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I' I? (FRL-2706-5)
PROPOSED GUIDELINES FOR EXPOSURE ASSESSMENT
f/9
v AGENCY: U.S. Environmental Protection Agency (EPA)
ACTION: Proposed Guidelines for Exposure Assessment
and Request for Comments
i SUMMARY: The U.S. Environmental Protection Agency is proposing Guidelines
for Exposure Assessment (Guidelines). These Guidelines are proposed for
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C* use within the policy and procedural framework provided by the various
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FOR FURTHER INFORMATION CONTACT: Dr. James W. Falco
Telephone: 202-475-8909
SUPPLEMENTARY INFORMATION:
Preliminary drafts of these Guidelines were sent out for review to
15 scientists and engineers In the field of exposure assessment within
government, universities in the United States and abroad, and the private
sector. Comments received from these reviews, generally favorable, were
taken into account in developing the Guidelines proposed here.
In addition, as a result of the reviews, four areas requiring further
research were identified as follows:
(1) Development of Mathematical Model Selection Criteria.
A large number of mathematical models are used to estimate a wide
variety of parameters needed for estimating exposures. Guidance in the
form of selection criteria are needed to ensure that the most appropriate
mathematical model is used for each exposure parameter estimate.
(2) Development of Guidance for Analysis of Metabolism Data.
Guidance is needed to provide appropriate consideration of metabolism
data in the calculation of whole body dose and in the extrapolation of
whole organism dose from one species to another.
(3) Definition of the Relationship Between Exposure Assessment and
Epidemiology.
Guidance is needed to ensure that pertinent parameters of exposure are
measured in prospective epidemiologic studies. Methods providing the
best estimates of exposure for retrospective and historical epidemiologic
studies must be defined.
(4) Development of Methods to Relate Exposures Measured by Personal
Monitoring to Source Contributions.
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Guidance is needed to establish methods to relate exposures as
measured by personal monitoring to controllable sources and to discriminate
among possible sources and between background and anthropogenic sources.
It is the Agency's intent to revise the Guidelines periodically to
incorporate the results obtained in the four research areas defined above
as they become available.
In addition to the publication of the Guidelines, the Agency also
will provide technical support documents that contain detailed technical
information needed to implement the Guidelines. Two of these technical
reports entitled "Development of Statistical Distribution or Ranges of
Standard Factors Used in Exposure Assessments" and "Methodology for
Characterization of Uncertainty in Exposure Assessments" are currently
available. Technical reports for the four new guideline areas described
above will be available at the time of publication of the corresponding
guideline section. These technical support documents will be revised
periodically to reflect improvements in exposure assessment methods and
new information or experience.
Support documents used in the preparation of these Guidelines as
well as comments received are available for inspection and copying at
the Public Information Reference Unit (202-382-5926), EPA Headquarters
Library, 401 M Street S.W., Washington, DC, between the hours of 8:00
a.m. and 4:30 p.m.
NOV9 1984
Date
Administrator
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CONTENTS
I. INTRODUCTION 5
II. GENERAL GUIDELINES AND PRINCIPLES 6
A. EXPOSURE AND DOSE 6
B. DECISION PATH TO DETERMINE SCOPE OF THE ASSESSMENT 7
C. UNCERTAINTY 9
III. ORGANIZATION AND CONTENTS OF AN EXPOSURE ASSESSMENT 9
A. OVERVIEW 9
B. DETAILED EXPLANATION OF OUTLINE 10
1. Executive Summary 10
2. Introduction 10
3. General Information 13
4. Sources 13
5. Exposure Pathways and Environmental Fate .15
6. Monitored or Estimated Concentration Levels 17
7. Exposed Populations 19
8. Integrated Exposure Analysis 20
9. References 36
10. Appendices 36
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I. INTRODUCTION
These guidelines provide the Agency with a general approach and
framework for carrying out human or nonhuman exposure assessments for
specified pollutants. The guidelines have been developed to assist future
assessment activities and encourage improvement in those EPA programs
that require, or could benefit from, the use of exposure assessments.
The guidelines are procedural. They should be followed to the extent
possible in instances where exposure assessment is a required element in
the regulatory process or where exposure assessments are carried out on
a discretionary basis by EPA management to support regulatory or program-
matic decisions.
This document, by laying out a set of questions to be considered in
carrying out an exposure assessment, should help avoid inadvertent mistakes
of omission. EPA recognizes that gaps in data will be common, but the
guidelines will nevertheless serve to assist in organizing the data that
are available, including any new data developed as part of the exposure
assessment. It is understood that exposure assessments may be performed
at many different levels of detail depending on the scope of the assessment.
These guidelines should also promote consistency among various
exposure assessment activities that are carried out by the Agency.
Consistency with respect to common physical, chemical, and biological
parameters, with respect to assumptions about typical exposure situations,
and with respect to the characterization of uncertainty of estimates,
will enhance the comparability of results and enable the Agency to improve
the state-of-the-art of exposure assessment over time through the.sharing
of common data and experiences. /
It is recognized that the main objective of an exposure assessment
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1s to provide reliable data and/or estimates for a risk assessment. Since a
risk assessment requires the coupling of exposure information and toxicity
or effects information, the exposure assessment process should be
coordinated with the toxicity/effects assessment. This document provides
a common approach to format, which should simplify the process of reading
and evaluating exposure assessments and thereby increase their utility in
assessing risk.
