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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON D C 20460'
FEB 26 1992
LIBRARY, AWBERC, CINCINNATI
U. S. EPA
26 W. MARTIN LUTHER KING DRIVE
CINCINNATI. OHSCb 45268 >'
MEMORANDUM
SUBJECT:
FROM:
TO:
Guidance on Risk Characterization for Risk Managers
and Risk Assessors
F. Henry Habicht I
Deputy Administrate
Assistant Administrators
Regional Administrators
JBRARY, AWBERC, CINCINNAT.
U. S. EPA
26 W. MARTIN LUTHER KING DRIVE
CINCINNATI. OHIO 45268
INTRODUCTION
This memorandum provides guidance for managers and assessors
on describing risk assessment results in EPA reports,
presentations, and decision packages. The guidance addresses a
problem that affects public perception regarding the reliability
of EPA's scientific assessments and related regulatory decisions.
EPA has talented scientists, and public confidence in the quality
of our scientific output will be enhanced by our visible
interaction with peer scientists and thorough presentation of
risk assessments and underlying scientific data.
Specifically, although a great deal of careful analysis and
scientific judgment goes into the development of EPA risk
assessments, significant information is often omitted as the
results of the assessment are passed along in the decision-ciaki.-.g
process. Often, when risk information is presented to the
ultimate decision-maker and to the public, the results have been
boiled down to a point estimate of risk. Such "short hand"
approaches to risk assessment do not fully convey the range of
information considered and used in developing the assessment. In
short, informative risk characterization clarifies the scientific
basis for EPA decisions, while numbers alone do not give a true
picture of the assessment.
This problem is not EPA's alone. Agency contractors,
industry, environmental groups, and other participants in the
overall regulatory process use similar "short hand" approaches.
We must do everything we can to ensure that critical
information from each stage of the risk assessment is
communicated from risk assessors to their managers, from middle
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to upper management, from EPA to the public, and from others to
EPA. The Risk Assessment Council considered this problem over
many months and reached several conclusions: 1) We need to
present a full and complete picture of risk, including a
statement of confidence about data and methods used to develop
the assessment; 2) we need to provide a basis for greater
consistency and comparability in risk assessments across Agency
programs; and 3) professional scientific judgment plays an
important role in the overall statement of risk. The Council
also concluded that Agency-wide guidance would be useful.
BACKGROUND
Principles emphasized during Risk Assessment Council
discussions are summarized below and detailed in the attached
Appendix.
Full Characterization of Risk
EPA decisions are based in part on risk assessment, a
technical analysis of scientific information on existing and
projected risks to human health and the environment. As
practiced at EPA, the risk assessment process depends on many
different kinds of scientific data (6.3., exposure, toxicity,
epidemiology), all of which are used to "characterize" the
expected risk to human health or the environment. Informed use
of reliable scientific data from many different sources is a
central feature of the risk assessment process.
Highly reliable data are available for many aspects of an
assessment. However, scientific uncertainty is a fact of life
for the risk assessment process as a whole. As a result, agency
managers make decisions using scientific assessments that are
less certain than the ideal. The issues, then, become when ia
scientific confidence sufficient to use the assessment for
decision-making, and how should the assessment be used? In or^er
to make these decisions, managers need to understand the
strengths and the limitations of the assessment.
On this point, the guidance emphasizes that informed EPA
risk assessors and managers need to be completely candid about
confidence and uncertainties in describing risks and in
explaining regulatory decisions. Specifically, the Agency's risk
assessment guidelines call for full and open discussion of
uncertainties in the body of each EPA risk assessment, including
prominent display of critical uncertainties in the risk
characterization. Numerical risk estimates should always be
accompanied by descriptive information carefully selected to
ensure an objective and balanced characterization of risk in risk
assessment reports and regulatory documents.
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Scientists call for fully characterizing risk not to
question the validity of the assessment, but to fully inform
others about critical information in the assessment. The
emphasis on "full" and "complete" characterization does not refer
to an ideal assessment in which risk is completely defined by
fully satisfactory scientific data. Rather, the concept of
complete risk characterization means that information that is
needed for informed evaluation and use of the assessment is
carefully highlighted. Thus, even though risk characterization
details limitations in an assessment, a balanced discussion of
reliable conclusions and related uncertainties enhances, rather
than detracts/ from the overall credibility of each assessment.
This guidance is not new. Rather, it re-states, clarifies,
and expands upon current risk assessment concepts and practices,
and emphasizes aspects of the process that are often incompletely
developed. It articulates principles that have long guided
experienced risk assessors and well-informed risk managers, who
recognize that risk is best described not as a classification or
single number, but as a composite of information from many
different sources, each with varying degrees of scientific
certainty.
Comparability and Consistency
The Council's second finding, on the need for greater
comparability, arose for several reasons. One was confusion --
for example, many people did not understand that a risk estimate
of 10~6 for an "average" individual should not be compared to
another 10~6 risk estimate for the "most exposed individual".
Use of such apparently similar estimates without further
explanation leads to misunderstandings about the relative
significance of risks and the protectiveness of risk reduction
actions. Another catalyst for change was the SAB's report,
Reducing Risk: Setting Priorities and Strategies for
Environmental Protection. In order to implement the SAB's
recommendation that we target our efforts to achieve the greatest
risk reduction, we need common measures of risk.
