&EPA
EPA/600/R-08/135 | August 2009 | www.epa.gov/ncea
United States
Environmental Protection
Agency
Highlights of the Child-Specific
Exposure Factors Handbook
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EPA/600/R-08/135
August 2009
www. epa. gov/ncea
Highlights of the Child-Specific
Exposure Factors Handbook
National Center for Environmental Assessment
Office of Research and Development
U.S. Environmental Protection Agency
Washington, DC 20460
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Highlights of the Child-Specific Exposure Factors Handbook
CSEFH
NOTICE
The U.S. Environmental Protection Agency through its Office of Research and Development, National Center for
Environmental Assessment, funded the research described here under contract no. EP-W-04-035 with Versar, Inc.
This document contains a brief overview of the contents of the 2008 version of the Child-Specific Exposure Factors
Handbook, which was published earlier, i.e., 2002 and revised in 2005. This highlights document has been
subjected to the Agency's administrative review and has been approved for publication as an EPA document.
Preferred Citation:
U.S. Environmental Protection Agency (EPA). (2009) Highlights of the child-specific exposure factors handbook.
National Center for Environmental Assessment, Washington, DC; EPA/600/R-08/135. Available from the National
Technical Information Service, Springfield, VA and online at http://www.epa.gov/ncea.
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FOREWORD
In 2008, the U.S. Environmental Protection Agency (EPA), Office of Research and Development, National Center
for Environmental Assessment (NCEA) published a revised version of its original 2002 Child-Specific Exposure
Factors Handbook. Its purpose is to provide exposure/risk assessors with information on behavioral and
physiological factors that can be used in assessing exposures among children. The Handbook presents information
on children's exposure factors, based on selected studies published through July 2008. It uses a standard set of age
categories for children, ages 0 to <21 years old, to permit comparison of data among multiple sources and to provide
consistency among different types of exposure factors. The Handbook provides recommended values for the
various exposure factors based on these standard age groups. These revisions assist exposure assessors with the
implementation of the recommendations presented in the EPA's 2005 Guidelines for Carcinogen Risk Assessment
and the Supplemental Guidance for Assessing Susceptibility from Early-Life Exposure to Carcinogens.
Specifically, the 2005 Guidelines emphasized the need to consider childhood as a series of life stages rather than
children as subpopulations and to sum exposures and risks across life stages rather than relying on the use of a
lifetime average adult exposure to calculate risk.
The goals for revising the Handbook were to
(1) most importantly, reanalyze data and present the information using the standardized set of childhood age
groups as recommended in EPA's 2005 Guidance on Selecting Age Groups for Monitoring and Assessing
Childhood Exposures to Environmental Contaminants', and
(2) incorporate new exposure factors data/research that had become available since the early 2000s.
This Highlights document was developed to provide a brief overview of the contents of the Child-Specific Exposure
Factors Handbook and to facilitate access to its exposure factors recommendations. As such, it contains a subset of
the information provided in the complete Handbook. This Highlights document is a product of the EPA's Exposure
Factors Program. NCEA established the Exposure Factors Program to develop tools and databases that improve the
scientific basis of exposure and risk assessment by (1) identifying exposure factors needs in consultation with
clients, and exploring ways for filling data gaps; (2) compiling existing data on exposure factors needed for
assessing exposures/risks; and (3) assisting clients in the use of exposure factors data. These activities are supported
by and respond to the needs of the various EPA program offices.
EPA invites you to visit http://epa.gov/risk/guidance.htm where you can view and download chapters from the
Child-Specific Exposure Factors Handbook as well as the Exposure Factors Handbook. Each chapter in these
handbooks presents recommended values for exposure factors as well as a discussion of the underlying data used to
develop the recommendations. NCEA intends to update its Web site periodically so that the information provided
by the Exposure Factors Program is current and relevant.
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Highlights of the Child-Specific Exposure Factors Handbook
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CONTENTS
ACRONYMS AND ABBREVIATIONS vi
ABOUT THE HANDBOOK vii
INTENDED AUDIENCE 1
BACKGROUND 1
SELECTION OF STUDIES FOR THE HANDBOOK 3
General Assessment Factors 3
Selection Criteria 3
APPROACH USED TO DEVELOP RECOMMENDATIONS FOR EXPOSURE FACTORS 5
SUGGESTED REFERENCES FOR USE IN CONJUNCTION WITH THE HANDBOOK 6
CONSIDERING LIFESTAGE WHEN CALCULATING EXPOSURE AND RISK 8
FUNDAMENTAL PRINCIPLES OF EXPOSURE ASSESSMENT 8
Dose Equations 9
Use of Exposure Factors Data in Probabilistic Analyses 11
CUMULATIVE EXPOSURES 12
REFERENCES 13
LIST OF TABLES
Table 1. Summary of Recommended Exposure Factors for Children 17
Table 2. Characterization of Variability inExposure Factors 25
Table 3. Considerations Used to Rate Confidence in Recommended Values 26
Table 4. Summary of Confidence Ratings for Exposure Factor Recommendations 27
Table 5. Age-Dependent Potency Adjustment Factor (ADAF) by Exposure Age Group 28
LIST OF FIGURES
Figure 1. The Exposure-Dose Effect Continuum 29
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ACRONYMS AND ABBREVIATIONS
ADAF
ADD
BMD
C
cm2
ED
g
GAP
Hc
IR
kg
LADD
m2
m3
mg
mL
NCEA
OCHP
OCHPEE
ORD
PBPK
RfD
RfC
SPC
USDA
U.S. EPA
Age Dependent Adjustment Factors
Average Daily Dose
Benchmark Dose
Contaminant Concentration
Square Centimeter
Exposure Duration
Gram
General Assessment Factor
Human Equivalent Concentration
Intake Rate
Kilogram
Lifetime Average Daily Dose
Square Meter
Cubic Meter
Milligram
Milliliter
National Center for Environmental Assessment
Office of Children's Health Protection
Office of Children's Health Protection and Environmental Education
Office of Research and Development
Physiologically-Based Pharmacokinetic
Reference Dose
Reference Concentration
Science Policy Council
United States Department of Agriculture
U.S. Environmental Protection Agency
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ABOUT THE HANDBOOK
This Highlights document presents an overview
of the information provided in the U.S. EPA's Child-
Specific Exposure Factors Handbook (U.S. EPA
2008a). The Handbook reviews and summarizes data
on the various factors used in the exposure
assessment of children (i.e., individuals <21 years
old) and provides recommendations for the exposure
assessment community. The Handbook contains 17
chapters: an introduction (Chapter 1), a discussion
about the variability and uncertainty in assessing
exposure factors (Chapter 2), and
nonchemical-specific data on exposure factors for the
U.S. EPA recommended set of childhood age groups
in the following areas:
• ingestion of water and other select liquids
(Chapter 3);
• non-dietary ingestion factors (Chapter 4);
• ingestion of soil and dust (Chapter 5);
• inhalation rates (Chapter 6);
• dermal exposure factors (Chapter 7);
• body weight (Chapter 8);
• intake of fruits and vegetables (Chapter 9);
• intake offish and shellfish (Chapter 10);
• intake of meat, dairy products, and fats (Chapter
ii);
• intake of grain products (Chapter 12);
• intake of home-produced foods (Chapter 13);
• total food intake (Chapter 14);
• human milk intake (Chapter 15);
• activity factors (Chapter 16); and
• consumer products (Chapter 17).
The Child-Specific Exposure Factors Handbook
was first published in 2002 (U.S. EPA, 2002a).
Subsequently, the U.S. EPA published its Guidance
on Selecting Age Groups for Monitoring and
Assessing Childhood Exposures to Environmental
Contaminants (U.S. EPA, 2005a). To the extent
possible, source data for the independent studies cited
in the earlier version of the Handbook were obtained
and reanalyzed to conform to the standard age
categories: birth to <1 month, 1 to <3 months, 3 to <6
months, 6 to <12 months, 1 to <2 years, 2 to <3
years, 3 to <6 years, 6 to <11 years, 11 to < 16 years,
and 16 to <21 years.
The data presented in the Child-Specific Exposure
Factors Handbook were compiled from various
sources including the U.S. EPA's Exposure Factors
Handbook (U.S. EPA, 1997a), government reports,
and information presented in the scientific literature,
published through July 2008. The data presented are
generally the result of analyses by the individual
study authors. However, in some cases, the U.S. EPA
conducted analysis of published primary data to
present results for the recommended age groups.
Studies presented in the Handbook were chosen
because they were seen as useful and appropriate for
estimating exposure factors based on the following
evaluation elements: (1) soundness; (2) applicability
and utility; (3) clarity and completeness; (4)
variability and uncertainty; and (5) evaluation and
review.
