the best studies
-------
I1IH1II' •
en <\
'./'iiiiiii'ioi;"1".;!!!,;.]
pi 4r' / : ' ll
I", +"iil" If"1 1' I III1,.
84 CEELS FOR HAZARDOUS SUBSTANCES
have sufficient exposure monitoring data and a large enough cohort to
grade responses over a range of exposure levels. It should be noted,
however, that the procedure for setting CEELs for threshold toxicity
might involve mathematical modeling rather than use of NOAELs or
LOAELs (see discussion of benchmark dose below).
Also of importance in setting CEELs is the question of whether the
severity of a toxic response is related to the cumulative dose—i.e., the
response is a function of concentration and time—or to the absolute
exposure concentration—i.e., the response is independent of time. One
relationship that describes the time dependence of severity of response
is Haber's law, which states simply that the severity of effect varies
directly as the product of concentration times exposure time. That is, of
course, most often an oversimplification of the true relationship, but if
some information can be obtained on the relationship between severity
of response and concentration and time, that information should be incor-
porated into the evaluation of the data.
MI'1,1.
i. ffVS"! 'i 1
: . -. '. ' ;, i- i .•,' • ' • r - .:'' » PkESENf ATl<5N "AND '
;• .• ,; " ' TREATMENT OF DATA ............ ' _' ........
;. • '''. | 1 1 '' ii iV . , ' ,;' "'"I* i ij'iwi1'- K','!i i
Data derived from literature review should be summarized to expedite
assimilation of the material by those involved in setting CEELs. The
following types of data should be presented:
• t". '
i • ii i ii. i ni illl ........ • ' ,,,,,, ,j , i ^ ^ , , i ii
• Human experience: epidemiology and case histories.
• Animal toxicology .
• Toxicologicai properties of structurally related compounds.
• Current exposure standards and guidelines.
Checklists for 'the presentation of data from human epidemiological
and case-history studies and from animal experiments are presented
below.
Human Experience Data
,i' '' I,*! M '!' .' • .. fiam ,• ii .if ', - •«, ,!• i • ,111 , n '
Data should include information on groups who might be more suscep-
-------
DEVELOPMENT OF CEELS 85
tible to chemical exposure—e.g., the elderly, the young, the pregnant,
and those with minor illnesses compatible with participation in normal
daily activities. In addition, subjective response information should be
included, such as vapor concentrations that caused eye or respiratory-
tract discomfort or produced objectionable odors. The following infor-
mation should be presented:
• Odor data (include odor threshold and odor description).
• Toxicity data (include data on humans who were exposed to known
concentrations).
• Clinical-, workplace-, and accidental-exposure experience.
• Epidemiological data.
Animal Data
Each animal study should be summarized and critically reviewed with
respect to adequacy and relevance. Duration, route, species, sex, and
effects at each dose should be given, and NOELs, NOAELs, and LOELs
should be reported for important effects. Negative and positive findings
should be included. Lengthy detail is not required for studies judged to
be of minimal relevance for establishing CEELs. The following types
of animal studies should be reviewed:
• Acute toxicity (include studies of less than 5-day exposures), e.g.,
oral LD50s and eye and skin irritation (provide exposure duration for all
LC50s; state whether nominal or actual concentration; include pathology
if available; give lowest lethal concentrations and highest nonlethal con-
centrations if available).
• Subacute toxicity (include 5- to 15-day exposure studies).
* Subchronic toxicity (include 15-day to 6-month exposure studies).
• Chronic toxicity and carcinogenicity (include studies of exposures
of greater than 6 months). Provide regulatory classifications based on
chronic effects (e.g., EPA and OSHA) and other agency classifications
(e.g., IARC and ACGIH). Mathematical models adopted by NRC might
be useful in assessing the risk of developing cancer from a single expo-
sure (see Chapter 5).
• Reproductive and developmental toxicity (include developmental
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86 CEELS FOR HAZARDOUS SUBSTANCES
toxicity and reproductive studies). Provide regulatory and other classifi-
cations based on those effects (e.g., EPA).
• Genotoxicity (include short-term In vivo or in vitro studies).
• Metabolism and pharmacokinetics.
Other Air Standards and Guidelines
As previously noted, the use of standards and guidelines recommended
by other governmental and private-sector organizations to set CEELs is
not recomniended. Nevertheless, the values published by these organiza-
tions along with the rationale for the limit set should be summarized and
presented to provide some indication of how those organizations consid-
ered the data and to assure that the CEELs set are not widely divergent
from the existing values when proper adjustment is made for differing
contexts: Some of the standards arid guidelines are presented in Table
3.