EPA's newly revised Exposure Assessment Guidelines provide
standard descriptors of exposure and risk. Use of these terms in
all Agency risk assessments will promote consistency and
comparability. Use of several descriptors, rather than a single
descriptor, will enable us to present a more complete picture of
risk that corresponds to the range of different exposure
conditions encountered by various populations exposed to most
environmental chemicals.
Professional Judgment
The call for more extensive characterization of risk has
obvious limits. For example, the risk characterization includes
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only the most significant data and uncertainties from the
assessment (those that define and explain the main risk
conclusions) so that decision-makers and the public are not
overwhelmed by valid but secondary information.
The degree to which confidence and uncertainty are addressed
depends largely on the scope of the assessment and available
resources. When special circumstances (e.g.., lack of data,
extremely complex situations, resource limitations, statutory
deadlines) preclude a full assessment, such circumstances should
be explained. For example, an emergency telephone inquiry does
not require a full written risk assessment, but the caller must
be told that EPA comments are based on a "back-of-the-envelope"
calculation and, like other preliminary or simple calculations,
cannot be regarded as a risk assessment.
GUIDANCE PRINCIPLES
Guidance principles for developing, describing, and using
EPA risk assessments are set forth in the Appendix. Some of
these principles focus on differences between risk assessment and
risk management, with emphasis on differences in the information
content of each process. Other principles describe information
expected in EPA risk assessments to the extent practicable,
emphasizing that discussion of both data and confidence in the
data are essential features of a complete risk assessment.
Comments on each principle appear in the Appendix; more detailed
guidance is available in EPA's risk assessment guidelines (e.g.,
51 Federal Register 33992-34054, 24 September 1986).
Like EPA's risk assessment guidelines, this guidance applies
to the development, evaluation, and description of Agency risk
assessments for use in regulatory decision-making. This
memorandum does not give guidance on the use of completed risk
assessments for risk management decisions, nor does it address
the use of non-scientific considerations (e.g., economic or
societal factors) that are considered along with the risk
assessment in risk management and decision-making. While some
aspects of this guidance focus on cancer risk assessment, the
guidance applies generally to human health effects (e.g..,
neurotoxicity, developmental toxicity) and, with appropriate
modifications, should be used in all health risk assessments.
Guidance specifically for ecological risk assessment is under
development.
TMPLgMEHTATIQlf
Effective immediately, it will be Agency policy for each EPA
office to provide several kinds of risk assessment information in
connection with new Agency reports, presentations, and decision
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packages. In general, such information should be presented as
carefully selected highlights from the overall assessment. In
this regard, common sense regarding information needed to fully
inform Agency decision-makers is the best guide for determining
the information to be highlighted in decision packages and
briefings.
1. Regarding the interface between risk assessment and
risk management, risk assessment information must be
clearly presented, separate from any non-scientific
risk management considerations. Discussion of risk
management options should follow, based on
consideration of all relevant factors, scientific and
non-scientific.
2. Regarding risk characterization, key scientific
information on data and methods (e.g., use of animal or
human data for extrapolating from high to low doses,
use of pharmacokinetics data) must be highlighted. We
also expect a statement of confidence in the assessment
that identifies all major uncertainties along with
comment on their influence on the assessment,
consistent with guidance in the attached Appendix.
3. Regarding exposure and risk characterization, it is
Agency policy to present information on the range of
exposures derived from exposure scenarios and on the
use of multiple risk-descriptors (i.e., central
tendency, high end of individual risk, population risk,
important subgroups, if known) consistent with
terminology in the attached Appendix and Agency
guidelines.
This guidance applies to all Agency offices. It applies tc
assessments generated by EPA staff and to those generated by
contractors for EPA's use. I believe adherence to this Agency-
wide guidance will improve understanding of Agency risk
assessments, lead to more informed decisions, and heighten the
credibility of both assessments and decisions.
From this time forward, presentations, reports, and decision
packages from all Agency offices should characterize risk and
related uncertainties as described here. Please be prepared to
identify and discuss with me any program-specific modifications
that may be appropriate. However, we do not expect risk
assessment documents that are close to completion to be
rewritten. Although this is internal guidance that applies
directly to assessments developed under EPA auspices, I also
encourage Agency staff to use these principles as guidance in
evaluating assessments submitted to EPA from other sources, and
in discussing these submissions with me and with the
Administrator.
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This guidance is intended for both management and technical
staff. Please distribute this document to those who develop or
review assessments and to your managers who use them to implement
Agency programs. Also, I encourage you to discuss the principles
outlined here with your staff, particularly in briefings on
particular assessments.
In addition, I expect that the Risk Assessment Council will
endorse new guidance on Agency-wide approaches to risk
characterization now being developed in the Risk Assessment Forum
for EPA's risk assessment guidelines, and that the Agency and the
Council will augment that guidance as needed.
The Administrator and I believe that this effort is very
important. It furthers our goals of rigor and candor in the
preparation, presentation, and use of EPA risk assessments. The
tasks outlined above may require extra effort from you, your
managers, and your technical staff, but they are critical to full
implementation of these principles. We are most grateful for the
hard work of your representatives on the RAC and other staff in
pulling this document together. I appreciate your cooperation in
this important area of science policy, and look forward to our
discussions.