As the Agency performs more exposure assessments, the guidelines
will be revised to reflect the benefit of experience.
II. GENERAL GUIDELINES AND PRINCIPLES
A. EXPOSURE AND DOSE
Exposure has been defined by Committee E-47, Biological Effects and
Environmental Fate, of the American Society for Testing and Materials, as
the contact with a chemical or physical agent. The magnitude of the
exposure is determined by measuring or estimating the amount of an agent
available at the exchange boundaries, i.e., lungs, gut, skin, during some
specified time. Exposure assessment is the determination or estimation
(qualitative or quantitative) of the magnitude, frequency, duration, and
route of exposure. Exposure assessments may consider past, present, and
future exposures with varying techniques for each phase, i.e., modeling
of future exposures, measurements of existing exposure, and biological
accumulation for past exposures. Exposure assessments are generally
combined with environmental and health effects data in performing risk
assessments.
In considering the exposure of a subject to a hazardous agent, there
are several related processes. The contact between the subject of concern
and the agent may lead to the intake of some of the agent. If absorption
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occurs, this constitutes an uptake (or an absorbed dose) which then may
lead to health effects. When biological tissue or fluid measurements
indicate the presence of a chemical, exposures can be estimated from
these data. Presence of a chemical in such biological samples is the most
direct indication that an exposure has occurred. The route of exposure
generally impacts the overall exposure and should be considered in perform-
ing risk assessments.
B. DECISION PATH TO DETERMINE SCOPE OF THE ASSESSMENT
The first step in preparing an exposure assessment should be the
circumscription of the problem at hand to minimize effort by use of a
narrowing process. A decision logic path that describes this process
is shown in Figure 1. As illustrated in Figure 1, the preliminary
assessment and the in-depth assessment are two major phases in this
logic path.
The preliminary assessment phase should commence by considering what
risk is under study and what law might regulate the exposure to the agent.
Within this framework, a preliminary data base should be compiled from
readily available scientific data and exposure information based on
manufacturer, processor, and user practices. Next, the most likely areas
of exposure (manufacturing, processing, consumer, distribution, disposal,
ambient, water and food, etc.) should be identified. Since a complete
data search has not been conducted, well-identified assumptions and order
of magnitude estimates are used to further narrow the exposure areas of
concern.
Data from this preliminary exposure assessment can then be coupled
with toxicity information to perform a preliminary risk analysis. As a
result of this analysis, a decision will be made that either an in-depth
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PRELIMINARY EXPOSURE
ASSESSMENT
IN-DEPTH EXPOSURE
ASSESSMENT
REGULATORY
RESPONSE
REGULATORY CONCERN
\
SCIENTIFIC DATA:
POPULATION
EXPOSURE
PRODUCT LIFE CYCLE
GENERAL INFORMATION
GATHERING
*
PROBABLE AREAS OF EXPOSURE!
IPRELIMINARY
HAZARD IDENTIFICATION
TOX1CITY
ENV. CONC., ETC.
_ . - .
PRELIMINARY RISK ANALYSIS]
I DECISIONi
BEGIN IN-DEPTH
ASSESSMENT
NO NEED FOR FURTHER
EXPOSURE ASSESSMENT
MULTI-DISCIPLINARY
PEER REVIEW
[DESIGN ASSESSMENT STUDY PLAN!
[COMPREHENSIVE DATA GATHERING)
i
[CONDUCT REFINEDEXPOSURE MODELiNgJ
......
| IN-DEPTH EXPOSURE ASSESSMENT |
DECISION |
SCIENCE PANEL
REVIEW.
HAZARD INPUT
FORMAL RISK
ASSESSMENT
! DECISION
REGULATORY
PROPOSAL
EXAMINED EXPOSURES
PRESENT NO
UNREASONABLE RISK
FIGURE 1. DECISION PATH FOR EXPOSURE ASSESSMENT
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exposure assessment Is necessary or that there is no need for further
exposure information. The organization and contents of an in-depth
exposure assessment are given in the following section.
In assembling the information base for either a preliminary assessment
or a more detailed assessment, its adequacy should be ascertained by
addressing the following considerations:
- availability of information in every area needed for an adequate
assessment;
- quantitative and qualitative nature of the data;
- reliability of information;
- limitations on the ability to assess exposure.
C. UNCERTAINTY
Exposure assessments are based on monitoring data, simulation
model estimates, and assumptions about parameters used in approximating
actual exposure conditions. Both data and assumptions contain varying
degrees of uncertainty which influence the accuracy of exposure assessments,
An evaluation of these uncertainties is important when the assessment is
the basis for regulatory action.
The uncertainty analyses performed will vary depending on the scope
of the assessment, the quantity and quality of monitoring data collected,
and the type and complexity of mathematical models used. A discussion
of the types of analysis used for quantifying uncertainties in exposures
is presented in the next section.