Generally, studies were designated as "key" or
"relevant" studies. Key studies were considered the
most useful for deriving recommendations, while
relevant studies provided applicable or pertinent data,
but not necessarily the most important for a variety of
reasons (e.g., data were outdated, limitations in study
design). The Handbook provides recommended
values for exposure factors based on its interpretation
of the key studies. Key recommendations from the
Handbook are summarized in Table 1 (see pages 17-
24) of this Highlights document. Additional
recommendations and detailed supporting
information can be found in the individual chapters
of the Handbook. These recommendations are not
legally binding and should be interpreted as
suggestions that U.S. EPA Program Offices or
individual exposure/risk assessors can consider and
modify as needed based on their own evaluation of a
given risk-assessment situation. In certain cases,
different values may be appropriate in consideration
of policy, precedent, strategy, or other factors (e.g.,
more up-to-date data of better quality or more
representative of the population of concern). The
U.S. EPA also assigned confidence ratings of low,
medium, or high to each recommended value based
on the evaluation elements described above. These
ratings are not intended to represent uncertainty
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analyses; rather, they represent the U.S. EPA's
judgment on the quality of the underlying data used
to derive the recommendations.
All tables and figures have been placed at the end
of the Handbook.
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INTENDED AUDIENCE
The Child-Specific Exposure Factors Handbook
(U.S. EPA 2008a) is intended for use by exposure
and risk assessors both within and outside the U.S.
EPA as a reference tool and primary source of
exposure factor information. It may be used by
exposure and risk assessors, economists, and other
interested parties as a source for data and/or U.S.
EPA recommendations on numeric estimates for
behavioral and physiological characteristics needed
to estimate childhood exposure to toxic contaminants
and other environmental stressors.
BACKGROUND
Because of physiological and behavioral
differences, environmental exposures among children
differ from exposures among adults. Children may be
more exposed to some environmental contaminants
because (1) they consume more of certain foods and
water per unit of body weight than adults; (2) they
have a higher ratio of body surface area to volume
than adults; and (3) they experience important, rapid
changes in behavior and physiology that may lead to
differences in exposure. Many studies have shown
that young children can be exposed to various
contaminants, including pesticides, during normal
oral exploration of their environment (i.e.,
hand-to-mouth behavior) and by touching floors,
surfaces, and objects such as toys (Eskenazi et al.,
1999; Gurunathan et al., 1998; Lewis et al., 1999;
Nishioka et al., 1999; Garry, 2004). Dust and
tracked-in soil accumulate in carpets, where young
children spend a significant amount of time (Lewis et
al., 1999). Children living in agricultural areas may
experience higher exposures to pesticides than do
other children (Curwin et al., 2007). They may play
in nearby fields or be exposed via consumption of
contaminated human milk from their farmworker
mothers (Eskenazi et al., 1999).
In terms of risk, children may also differ from
adults in their vulnerability to environmental
pollutants because of toxicodynamic differences
(e.g., when exposures occur during periods of
enhanced susceptibility) and/or toxicokinetic
differences (i.e., differences in absorption,
metabolism, and excretion) (U.S. EPA, 2000a). The
immaturity of metabolic enzyme systems and
clearance mechanisms in young children can result in
longer half-lives of environmental contaminants
(Ginsberg et al., 2002; Clewell et al., 2004). The
cellular immaturity of children and the ongoing
growth processes account for elevated risk (AAP,
1997). Toxic chemicals in the environment can cause
neurodevelopmental disabilities, and the developing
brain can be particularly sensitive to environmental
contaminants. For example, elevated blood lead
levels and prenatal exposures to even relatively low
levels of lead can result in behavior disorders and
reductions of intellectual function in children
(Landrigan et al., 2005). Exposure to high levels of
methylmercury can result in developmental
disabilities (e.g., intellectual deficiency, speech
disorders, and sensory disturbances) among children
(Myers et al., 2000). Other authors have described the
importance of exposure timing (i.e., preconceptional,
prenatal, and postnatal) and how it affects the
outcomes observed (Selevan et al., 2000). It has also
been suggested that higher levels of exposure to
indoor air pollution and allergens among inner-city
children compared to non-inner-city children may
explain the difference in asthma levels between these
two groups (Breysee et al., 2005). With respect to
contaminants that are carcinogenic via a mutagenic
mode of action, the U.S. EPA has found that
childhood is a particularly sensitive period of
development in which cancer potencies per year of
exposure can be an order of magnitude higher than
during adulthood (U.S. EPA, 2005c).
Executive Order 13045: Protection of Children
from Environmental Health Risks and Safety Risks,
signed in 1997, requires all federal agencies to
address health and safety risks to children, to
coordinate research priorities on children's health,
and to ensure that their standards take into account
special risks to children (EO, 1997). To implement
the Order, the U.S. EPA established the Office of
Children's Health Protection (OCHP) (renamed the
Office of Children's Health Protection and
Environmental Education [OCHPEE] in 2005), who
works with Program and regional offices within the
U.S. EPA to promote a safe and healthy environment
for children by ensuring that all regulations,
standards, policies, and risk assessments take into
account risks to children. Legislation, such as the
Food Quality Protection Act and the Safe Drinking
Water Act amendments of 1996, has made coverage
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of children's health issues more explicit, and research
on children's health issues is continually expanding.
As a result of the emphasis on children's risk, the
U.S. EPA's ORD developed a Strategy for Research
on Environmental Risks to Children (U.S. EPA,
2000a). The goal of the Strategy is to improve the
quality of risk assessments for children. The Child-
Specific Exposure Factors Handbook (U.S. EPA,
2008a) is intended to support the U.S. EPA/ORD's
efforts to improve exposure and risk assessments for
children.
In 1997, the U.S. EPA/ORD/NCEA published
the Exposure Factors Handbook (U.S. EPA, 1997a).
The Handbook includes exposure factors and related
data on both adults and children. Subsequently, the
U.S. EPA Program Offices identified the need to
consolidate all children's exposure data into a single
document, and the interim final Child-Specific
Exposure Factors Handbook was published in 2002
to fulfill this need (U.S. EPA, 2002a). The 2008
Handbook (U.S. EPA, 2008a) updates the 2002
edition of the Child-Specific Exposure Factors
Handbook. It provides nonchemical-specific data on
exposure factors that can be used to assess
contributions from dietary and non-dietary ingestion
exposure, dermal exposure, and inhalation exposure
among children. Although the preconceptional and
prenatal (fetal) life stages are important to consider,
they are not covered in the Handbook.
Preconceptional exposures are included in the
Exposure Factors Handbook (U.S. EPA, 1997a)
since they relate to maternal and paternal exposures,
and exposure factors for pregnant and lactating
women are being developed as part of a separate
effort. The Handbook also highlights the changes in
risk-assessment practices that were first presented in
the U.S. EPA's Cancer Guidelines (U.S. EPA,
2005b), regarding the need to consider children as
life stages rather than as subpopulations. It also
emphasizes a major recommendation in U.S. EPA's
Supplemental Guidance for Assessing Susceptibility
from Early-Life Exposure to Carcinogens (U.S. EPA,
2005c) to sum exposures and risks across life stages
rather than relying on the use of a lifetime average
adult exposure to calculate risk.
The Child-Specific Exposure Factors Handbook
(U.S. EPA, 2008a) does not include
chemical-specific data or information on
physiological parameters that may be needed for
exposure assessments involving
physiologically-based pharmacokinetic (PBPK)
modeling. The U.S. EPA has developed guidance on
how to use and applications of PBPK information in
risk assessment in the report titled Use of
Physiologically Based Pharmacokinetic (PBPK)
Models to Quantify the Impact of Human Age and
Interindividual Differences in Physiology and
Biochemistry Pertinent to Risk (U.S. EPA, 2006a).
With very few exceptions, the data presented in
the Handbook were derived from the analyses of the
individual study authors. Because the studies
included in the Handbook vary in terms of their
objectives, design, scope, presentation of results, etc.,
the level of detail, statistics, and terminology may
vary from study to study and from factor to factor.
For example, some authors used geometric means to
present their results, while others used arithmetic
means or distributions. Authors sometimes used
different age ranges to describe data for children. In
most cases, the original data were unavailable, and
the study results could not be reallocated into the
standard age groups used in the Handbook. When
adequate detailed data were available, efforts were
made to reallocate source data into the standard age
groups recommended by the U.S. EPA in the report
titled Guidance on Selecting Age Groups for
Monitoring and Assessing Childhood Exposures to
Environmental Contaminants (U.S. EPA, 2005a).