TRANSLATION OF ANIMAL
DATA TO HUMANS
Interpretation of data derived from animal experiments and their ex-
trapolation to humans require experienced scientific judgment in a variety
of disciplines. The evaluation should consider the conditions under
which the data were obtained and, in particular, their relevance to the
conditions of human exposure. In addition, the evaluation should consid-
er the similarity of the test species to humans with respect to target or-
gans, metabolism, and disposition of the agent.
Development of CEELs requires that animal data be extrapolated
quantitatively to humans. As noted earlier, the lexicological, biological,
and pharmacokinetic characteristics of the species most representative of
humans should be used to set CEELs if adequate data on that species are
available. If data on that species are not available or if the most appro-
priate species cannot be identified from the data, it is prudent to use data
from the most sensitive species to set the appropriate limits.
-------
87
;Ub ana uuiuelmes
1
§
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ss CEELS FOR mz^ous SUBSTANCES
THRESHOLD TOXICITY
ii i i 1 1 i
1 ' 'I V i II i
Use of Uncertainty
And Modifying Factors
ill II II 'i ...... !/." •. nL ini'i f, ,); 1,1 ill 'ill,!' '' i",!!!';/1
When appropriate for threshold toxicity (usually noncarcinogens and
nongenotoxic carcinogens), uncertainty factors should be applied to
NOAELs (or NOELs, LOELs, LOAELs, FELs) or to benchmark doses
if they are available. In the application of uncertainty factors, a factor
must be incorporated that approximates the likely range of susceptibilities
among humans. People at increased risk include those at either extreme
of age those with poor nutritional status, those with preexisting diseases
such as certain heart diseases, that are fairly widespread in the general
population, those with enlancid hereditary susceptibility, or those who
are Overexposed because of unusual physical exertion. Nevertheless, the
intent of CEELs is not necessarily to provide absolute assurance that
everyone at risk will be protected under all circumstances, and thus the
uncertainty factors should be chosen with the understanding that a few
hypersusceptible persons might not be protected. In the case of CEEL-Z
uncertainty factors must be balanced against the inherent risk associated
with" actions, such as "evacuation, that might be taken as a result of appli-
cation of (JEELs; Large uncertainty factors, which might be appropriate
with chronic exposure limits, such as PELs, might be associated with
increased risk to the community in the application of CEEL-2.
In addition, the CEEL is derived from the NOAEL through the appli-
cation of uncertainty factors that reflect various specific recognized un-
certainties in extrapolating from the animal or human studies. An addi-
tional modifying factor also is applied that reflects professional judgment
of m"e entire data base available on the specific agent.
The CEEL can be derived from the NOAEL (or other effect level) as
follows : ........... „ ...... ' ...... \ ..........................
. :': . 'i' ; • ' ' ..-•' ." ;; CEEL" ="NOAEL/(UF x MF), ' ' "_' _ ^ '' ^ ' '"
• , j •; ...........
where UF is the product of uncertainty factors described below, and MF
is the modifying factor. ,
Generally, a 10-fold UF will be applied when extrapolating from valid
experimental' results of studies involving appropriate exposures to aver-
-------
DEVELOPMENT OF CEELS 89
age healthy humans or to experimental animals. That factor is intended
to account for the variation in sensitivity among the human population.
If data are available on a sensitive subpopulation, the factor might be as
low as 1. .
An additional 10-fold UF generally is applied when extrapolating from
valid results of studies on experimental animals because results of studies
of human exposure are not available or are inadequate. That factor is
intended to account for the uncertainty in extrapolating animal data to
humans.
An additional 10-fold UF may be introduced when deriving a CEEL-1
from a LOAEL instead of a NOAEL or a CEEL-2 or CEEL-3 from
appropriate LOAELs or FELs. That factor is intended to account for the
uncertainty in extrapolating from a LOAEL to a NOAEL and so forth.
An intermediate factor may be applied when the data are very strong and
relevant animal-to-human extrapolation factors have been derived, when
limited data are available for a sensitive human subpopulation, and when
dosimetric adjustments have been made on the basis of species consider-
ations. The intermediate factor is usually 3—i.e., the geometric mean of
1 and 10, rounded to one significant figure.