III. ORGANIZATION AND CONTENTS OF AN EXPOSURE ASSESSMENT
A. OVERVIEW
A suggested outline for an exposure assessment document is given in
Exhibit 1. The five major topics to be addressed within most exposure
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assessments are as follows: Source(s); Exposure Pathways; Monitored or
Estimated Concentration Levels and Duration; Exposed Population(s); and
Integrated Exposure Analysis. These five topics are appropriate for
exposure assessments in general, whether the assessments are of global,
national, regional, local, site-specific, workplace-related, or other
scope. The topics are appropriate for exposure assessments on new or
existing chemicals and radionuclides. They are also applicable to both
single media and multimedia assessments. Since exposure assessments are
performed at different levels of detail, the extent to which any assessment
contains items listed in Exhibit 1 depends upon its scope. The outline is
a guide to organize the data whenever they are available.
B. DETAILED EXPLANATION OF OUTLINE
1. EXECUTIVE SUMMARY
The "Executive Summary" should be written so that it can stand on
its own as a miniature report. Its main focus should be on a succinct
description of the procedures used, assumptions employed, and summary
tables or charts of the results. A brief discussion of the uncertainties
associated with the results should be included.
2. INTRODUCTION (Purpose and Scope)
This section should state the intended purpose of the exposure
assessment and identify the agent being investigated, the types of sources
and exposure routes included, and the populations of concern.
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Exhibit 1
SUGGESTED OUTLINE FOR AN EXPOSURE ASSESSMENT
1. EXECUTIVE SUMMARY
2. INTRODUCTION
a. Purpose
b. Scope
3. GENERAL INFORMATION
a. Identity .
(1) Molecular formula and structure, CAS number, TSL number
(2) Description of technical grades, contaminants, additives
(3) Other identifying characteristics
b. Chemical and Physical Properties
4. SOURCES
a. Characterization of Production and Distribution
(1) Production and processing
(2) Distribution in commerce
b. Uses
c. Disposal
d. Summary of Environmental Releases
5. EXPOSURE PATHWAYS AND ENVIRONMENTAL FATE
a. Transport and Transformation
b. Identification of Principal Pathways of Exposure
c. Predicting Environmental Distribution
6. MONITORED OR ESTIMATED CONCENTRATION LEVELS
a. Summary of Monitoring Data
b. Estimation of Environmental Concentrations
c. Comparison of Concentration Estimates with Monitoring Data
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7. EXPOSED POPULATIONS
a. Human Populations (Size, Location, and Habits)
(1) Population size and characteristics
(2) Population location
(3) Population habits
b. Nonhuman Populations (where appropriate)
(1) Population size and characteristics
(2) Population location
(3) Population habits
8. INTEGRATED EXPOSURE ANALYSIS
a. Calculation of Exposure
(1) Identification and characterization of the exposed
populations and critical elements of the ecosystem
(2) Pathways of exposure
b. Human Dosimetry and Monitoring
c. Development of Exposure Scenarios and Profiles
d. Evaluation of Uncertainty
9. REFERENCES
10. APPENDICES
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3. GENERAL INFORMATION
a. Identity - —
(1) Molecular formula and structure, synonyms, Chemical Abstract
Service number, Toxic Substance List number.
(2) Description of technical grades, contaminants, additives.
(3) Other identifying characteristics.
b. Chemical and Physical Properties
This subsection should provide a summary description of the chemical
and physical properties of the agent. Particular attention should be
paid to the features that would affect its behavior in the environment.
Examples of factors to be included are molecular weight, density, boiling
point, melting point, vapor pressure, solubility, pKa, partition coeffi-
cients, and half-lives.
4. SOURCES
The points at which a hazardous substance is believed to enter the
environment should be described, along with any known rates of entry.
Points of entry may be indoors as well as outdoors, and environments
include indoor settings such as offices as well as outdoor environments.
A detailed exposure assessment should include a study of sources,
production, uses, destruction/disposal, and environmental release of a
substance. The studies should include a description of human activities
with respect to the substance and the environmental releases resulting
from those activities. It should account for the controlled mass flow of
the substance from creation to destruction and provide estimates of
environmental releases at each step in this flow. Seasonal variations in
environmental releases should also be examined. All sources of the
substances should be accounted for with the sum of the uses, destruction,
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and the environmental releases. The environmental releases can be described
in terms of geographic and temporal distribution and the receiving environ-
mental media, with the form identified at the various release points.
a. Character!zation of Productioh and Pistribiition
All sources of the substance's release to the environment, consistent
with the scope of the assessment, should be included, such as production,
extraction, processing, imports, stockpiles, transportation, accidental/
incidental production as a side reaction, and natural sources. The
sources should be located, and activities involving exposure to the
substance should be identified.
b. Uses
The substance should be traced from its sources through various
uses (with further follow-up on the products made to determine the presence
of the original material as an impurity), exports, stockpile increases,
etc.
