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5
Risk Characterization

Introduction

Characterization of risk is the final step in health risk assessment. This chapter discusses the methods used by the Environmental Protection Agency (EPA) to characterize the public-health risk associated with an emission source. In risk characterization, the assessor takes the exposure information from the exposure-assessment stage (discussed in Chapter 3) and combines it with information from the dose-response assessment stage (discussed in Chapter 4) to determine the likelihood that an emission could cause harm to nearby individuals and populations. The results of this risk characterization are then communicated to the risk manager with an overall assessment of the quality of the information in that analysis. The goal of risk characterization is to provide an understanding of the type and magnitude of an adverse effect that a particular chemical or emission could cause under particular circumstances. The risk manager then makes decisions on the basis of the public-health impact as determined by the risk characterization and other criteria outlined in the appropriate statute.

The elements of risk characterization are discussed here on the basis of several EPA documents, including EPA's Risk Assessment Guidelines of 1986 (EPA, 1987a); Guidelines for Exposure Assessment (EPA, 1992a); a memorandum from Henry Habicht II, deputy administrator of EPA, dated February 26, 1992 (EPA, 1992c) (see Appendix B) (known hereafter as the ''risk-characterization memorandum"); and Risk Assessment Guidance for Superfund (EPA, 1989a) (the "Superfund document").



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Page 68 5 Risk Characterization Introduction Characterization of risk is the final step in health risk assessment. This chapter discusses the methods used by the Environmental Protection Agency (EPA) to characterize the public-health risk associated with an emission source. In risk characterization, the assessor takes the exposure information from the exposure-assessment stage (discussed in Chapter 3) and combines it with information from the dose-response assessment stage (discussed in Chapter 4) to determine the likelihood that an emission could cause harm to nearby individuals and populations. The results of this risk characterization are then communicated to the risk manager with an overall assessment of the quality of the information in that analysis. The goal of risk characterization is to provide an understanding of the type and magnitude of an adverse effect that a particular chemical or emission could cause under particular circumstances. The risk manager then makes decisions on the basis of the public-health impact as determined by the risk characterization and other criteria outlined in the appropriate statute. The elements of risk characterization are discussed here on the basis of several EPA documents, including EPA's Risk Assessment Guidelines of 1986 (EPA, 1987a); Guidelines for Exposure Assessment (EPA, 1992a); a memorandum from Henry Habicht II, deputy administrator of EPA, dated February 26, 1992 (EPA, 1992c) (see Appendix B) (known hereafter as the ''risk-characterization memorandum"); and Risk Assessment Guidance for Superfund (EPA, 1989a) (the "Superfund document").

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Page 69 Elements Of Risk Characterization EPA's risk-characterization step has four elements: generation of a quantitative estimate of risk, qualitative description of uncertainty, presentation of the risk estimate, and communication of the results of risk analysis. Quantitative Estimates of Risk To determine the likelihood of an adverse effect in an exposed population, quantitative information on exposure—i.e., the dose (determined from the analysis in Chapter 3)—is combined with information on the dose-response relationship (determined from the analysis in Chapter 4). This process is different for carcinogens and for noncarcinogens. For noncarcinogens, the dose estimate is divided by the RfD to obtain a hazard index. If the hazard index is less than 1, the chemical exposure under consideration is regarded as unlikely to lead to adverse health effects. If the hazard index is greater than 1, adverse health effects are more likely and some remedial action is called for. The hazard index is thus not an actual measure of risk; it is a benchmark that can be used to estimate the likelihood of risk. For carcinogens, excess lifetime risk is calculated by multiplying the dose estimate by a potency factor. The result is a value that represents an upper bound on the probability that lifetime exposure to an agent, under the specified conditions of exposure, will lead to excess cancer risk. This value is usually expressed as a population risk, such as 1 × 10-6, which means that no more than one in 1 million exposed persons is expected to develop cancer. Risk estimates obtained in this way are not scientific estimates of actual cancer risk; they are upper bounds on actual cancer risk that are useful to regulators for setting priorities and for setting exposure limits. When exposure to more than one agent occurs simultaneously, the cancer risk estimates obtained for each agent can be combined in an additive manner for each route of exposure. Hazard indexes for noncarcinogens may be combined when the agents of concern elicit similar end points of toxicity. Sometimes, this risk-characterization technique is used to estimate an upper bound on excess lifetime cancer risk to exposed individuals, instead of populations. EPA's Guidelines for Exposure Assessment (EPA, 1992a) (not yet implemented) lists some of the questions that should be answered when considering individual versus population risk. These questions are stated by EPA as follows: Individual Risk • Are individuals at risk from exposure to the substances under study? Although for substances, such as carcinogens, that are assumed to have no threshold, only a zero dose would result in nonexcess risk for noncarcinogens, this