Within the constraint of presenting the original
material as accurately as possible, the U.S. EPA
made an effort to present discussions and results in a
consistent manner. The strengths and limitations of
each study were discussed to provide the reader with
a better understanding of the uncertainties associated
with the values derived from the study.
Most of the data presented in the Handbook were
derived from studies that targeted (1) the general
national population (e.g., USDA food consumption
surveys) or (2) a sample population from a specific
area or group (e.g., soil ingestion in children from a
three-city area in southeastern Washington State). If
it is necessary to characterize a population that is not
directly covered by the data in the Handbook, the risk
or exposure assessor should evaluate whether these
data may be used as suitable substitutes for the
population of interest or whether there is a need to
seek additional population-specific data. The
decision as to whether to use site-specific or national
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values for an assessment depends both on the quality
of the competing data sets as well as on the purpose
of the specific assessment. If information is needed
for identifying and enumerating populations who
may be at risk for greater contaminant exposures or
who exhibit a heightened sensitivity to particular
chemicals, the reader is referred to
Socio-demographic Data Used for Identifying
Potentially Highly Exposed Populations (U.S. EPA,
1999).
In conjunction with the Guidance on Selecting
Age Groups for Monitoring and Assessing Childhood
Exposures to Environmental Contaminants (U.S.
EPA, 2005a), the Handbook adopted the age group
notation "X to < Y" (e.g., the age group 3 to <6 years
is meant to span a 3-year time interval from a child's
3rd birthday up until the day before his or her 6th
birthday).
SELECTION OF STUDIES FOR THE
HANDBOOK
Information in the Handbook was summarized
from studies documented in the scientific literature
and other available sources. Studies were chosen that
were seen as useful and appropriate for estimating
exposure factors for children. The Handbook contains
summaries of selected studies published through July
2008.
Certain studies described in the Handbook are
designated as "key," that is, the most useful for
deriving exposure factors. The recommended values
for most exposure factors are based on the results of
the key studies. Other studies are designated
"relevant," meaning applicable or pertinent, but not
necessarily the most important. This distinction was
made on the strength of the attributes listed in the
"General Assessment Factors" listed below.
General Assessment Factors
The U.S. EPA recognizes the need to evaluate
the quality and relevance of scientific and technical
information used in support of Agency actions (U.S.
EPA, 2002b, 2003a, 2006b). When evaluating
scientific and technical information, the U.S. EPA's
Science Policy Council (SPC) recommends using
five General Assessment Factors (GAFs): (1)
soundness, (2) applicability and utility, (3) clarity and
completeness, (4) uncertainty and variability, and (5)
evaluation and review (U.S. EPA, 2003a). These
GAFs were adapted and expanded to include specific
considerations deemed to be important during
evaluation of exposure factors data, and were used to
judge the quality of the underlying data used to
derive recommendations.
Selection Criteria
The selection of key studies that form the basis
for the exposure factor recommendations provided in
the Child-Specific Exposure Factors Handbook (U.S.
EPA, 2008a) as well as the confidence ratings for
these recommendations, were based on specific
criteria within each of the five GAFs, as follows:
(1) Soundness: Scientific and technical
procedures, measures, methods, or models
employed to generate the information are
reasonable for, and consistent with, the intended
application.
Adequacy of the Study Approach: In
general, more confidence was placed on
experimental procedures or approaches that
more likely or closely captured the desired
measurement. Direct exposure data collection
techniques, such as direct observation,
personal monitoring devices, or other known
methods were preferred where available. If
studies utilizing direct measurement were not
available, studies were selected that relied on
validated indirect measurement methods.
Studies were also deemed preferable if based
on primary data, but studies based on
secondary sources were also included where
they offered an original analysis. In general,
higher confidence was placed on exposure
factors based on primary data.
Minimal (or Defined) Bias in Study Design:
More confidence was placed on exposure
factors based on studies that minimized bias.
Studies were sought that were designed with
minimal bias, or at least if biases were
suspected to be present, the direction of the
bias (i.e., an over or underestimate of the
parameter) was either stated or apparent from
the study design.
(2) Applicability and Utility: The information is
relevant for the Agency's intended use.
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Focus on Exposure Factor of Interest:
Studies were preferred that directly addressed
the exposure factor of interest, or addressed
related factors that have significance for the
factor under consideration.
Representativeness of the Population: More
confidence was placed in studies that
specifically addressed the United States
population. Data from populations outside the
United States were sometimes included if
behavioral patterns or other characteristics of
exposure were similar. Additionally, studies
seeking to characterize a particular region or
population were selected, if appropriately
representative of that population.
Currency of Information: More confidence
was placed in studies that were sufficiently
recent to represent current exposure
conditions. This is an important consideration
for those factors that change with time. Older
data were evaluated and considered in
instances where the variability of the exposure
factor over time was determined to be
insignificant or unimportant. In some cases,
recent data were very limited. Therefore, the
data provided in these instances were the only
available data. Limitations on the age of the
data were noted. Recent studies are more
likely to use state-of-the-science
methodologies that reflect advances in the
exposure assessment field. Consequently,
exposure factor recommendations based on
current data were given higher confidence
ratings than those based on older data—except
in cases where the age of the data would not
affect the recommended values.
Adequacy of Data Collection Period:
Because most users of the Handbook are
primarily addressing chronic exposures,
studies were sought that utilized the most
appropriate techniques for collecting data to
characterize long-term behavior. Higher
confidence ratings were given to exposure
factor recommendations that were based on an
adequate data collection period.
(3) Clarity and Completeness: The degree of
clarity and completeness with which the data,
assumptions, methods, quality assurance,
sponsoring organizations, and analyses employed
to generate the information are documented.
Accessibility: Studies that the user could
access in their entirety, if needed, were
preferred.
Reproducibilitv: Studies that contained
sufficient information so that methods could
be reproduced, or could be evaluated, based
on the details of the author's work, were
preferred.
Quality Assurance: Studies with documented
quality-assurance/quality-control measures
were preferred. Higher confidence ratings
were given to exposure factors that were
based on studies where appropriate quality
assurance/quality control measures were used.
(4) Variability and Uncertainty: The variability
and uncertainty (quantitative and qualitative) in
the information or the procedures, measures,
methods, or models are evaluated and
characterized.
Variability in the Population: Variability
arises from true heterogeneity across people,
places, or time and can affect the precision of
exposure estimates and the degree to which
they can be generalized. The types of
variability include spatial, temporal, and
interindividual. Studies were sought that
characterized any variability within
populations. Higher confidence ratings were
given to exposure factors that were based on
studies where variability was well
characterized.
Uncertainty: Uncertainty represents a lack of
knowledge about factors affecting exposure or
risk and can lead to inaccurate or biased
estimates of exposure. The types of
uncertainty include scenario, parameter, and
model. Studies were sought with minimal
uncertainty in the data, which was judged by
evaluating all the considerations listed above.
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Studies were preferred that identified
uncertainties, such as those due to inherent
variability in environmental and
exposure-related parameters or possible
measurement error. Higher confidence ratings
were given to exposure factors based on
studies where uncertainty had been
minimized.
(5) Evaluation and Review: The information or
the procedures, measures, methods, or models are
independently verified, validated, and peer
reviewed.
Peer Review: Studies selected were those
from the peer-reviewed literature and final
government reports. Unpublished and internal
or interim reports were avoided.
Number and Agreement of Studies: Higher
confidence was placed on recommendations
where data were available from more than one
key study and there was good agreement
between studies.
APPROACH USED TO DEVELOP
RECOMMENDATIONS FOR EXPOSURE
FACTORS
As a first step to develop recommendations, the
U.S. EPA reviewed the literature pertaining to a
factor and determined key studies. These key studies
were used to derive recommendations for the values
of each factor. The recommended values were
derived solely from the U.S. EPA's interpretation of
the available data. Different values may be
appropriate for the user in consideration of policy,
precedent, strategy, or other factors such as
site-specific information.
In providing recommendations for the various
exposure factors, an attempt was made to present
percentile values that are consistent with the exposure
estimators defined in Guidelines for Exposure
Assessment (i.e., mean, 50th, 90th, 95th, 98th, and
99.9th percentiles) (U.S. EPA. 1992a). However, this
was not always possible because the data available
were limited for some factors, or the study authors
did not provide such information. It is important to
note, however, that these percentiles were discussed
in the Guidelines within the context of risk
descriptors and not individual exposure factors. For
example, the guidelines state that the assessor may
derive a high-end estimate of exposure by using
maximum or near maximum values for one or more
sensitive exposure factors, leaving others at their
mean value. The term "upper percentile" is used
throughout the Handbook, and it is intended to
represent values in the upper tail (i.e., between 90th
and 99.9th percentiles) of the distribution of values
for a particular exposure factor.