c. Disposal
This subsection should contain an evaluation of disposal sites and
destruction processes, such as incineration of industrial chemical waste,
incineration of the substance as part of an end-use item in municipal
waste, landfilling of wastes, biological destruction in a secondary
wastewater treatment plant, or destruction in the process of using the
end product. Hazardous contaminants of the substance may be included,
and products containing the substance as a contaminant may be followed
from production through destruction/disposal.
d. Summary of Environmental Releases
Estimates should be made of the quantities of the substances released
to the various environmental media. Sources of release to the environment
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Include production, use, distribution/transport, natural sources, disposal,
and contamination of other products. Environmental releases should be
presented at a reasonable level of detail. Extremely detailed exposure
estimates would attempt to specify the following information for each
significant emission source: location, amount of the substances being
released as a function of time to each environmental medium, physical
characteristics of the emission source, and the physical and chemical
form of the substance being released. Evaluation of the uncertainties
associated with the emission estimates should be given. A detailed
discussion of procedures for estimating uncertainty is presented in
section 8.d.
5. EXPOSURE PATHWAYS AND ENVIRONMENTAL FATE
The exposure pathways section should address how a hazardous agent
moves from the source to the exposed population or subject. For a less
detailed assessment, broad generalizations on environmental pathways and
fate may be made. In the absence of data, e.g., for new substances,
fate estimates may have to be predicted by analogy with data from other
substances. Fate estimates may also be made by using models and/or
monitoring data and laboratory-derived process rate coefficients. At
any level of detail, certain pathways may be judged insignificant and
not pursued further.
For more detailed assessments involving environmental fate, the
sources analysis described previously should provide the amount and rate
of emissions to the environment, and possibly the locations and form of
the emissions. The environmental pathways and fate analysis follows the
substance from its point of initial environmental release, through the
environment, to its ultimate fate. It may result in an estimation of the
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geographic and temporal distribution of concentrations of the substance
in the various contaminated environmental media. _
a. Transport and Transformati oji
The substance, once released to the environment, may be transported
(e.g., convected downstream in water or on suspended sediment, through
the atmosphere, etc.) or physically transformed (e.g., volatilized,
melted, absorbed/desorbed, etc.); may undergo chemical transformation such
as photolysis, hydrolysis, oxidation, reduction; may undergo biotransformation
such as biodegradation; or may accumulate in one or more media. Thus, the
environmental behavior of a substance should be evaluated before exposures
are assessed. Factors that should be addressed include:
o How does the agent behave in air, water, soil, and biological
media? Does it bioaccumulate or biodegrade? Is it absorbed or
taken up by plants?
o What are the principal mechanisms for change or removal in each
of the environmetal media.
o Does the agent react with other compounds in the environment?
o Is there intermedia transfer? What are the mechanisms for
intermedia transfer? What are the rates of the intermedia transfer
or reaction mechanisms?
o How long might the agent remain in each environmental medium?
How does its concentration change with time in each medium?
o What are the products into which the agent might degrade or change
in the environment? Are any of these degradation products
ecologically or biologically harmful? What is the environmental
behavior of the harmful products?
o Is a steady-state concentration distribution in the environment,
or in specific segments of the environment, achieved? If not, can
the nonsteady-state distribution be described?
o What is the resultant distribution in the environment--for
different media, different types or forms of the agent, for
different geographical areas, at different times or seasons?
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b. Identification of Principal Pathways of Exposure
The principal pathway analysis should evaluate the sources, locations,
and types of environmental releases, together with environmental behavioral
factors, to determine the significant routes of human and environmental
exposure to the substance. Thus, by listing the important characteristics
of the environmental release {entering media, emission rates, etc.) and
the agent's behavior (intermedia transfer, persistence, etc.) after
release to each of the entering media, it should be possible to follow
the movement of the agent from its initial release to its subsequent fate
in the environment. At any point in the environment, human or environmental
exposure may occur. Pathways that result in major concentrations of the
agent and high potential for human or environmental contact are the principal
exposure pathways.
c. Predicting Environmental Distribution
Models may be used to predict environmental distributions of chemicals.
Many modeling estimates of environmental distribution of chemicals are
based in part on monitoring data. In predicting environmental distributions
of chemicals, available monitoring data should be considered.
In this section an estimation is made, using appropriate models, of
representative concentrations of the agent in different environmental
media, and its time-dependence in specific geographical locations (e.g.,
river basins, streams, etc.).
6." MONITORED OR ESTIMATED CONCENTRATION LEVELS
a. Summary of Monitoring Data
Monitoring data are used to identify releases (source terms) and, in
the exposure pathways and fate assessments, to quantitatively estimate both
release rates and environmental concentrations. Some examples of uses of
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monitoring data are: sampling of stacks or discharge pipes for emissions
to the environment; testing of products for chemical or radionuclide
content; testing of products for chemical or radioactive releases; sampling
of appropriate points within a manufacturing plant to determine releases
from industrial processes or practices; and sampling of solid waste for
chemical or radionuclide content. These data should be characterized as
to accuracy, precision, and representativeness. If actual environmental
monitoring data are unavailable, concentrations can be estimated by
various means, including the use of fate models (see previous section) or,
in the case of new chemicals, by analogy with existing chemicals.