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Page 70   question can often be addressed. In the case of the use of hazard indices, where exposure or doses are compared to a reference dose or some other acceptable level, the risk descriptor would be a statement based on the ratio between the dose incurred and the reference dose. • To what risk levels are the persons at the highest risk subjected? Who are these people, what are they doing, where do they live, etc., and what might be putting them at this higher risk? • Can people with a high degree of susceptibility be identified? • What is the average individual risk? Population Risk • How many cases of a particular health effect might be probabilistically estimated for a population of interest during a specified time period? • For noncarcinogens, what portion of the population exceed the reference dose (RfD), the reference concentration (RfC), or other health concern level? For carcinogens, how many persons are above a certain risk level such as 10-6 or a series of risk levels such as 10-5, 10-4, etc. • How do various subgroups fall within the distributions of exposure, dose, and risk? • What is the risk for a particular population segment? • Do any particular subgroups experience a high exposure, dose, or risk? Description of Uncertainty Analysis of the uncertainty associated with a health risk estimate involves each step of the risk-assessment process: it brings together the uncertainty in emissions and exposure estimates with that of the toxicity dose-response assessment. Table 5-1 lists the uncertainty issues to be addressed at each step of a health risk assessment. Uncertainty analysis can take place at the time of each of those analyses, but because it affects the eventual risk estimate, it is considered part of the final step of risk assessment—risk characterization. Several recent documents illustrate EPA's current approach to the analysis of uncertainty associated with health risk assessment, including the Superfund document (EPA, 1989a), the background information document for NESHAPS for radionuclides (EPA, 1989b), the Guidelines for Exposure Assessment (EPA, 1992a), and the risk-characterization memorandum (Appendix B). Superfund Risk-Assessment Guidance The Superfund document provides guidance to EPA and other government employees and contractors who are risk assessors, risk-assessment reviewers, remedial project managers, or risk managers involved in Superfund-site cleanup.

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Page 71 TABLE 5-1 Uncertainty Issues To Be Addressed in Each Risk Assessment Step A. Hazard Identification: What do we know about the capacity of an environmental agent for causing cancer (or other adverse effects) in laboratory animals and in humans?   1. the nature, reliability, and consistency of the particular studies in humans and in laboratory animals;   2. the available information on the mechanistic basis for activity; and   3. experimental animal responses and their relevance to human outcomes. B. Dose-Response Assessment: What do we know about the biological mechanisms and dose-response relationships underlying any effects observed in the laboratory or epidemiology studies providing data for the assessment?   1. relationship between extrapolation models selected and available information on biological mechanisms;   2. how appropriate data sets were selected from those that show the range of possible potencies both in laboratory animals and humans;   3. basis for selecting interspecies dose scaling factors to account for scaling dose from experimental animals to humans; and,   4. correspondence between the expected route(s) of exposure and the exposure route(s) utilized in the hazard studies, as well as the interrelationships of potential effects from different exposure routes. C. Exposure Assessment: What do we know about the paths, patterns, and magnitudes of human exposure and number of persons likely to be exposed?   1. The basis for the values and input parameters used in each exposure scenario. If based on data, information on the quality, purpose, and representatives of the database is needed. If based on assumptions, the source and general logic used to develop the assumption (e.g., monitoring, modeling, analogy, professional judgment) should be described.   2. The 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. These risk descriptors should include: (1) individual risk including the central tendency and high end portions of the risk distribution, (2) important subgroups of the population such as highly exposed or highly susceptible groups or individuals (if known), and (3) population risk. This issue includes the conservatism or non-conservatism of the scenarios, as indicated by the choice of descriptors. In addition, information that addresses the impact of possible low probability but possibly high consequence events should be addressed.     For individual risk, information such as the people at highest risk, the risk levels these individuals are subject to, the activities putting them at higher risk, and the average risk for individuals in the population of interest should be addressed. For population risk, information as to the number of cases of a particular health effect that might be probabilistically estimated in this population for a specific time period, the portion of the population that are within a specified range of some benchmark level for non-carcinogens; and, for carcinogens, the number of persons above a certain risk level should be included. For subgroups, information as to how exposure and risk impact the various subgroups and the population risk of a particular subgroup should be provided. (Table continues on following page.)