The U.S. EPA's procedure for developing
recommendations was as follows:
(1) Study Review and Evaluation: Key studies
were evaluated in terms of both quality and
relevance to specific populations (general U.S.
population, age groups, gender, etc.). The GAFs
described earlier were used as criteria for
assessing the quality of studies.
(2) Single Versus Multiple Key Studies: If only
one study was classified as key for a particular
factor, the mean value from that study was
selected as the recommended central value for
that population. If multiple key studies with
reasonably equal quality, relevance, and study
design information were available, a weighted
mean (if appropriate, considering sample size and
other statistical factors) of the studies was chosen
as the recommended mean value. If the key
studies were judged to be unequal in quality,
relevance, or study design, the range of means
was presented, and the user of the Handbook must
employ judgment in selecting the most
appropriate value for the population of interest.
Recommendations for upper percentiles, when
multiple studies were available, were calculated
as the midpoint of the range of upper percentile
values of the studies for each age group where
data were available.
(3) Variability: The variability of the factor
across the population was described. For
recommended values, as well as for each of the
studies on which the recommendations are based,
variability was characterized in one or more of
three ways: (1) as a table with various percentiles
or ranges of values; (2) as analytical distributions
with specified parameters; and/or (3) as a
qualitative discussion. Analyses to fit standard or
parametric distributions (e.g., normal, lognormal)
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to the exposure data were not performed by the
authors of the Handbook, but they have been
reproduced as they were found in the literature.
Recommendations on the use of these
distributions were made where appropriate based
on the adequacy of the supporting data. The list of
exposure factors and the way in which variability
was characterized throughout the Handbook (i.e.,
average, median, upper percentiles, multiple
percentiles, and fitted distribution) are presented
in Table 2.
(4) Uncertainty: Uncertainties were discussed in
terms of data limitations. Such limitations
include the range of circumstances over which the
estimates were (or were not) applicable, possible
biases in the values themselves, a statement about
parameter uncertainties (measurement error,
sampling error) and model/scenario uncertainties,
if models/scenarios were used to derive the
recommended value. Chapter 2 of the Handbook
presents a discussion of variability and
uncertainty for exposure factors.
(5) Confidence Ratings: Finally, the U.S. EPA
assigned a confidence rating of law, medium, or
high to each recommended value. This rating is
not intended to represent an uncertainty analysis;
rather, it represents the U.S. EPA's judgment on
the quality of the underlying data used to derive
the recommendation. This judgment was made
using the GAFs described earlier. Table 3
provides an adaptation of the GAFs as they
pertain to the confidence ratings for the exposure
factor recommendations. Clearly, there is a
continuum from law to high. Therefore, the
assignment of a rating to a particular factor
involves professional judgment.
Recommendations given in the Handbook are
accompanied by a discussion of the rationale for
their rating.
It is important to note that the study elements
listed in Table 3 do not have the same weight
when arriving at the overall confidence rating for
the various exposure factors. The relative weight
of each of these elements for the various factors is
subjective and based on the professional
judgment of the authors of the Handbook. Also,
the relative weights depend on the exposure factor
of interest. For example, the adequacy of the data
collection period may be more important when
determining usual intake of foods in a population,
but it is not as important for factors where
long-term variability may be small, such as tap
water intake. In the case of tap water intake, the
currency of the data was a critical element in
determining the final rating. In general, most
studies ranked high with regard to "level of peer
review," "accessibility," "focus on the factor of
interest," and "data pertinent to the United States"
because the U.S. EPA specifically sought studies
for the Handbook that met these criteria.
The elements in Table 3 were important
considerations for inclusion of a study in the
Handbook. However, a high score for these
elements does not necessarily translate into a high
overall rating. Other considerations also informed
the assigned confidence ratings. One such
consideration was the ease at which the exposure
factor of interest could be measured. For example,
soil ingestion by children can be estimated by
measuring, in the feces of children, the levels of
certain elements found in soil. Body weight,
however, can be measured directly, and it is
therefore a more reliable measurement. The fact
that soil ingestion is more difficult to measure
than body weight is reflected in the overall
confidence rating given to both of these factors.
In general, the better the methodology used to
measure the exposure factor, the higher the
confidence in the value.
(6) Recommendation Tables: The U.S. EPA
developed a table at the beginning of each chapter
of the Child-Specific Exposure Factors Handbook
(U.S. EPA, 2008a) that summarizes the
recommended values for the relevant factor.
Table 1 summarizes the principal exposure factors
addressed in the Handbook. Table 4 summarizes
the confidence ratings assigned to the various
factors.
SUGGESTED REFERENCES FOR USE IN
CONJUNCTION WITH THE HANDBOOK
The main steps for performing an exposure
assessment are (1) identifying the source of the
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environmental contamination and the media that
transports the contaminant; (2) determining the
contaminant concentration; (3) determining the
exposure scenarios—including pathways and routes
of exposure; (4) determining the exposure time,
frequency, and duration; and (5) identifying the
exposed population. Many of the issues related to
characterizing exposure from selected exposure
pathways have been addressed in a number of
existing U.S. EPA documents. Some of these provide
guidance while others demonstrate various aspects of
the exposure process. These documents include, but
are not limited, to the following, which are listed in
chronological order:
• Methods for Assessing Exposure to Chemical
Substances, Volumes 1-13 (U.S. EPA,
1983-1989)
• Standard Scenarios for Estimating Exposure
to Chemical Substances During Use of
Consumer Products (U.S. EPA, 1986)
• Selection Criteria for Mathematical Models
Used in Exposure Assessments: Surface
Water Models (U.S. EPA, 1987)
• Selection Criteria for Mathematical Models
Used in Exposure Assessments: Groundwater
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Assessing Susceptibility from Early-Life
Exposure to Carcinogens (U.S. EPA, 2005b)
• Supplemental Guidance for Assessing
Susceptibility from Early-Life Exposure to
Carcinogens (U.S. EPA, 2005c)
• Protocol for Human Health Risk Assessment,
Protocol for Hazardous Waste Combustion
Facilities (U.S. EPA, 2005d)
• A Framework for Assessing Health Risk of
Environmental Exposures to Children (Final)
(U.S. EPA, 2006c)
• Concepts, Methods, and Data Sources for
Cumulative Health Risk assessment of
Multiple Chemicals, Exposures and Effects:
A Resource Document (Final) (U.S. EPA,
2008b)
These documents may serve as valuable information
resources to assist in the assessment of exposure. The
reader is encouraged to refer to them for more
detailed discussion.
CONSIDERING LIFESTAGE WHEN
CALCULATING EXPOSURE AND RISK
A key component of U.S. EPA's Guidance on
Selecting Age Groups for Monitoring and Assessing
Childhood Exposures to Environmental
Contaminants (U.S. EPA, 2005a) involves the need
to sum age-specific differences in exposure across
time when assessing long-term exposure, as well as
integrating these age-specific exposures with
age-specific differences in toxic potency in those
cases where information exists to describe such
differences: an example is carcinogens that act via a
mutagenic mode of action (Supplemental Guidance
for Assessing Susceptibility from Early-Life Exposure
to Carcinogens [U.S. EPA, 2005c]). When assessing
chronic risks (i.e., exposures greater than 10% of
human lifespan), rather than assuming a constant
level of exposure for 70 years (usually consistent
with an adult level of exposure), the Agency is now
recommending that assessors calculate chronic
exposures by summing time-weighted exposures that
occur at each life stage; the Handbook provides data
arrayed by childhood age in order to follow this new
guidance. This approach is expected to increase the
accuracy of risk assessments because it will account
for life-stage differences in exposure. Depending on
whether body-weight-adjusted childhood exposures
are either smaller or larger compared to those for
adults, calculated risks could either decrease or
increase when compared with the historical approach
of assuming a lifetime of a constant adult level of
exposure.
The Supplemental Guidance for Assessing
Susceptibility from Early-Life Exposure to
Carcinogens also recommends that in those cases
where age-related differences in toxicity also occur,
differences in both toxicity and exposure should be
integrated across all relevant age intervals. This
guidance describes such a case for carcinogens that
act via a mutagenic mode of action, where age
dependent potency adjustments factors (ADAFs) of
lOx and 3x are recommended for children ages birth
to <2 years and 2 to <16 years, respectively, when
there is exposure during those years and available
data are insufficient to derive chemical-specific
adjustment factors.