The analysis of monitoring data should be considered a complement to
environmental pathway and fate analysis for the following reasons: for
most pollutants, particularly organic and new chemicals, monitoring data
are limited; analysis of monitoring data does not often yield relationships
between environmental releases and environmental concentration distribution
in media or geographic locations that have not been monitored; analysis
of monitoring data does not provide information on how and where biota
influence the environmental distribution of a pollutant; and monitored
concentrations may not be traceable to individual sources that EPA can
regulate. Monitoring data are, however, a direct source of information
for exposure analysis and, furthermore, they can be used to calibrate or
extrapolate models or calculations to assess environmental distribution.
b. Estimation of Environmental Concentrations
Concentrations of agents should be estimated for all environmental
media that might contribute to significant exposures. Generally, the
environmental concentrations are estimated from monitoring data, mathe-
matical models, or a combination of the two.
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The concentrations must be estimated and presented in a format
consistent with available dose-response information. In some cases an
estimate of annual average concentration will be sufficient, while in
other cases the temporal distribution of concentrations may be required.
Future environmental concentrations resulting from current or past releases
may also be projected. In some cases, both the temporal and geographic
distributions of the concentration may be assessed. Moreover, if the
agent has natural sources, the contribution of these to environmental
concentrations may be relevant. These "background" concentrations may
be particularly important when the results of tests of toxic effects
show a threshold or distinctly nonlinear dose-response.
The uncertainties associated with the estimated concentrations should
be evaluated by an analysis of the uncertainties of the model parameters
and input variables. When the estimates of the environmental concentrations
are based on mathematical models, the model results should be compared to
available monitoring data, and any significant discrepancies should be
discussed. Reliable, analytically-determined values should be given
precedence over estimated values whenever significant discrepancies are
found.
7. EXPOSED POPULATIONS
Populations selected for study may be done a priori, but frequently
the populations will be identified as a result of the sources and fate
studies. From an analysis of the distribution of the agent, populations
convected and subpopulations (i.e., collections of subjects) at potentially
high exposure can be identified, which will then form the basis for the
populations studied. Subpopulations of high sensitivity, such as pregnant
women, infants, chronically ill, etc., may be studied separately.
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In many cases, exposed populations can be described only generally.
In some cases, however, more specific information may be available on
matters such as the following:
a. Human Populations
(1) Population size and characteristics (e.g., trends, sex/age
distribution)
(2) Population location
(3) Population habits - transportation habits, eating habits,
recreational habits, workplace habits, product use habits, etc.
b. Nonhuman Populations (where appropriate)
(1) Population size and characteristics (e.g., species, trends)
(2) Population location
(3) Population habits
Census and other survey data may be used to identify and describe
the population exposed to various contaminated environmental media.
Depending on the characteristics of available toxicological data, it may
be appropriate to describe the exposed population by other characteristics
such as species, race-age-sex distribution, and health status.
8. INTEGRATED EXPOSURE ANALYSIS
The integrated exposure analysis combines the estimation of environ-
mental concentrations (sources and fate information) with the description
of the exposed population to yield exposure profiles. Data should be
provided on the size of the exposed populations; duration, frequency,
and intensity of exposure; and routes of exposure. Exposures should be
related to sources.
For more detailed assessments, the estimated environmental concentrations
should be considered in conjunction with the geographic distribution of the
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human and environmental populations. The behavioral and biological
characteristics of the exposed populations should be considered and the
exposures of populations to various concentration profiles should be
estimated. The results can be presented in tabular or graphic form, and
an estimate of the uncertainty associated with them should be provided.
a. Calculations of Exposure
The calculation of exposure involves two major aspects:
(1) Identification of the Exposed Population and Critical Elements
of the Ecosystem
The estimate of environmental concentrations also should give the
geographical areas and environmental media contaminated. The stated
purpose of the assessment should have prescribed the human and environmental
subjects for which exposures are to be calculated. If the subjects are
not listed, the contaminated geographical areas and environmental media
can be evaluated to determine subject populations. The degree of detail
to be used in defining the exposed population distribution depends on the
concentration gradient over geographic areas.
(2) Identification of pathways of exposure
(a) Identification and description of the routes by which the
substances travel from production site, through uses,
through environmental releases/sources, through transport
and fate processes, to the target population.
(b) Quantitative estimates of the amounts of the chemical
following each exposure pathway. Such estimates allow the
various pathways to be put in the perspective of relative
importance.
From the geographic and temporal distribution of environmental
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concentrations, the exposed population, the behavioral characteristics,
and the critical elements of the ecosystem, exposure distributions can be
estimated. The results of exposure calculation should be presented in
a format that is consistent with the requirements of the dose-response
functions which may later be used in a risk assessment. For example,
when health risks caused by exposure over extended durations are considered,
average daily exposure over the duration of exposure usually is calculated.