Attachment
cc: The Administrator
Risk Assessment Council
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GUIDANCE FOR RISK ASSESSMENT
Section 1. Risk Assessment-Risk Management
Interface
Section 2. Risk Characterization
Section 3. Exposure and Risk Descriptors
U.S. Environmental Protection Agency
Risk Assessment Council
November, 1991
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1. RISK ASSESSMENT - RISK MAHAGEMENT
Recognizing that for many people the term risk assessment
wide meaning, the National Research Council's 1983 report on
risk assessment in the federal government (hereafter
report") distinguished between risk assessment and risk
management.
Broader uses of the [risk assessment] than ours
also embrace analysis of perceived risks,
comparisons of risks associated with different
regulatory strategies, occasionally analysis
of economic social implications of
regulatory decisions —
t^__risk_manaaement (emphasis added). (1)
In 1984, EPA endorsed these distinctions between risk
risk management for Agency use (2), later relied on
in'developing risk guidelines (3),
This distinction suggests that EPA participants in the
can be grouped into two main categories, each with
different responsibilities, based on their roles with
to risk assessment risk management.
" *
One group the risk assessment by collecting,
analyzing, and synthesizing scientific data to produce
the hazard identification, dose-response,
assessment portion of the risk assessment to
characterize risk* This group relies in part on Agency
risk assessment guidelines to address science policy
issues scientific uncertainties.
Generally, this group includes scientists
statisticians in the Office of Research
Development, the Office of Pesticides Toxic
Substances other offices-, the Carcinogen
Risk Assessment Verification Endeavor (CRAVE),
RfD/RfC Workgroups.
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Others iase analyses produced by the first group to
generate site- or media-specific exposure assessments
and risk characterizations for use In regulation
development. These assessors rely on existing
databases (e.g., IRIS, ORD Health Assessment Documents,
and R£D/R£C Workgroup documents) to develop
regulations and evaluate alternatives.
Generally, this group Includes scientists analysts
in program offices, regional offices, and the Office of1
Research Development.
Risk__Managenient
A third group integrates the risk characterization with
other non-scientific considerations specified in
applicable statutes to make and justify regulatory
decisions.
Generally, this group includes Agency managers
decision-makers.
Each group has different responsibilities for observing
distinction between risk assessment risk management. At
time, the risk assessment process involves regular
interaction between each of the groups, with overlapping
responsibilities at various stages In the overall process.
guidance to follow outlines principles specific for
generate, review, use, Integrate risk assessments
for decision-making*
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1. ^Risk assessors risk managers should b€ s«nsitiv® to
distinctions b«tw««n risk assessment and risk management.
The major participants In the risk assessment process have
responsibilities. Where responsibilities differ, It
Is important that participants confine themselves to tasks In
of responsibility not inadvertently obscure
differences between risk assessment and risk management.
J hared responsibilities of assessors managers Include
Initial decisions regarding the planning conduct of an
assessment, discussions as the assessment develops, decisions
to complete an assessment to
significant uncertainties. At critical junctures In
assessment, such consultations the nature of, schedule
for, assessment.
For the distinguishing between-
'risk that scientific
information Is selected, evaluated, presented without
considering non-scientific factors Including how scientific
analysis might Influence regulatory decision. Assessors
with (1) generating a credible, objective," realistic,
analysis; (2) presenting information on hazard, dose-
response, exposure risk; (3) explaining confidence In
assessment by clearly delineating uncertainties
assumptions along with the of these factors (e.g.,
confidence limits, use of conservatlve/non-conservatlve
assumptions) on the overall assessment. They do not
decisions on the acceptability 'of any risk level for protecting
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public, health or selecting procedures for reducing risks.
For itlJ2|Ls__oJLj^^ who
Integrate these assessments into regulatory decisions, the
distinction between risk assessment risk management
refraining Influencing risk description through
consideration of non~scientific factors -- e.g., the regulatory
~ attempting to the risk assessment to
statutory constraints, regulatory objectives, or
political purposes. Such management considerations often
considerations for the overall regulatory decision
principle), bat they have no role In estimating or
risk.
However, decision-makers establish policy directions that
the overall nature of Agency risk
as-appropriate, provide policy guidance on difficult
controversial . as risk
priorities, of conservatism, acceptability
of particular risk levels reserved for decision-makers who
with making decisions regarding protection of public
health.
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2. Tfa« risk assessment product, that is, the risk
characterization, Is only on® of several kinds of Information
used for regulatory decision-making.
Risk characterization, the last step In risk assessment, Is
the starting point for risk management considerations the
foundation for regulatory decision-making, but It Is only of
several Important components in such decisions. Each of
environmental laws administered by EPA calls for consideration of
non-scientific factors at various stages In the regulatory
process* As authorized by different statutes, decision-makers
evaluate technical feasibility (e.g., treatability, detection
limits), economic, social, political, legal factors as
of analysis of whether or not to regulate and, If so, to what
extent. Thus, regulatory decisions are usually based on a
combination of technical analysis to develop risk
information from other fields.