Table 5, along with Chapter 6 of the
Supplemental Guidance, has been developed to help
the reader understand how to use the new sets of
exposure and potency age groupings when
calculating risk through the integration of life-stage-
specific changes in exposure and potency.
Thus, Lifetime Cancer Risk (for a population with
average life expectancy of 70 years) = S (Exposure x
Duration/70 yrs x Potency x ADAF) summed across
all the age groups presented in Table 5. This is a
departure from the way cancer risks have historically
been calculated which was based upon the premise
that risk is proportional to the daily average of the
long-term adult dose.
FUNDAMENTAL PRINCIPLES OF
EXPOSURE ASSESSMENT
The definition of exposure as used by the
International Programme on Chemical Safety is the
"contact of an organism with a chemical or physical
agent, quantified as the amount of chemical available
at the exchange boundaries of the organism and
available for absorption." This means contact with
the visible exterior of a person such as the skin, and
openings such as orifices and lesions. The process of
a chemical entering the body can be described in two
steps: contact (exposure) followed by entry
(crossing the boundary). In the context of
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environmental risk assessment, risk to an individual
or population can be represented as a continuum from
the source through exposure to dose to effect as
shown in Figure 1 (U.S. EPA, 2003d; IPCS, 2006).
The process begins with a chemical or agent released
from a source into the environment. Once in the
environment, the chemical or agent can be
transformed and transported through the environment
via air, water, soil, dust, and diet. Individuals come in
contact with the chemical through inhalation,
ingestion, or skin/eye contact. The individual's
activity patterns as well as the concentration of the
chemical will determine the magnitude, frequency,
and duration of the exposure. The exposure becomes
an absorbed dose when the chemical crosses an
absorption barrier. When the chemical or its
metabolites interact with a target tissue, it becomes a
target tissue dose, which may lead to an adverse
health outcome. The text under the boxes in Figure 1
indicates the specific information that may be needed
to characterize each box.
Dose Equations
Starting with a general integral equation for
exposure (U.S. EPA, 1992a), several dose equations
can be derived depending upon boundary
assumptions. One of the more useful of these derived
equations is the Average Daily Dose (ADD). The
ADD, which is used for many noncancer effects,
averages an external dose over the period of time
exposure occurred, and it is normalized by body
weight (ADDpot)(see equation 1).
ADDpot =
External Dose
Body Weight x Averaging Time
The exposure can be expressed in as follows:
External Dose = C x IR x ED (2)
Where
C
IR
ED
= Contaminant Concentration
= Intake Rate
= Exposure Duration
Contaminant concentration is the concentration
of the contaminant in the medium (e.g., air, food, and
soil) contacting the body and has units of
mass/volume or mass/mass.
The intake rate refers to the rates of inhalation,
ingestion, and dermal contact, depending on the route
of exposure. For ingestion, the intake rate is simply
the amount of food containing the contaminant of
interest that an individual ingests during some
specific time period (units of mass/time). Much of the
Handbook is devoted to rates of ingestion for some
broad classes of food. For inhalation, the intake rate
is the rate at which contaminated air is inhaled.
Factors presented in the Handbook that affect dermal
exposure are skin surface area and estimates of the
amount of soil that adheres to the skin.
The exposure duration is the length of time of
contaminant contact. The length of time a person
lives in an area, frequency of bathing, time spent
indoors versus outdoors, etc., all affect the exposure
duration. Chapter 16, Activity Factors, describes
examples of population behavior/activity patterns that
may be useful for estimating exposure durations.
When the parameter values IR and ED remain
constant over time, they are substituted directly into
the exposure equation. When they change with time,
a summation approach is needed to calculate
exposure. In either case, the exposure duration is the
length of time exposure occurs at the concentration
and the intake rate specified by the other parameters
in the equation.
Note that the advent of childhood age groupings
means that separate ADDs should be calculated for
each age group considered. Chronic exposures can
then be calculated by summing across each
life-stage-specific ADD.
Cancer risks have traditionally been calculated in
those cases where a linear nonthreshold model is
assumed, in terms of lifetime probabilities, by
utilizing dose values presented in terms of lifetime
ADDs (LADDs). The LADD takes the form of
Equation 1, with lifetime replacing averaging time.
While the use of LADD may be appropriate when
developing screening level estimates of cancer risk,
as discussed above, the U.S. EPA is now
recommending that risks should be calculated by
integrating exposures or risks throughout all life
stages (U.S. EPA, 1992a).
For some types of analyses, dose can be
expressed as a total amount (with units of mass, e.g.,
mg) or as a dose rate in terms of mass/time (e.g.,
mg/day), or as a rate normalized to body mass (e.g.,
with units of mg of chemical per kg of body weight
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per day [mg/kg-day]). The LADD is usually
expressed in terms of mg/kg-day or other
mass/mass-time units.
In most cases (inhalation and ingestion
exposures), the dose-response parameters for
carcinogenic risks have been adjusted for the
difference in absorption across body barriers between
humans and the experimental animals used to derive
such parameters. Therefore, the exposure assessment
in these cases is based on the potential dose, with no
explicit correction for the fraction absorbed.
However, the exposure assessor needs to make such
an adjustment when calculating dermal exposure and
in other specific cases when current information
indicates that the human absorption factor used in the
derivation of the dose-response factor is
inappropriate.
For carcinogens, the duration of a lifetime has
traditionally been assigned the nominal value of 70
years as a reasonable approximation. For exposure
estimates to be used for assessments other than
carcinogenic risk, various averaging periods have
been used. For acute exposures, the doses are usually
averaged over a day or a single event. For nonchronic
noncancer effects, the time period used is the actual
period of exposure (exposure duration). The objective
in selecting the exposure averaging time is to express
the exposure in a way that can be combined with the
dose-response relationship to calculate risk.
The body weight to be used in the exposure
equation (see Equation 1) depends on the units of the
exposure data presented in the Handbook. For
example, for food ingestion, the body weights of the
surveyed populations were known in the USDA
surveys, and they were explicitly factored into the
food intake data in order to calculate the intake as
g/kg body weight-day. In this case, the body weight
has already been included in the "intake rate" term in
Equations 1-2, and the exposure assessor does not
need to explicitly include body weight.
The units of intake in the Handbook for the
incidental ingestion of soil and dust are not
normalized to body weight. In this case, the exposure
assessor will need to use the average weight of the
exposed population during the time when the
exposure actually occurs (shown in Equation 1).
When making body weight assumptions, care must
be taken that the values used for the population
parameters in the dose-response analysis are
consistent with the population parameters used in the
exposure analysis. Intraspecies adjustments based on
lifestage can be made using a scaling factor of BW 4
(U.S. EPA, 2006c; 2006d). Some of the parameters
(primarily concentrations) used in estimating
exposure are exclusively site specific, and, therefore,
default recommendations should not be used. It
should be noted that body weight is correlated with
food consumption rates and inhalation rates.
The link between the intake rate value and the
exposure duration value is a common source of
confusion in defining exposure scenarios. It is
important to define the duration estimate so that it is
consistent with the intake rate:
The intake rate can be based on an individual
event (e.g., serving size per event). The
duration should be based on the number of
events or, in this case, meals.
The intake rate also can be based on a
long-term average, such as 10 g/day. In this
case, the duration should be based on the total
time interval over which the exposure occurs.
The objective is to define the terms so that, when
multiplied, they give the appropriate estimate of mass
of contaminant contacted. This can be accomplished
by basing the intake rate on either a long-term
average (chronic exposure) or an event (acute
exposure) basis, as long as the duration value is
selected appropriately.
Inhalation dosimetry is employed to derive the
human equivalent concentration (Hc) on which
inhalation unit risks, and reference concentrations,
are based (U.S. EPA, 1994b). U.S. EPA has
traditionally approximated children's respiratory
exposure by using adult values, although a recent
review (Ginsberg et al., 2002) concluded that there
may be some cases where young children's greater
inhalation rate per body weight or pulmonary surface
area as compared to adults can result in greater
exposures than adults. The implications of this
difference for inhalation dosimetry and children's
risk assessment were discussed at a peer involvement
workshop hosted by the U.S. EPA in 2006 (Foos et
al., 2008).
Consideration of life-stage-particular
physiological characteristics in the dosimetry analysis
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may result in a refinement to the human equivalent
concentration (Hc) to insure relevance in risk
assessment across life stages, or might conceivably
conclude with multiple Hcs, and corresponding
inhalation unit risk values (e.g., separate for
childhood and adulthood). The RfC methodology,
which is described in Methods for Derivation of
Inhalation Reference Concentrations and
Applications of Inhalation Dosimetry (U.S. EPA,
1994b), allows the user to incorporate
population-specific assumptions into the models. The
reader is referred to U.S. EPA guidance (U.S. EPA,
1994b) on how to make these adjustments.