When lifetime risks are considered, average daily exposure over a lifetime
usually is calculated. In contrast, when health risks caused by exposures
over short durations are considered, exposure rates are calculated over
short time intervals to ensure that peak risks are defined. Many exposure
assessments are based on the average exposure occurring over the exposure
period. The range of possible exposures is usually divided into intervals,
and the exposures within each interval are counted. The results can be
presented in tabular form or as a histogram.
The population residing in a specific geographic area may be exposed
to a substance from several exposure routes. For each exposure route,
exposure of individuals in these populations may be determined by summing
the contribution of all sources to the exposure route. When exposures
involve more than one exposure route, the relative amounts of a substance
absorbed is usually route dependent. Consequently, total absorbed dose
estimates must account for these differences. Because EPA regulates
sources of releases, the contribution to exposures from each type of
source being considered should be displayed. Exposure estimates should
be presented for each significant exposure route (i.e., those routes
consistent with the regulatory purpose), and the results should be tabulated
in such a way that total externally applied and absorbed dose can be
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determined.
b. Human Dosimetry and Monitoring
Biological monitoring of human body fluids and tissues for substances
or their metabolites can be used to estimate current or past exposure to
chemicals. When analytical methods are available, chemicals that have
been absorbed into the body can be measured in body tissue and fluid.
Such measurements can be used to estimate exposure. However, the substances
to which humans are exposed are highly variable in the degree to which
they leave in the body reliable indicators of exposure. Furthermore,
although a compound may be relatively easy to detect in body tissue, for
some compounds, attributing body burdens to specific environmental releases
may be difficult because of limited ability to obtain environmental
monitoring data.
c. Development of Exposure Scenarios and Profiles
Depending on the scope of the exposure assessment, the total exposure
may be fractionated into one or more "exposure scenarios" to facilitate
quantification. As an example, Table 1 lists seven very broad scenarios:
Occupational, Consumer, Transportation, Disposal, Food, Drinking Water, and
Ambient. For each of the scenarios, the major topics necessary to quantify
exposure include sources, pathways, monitoring, and population character-
istics. Investigation of only one scenario may be necessary for the
scope of some assessments. For example, a pesticide application exposure
assessment may consider the occupational scenario which would address
the exposure to applicators and populations in the vicinity of the site.
An exposure assessment around a hazardous waste site may focus on the
disposal scenario. The exposure assessment also may consider other
scenarios. The more extensive and comprehensive the scope, the more
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scenarios are usually involved.
It will usually be advantageous in performing an exposure assessment
to identify exposure scenarios, quantify the exposure in each scenario,
and then integrate the scenarios to estimate total exposure. In this
"integrated exposure analysis," summation of independent exposures from
different scenarios (keeping exposure routes separate) often will result
in a breakout of exposure by subpopulations, since the individual scenarios
usually treat exposure by subpopulation. Therefore, the integration of
the scenarios, or integrated exposure analysis, will often result in an
exposure profile.
For each exposed subpopulation, exposure profiles should include the
size of the group, the make-up of the group (age, sex, etc.), the source
of the agent, the exposure pathways, the frequency and the intensity of
exposure by each route (dermal, inhalation, etc.), duration of exposure,
and the form of the agent when exposure occurs. Assumptions and uncer-
tainties associated with each scenario and profile should be clearly
discussed.
d. Evaluation of Uncertainty
(1) Introduction
Often an exposure assessment progresses through several stages of
refinement. The purpose of these Guidelines is to present methods
appropriate for characterization of uncertainty for assessments at various
stages of refinement, from assessments based upon limited initial data
to those based upon extensive data.
The appropriate method for characterizing uncertainty for an exposure
assessment depends upon the underlying parameters being estimated, the
type and extent of data available, and the estimation procedures utilized.
25
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The uncertainty of interest is always with regard to the population
characteristic being estimated. For example, when the population
distribution of exposures is being estimated, characterization of
uncertainty addresses the possible differences between the estimated
distribution of exposure and the true population distribution of exposure.
An exposure assessment quantifies contact of a substance with affected
population members (human or nonhuman subjects). The measure of contact
(e.g., environmental level or absorbed dose) depends upon what is needed
to predict risk. An integrated exposure assessment quantifies this
contact via all routes of exposure (inhalation, ingestion, and dermal)
and all exposure pathways (e.g., occupational exposure, exposure from
consumption of manufactured goods, etc.). The exposed population generally
is partitioned into subpopulations such that the likely exposure of all
members of a subpopulation is attributable to the same sources. The exposure
for each member of a subpopulation is then the sum of exposures over a
fixed set of sources and pathways. The measured or estimated exposures
for members of a subpopulation are ideally used to estimate the subpopulation
distribution of exposure or characteristics thereof. However, a lack of
sufficient information sometimes precludes estimation of the subpopulation
distributions of exposure and only summary measures of this distribution,
such as the mean, minimum, maximum, etc., are estimated. In each case
characterization of uncertainty for the exposure assessment primarily
addresses limitations of the data and the estimation procedures. The
proportions of the population members in the individual subpopulations
are usually estimated and can be used (by combining estimated distributions
for the subpopulations) to estimate the distribution of exposure for the
total population. Uncertainty concerning the sizes of the subpopulations
26
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should be addressed by discussing limitations of the data and estimation
methods as well as by tabulating confidence interval estimates for the
population sizes whenever possible.