For this reason, risk managers should
that regulatory decision is usually
solely by -outcome of risk assessment. That is, the '
analysis of overall regulatory problem may be as
the picture presented by the risk analysis alone. For example, a
pesticide risk assessment may describe risk to
populations but, if the agricultural benefits of its use
important for the nation'a food supply, product may be
allowed to remain on with certain restrictions on
to possible exposure. Similarly, assessment efforts may
an RfD for a particular chemical, but other
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considerations may result In a regulatory level that is more or
protective than the RfD Itself,
For decision-makers, this means that societal considerations
(e.g., costs, benefits) that, along with the risk assessment,
regulatory decision should be described as fully as the
scientific Information forth In the risk characterization.
Information on analyses, their strengths
limitations, In assessment* uncertainties,
alternative analyses as Important as they the
scientific components of regulatory decision. Decision-
should be able to expect, for «3tampie, the level of
analysis as they receive from risk
analysis.
Decision-makers not "captives of numbers." On
contrary, quantitative qualitative risk characterization
Is only one.of Important factors that be considered In
final decision — a difficult distinctly
different task' risk se« Risk
Involve numerous assumptions uncertainties--
technology, factors, which to
be explicitly identified decIsion-makers public.
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2. RISK CHARACTERIZATION
EPA risk assessment principles and practices draw on many
sources. The environmental laws administered by EPA, the
National Research Council's 1983 report on risk assessment (1),
Agency's Risk Assessment Guidelines (3)/ various program-
specific guidance (e*ge, the Risk Assessment Guidance for
Superfund) obvious sources. Twenty years of EPA experience
in developing,, defending/ enforcing risk assessment-based
regulation is another. Together these various sources stress
importance of a clear explanation of Agency processes for
evaluating hazard, dose-response, exposuref other that
scientific foundation for characterizing risk.
This section focuses on two requirements for full
characterization of risk* First/ the characterization
qualitative and quantitative features of the assessment.
Second, it must identify any important uncertainties in
as part of a discussion on confidence in the
assessment.
This on a full description of all elements'- of the
assessment'draws attention to the importance of qualitative
as well as the quantitative dimensions of the assessment* The
1983 carefully distinguished qualitative risk
from quantitative assessments, preferring risk
statements that are not strictly numerical.
The term is often given
narrower and broader meanings than we
have here. For observers,
the term is synonymous with guantitatj.ve_
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emphasizes reliance
on numerical results . Our broader definition
includes quantification, but also includes
qualitative expressions of risk. Quantitative
estimates of risk not always feasible, and
they may be eschewed by agencies for policy
reasons. (Emphasis in original) (1)
recently, an Ad Study Group (with representatives
•from .EPA, HHS, private sector) on Risk Presentation
expanded upon these principles by specifying
"attributes" for risk characterization.
1. major components of risk (hazard
identification, dose-response,
exposure assessment) are presented in
statements, along with quantitative
estimates of risk, to give a combined
Integrated view of evidence*
2. The report clearly identifies
assumptions , their rationale, and
extent of scientific consensus?
uncertainties thus accepted! the
effect of reasonable alternative
assumptions on conclusions estimates.
3. report outlines specific ongoing or
potential research projects that would
probably clarify significantly the extent
of «nc«rtalaty In risk estimation.
.... (4)
Particularly critical to foil characterization of risk Is a
discussion of the uncertainty in the overall
in of its components. The uncertainty
Is • several reasons .
different carries
of uncertainty knowledge of differences
Is Important uncertainties for
characterizing risk.
Decisions must be on expending resources to
additional information to
uncertainties.
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A^clear and explicit statement of the Implications and
limitations of a risk assessment requires a clear and
explicit statement of related uncertainties.
Uncertainty analysis gives the decision-maker a better
understanding of the implications and limitations of
the assessments.
A discussion of uncertainty requires comment on issues
as quality quantity of available data, gaps in data
for specific chemicals, Incomplete understanding of general
biological phenomena, scientific judgments or science policy
positions that employed to bridge information gaps.
In short, broad agreement exists on"the Importance of a full
picture of risk, particularly.including a statement of confidence
In that uncertainties within reason.
This section discusses information content and uncertainty
of risk characterization^ while Section 3
descriptors In risk characterization.
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1. Th® risk assessment procass calls for charact®rizing
risk aa a combination of qualitativ* information, quantitative
information, and information regarding unc@rtainti@s.
Risk assessment Is on a series of questions that the
the the Implications of the data 'for
risk- question calls for analysis Interpretation
of available studies, selection of that
scientifically reliable relevant to problem at hand,
scientific conclusions regjtrding the question presented* As
below, because the questions analyses complex,
a characterization Includes several different kinds of
information, carefully selected for reliability relevance.
a« -- What do we the
capacity of an environmental agent for causing cancer
(or other effects) In laboratory animals In
identification Is a qualitative description on
as kind quality of data on or
animals, availability of ancillary information
(e.g., structure-activity analysis, toxicity,
kinetics) other studies, the weight~o£-the evidence
allvof sources. For example, to develop this
description^ th« Issues addressed Includei
1. nature, reliability, consistency of the
particular studies In In laboratory animals;
2* available information on mechanistic basis
activity?