There are no specific exposure factor
assumptions in the derivation of Reference Doses
(RfDs). The assessment of the potential for adverse
health effects in infants and children is part of the
overall hazard and dose-response assessment for a
chemical. Available data pertinent to children's
health risks are evaluated along with data on adults
and the no-observed-adverse-effect-level (NOAEL)
or benchmark dose (BMD) for the most sensitive
critical effect(s), based on consideration of all health
effects. By doing this, protection of the health of
children will be considered along with that of other
sensitive populations. In some cases, it is appropriate
to evaluate the potential hazard to children separately
from the assessment for the general population or
other population subgroups.
Use of Exposure Factors Data in Probabilistic
Analyses
Although the Handbook is not intended to
provide complete guidance on the use of Monte Carlo
and other probabilistic analyses, some of the data in
the Handbook may be appropriate for use in
probabilistic assessments. The use of Monte Carlo or
other probabilistic analysis requires characterization
of the variability of exposure factors and requires the
selection of distributions or histograms for the input
parameters of the dose equations presented earlier.
The following suggestions are provided for
consideration when using such techniques:
The exposure assessor should only consider
using probabilistic analysis when there are
credible distribution data (or ranges) for the
factor under consideration. Even if these
distributions are known, it may not be
necessary to apply this technique. For
example, if only average exposure values are
needed, these can often be computed
accurately by using average values for each of
the input parameters unless a nonlinear model
is used. Probabilistic analysis is also not
necessary when conducting assessments for
screening purposes, i.e., to determine if
unimportant pathways can be eliminated. In
this case, bounding estimates can be calculated
using maximum or near maximum values for
each of the input parameters. Alternatively, the
assessor may use the maximum values for
those parameters that have the greatest
variance.
It is important to note that the selection of
distributions can be highly site specific and
dependent on the purpose of the assessment. In
some cases, the selection of distributions is
driven by specific legislation. It will always
involve some degree of judgment.
Distributions derived from national data may
not represent local conditions. The assessor
needs to evaluate the site-specific data, when
available, to assess their quality and
applicability. The assessor may decide to use
distributional data drawn from the national or
other surrogate population. In this case, it is
important that the assessor address the extent
to which local conditions may differ from the
surrogate data.
It is also important to consider the
independence/dependence of variables and
data used in a simulation. For example, it may
be reasonable to assume that ingestion rate and
contaminant concentration in foods are
independent variables, but ingestion rate and
body weight may or may not be independent.
In addition to a qualitative statement of
uncertainty, the representativeness assumption should
be appropriately addressed as part of a sensitivity
analysis.
Distribution functions to be used in
probabilistic analysis may be derived by fitting
an appropriate function to empirical data. In
doing this, it should be recognized that in the
lower and upper tails of the distribution the
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data are scarce, so that several functions, with
radically different shapes in the extreme tails,
may be consistent with the data. To avoid
introducing errors into the analysis by the
arbitrary choice of an inappropriate function,
several techniques can be used. One technique
is using the empirical data itself rather than an
analytic function. Another is to do separate
analyses with several functions that have
adequate fit but form upper and lower bounds
to the empirical data. A third way is to use
truncated analytical distributions. Judgment
must be used in choosing the appropriate
goodness-of-fit test. Information on the
theoretical basis for fitting distributions can be
found in a standard statistics text. Off-the-shelf
computer software can be used to statistically
determine the distributions that fit the data.
Other software tools are available to identify
outliers and for conducting Monte Carlo
simulations.
If only a range of values is known for an
exposure factor, the exposure assessor has
several options:
keep that variable constant at its central
value;
assume several values within the range of
values for the exposure factor.; calculate
a point estimate(s) instead of using
probabilistic analysis; or
assume a distribution. (The rationale for
the selection of a distribution should be
discussed at length.)
There are, however, cases where assuming
a distribution is not recommended. These
include the following:
~ data are missing or very limited for a
key parameter;
~ data were collected over a short time
period and may not represent
long-term trends (the respondent's
usual behavior) - examples include
food consumption surveys; activity
pattern data;
~ data are not representative of the
population of interest because sample
size was small or the population
studied was selected from a local area
and was therefore not representative
of the area of interest; for example,
soil ingestion by children; and
~ ranges for a key variable are
uncertain due to experimental error or
other limitations in the study design
or methodology; for example, soil
ingestion by children.
CUMULATIVE EXPOSURES
The U.S. EPA recognizes that children may be
exposed to mixtures of chemicals both indoors and
outdoors through more than one pathway. New
directions in risk assessments in the U.S. EPA put
more emphasis on total exposures of multiple
chemicals through multiple pathways (U.S. EPA,
1986a; 2000c). Over the last several years, the U.S.
EPA has developed a methodology for assessing risk
from multiple chemicals. For more information, the
reader is referred to the U.S. EPA's Framework for
Cumulative Risk Assessment (U.S. EPA, 2003b).
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Breysee, P.N.; Buckley, T.J.; Williams, D.; Beck,
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Clewell, H.J.; Gentry, P.R.; Covington, T.R.;
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Curwin, B.D.; Hein, M.J.; Sanderson, W.T.; Striley,
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Eskenazi, B.; Bradman, A.; Castriona, R. (1999)
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Ginsberg, G.; Hattis, D.; Sonawame, B.; Russ, A.;
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IPCS (2006) Principles for evaluating health risks in
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Landrigan, P.J.; Sonawane, B.; Butler, R.N.;
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Lewis, R. G.; Fortune C.; Willis, R. D.; Camann, D.
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models used in exposure assessments: groundwater
models. Exposure Assessment Group, Office of
Health and Environmental Assessment, Washington,
DC; EPA/600/88/075. Available from NTIS,
Springfield, VA; PB-88-248752/AS.
U.S. EPA. (1989) Risk assessment guidance for
Superfund. Volume I. Human health evaluation
manual: Part A. Interim final. Office of Solid Waste
and Emergency Response, Washington, DC;
EPA/540-189/002. Available online at
http://www.epa.gov/oswer/riskassessment/ragsa
/index, htm.
U.S. EPA. (1990) Methodology for assessing health
risks associated with indirect exposure to combustor
emissions. Office of Health and Environmental
Assessment, Washington, DC; EPA 600/6-90/003.
Available from NTIS, Springfield, VA; PB-90-
187055/AS.
U.S. EPA. (1991a) Risk assessment guidance for
Superfund. Volume I. Human health evaluation
manual: Part B, Development of preliminary
remediation goals. Office of Solid Waste and
Emergency Response, Washington, DC; EPA/540/R-
92/003. Available online at
http://www.epa.gov/oswer/riskassessment/ragsb
/index, htm.
U.S. EPA. (1991b) Risk assessment guidance for
Superfund. Volume I. Human health evaluation
manual: Part C, Risk evaluation of remedial
alternatives. Office of Solid Waste and Emergency
Response, Washington, DC. Publication 9285.7-01C.
Available online at
http://www.epa.gov/oswer/riskassessment/ragsc
/index, htm.
U.S. EPA (1992a) Guidelines for exposure
assessment. Office of Research and Development,
Office of Health and Environmental Assessment,
Washington, DC; EPA/600/Z-92/001.
U.S. EPA. (1992b) Dermal exposure assessment:
principles and applications. Office of Health and
Environmental Assessment, Washington, DC;
EPA/600/8-9/01 IF. Available online at
http://www.epa.gov/ncea/pdfs/efh/references
/derexp.pdf.
U.S. EPA. (1994a) Estimating exposures to dioxin-
like compounds. Draft Report. Office of Research
and Development, Washington, DC; EPA/600/6-
88/005Cb.
U.S. EPA. (1994b) Methods for derivation of
inhalation reference concentrations and applications
of inhalation dosimetry. Office of Health and
Environmental Assessment, Washington, DC;
EPA/600/8-90/066F.
U.S.EPA (1996a) Soil screening guidance. Office of
Solid Waste and Emergency Response, Washington,
DC; EPA/540/F-95/041. Available online at
http ://www. epa. gov/superfund/health/conmedia/soil
/index, htm.
U.S.EPA. (1996b) Series 875 occupational and
residential exposure test guidelines - final guidelines
- Group A - application exposure monitoring test
guidelines. Office of Pesticides and Toxic
Substances, Washington, DC; EPA/712/C96/261.