(2) Assessments Based Upon Limited Initial Data
The initial exposure assessment for a substance may be based upon
limited data for exposure and/or input variables for an exposure prediction
model (i.e., an equation that expresses exposure as a function of one or
more input variables). These data might be either extant data or data
produced by an initial small-scale study. The initial limited data frequently
are insufficient to permit estimation of the entire distribution of
exposure. Instead, summary measures of this distribution, such as the
mean, minimum, and maximum, are usually estimated.
If the assessment is based upon measured exposures, the methods used
to characterize uncertainty depend mainly upon whether or not the data
result from a probability sample for which the probability of inclusion
is known for each sample member. Characterization of uncertainty for an
assessment based upon a probability sample of exposures is discussed later
in section 8. d. (5). If the measured exposures are not based upon a
probability sample, acknowledgement that no strictly valid statistical
inferences can be made beyond the units actually in the sample is one
aspect of the characterization of uncertainty. If inference procedures
are implemented, the assumptions upon which these inferences are based
(e.g., treatment of the sample as if it was a simple random sample, or
assumption of an underlying model) should be explicitly stated and
justified. The data collection methods and inherent limitations of the
data should also be discussed.
An initial exposure assessment also may be based upon limited data,
27
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such as estimated ranges, for input variables for an exposure prediction
model. The exposure prediction model would be derived from a postulated
exposure scenario that describes the pathways from sources to contact
with population members. If the data were only sufficient to support
estimates of the ranges of the input variables, the exposure assessment
might be limited to a sensitivity analysis. The purpose of the sensitivity
analysis would be to identify influential model input variables and
develop bounds on the distribution of exposure. A sensitivity analysis
would estimate the range of exposures that would result as individual
model input variables were varied from their minumum to their maximum
possible values with the other input variables held at fixed values,
e.g., their midranges. The overall minimum and maximum possible exposures
usually would be estimated also. For an exposure assessment of this
type, the uncertainty would be characterized by describing the limitations
of the data used to estimate plausible ranges of model input variables
and by discussing justification for the model. Justification of the
model should include a description of the exposure scenario, choice of
model input variables, and the functional form of the model. Sensitivity
to the model formulation also can be investigated by replicating the
sensitivity analysis for plausible alternative models.
If the maximum possible exposure estimated by the sensitivity analysis
presented no significant health risk, there might be no need to refine
the assessment. If both the minimum and maximum exposures presented a
potentially significant health risk, it would be known that the exposure
scenario represented a significant health problem without refining the
assessment. When the minimum exposure estimate does not present a potentially
significant health risk and maximum dose, then greater importance is
28
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placed on choosing a summary parameter of the exposure distribution
(e.g., the mean or percentile) as the basis for a regulatory decision.
Refining the exposure assessment to estimate the distribution of exposure
permits selection of any summary parameter (minimum, maximum, mean, or
percentile, etc.) as the basis for regulatory decision.
The sensitivity analysis can be enhanced by computing the predicted
exposures that result from all possible input variable combinations. If
each input variable has only a finite set of possible values, the set of
all possible combinations of the input variables can be formed, and the
predicted exposure can be computed for each combination. These exposure
predictions can be used to form a distribution of exposures by counting
the number of occurrences of each exposure level or interval of exposures.
This is equivalent to estimating the distribution of exposures that
results from treating all input variable combinations as equally likely.
This procedure can also be applied by discretizing continuous input
variables and representing them by equally-spaced points. In the limit,
as the equal spaces become small and the number of points becomes large,
the distribution of exposure that results from counting occurrences of
exposure levels is equivalent to estimating the distribution of exposures
that results from statistically-independent, continuous input variables
with uniform distributions on the estimated ranges. This estimated
distribution of exposure values can be produced by the methods of
mathematical statistics or Monte Carlo simulation. The Monte Carlo method
consists of randomly generating input variate values and using these to
compute corresponding exposure levels, generating an exposure distribution
via many iterations. Interpretation of statistics based upon this exposure
distribution would be in terms of the equally likely input variable
29
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combinations. For example, the 95th percent!le of this distribution would
be the exposure level exceeded by only 5% of the exposures resulting
from treating all combinations of input variable values as equally likely.
Although this distribution of exposures cannot be interpreted as an
estimate of the population distribution (unless the input variables
actually are statistically independent and uniformly distributed), it
provides additional information for making regulatory decisions.
Characterization of uncertainty would include a discussion of limitations
of the data and justification for the model as discussed above. Sensi-
tivity to model formulation could also be investigated by estimating the
distribution of exposure that results from using the same uniform input
variable distributions with plausible alternative models and comparing
the estimated percent!les.