3. experimental animal responses their relevance to
human outcomes.
clear that the task of hazard
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identification is characterized by describing the full range .of
available information the Implications of that information
for human health.
b. ~~ What do we know about the
biological mechanisms dose-response relationships
underlying any effects observed in the laboratory or
epidemiology studies providing data for the
The dose-response quantitative
relationships between* exposure (or dose) effects in the
to identify define effects of concern. This
Information is later used along with "real world" exposure
information (see below) to develop of likelihood of
effects In populations potentially at risk* •
for establishing dose-response relationships often
on various assumptions used In lieu of a complete
method chosen can strongly Influence the overall
assessment. This relationship that careful attention to
the choice of a high-to-low extrapolation procedure Is very
Important* As, a result, an assessor who Is characterizing a,
dose-response relationship considers several Issues:
1. relationship between extrapolation models selected
available Information on biological mechanisms;
2. how appropriate data were selected from those
show the range of possible potencies both In laboratory
animals and humans;
3. basis for selecting interspeciea scaling factors
to account for scaling from experimental animals
to humans; and
4. correspondence the route(s) of .
• exposure the exposure route(s) utilized In. the
hazard studies, as'well as the -Interrelationships of
potential effects from different exposure routes.
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EPA's Integrated Risk Information System (IRIS) Is a primary
source of this information. IRIS Includes data summaries
representing Agency consensus on specific chemicals, on a'
careful review of the scientific issues listed above. For
risk on in IRIS and on other
sources, risk assessors should carefully review information
presented, emphasizing confidence In the database and
uncertainties (see subsection d below). The IRIS statement of
should be included as part of the risk
characterization for hazard dose-response Information.
c. •- What do we know about the ,
patterns, magnitudes, of human exposure
of likely to be
a wide range of
pertaining to "real world" environmental
of be to the study. The
for exposure assessment range froa monitoring
of chemical concentration* In environmental media, food,
materials to information, on activity patterns vof
different population subgroups. An assessor who characterizes
should several Issues.
1. basis for values Input parameters
exposure scenario* If on data, Information
on the quality, purpose, representativeness of
database la needed. If on assumptions,
general logic used to develop the assumption
(e.g., monitoring »> modeling, analogy, professional
judgment) should be described.
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2. major factor or factors (e.g., concentration, body
uptake, duration/frequency of exposure} thought to
account for the greatest uncertainty In the exposure
estimate, due either to sensitivity or lack of data.
3. The link of the exposure Information to the risk
descriptors discussed in Section 3 of this Appendix.
This Issue includes the conservatism or non-
conservatism of the scenarios, as Indicated by the
choice of descriptors.
In summary, confidence in the Information used to
characterize risk Is variable, with the result that risk
characterization requires a statement regarding the assessor's
confidence in each of the assessment.
d. — What do other assessors,
decision-makers, the public need to know about
primary conclusions assumptions, .the
balance between confidence and uncertainty In the
assessment?
In risk characterization, conclusions about hazard
are integrated with from'the
assessment. In addition^ confidence about these conclusions,
Including Information the uncertainties associated with
final risk summary, is highlighted. As summarized below,
characterization Integrates all of preceding Information to
communicate the overall meaning of, confidence In,
hazard, exposure, risk conclusions.
Generally, risk carry two categories of .
uncertainty, each merits consIderation. Measurement
•uncertainty refers to the usual variance that accompanies
scientific (such as range around an
estimate) reflects the accumulated variances around the
Individual values used to develop estimate. A
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different kind of uncertainty from data gaps -- that is,
Information needed to complete the data base' for the assessment.
Often, the gap is broad, such as the absence of information
on effects of exposure to a chemical on humans or on the
biological mechanism of action of an .
The to which confidence uncertainty in of
is largely on the of the
resources available. For example, the Agency
not an to evaluate assess every
conceivable scenario for every possible pollutant, to
all susceptible populations potentially at risk, or to
every possible environmental scenario to determine
effect relationships exposure to pollutants
health effects. Rather, the uncertainty analysis
reflect type and complexity of risk
level of for analysis discussion of
uncertainty corresponding to level of effort for
of of uncertainty
below.
Often risk assessors managers simplify discussion of
risk by speaking only of the numerical of an
That is, they refer to ch« weight-of-evidence, unit
risk, risk-specific or ql* for cancer risk/
'RfD/RfC health effects other than cancer, to the exclusion of
Information bearing on risk case* However,
carries uncertainties, a simplified numerical
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presentation of risk Is always Incomplete and often misleading.
For this reason, the (1) and EPA risk assessment guidelines
(2} call 'for "characterizing" risk to include qualitative
information, a related numerical risk estimate and a discussion
of uncertainties, limitations/ assumptions.
Qualitative Information on methodology, alternative
interpretations, working assumptions Is an important
component of risk characterization. For example, specifying
studies rather than human studies were used in an
tells others that the risk estimate ia on
assumptions about human to a particular chemical rather
data. Information that human estimates
on the subjects' In the vicinity of a chemical
accident rather than tissue measurements defines known
unknown aspects of the exposure component of the study*
Qualitative descriptions of this kind provide crucial
Information that augments understanding of numerical risk
estimates. Uncertainties such as expected In
scientific studies In any risk on 'tfiese
studies. Such uncertainties do not the validity of
assessment* Rather, they are highlighted along with other
risk assessment conclusions to Inform others fully on
results of the assessment.