Available online at
http://www.epa.gov/opptsfrs/publications/OPPTS_
Harmonized/875_Occupational_and_Residential_
Exposure_Test_Guidelines/Series/875_OOO.pdf.
U.S. EPA. (1996c) Series 875 occupational and
residential exposure test guidelines - group B - post
application exposure monitoring test guidelines.
Office of Pesticides and Toxic Substances,
Washington, DC; EPA/712/C96/266. Available
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online at
http://www.epa.gov/opptsfrs/publications/OPPTS_
Harmonized/875_Occupational_and_Residential_
Exposure_Test_Guidelines/Series/875-2000.pdf.
U.S. EPA (1997a) Exposure factors handbook.
National Center for Environmental Assessment,
Office of Research and Development, Washington,
DC; EPA/600/P-95/002Fa,b,c.
U.S. EPA. (1997b) Policy for use of probabilistic
analysis in risk assessment at the U.S. Environmental
Protection Agency. Science Policy Council.
Available online at
http://www.epa.gov/OSA/spc/pdfs/probpol.pdf.
U.S. EPA. (1997c) Guiding principles for Monte
Carlo analysis. Office of Research and Development,
Risk Assessment Forum, Washington, DC;
EPA/600/R-97/001. Available online at
http ://cfpub .epa. gov/ncea/cfm/recordisplay. cfm?deid
=29596.
U.S. EPA. (1999) Sociodemographic data used for
identifying potentially highly exposed populations.
National Center for Environmental Assessment,
Washington, DC; EPA/600/R-99/060. Available
online at
http ://cfpub .epa. gov/ncea/cfm/recordisplay. cfm?deid
=22562.
U.S. EPA. (2000a) Strategy for research on risks to
children. Office of Research and Development,
Washington, DC; EPA/600/R-00/068.
U.S. EPA. (2000b) Options for developing
parametric probability distributions for exposure
factors. National Center for Environmental
Assessment, Office of Research and Development,
Washington, DC; EPA/600/R-00/058. Available on
line at http://www.epa.gov/ncea/paramprob4ef.htm.
U.S. EPA. (2000c) Supplementary guidance for
conducting health risk assessment of chemical
mixtures. Risk Assessment Forum, Washington, DC;
EPA/630/R-00/002F. Available online at
http://www.epa.gov/NCEA/raf/pdfs/chem_mix/chem
_mix_08_2001.pdf.
U.S. EPA. (200la) Risk assessment guidance for
Superfund. Volume I. Human health evaluation
manual: Part D. Standardized planning, reporting and
review of Superfund risk assessments. Office of Solid
Waste and Emergency Response, Washington, DC.
Publication 9285.7-47. Available online at
http://www.epa.gov/oswer/riskassessment
/ragsd/tara.htm.
U.S. EPA. (200Ib) Risk assessment guidance for
Superfund. Volume III. Part A. Process for
conducting probabilistic risk assessment. Office of
Solid Waste and Emergency Response, Washington,
DC; EPA/540/R-02/002. Available online at
http://www.epa.gov/oswer/riskassessment/rags3adt/.
U.S. EPA. (2002a) Child-specific exposure factors
handbook. Interim Final. National Center for
Environmental Assessment, Washington, DC;
EPA/600/P-00/002B.
U.S. EPA (2002b). Overview of the EPA quality
system for environmental data and technology. Office
of Environmental Information, Washington DC;
EPA/240/R-02/003. Available online at
http ://www. epa. gov/quality/qs-docs/overview-
final.pdf.
U.S. EPA (2003a). A summary of general assessment
factors for evaluating the quality of scientific and
technical information. Science Policy Council,
Washington DC; EPA/100/B-03/001. Available
online at
http://www.epa.gov/osa/spc/pdfs/assess2.pdf.
U. S. EPA. (2003b) Framework for cumulative risk
assessment. Risk Assessment Forum, Washington,
DC; EPA/630/P-02/001F. Available online at
http ://cfpub .epa. gov/ncea/raf/recordisplay. cfm?deid=
54944.
U.S. EPA. (2003c) Example exposure scenarios.
National Center for Environmental Assessment,
Washington, DC. EPA/600/R-03/036. Available
online at
http ://cfpub .epa. gov/ncea/cfm/recordisplay. cfm?deid
=85843.
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U.S. EPA. (2003d) Human health research strategy.
Office of Research and Development, Washington,
DC; EPA/600/R-02/050. Available online at
http://www.epa.gov/OPxD/htm/researchstrategies.htm
#rs01.
U.S. EPA. (2004) Risk assessment guidance for
Superfund. Volume I. Human health evaluation
manual: Part E. Supplemental guidance for dermal
risk assessment. Interim. Office of Solid Waste and
Emergency Response, Washington, DC; EPA/540/R-
99/005. Available online at
http://www.epa.gov/oswer/riskassessment/ragse
/index, htm.
U.S. EPA (2005a) Guidance on selecting age groups
for monitoring and assessing childhood exposures to
environmental contaminants Office of Research and
Development, Washington, DC; EPA/630/P-03/003F.
U.S. EPA. (2005b) Guidelines for carcinogen risk
assessment. Risk Assessment Forum, Washington,
DC; EPA/630/P-03/001F. Available online at
http ://cfpub .epa. gov/ncea/cfm/recordisplay. cfm?deid
=116283.
U.S. EPA. (2005c) Supplemental guidance for
assessing susceptibility from early-life exposure to
carcinogens. Risk Assessment Forum, Washington,
DC; EPA/630/R03/003F. Available online at:
http ://cfpub .epa. gov/ncea/cfm/recordisplay. cfm?deid
=116283.
U.S. EPA. (2005d) Protocol for human health risk
assessment protocol for hazardous waste combustion
facilities. U.S. EPA, Washington, DC; EPA/530/R-
05/006. Available online at
http://www.epa.gov/combustion/risk.htm.
U.S. EPA. (2006a) Use of physiologically-based
pharmacokinetic (PBPK) models to quantify the
impact of human age and interindividual differences
in physiology and biochemistry pertinent to risk
(Final Report). Office of Research and Development,
Washington, DC; EPA/600/R-06/014A. Available
online at
http ://cfpub .epa. gov/ncea/CFM/recordisplay. cfm
?deid=151384
U.S. EPA (2006b) Guidance on systematic planning
using the data quality objectives process. Office of
Environmental Information, Washington DC;
EPA/240B/06/001. Available online at
http://www.epa.gov/QUALITY/qs-docs/g4-final.pdf.
U.S. EPA. (2006c) A framework for assessing health
risk of environmental exposures to children (Final).
Office of Research and Development, Washington,
DC; EPA/600/R-05/093F. Available online at
http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm7deid
= 158363.
U.S. EPA (2006d) Harmonization in interspecies
extrapolation: use of BW3/4 as default method in
derivation of the oral RfD (External Review Draft).
Risk Assessment Forum, Washington, DC;
EPA/630/R-06/001. Available online at
http://cfpub.epa. gov/ncea/raf/recordisplay.cfm?deid=
148525.
U.S. EPA (2008a) Child-specific exposure factors
handbook. National Center for Environmental
Assessment, Washington, DC; EPA/600/R-06/096F.
Available online at www.epa.gov/ncea.
U.S. EPA (2008b) Concepts, methods, and data
sources for cumulative health risk assessment of
multiple chemicals, exposures and effects: a resource
document (final). Office of Research and
Development, Washington, DC; EPA/600/R-
06/013F. Available online at
http ://cfpub. epa. gov/ncea/cfm/recordisplay. cfm?deid
= 190187.