Professional judgment must be used to determine another UF, the MF,
which is more than 0 and less than 10. The magnitude of the MF de-
pends on the professional assessment of the scientific uncertainties of the
study and data base that are not explicitly treated by the UF (e.g., the
completeness of the overall data base and the number of species tested).
In selecting UFs for deriving CEELs, it is important to recognize that
the intent is to avoid unnecessary conservatism that might result in expo-
sure levels with little or no biological plausibility. An example of that
would be the application of several levels of uncertainty to a concentra-
tion of an agent that produced irritation in an appropriate and adequate
animal inhalation study. Although it is desirable to consider the uncer-
tainty around varying sensitivities and extrapolations, it is not practical
to establish an inflexible system of UFs that simply become multipli-
cative in their application. Therefore, UFs must be determined case by
case. Of course, the determinations would be associated closely with the
quantity and quality of the data and the end points in question.
It has long been recognized, however, that there are two major prob-
lems with the use of NOAELs (or LOAELs, etc.) in deriving reference
concentrations (RfCs) (and derivative values such as CEELs). One prob-
-------
"I" I" "
'I11. '•• - ; "ii .'[.ni i'. .• -;, ' ' • •:" i ?•;•?. • j i ii i I lii
i i
90 CEELS FOR HAZARDOUS SUBSTANCES
iem is that the use of NOAEL's does not readily account for the number
of ammate used to determine' the NOAEL. It might be useftil if the
number of animus used to determine the NOAEL would affect the value
of the resulting RfC in some way other than through the level of confi-
dence (as reflected in the MF). A second major problem with the use of
NOAELs is that the slope of the dose-response curve for the critical toxic
effect is generally ignored in the estimation of the RfC. Many scientists
have argued that the slope should directly affect the resulting RfC; steep
'"effect' slopes would presumably yield higher values. Thus, in some
cas&s; the use of some other procedure for quantitative risk assessment
for noncancer end points might have greater validity.
The current principal alternative to NOAELs is the benchmark dose
(BD) procedure, the BD has certain advantages over the NOAEL ap-
proach The BD makes appropriate use of the sample size, as reflected
in me magnitude of the confidence limit. It exploits the shape (steepness)
of the dose-response curve in the experimental range but does not depend
strongly on the particular mathematical model used, because the model
is not followed below me 1% response level. EPA's "Guidelines for
Developmental Toxicity Risk Assessment" (EPA, 1991) recommends that
a BD be calculated to supplement the NOAEL or LOAEL used in deter-
mining a reference dose for developmental toxicity.
EPA considers the BD to be useful for comparison with RfC results
or as a possible alternative to NdAELor LOAEL. The use of the BD
in the risk assessment of noncarcinogenic toxic effects should be encour-
1 ' ", „ ''^ '" ' '' ' '''" '' ''! I 1|""
III I II ill I , 5" ", •• ' ,'ili! •:•!
I II . j'l I ., , ',!! ; SB
One unresolved issue with the BD procedure, however, is the size of
the UF that should be applied to any particular BD to account for varia-
tion in sensitivity among members of a population. For example, would
applying a 10-fold UF to the BD01 be equivalent to applying a 10-fold
UF to a well-defined NOAEL, or would it be more or less protective?
That question will be answered only after more experience is gained with
the BP procedure.
Cross-Species Extrapolation:
1 :;'•,';.;•• ,'^::r:"' Specific Dose Adjustment
•,'.:" ' I " ' i
The extrapolation of quantitative animal data to humans, whether for
-------
DEVELOPMENT OF CEELS 91
threshold toxicity or for assumed nonthreshold toxicity (e.g., genotoxic
carcinogens) requires cross-species extrapolation. Traditionally, cross-
species extrapolation has been based on body weight (e.g., milligram per
kilogram of body weight in animals is equivalent to milligram per kilo-
gram of body weight in humans). The U.S. Food and Drug Administra-
tion has used this formula until the present time. Many physiological
factors (e.g., consumption of water, food, or oxygen), however, vary as
a fractional power of the body weight. Dourson and Stara (1983) in a
review of the data pointed out that the toxicity of many compounds ap-
pears to be more closely related to body-surface area than to body weight
(Pinkel, 1958; Freireich et al., 1966). Because body-surface area varies
approximately in proportion to the 2/3 power of body weight (as it would
for spherical bodies of equal densities), that value has often been used.