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2-. **®li-balanc€d risk characterization pr«s®nts information
for oth«r risk assessor®, EPA dscision-makera,, and th® public
regarding th« strengths limitations of th« assessment.
risk assessment process calls for identifying
highlighting significant risk conclusions and related
partly to full communication risk
partly to that decision-Bakers fully
informed. Issues identified by acknowledging noteworthy
qualitative quantitative factors that a difference in
overall assessment of hazard and risk, and hence in the
ultimate regulatory decision.
word is "noteworthy": information that
significantly influences analysis is retained •— that is,
—- in all future presentations of the risk
in decision. Uncertainties
strongly influence confidence in risk
attention.
As earlier, two major of uncertainty
variability in factors upon which
of fundamental gaps. This distinction is "relevant
for of risk characterization. For example,
central high individual exposure estimates
to in exposure, lifestyles,
lead to a distribution of risk across a
population. Key considerations underlying risk
be fully described. In contrast, scientific
to knowledge gaps such as of scaling or
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extrapolation factors the use of a particular upper
confidence limit around a dose-response estimate. Such
assumptions need to be discussed separately, along with the
implications of using alternative assumptions.
For users of others who rely on
assessment, numerical estimates should never be separated from
descriptive information that is integral to risk
characterization. All documents presentations should include
both? in short reports,, this information is abbreviated but never
omitted.
For decision-makers, a complete characterization (key
descriptive elements along with numerical estimates) should be
retained in all discussions and papers relating to an assessment
in decision-making. Fully visible information assures that
important features of assessment immediately available at
level of decision-aaking for evaluating whether risks
acceptable or unreasonable. In short, differences in
uncertainties, coupled with non-scientific considerations
called for in various environmental statutes, can clear 1-y to
different risk management decisions in ca-ses with ostensibly
identical quantitative risks; i.e., the "number™ alone not
determine the decision.
Consideration of alternative approaches involves
selected plausible options for addressing a given uncertainty.
The words are "selected" "plausible;" listing all
options, regardless of their merits would be superfluous.
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Generators of the assessment should outline the strengths and
weaknesses of each alternative approach as appropriate,
of central tendency variability (e.g., mean,
percentll.es,, range, variance.)
Describing option chosen Involves several statements,
1. A rationale for choice.
2. Effects of option selected on the assessment.
3. Comparison with plausible options.
4. Potential of (on-going,
potential near-term and/or long-term studies),
of assessmentr giving attention to uncertainties In
all discussions'involving the assessment*
statement of confidence In all presentations is
For decision-maker s i> understanding effect of
on overall explaining
of uncertainties on regulatory
.decision.
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SECTION 3, AMD RISK DESCRIPTORS
The results of a risk assessment are usually communicated to
the risk manager in the risk characterization portion of the
assessment. This communication is often accomplished through
risk_descri£tors which convey information and answer questions
about risk, each descriptor providing different information
insights. Exposure assessment plays a key role in developing
risk descriptors, since each descriptor is in on
the distribution within the population of interest. The
Council (RAC) discussing of risk
descriptors from to over past two years.
recent RAC efforts laid foundation for
discussion to follow. Pirstr as a result of a discussion
on the comparability of risk assessments across the Agency
programs? the RAC discussed how the program presentations of risk
to ambiguity when risk assessments were compared
programs. Because different assessments presented different
descriptors of risk without always making clear what was
described, the RAC discussed the advisability of using-
descriptors for population risk, individual risk,
identification of sensitive or highly exposed population
segments* also discussed for consistency
the advisability of requiring risk assessments to
provide roughly comparable information to risk' managers the
public through the use of a consistent of risk descriptors.
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The following guidance outlines the different descriptors In
a convenient order that should not be construed as a hierarchy of
importance. These descriptors should be used to. describe risk In
a variety of ways for a given assessment, consistent with
assessment's purpose, the data available^ and the Information the
risk needs* Use of a range of descriptors Instead of a
single descriptor enables Agency programs to present a picture of
risk corresponds to range of different exposure
conditions encountered for most environmental chemicals. This
analysis, In turn* allows risk managers to Identify populations
at greater lesser risk to shape regulatory solutions
accordingly.
EPA risk assessments will be expected to address or provide
descriptions of (1) Individual risk to Include the central
high portions of the risk distribution,
(2) subgroups of population such as highly
or highly susceptible groups or Individuals, if known,
(3) population risk* Assessors may also use additional
descriptors of risk as needed when these add to the clarity of
presentation* With exception of
particular descriptors clearly do not apply, form of
of descriptors should be routinely developed
•presented for EPA risk assessments. Furthermore, presenters of
risk assessment Information should be prepared to routinely
answer questions by risk managers concerning these descriptors.
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Tt is essential that presenters not only communicate the
results of the assessment by addressing each of the descriptors
where appropriate, but they also communicate their confidence
that results portray a reasonable picture of the actual or
projected exposures. This task will usually be accomplished by
highlighting the key assumptions parameters that have
greatest on the results, the basis or rationale for
choosing assumptions/parameters/ the consequences of
choosing other assumptions.