(3) Assessments Based Upon Subjective Estimates of Input Variable
Distributions
If a model has been formulated that expresses exposure as a function
of one or more input variables, the methods of mathematical statistics or
Monte Carlo simulation can be used to estimate the population distribution
of exposure from an estimate of the joint distribution of the model input
variables. Ideally model input variables should be represented by
empirically validated probability distributions. In some cases, it may
be possible to formulate an estimate of the joint distribution of model
input variables from discussions with subject-matter experts (e.g., via
histograms for statistically-independent input variables). The estimated
population distribution of exposure will be equivalent to the distribution
discussed in section 8. d. (2) for equally likely combinations of input
variable values only when the input variable distributions supported are
30
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Independent uniform distributions. When qualitative knowledge of input
variable distributions is used to estimate the population distribution of
exposure, uncertainty is characterized by discussing justification for
the presumed model and input variable distributions. Alternative models
and/or alternative input variable distributions also should be discussed.
Sensitivity to these alternatives can be investigated by estimating the
distributions of exposure that result from plausible alternatives and
comparing the percentiles of the estimated exposure distributions. All
available data, even if data are limited, should be used to validate the
presumed input variable distributions and the predicted distribution of
exposure.
(4) Assessments Based Upon Data for Model Input Variables
The exposure assessment based upon an estimate of the joint probability
distribution for model input variables can be refined by collecting sample
survey data for model input variables for a sample of population members.
The population distribution of exposure can then be estimated by computing
the expected exposure for each sample member based upon the model. These
expected exposures can be used to directly compute confidence interval
estimates for percentiles of the exposure distribution. Alternatively,
the sample survey data can be used to compute joint confidence interval
estimates for percentiles of the input variable distribution, which can
then be used to generate confidence interval estimates for percentiles of
th'e exposure distribution. In either case, the interval estimates for
percentiles of the exposure distribution are a useful quantitative
characterization of uncertainty.
Characterization of uncertainty for the exposure assessment would
contain a thorough discussion of limitations of the data and justification
31
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for the model used to compute expected exposures. The design of the
sample survey used to produce the data base should also be discussed. If
a probability sample were not used, the lack of a probability sample
would be an additional source of uncertainty. Any assumptions used in
computing the confidence interval estimates, such as independence of
model input variables, should be explicitly stated and justified.
Sensitivity to model formulation can be investigated by estimating the
distribution of exposure for plausible alternative models and comparing
the estimated percentiles, if sample survey data have been collected for
the input variables of the alternative models. Appropriate available
data for exposure should be used to validate the predicted distribution
of exposure. If specific probability distributions have been presumed
for any model input variables, the data for these variables should be
used to test for goodness of fit for these distributions.
(5) Assessments Based Upon Data for Exposure
A major reduction in the uncertainty associated with an exposure
assessment can be achieved by directly measuring the exposure for a
sufficiently large sample of members of the affected population. This
reduction in uncertainty is achieved by eliminating the use of a model
to predict exposure. The measured exposure levels can be used to directly
estimate the population distribution of exposure and confidence interval
estimates for percentiles of the exposure distribution. Direct confidence
interval estimates also can be computed for other characteristics of the
exposure distribution, such as the mean exposure.
These confidence interval estimates are then the primary characteri-
zation of uncertainty for the exposure assessment. Limitations of the
data and design of the sample survey used to collect the data also should
32
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be discussed. If the sample was not a probability sample, this would
again be an additional source of uncertainty.
. (6) Summary
A summary of the primary methods recommended for characterizing
uncertainty in exposure assessments is presented in Table 2. Virtually
all exposure assessments, except those based upon measured exposure levels
for a probability sample of population members, rely upon a model to
predict exposure. The model may be any mathematical function, simple or
complex, that expresses an individual's exposure as a function of one or
more input variables. Whenever a model that has not been validated is
used as the basis for an exposure assessment, the uncertainty associated
with the exposure assessment may be substantial. The primary characterization
of uncertainty is at least partly qualitative in this case, i.e., it
includes a description of the assumptions inherent in the model and
their justification. Plausible alternative models should be discussed.
Sensitivity of the exposure assessment to model formulation can be
investigated by replicating the assessment for plausible alternative
models.
When an exposure assessment is based upon directly measured exposure
levels for a probability sample of population members, uncertainty can be
greatly reduced and described quantitatively. In this case, the primary
sources of uncertainty are measurement errors and sampling errors. The
effects of these sources of error are measured quantitatively by confidence
interval estimates of percent!les of the exposure distribution. Moreover,
the sampling errors can be limited by taking a large sample.
Whenever the latter is not feasible, it is sometimes possible to
obtain at least some data for exposure and model input variables. These
33
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35
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data should be used to assess goodness of fit of the model and/or presumed
distributions of input variables. This substantially reduces the amount
of quantitative uncertainty for estimation of the distribution of exposure
and is strongly recommended. It is recognized, however, that it may not
be feasible to collect such data.
9. REFERENCES
The references should contain a listing of all reports, documents,
articles, memoranda, contacts, etc. that have been cited in the report.
10. APPENDICES
The appendices may contain such items as memoranda and letters that
are not readily accessible, other tables of monitoring data, detailed
lists of emission sources, detailed tables of exposures, process flow
diagrams, mathematical model formulations, or any other item that may be
needed to describe or document the exposure assessment.
36
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