Extrapolation on the basis of concentration of an agent in a medium
(e.g., in diet or air) generally yields a factor that falls between one calcu-
lated on the basis of body weight and one calculated on the basis of the
2/3 rule. After examining the data of Freireich et al. (1966), Travis and
White (1988) suggested that body weight to the 3/4 power might be more
appropriate for interspecies dose conversion, but other values between
0.67 and 0.75 have also been suggested. Recently, three federal agen-
cies proposed the use of body weight to the 3/4 power as a default
interspecies dose scaling factor for carcinogens (EPA, 1992). In some
instances, physiologically based pharmacokinetic models can be used for
cross-species extrapolation. Although such models or the data needed to
create the models usually are available only for well-studied genotoxic
and nongenotoxic carcinogens, use of the models should be considered
whenever they are available for either threshold or nonthreshold toxicity.
If cross-species extrapolation is made on the basis of pharmacokinetic
data in humans and the experimental animal, the cross-species extrapola-
tion factor (maximum value =10) may be adjusted appropriately.
Cross-Species Extrapolation: Inhaled Dose
In Animals to Inhaled Dose in Humans
The effect that an agent has on an animal or human might be a func-
tion of concentration alone or a function of concentration and time (or
something in between). If the effect is directly on the surfaces of the
-------
92 CEELS FOR HAZARDOUS SUBSTANCES
lung then the effect might well depend on concentration, although the
effect also will depend on the breaming rate and the geometry of the
airways and lungs of each species. Ways to approach these situations
have been proposed by EPA. For systemic toxicants, on the other hand,
the important consideration is the dose of the active compound (possibly
a metabolite) to the site of action. It is assumed that the action of the
compduhd is a function of the specific dose in the body.
If data are sufficient, the percentage uptake of a chemical through the
lungs for the animal species and humans can be factored into the extrapo-
lation, but, in general, 100% absorption through the lungs is assumed.
In the absence of sufficient data, the amounts absorbed are assumed to
be determined by the respiratory rates of the subjects and the time of
exposure. The rates are available from various sources, although actual
data on the animals in question should be used if available. For exam-
ple, the accepted |^re!a^fng rate for a resting human is usually 7-10
L/min, whereas the value for a 0.113-kg rat is about 73 mL/mm. The
conversion of data in rats, expressed as concentration in milligrams per
cubic meter, to humans can be calculated on the basis of the total volume
breathed by the two species over the relevant time as follows:
•. 1J•'•':! iv!! '. i , ' • i .' :',n~ >;,., ';' -it:,' I"1"" " • ' • , • . •, i i • .? -
! .•"•/! . i' •• ••; ; , .;. i "i-ji" •;;!,?,,;. LI,: [_"_ : , .w i 'f . •: -,n , - .. ItW"1 ail,; ,,-
111 , :": •.':'J."1;?>ii xbAxBWH •'
BWA
'1! il'i'i
•• i! 3
where C is exposure concentration in milligrams per cubic meter, R is
respiratory rate in cubic meters per unit of time, BW is body weight, H
is human, and A is animal. Note that the interspecies conversion is a
one-to-one unit weight conversion.
Cross-Species and Dose-Route Extrapolation:
Oral Dose in Animals to Inhaled Dose in Humans
In some instances, the only data available for cross-species extrapola-
tion is that obtained from oral doses in animals. In that case, the extrap-
olation should be tjie same as that for inhalation extrapolation, except
that conversion from the air concentration for the animal will not have
to be made. As before, if toxicokinetic information on uptake of the
• ''i " • ' ! '
ii), i : j: ,! >
-------
DEVELOPMENT OF CEELS 93
compound of interest (or a similar compound) is available for humans,
the value should be corrected for the absorption ratio.
NONTHRESHCMLD EFFECTS
Decisions on the carcinogenicity of chemicals in humans should be
based on considerations of all relevant data, whether they are indicative
of a positive or negative response, and should embody sound biological
and statistical principles. Because animal carcinogens are not the same
with respect to potency, target organs, mechanism, and so forth, and thus
are not equally relevant to humans, hazard evaluation should be made on
a weight-of-evidence basis that includes consideration of all relevant
factors, giving appropriate weight to each factor on a case-by-case basis.
The weight-of-evidence evaluation of carcinogenic hazard to humans
provides the basis for carcinogen classification. In addition, it is the
initial step in determining the adequacy and appropriateness of the data
for quantitative cancer risk assessment.