In order for risk assessor to successfully develop
present various risk descriptors, the exposure
provide and information in a form that can be
with exposure-response or dose-response relationships to
risk* Although there will be differences among
individuals within a population as to absorption, intake rates,
susceptibility/ and other variables such that a high
not necessarily result in a high or risk, a moderate or
highly positive correlation among exposure/ dose/ and risk is
in the following discussion. Since the generation of all
descriptors is not appropriate in all risk assessments
of descriptor translates fairly directly into the of
analysis that the exposure assessor must perform/ the exposure
assessor needs to be aware of the ultimate goals of
assessment* The following.sections discuss what type of
information is necessary.
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r. Information about jLndiy_iduai ®xpo®ur€ risk is
important to communicating thm results of a risk
Individual risk descriptors intended to
questions dealing with risks borne by individuals within a
population. These questions can the of:
the people at the highest risk?
What risk levels are they subjected to?
What doing, where do they live, etc., that
be putting at this higher risk?
What Is average risk for individuals In the
population of interest?
Tr.T "h->- ar.d" cf the r.sk --f •' strf.b-rtitr. is, conceptually,
90th percent He of actual (either or
distribution. This conceptual Is to
the limits of this descriptor, but should be
by as a for characterizing "high
risk". Bounding worst scenarios1 should
be risk
high risk descriptor is a plausible
of individual risk
at upp^z1 of risk
distribution. The Intent of this descriptor
Is to convey an of risk In
of distribution, but to
avoid estimates which beyond the
1 High estimates focus on estimates of the or
in actual populations* "Bounding estimates," on
hand, purposely overestimate the exposure or dose In an
actual population.for the purpose of developing a statement that
risk Is "not greater than...." A "worst cas« scenario"
to a combination of events conditions such that,
together, the highest conceivable risk. Although It Is
that such an exposure, doser or sensitivity combination
In a given population of Interest, the probability of
an Individual receiving this combination of events conditions
Is usually small, often so small that such a combination will
occur In a particular, actual population.
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true distribution. Conceptually, high
end risk means risks' about the
90th percentile of the population
distribution, but not higher than the
individual in the population who
the highest risk.
This descriptor is intended to estimate risks that are
to in small definable "high end" of
subject population. The individuals with these risks may be
of a special population or individuals in the
general population who highly because of the inherent
stochastic nature of factors which give rise to exposure.
no particular difference in sensitivity can be identified
within population, high risk will be related to
high exposure or dose.
In those few where the complete data on the population
distributions of exposures available, high
or be represented by reporting
or at selected percentiles of the distributions,
as the 90th, 95th, or 98th percentile. High
or doses, as appropriate, can then be used to calculate high
risk estimates.
In majority of cas-ea where the complete distributions
available, several methods help estimate a high
or dose. If sufficient information about
.variability in lifestyles and factors available to
simulate the distribution through use of appropriate
modeling, e.g., Monte Carlo simulation, the estimate from the
simulated distribution may be used* As in the -method above,
risk manager should be told where in the high range the
24
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Is being by stating the percentile or the number of
this estimate. The assessor and risk manager
'should be aware, however, that unless a great deal is known about
at .the high of distribution,
will Involve considerable uncertainty which
will to describe.
If only limited information on distribution of
or la available, the assessor should
estimating high by Identifying the
sensitive using or near-maximum values for
or a of variables/ leaving others at their
values2. In doing this, the assessor to avoid
of values that Inconsistent, e.g.,, low
weight in combination with high Intake rates,
in ultimate objective of within the
distribution of actual doses,
it.
- If no available on
various parameters, It will be difficult to estimate' or
In high with confidence, to
risk estimate* One that in
is to start with a "back off"
.limits until combination of values is, In
2 all variables will In virtually all cases
result In an estimate that Is above actual values in
population. When the principal of equation
(e.g., concentration, Intake rate, duration) broken out into
subcomponentsf It may be necessary to values for
than two of these subcomponent parameters, depending on a
sensitivity analysis.
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judgment of the assessor, clearly within the distribution of
exposure, and still lies within the upper 10% of persons
Obviously, this method results in a large uncertainty
requires explanation.
The risk descriptor addressing central •
tendency may be either the arithmetic
risk (Average Estimate) or
risk (Median Estimate), either
of which^should be clearly la- "ad.
Where both the arithmetic mean -,™*d
the median are available but tn~y
differ substantially, it is helpful
to present both.
The Average Estimate, to approximate arithmetic
mean, be derived by using values for all
factors. It not necessarily represent a particular
individual on the distribution. The Average Estimate is not very
meaningful when exposure across a population varies by several
of magnitude or when the population truncated,
e.g., at prescribed distance from a point source.
of the skewness of typical exposure profiles,
arithmetic is not necessarily a good indicator of
(median, 50th percentile) of a distribution* ' A Median
Estimate, e.g., geometric mean, is usually a valuable descriptor
for this type of distribution, since half the population will be
-and half below this value*
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2. Information about population ®xpo®ur® l®ad® to anoth@r
important way to d®»crib€ risk.
Population risk refers to an assessment of the extent of
population as a whole. In theory, It be
calculated by summing the Individual risks for all Individuals
within population. This taskr of coursef requires a
deal Information than Is normally, If ever* available.
questions by descriptors of population risk
Includet
How many of a particular health effect might be
probabilistically In this population for a
period?