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Table 1. Summary of Recommended Exposure Factors for Children
Age Group
0 to <1 1 to <3
mo mos
3 to <6
mos
6to<12
mos
1 to<2
yrs
2to<3
yrs
3 to <6
yrs
6 to <1 1
yrs
11 to <16
yrs
16 to <18
yrs
18to<21
yrs
Ingestion of Drinking Water (mL/day) — see Chapter 3
Mean per capita
95th percentile per capita
Mean consumer only
95th percentile consumer only
1 84 227
839 896
470 552
858 1,053
362
1,056
556
1,171
360
1,055
467
1,147
271
837
308
893
317
877
356
912
380
1,078
417
1,099
447
1,235
480
1,251
606
1,727
652
1,744
731
1,983
792
2,002
826
2,540
895
2,565
Ingestion of Drinking Water (mL/kg-day) — see Chapter 3
Mean per capita
95th percentile per capita
Mean consumer only
95th percentile consumer only
52 48
232 205
137 119
238 285
52
159
80
173
41
126
53
129
23
71
27
75
23
60
26
62
22
61
24
65
16
43
17
45
12
34
13
34
11
31
12
32
12
35
13
35
Ingestion of Water while Swimming (mL/hour) — see Chapter 3
Mean
Upper percentile
50
100
20
70
Hand-to-Mouth Frequency (contacts/hour) — see Chapter 4
Indoor Mean
95th percentile
Outdoor Mean
95th percentile
28
65
19
52
15
47
20
63
14
42
13
37
5
20
15
54
9
36
7
21
12
Object-to-Mouth Frequency (contacts/hour) — see Chapter 4
Mean
95th percentile
20
10
1
Object- to-Mouth Duration (minutes/hour) — see Chapter 4
Mean
95th percentile
_
11
26
8
22
13
16
_
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0 to <1 1 to <3
mo mos
3 to <6
mos
6to<12
mos
1 to<2
yrs
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mos
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mos
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mos
1 to<2
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2to<3
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Age Group
0 to <1 1 to <3
mo mos
3 to <6
mos
6to<12
mos
1 to<2
yrs
Total Meat Intake (g/kg-day)a —
Mean per capita
95th percentile per capita
Mean consumer only
95th percentile consumer
1.2
6.7
3.0
9.2
2to<3
yrs
3to<6
yrs
6to
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6to<12
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Table 2.
Exposure Factors
Ingestion of water and other select
liquids
Non-dietary ingestion
Soil and dust ingestion
Inhalation rate
Surface area
Soil adherence
Body weight
Intake of fruits and vegetables
Intake of fish and shellfish
Intake of meats, dairy products, and
Intake of grain products
Intake of home produced foods
Total food intake
Human milk intake
Time indoors
Time outdoors
Time showering
Time bathing
Time swimming
Time playing on sand/gravel
Time playing on grass
Time playing on dirt
Characterization of Variability in Exposure
Average
y
y
y
y
y
J
y
J
J
fats J
J
J
J
J
J
y
j
y
j
y
y
y
Median
y
y
y
y
y
y
y
y
y
y
y
y
y
y
y
y
y
y
Factors
Upper percentile
y
y
ya
y
y
y
y
y
y
y
y
y
y
y
y
y
y
y
y
Multiple
Percentiles
y
y
y
y
y
y
y
y
y
y
y
y
y
y
y
y
a Soil pica and geophagy.
y = Data available.
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Highlights of the Child-Specific Exposure Factors Handbook
Table 3. Considerations Used to Rate Confidence in Recommended Values
General Assessment Factors
Increasing Confidence
Decreasing Confidence
Soundness
Adequacy of Approach
Minimal (or defined) Bias
The studies used the best available
methodology and capture the measurement of
interest.
As the sample size relative to that of the
target population increases, there is greater
assurance that the results are reflective of the
target population.
The response rate is greater than 80% for in-
person interviews and telephone surveys, or
greater than 70 % for mail surveys.
The studies analyzed primary data.
The study design minimizes measurement
There are serious limitations with the approach used;
study design does not accurately capture the
measurement of interest.
Sample size is too small to represent the population
of interest.
The response rate is less than 40 %.
The studies are based on secondary sources.
Uncertainties with the data exist due to measurement
error.
Applicability and Utility
Exposure Factor of Interest
Representativeness
Currency
Data Collection Period
The studies focused on the exposure factor of The purpose of the studies was to characterize a
interest. related factor.
The studies focused on the U.S. population. Studies are not representative of the U.S. population.
The studies represent current exposure
conditions.
The data collection period is sufficient to
estimate long-term behaviors.
Studies may not be representative of current
exposure conditions.
Shorter data collection periods may not represent
long-term exposures.
Clarity and Completeness
Accessibility
Reproducibility
Quality Assurance
The study data could be accessed.
The results can be reproduced, or
methodology can be followed and evaluated.
The studies applied and documented quality
assurance/quality control measures.
Access to the primary data set was limited.
The results cannot be reproduced, the methodology
is hard to follow, and the author(s) cannot be located.
Information on quality assurance/control was limited
or absent.
Variability and Uncertainty
Variability in Population
Uncertainty
The studies characterize variability in the
population studied.
The uncertainties are minimal and can be
identified. Potential biases in the studies are
stated or can be determined from the study
design.
The characterization of variability is limited.
Estimates are highly uncertain and cannot be
characterized. The study design introduces biases in
the results.
Evaluation and Review
Peer Review
The studies received high level of peer
review (e.g., they are published in peer-
reviewed journals).
Number and Agreement of Studies The number of studies is greater than three.
The results of studies from different
researchers are in agreement.
The studies received limited peer review.
The number of studies is one. The results of studies
from different researchers are in disagreement.
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Table 4. Summary of Confidence Ratings For Exposure Factor Recommendations
Exposure Factor
Ingestion of drinking water — see Chapter 3
Ingestion of water while swimming — see Chapter 3
Mouthing frequency and duration — see Chapter 4
Soil and dust ingestion — see Chapter 5
Inhalation rates — see Chapter 6
Skin surface area — see Chapter 7
Soil adherence — see Chapter 7
Body Weight — see Chapter 8
Intake of Fruits and Vegetables — see Chapter 9
Intake of Fish and Shellfish — see Chapter 10
Intake of Meats, Dairy, and Fats — see Chapter 11
Intake of Grains — see Chapter 12
Intake of Home-produced Foods — see Chapter 13
Total Food Intake — see Chapter 14
Human Milk Intake — see Chapter 15
Activity Factors — see Chapter 16
Overall Confidence Rating
Medium to High
Low
Low
Low
Medium
Medium for Total Surface Area
Low for Surface Area of Individual Body Parts
Low
High
High for means
Low for long-term upper percentiles
High for mean
Medium for upper percentile
High for means
Low for long-term upper percentiles
High for means
Low for long-term upper percentiles
Low to Medium for means and short-term distributions
Low for long-term distributions
Medium
Medium
Medium for means
Low for upper percentiles
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Table 5. Age-Dependent Potency Adjustment Factor (ADAF) by Exposure Age Group a
Exposure Age Group3
Birth to < 1 mo
1 to <3 mos
3 to <6 mos
6 to <12 mos
1 to <2 yrs
2 to <3 yrs
3 to <6 yrs
6to21yrs(21to<70yr)
Exposure Duration (years)
0.083
0.167
0.25
0.5
1
1
3
5
5
5
49
ADAF (Age-Dependent Potency
Adjustment Factor)
lOx
lOx
lOx
lOx
lOx
3x
3x
3x
3x
Ix
Ix
a Integrating U.S. EPA's Guidance on Selecting Age Groups for Monitoring and Assessing Childhood
Exposures to Environmental Contaminants (U.S. EPA, 2005a) with U.S. EPA's Supplemental Guidance for
Assessing Susceptibility from Early-Life Exposure to Carcinogens (U.S. EPA, 2005c) for those
contaminants which act via a mutagenic mode of action.
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SOURCE/STRESSOR
FORMATION
DISEASE
Chemical
Physical
Microbia
Magnitude
Duration
Timin
TRANSPORT/
TRANSFORMATION
ALTERED STRUCTURE/
FUNCTION
Cancer
Asthma
Infertility
Dispersion
Kinetics
Thermodynamics
Distributions
Meteorolog
s
1
ENVIRONMENTAL
CHARACTERIZATION
EARLY BIOLOGICAL
EFFECT
Edema
Arrhythmia
Enzymuria
Necrosis
Air
Water
Diet
Soil and
EXPOSURE
Molecular
Biochemical
Cellular
Organ
Organis
I
\
Activity
Pattern
Pathway
Route
Duration
Frequency
Magnitud
Individual
Community
Population
Absorbed
Target
Internal
Biologically
Statistical Profile
Reference
Susceptible
Susceptible
Population
Figure 1. The Exposure-Dose-Effect Continuum
The exposure-dose-effect continuum depicts the trajectory of a chemical or agent from its
source to an effect. The chemical or agent can be transformed and transported through the
environment via air, water, soil, dust, and diet. Children can become in contact with the
chemical through inhalation, ingestion, or skin/eye contact. The child's physiology, behavior,
and activity patterns as well as the concentration of the chemical will determine the magnitude,
frequency, and duration of the exposure. The exposure becomes an absorbed dose once the
chemical crosses the absorption barrier (i.e., skin, lungs, eyes, gastrointestinal tract, placenta).
Interactions of the chemical or its metabolites with a target tissue may lead to an adverse health
outcome. The text under the boxes indicates the specific information that may be needed to
characterize each box in the exposure-dose-effect continuum.
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Agency
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Offfie of Research and Development
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