Data addressing the strength of the conclusions to be drawn from
animal bioassays and data addressing the appropriateness of the animal
model as a predictor for humans should include information on compara-
tive metabolic paths, pharmacokinetics, routes of exposure, mechanisms
of action, and organ or species differences in response, as well as infor-
mation on humans. If pharmacokinetic models for calculating delivered
dose and cross-species extrapolation have been developed, the
pharmacokinetic information should be incorporated into the quantitative
risk estimates.
When an agent under evaluation is determined to present a potential
carcinogenic risk to humans, a separate quantitative risk assessment is
undertaken in recognition of the fact that.even limited exposure to such
agents theoretically can increase the risk of cancer. Calculating CEELs
for chemical carcinogens is a complex process. Many carcinogenicity
studies involve high-dose, long-duration exposures to compensate for the
small number of animals used in the studies. Data on short-term or
single exposures that assess carcinogenicity in animals or humans are
virtually nonexistent for agents of interest to organizations that will set
CEELs (NRC, 1986b).
In the absence of human data, usually it is assumed that carcinogenic
-------
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(if • • i
V. ' ill1.!1'!; ;'f(V '«,, '" "i" Sr"!
94 CEELS FOR HAZARDOUS SUBSTANCES
risk derived from animal data is directly and quantitatively applicable to
humans. Extrapolation from high-dose animal exposures to low-dose
human exposures is often required. For known genotoxic carcinogens
and for carcinogens with mechanisms of action that are not well under-
stood^ setting CEELs should involve linear low-dose extrapolation from
an upper confidence limit on excess risk. Implementation might be
accomplished with a computer program such as GLOBAL82 (Howe and
Crump, 1982), whether one employs the basic procedure of Crump
(1984b) or Gaylor and Kodell (1980). In the equation below, if ft
denotes a computed upper confidence limit on the excess-risk low-dose
slope for a specified level of confidence (e.g., 95%) and P0 represents
a specified "acceptable" excess risk (e.g., 10"4 to lO"6), then the corre-
sponding dose, d0, is found by solving
Thus, PQ is an upper bound on excess risk corresponding to dose d0, and
d0 represents a lower bound on dose for risk level P0. See Chapter 5 for
greater details.
Most bioassay data suitable for low-dose extrapol ation reflect continu-
ous exposure to a carcinogen, necessitating the translation of risks calcu-
lated for continuous exposures to risks associated with short-term expo-
sures. Crump arid Howe (1984) developed a method for applying the
multistage "model to carcinogenic risk assessment when exposure to a
carcinogen is of short duration. Their results have been adapted by COT
for setting EEGLs and SPEGLs (NRC, 1986b). The determination of
CEELs for carcinogens generally will require extrapolating risks estimat-
ed for long-term exposure conditions to short-term exposure situations.
In the case of near-instantaneous exposure (e.g., single-day duration
or less), total dose D "at "time "fp ''that will give equivalent risk to that of
exposure at constant daily dose d from birth to age t is
D =
dt*
Expressions derived on the basis of specific multistage models can be
t/iiiin i , i , ,i .v mil:,,,
-------
DEVELOPMENT OF CEELS 95
useful for setting 1-hr and 8-hr CEELs, but generally information will
be insufficient to postulate the total number of stages in the cancer pro-
cess and the stages that are dose-related. In such situations, the limiting
case of the time-weighted-average dose can be used, whereby instanta-
neous dose D at time f0 having equivalent lifetime excess carcinogenic
risk as daily dose d up to time t is approximated by
D = dt.
As shown by Kodell et al. (1987), the actual risk at dose D will not
exceed k times the presumed risk, where k is the number of stages hi the
multistage model.
If prudence dictates conservatism, then the above approximation to D
may be reduced by a factor of 2 to 6 (the number of assumed stages in
the multistage model), according to Kodell et al. (1987). In determining
cancer-based CEELs, extrapolation from a daily lifetime exposure level
to near-instantaneous exposure levels should follow that approach based
on the multistage theory of carcinogenesis, unless there is evidence to the
contrary. See Chapter 5 for greater detail on risk-assessment techniques
for carcinogens and noncarcinogens.
CALCULATION OF CEELS
AND RATIONALE
CEELs are calculated on the basis of the principles embodied in the
previous discussions. The steps are outlined here. It is stressed, howev-
er, that sound scientific principles should always guide the calculation of
CEELs. Thus, if some other method for identifying levels of concern,
for example, the benchmark dose, can be substituted for NOELs and
LOAELs with scientific probity, then the alternative method should be
used. In addition, agents demonstrating nonthreshold toxicity should
undergo evaluation using both threshold and nonthreshold methods; the
value most protective of human health for any particular CEEL level
should be chosen as the CEEL.