For noncarclnogens, what portion of the population
within a of level* e.g.,
of (a doae), RfC (a
concentration), or other health level?
carcinogensr many above a certain
risk level such as 1Q~6 or a of risk levels
as 10""5, 10"4r
requires of
distribution in population. In
particular, third
risk distribution, questions to
of population risk.
first descriptor Is probabilistic
o£ health effect cases estimated
in population of Interest a
specified period.
This descriptor can be obtained either by (a)
Individual risks over all Individuals In population
Information la available, or (b) through the of a risk
model such as carcinogenic models or procedures which a
27
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linear non-threshold response to exposure. If risk varies
linearly with exposure, knowing the risk and the population
size can lead to an estimate of•the extent of harm for
population as a whole, excluding sensitive subgroups for which a
different dose-response curve needs to-be used.
Obviously, the information has, certain
estimate of this risk descriptor, but inherent uncertainties
in risk assessment methodology place limitations on accuracy
of estimate. With the current state of the science, explicit
should be taken to assure that this descriptor is not
confused with an actuarial prediction of cases in the population
(which is a statistical prediction on a great deal of
empirical data).
Although estimating population risk by calculating a
individual risk multiplying by population size is
appropriate for carcinogen using linear,
non-threshold models3, this is not appropriate for non-
carcinogenic effects or for other types of cancer models. For
non-linear cancer models, -an estimate of population risk be
calculated by individual risks. For non-cancer effects,
we generally have not developed the risk assessment techniques to
point of knowing how to add risk probabilities, so a second
.descriptor, below, is more appropriate.
Another descriptor of population risk
is an estimate of the percentage of
the population, or the number of
persons, above a specified level of
3 Certain important cautions apply. These cautions
explicitly spelled out in the Agency's Guidelines for Exposure
Assessment, tentatively scheduled to be published in late 1991.
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risk or within a specified range of
benchmark level, e.g., exceedance
of the RfD or the RfC, LOAEL, or other
specific level of interest.
This descriptor must be obtained through measuring or simulating
population distribution.
29
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3. Information about the distribution of ®xposur@ and risk
for dlf£«c«nt £ub3rou£§ of tfa® population arm important
component* of a risk assessment.
A risk manager might also ask questions about the
distribution of the risk burden among various segments of
subject population such as the followingi
How do risk various subgroups.?
What Is the population risk of a particular subgroup?
Questions the distribution of exposure risk such
population segments require additional risk descriptors.
Highly can be
identified, where possible, characterized
the magnitude of risk quantified.
This descriptor Is useful when there
Is (or Is to be) a subgroup
experiencing significantly different
or from that of the
larger population.
These subpopulations be identified by age, sex, life-
style, economic factors, or other demographic variables.
example, toddlers who play in contaminated soil and certain high
fish consumers represent subpopulations that may have
to certain agents.
Highly susceptible subgroups can also
be Identified, If possible,
characterized the magnitude of
risk quantified. This descriptor Is
.useful when the sensitivity or
susceptibility to the effect for
specific subgroups is (or Is
to be) significantly
different from that of larger
population. In order to calculate
risk for these subgroups, It will
be necessary to use a
different dose-response relationship.
30
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For example, upon to a chemical, women, elderly
people, children, and people with certain illnesses may each be
sensitive than the population as a whole.
Generally, selection of population is a matter
of a priori interest in subgroup, in which
risk risk manager can jointly agree on which
to highlight, or a of discovery of a sensitive
or highly during the assessment process. In
case, identified, can be as a
population in itself, characterized way as
population using the descriptors for population
individual risk.
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4. Situation-specific information adds p«rsp«ctiv® on
posslbl® futur® «v®nts or regulatory options.
These postulated questions are normally designed to answer
"what If" questions, which either directed at low probability
but possibly high consequence events or are intended to
candidate risk management options. Such questions might
.following form;
What if a pesticide applicator applies
this pesticide without using protective
equipment?
What if this site residential
in the future?
What risk level will occur if we set
the standard at 100 ppb?
The assumptions in answering these postulated questions
be confused with assumptions in developing a
estimate of exposure or with the adjustments in
values in performing a sensitivity analysis. The
to thes'e postulated questions do not give information
about how likely the combination of values might be in the actual
population or about how many (if any) persons might be -subjected
to calculated exposure or risk in real world.
A calculation of risk on specific
hypothetical or actual combinations
of factors postulated within the
exposure assessment can also be
useful as a risk descriptor. It
is often valuable to ask answer
specific questions of the "what if"
nature to perspective to the
risk assessment*
The'only information the answers to these questions convey
is that if conditions A, B, C assumed, then resulting
exposure or risk will be X, Y, or Z, respectively. The values
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for X/ Y, and Z are usually fairly straightforward to calculate
and be expressed as point estimates or ranges.
Each may have none, one, or several of these types of
descriptors. The answers d© not directly give information about
how likely that combination of.values might be in the actual
population, so there are some limits to the applicability of
these descriptors.
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Rjr5>pc«_5
National Research Council. Risk Assessment in the Federal
Governmenti Managing the Process. 1983
U.S. EPA. Risk Assessment Managementi Framework for
Decision Making, 1984,
UaS* EPA. Risk Assessment Guidelines. 51 Federal Register
33992-34054. 24, 1986.
Presentation of Risk Assessment of Carcinogens; Ad Stcdy
Group on Risk Assessment Presentation. American Industrie.!
Health Council. 1989.
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