The following list briefly outlines one rational approach to the calcula-
tion of CEELs; the procedure is diagrammed in Figure 1.
-------
96 CEELS FOR HAZARDOUS SUBSTANCES
ASh |
• 4-
Select
Physiological
Reference
Values
4-
Select Risk
Level
4r
Calculate
Acceptable
Total Dose
.: "" Select I
Averaging 1
Period 1
|
Review Overall
Data Base 1
4
Select Key I
Studies 1
^^V No Identify
reshold\^ ^ NOAEL or
ects ^ LOAEL
^r |
Determine
Absorption
Contribution
™"""^""""~™^
Select
Physiological
Reference
Values
Select
Extrapolation
Procedure
T
Select 1
Uncertainty 1
1 Factors |
Calculate
Recommended
CEELs Based
on Threshold
Select CEELs Most L
Protective of
Human Healthr
4- •
Present All Assumptions I
and Uncertainties |
i
r
Compare CEELs to Other 1
Exposure Levels |
"" !;' . . !,!: '"'
r
Present CEELs 1
FIGURE 1
process.
-------
DEVELOPMENT OF CEELS 97
1. Select exposure averaging time, e.g., 1 hr. CEELs may be
established for more than one period, e.g., 1 hr and 8 hr.
2. Review overall data base.
3. Select the key studies. Identify the most relevant or sensitive
species and target organ for each limiting toxicity and each time period.
4. Select most appropriate no-effect or threshold levels for each
limiting toxicity.
5. Determine contribution of absorption data to dose calculation.
6. Select physiological and anthropometric reference values, e.g.,
body weight, surface area, minute volume, and volume of inspired air,
7. Select extrapolation procedures.
8. For agents that are considered to produce threshold effects, select
an appropriate uncertainty factor of 1 or greater for end points affording
NOELs, NOAELs, LOELs, or thresholds.
9. Calculate the recommended CEELs on the basis of threshold
toxicities.
10. For agents that are considered to produce nonthreshold effects
(e.g., genotoxic carcinogens), select an appropriate model, appropriate
physiological reference values, and an appropriate risk level ranging
from 10"4 to 10"6 depending on the population potentially exposed. Cal-
culate the acceptable dose levels (1 and 8 hr, etc.). Note that for materi-
als that might cause threshold and nonthreshold effects, both approaches
should be used to determine the CEELs.
11. Select CEELs most protective of human health based on the
overall data base.
12. Present and discuss all assumptions and uncertainties used to
derive CEELs.
13. Compare CEELs with other recommended exposure levels for the
same potentially exposed individuals in the general population or for
different groups, such as workers.
14. Present CEELs, provide documentation, and briefly discuss
rationale.
PRESENTATION OF CEELS
The procedure used to derive CEELs should be fully documented. As
well as presenting all the calculations, the rationale for selecting the
-------
it. f
98 CEEIS FORtfAZARtiOUS SUBSTANCES
relevant studies, end points, and no-effect or threshold levels should be
discussed.
Furthermore, because risk assessments are based on the use ot many
assumptions to bridge gaps in understanding and data, the major assump-
tions, uncertainties, and their rationale are important and should be pro-
vided to the risk manager. It is important for agents with nonthreshold
effects that the range of values from different models or approaches be
presented along with an analysis of key assumptions and discussion of
key critical judgmental factors-e.g., the key assumptions that might bear
on the likelihood that the true risk is below the upper bound.
After CEELs have been calculated according to the procedures de-
scribed in this document; they must be submitted for review by a com-
mittee of experts (e.g., COT or a comparable group). If necessary, pro-
posed CEELs will be modified in response to recommendations of the re-
view committee before'being; finalized. Recommended CEELs for which
significant data gaps exist should Be considered preliminary and should
be reviewed and updated as new data become available. All chemical
substances studied thus far have toxicity data gaps; therefore, all recom-
mended CEELs should be considered for possible updating every 5-10
years, or even earlier if new data on the compounds under investigation
become available.
When toxicity data for a chemical under consideration are judged to
be inadequate to recommend CEELs with an acceptable level of confi-
dence, relevant testing should be recommended.
-------
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