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Page 72 TABLE 5-1 Continued D. Risk Characterization: What do other assessors, decision-makers, and the public need to know about the primary conclusions and assumptions, and about the balance between confidence and uncertainty in the assessment? What are the strengths and limitations of the assessment?   1. Numerical estimates should never be separated from the descriptive information that is integral to the risk assessment. For decisionmakers, a complete characterization (key descriptive elements along with numerical estimates) should be retained in all discussions and papers relating to an assessment used in decision-making. Differences in assumptions and uncertainties, coupled with non-scientific considerations called for in various environmental statutes, can clearly lead to different risk management decisions in cases with ostensibly identical quantitative risks; i.e., the "number" alone does not determine the decisions.   2. Consideration of alternative approaches involves examining selected plausible options for addressing a given uncertainty. The strengths and weaknesses of each alternative approach and as appropriate, estimates of central tendency and variability (e.g., mean, percentiles, range, variance). The description of the option chosen should include the rationale for the choice, the effect of option selected on the assessment, a comparison with other plausible options, and the potential impacts of new research. SOURCE: Risk-characterization memorandum (Appendix B). Section 8.4 of the document "discusses practical approaches to assessing uncertainty in Superfund site risk assessments and describes ways to present key information bearing on the level of confidence in quantitative risk estimates for a site." The document considers three categories of uncertainty associated with site risk assessments: selection of substances, toxicity values, and exposure assessments. Table 5-2 is EPA's uncertainty checklist for Superfund-site risk assessments. Risk assessors are to use the checklist to ensure that they describe adequately the uncertainty in a risk assessment. The document indicates that, although the uncertainty associated with each variable in a risk assessment would ideally be associated with the final risk estimate, a more practical approach is to describe qualitatively how the uncertainties might be magnified or the estimates of risk biased because of the risk models used. This document is being updated. Uncertainty Analysis for Radionuclide Risk EPA undertook a more comprehensive, integrated, quantitative approach to uncertainty characterization in the background document for its environmental impact statement on the National Emission Standards for Hazardous Air Pollutants (NESHAPS) for radionuclides (EPA, 1989b). This document includes an extensive presentation of estimates of fatal cancer risks associated with exposure to radionuclides. The estimates were "intended to be reasonable best estimates of risk; that is, to not significantly underestimate or overestimate risks and be of

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Page 73 TABLE 5-2 EPA Guidance for Uncertainty Analysis in Superfund Risk Assessments LIST PHYSICAL SETTING DEFINITION UNCERTAINTIES • For chemicals not included in the quantitative risk assessment, describe briefly:   — reason for exclusion (e.g., quality control), and   — possible consequences of exclusion on risk assessment (e.g., because of widespread contamination, underestimate of risk). • For the current land uses describe:   — sources and quality of information, and   — qualitative confidence level. • For the future land uses describe:   — sources and quality of information, and   — information related to the likelihood of occurrence. • For each exposure pathway, describe why pathway was selected or not selected for evaluation.   For each combination of pathways, describe any qualifications regarding the selection of exposure pathways considered to contribute to exposure of the same individual or group of individuals over the same period of time. CHARACTERIZE MODEL UNCERTAINTIES • List/summarize the key model assumptions. • Indicate the potential impact of each on risk:   — direction (i.e., may over- or underestimate risk); and   — magnitude (e.g., order of magnitude). CHARACTERIZE TOXICITY ASSESSMENT UNCERTAINTIES For each substance carried through the quantitative risk assessment, list uncertainties related to: • qualitative hazard findings (i.e., potential for human toxicity); • derivation of toxicity values, e.g.,   — human or animal data,   — duration of study (e.g., chronic study used to set subchronic RfD), and   — any special considerations; • the potential for synergistic or antagonistic interactions with other substances affecting the same individuals; and • calculation of lifetime cancer risks on the basis of less-than-lifetime exposures. For each substance not included in the quantitative risk assessment because of inadequate toxicity information, list: • possible health effects; and • possible consequences of exclusion on final risk estimates. RISK CHARACTERIZATION • confidence that the key site-related contaminants were identified and discussion of contaminant concentrations relative to background concentration ranges; (Table continues on following page.)

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Page 74 TABLE 5-2 Continued RISK CHARACTERIZATION—continued • a description of the various types of cancer and other health risks present at the site (e.g., liver toxicity, neurotoxicity), distinguishing between known effects in humans and those that are predicted to occur based on animal experiments; • level of confidence in the quantitative toxicity information used to estimate risks and presentation of qualitative information on the toxicity of substances not included in the quantitative assessment; • level of confidence in the exposure estimates for key exposure pathways and related exposure parameter assumptions; • the magnitude of the cancer risks and noncancer hazard indices relative to the Superfund site remediation goals in the NCP (e.g., the cancer risk range of 10-4 to 10-7 and noncancer hazard index of 1.0); • the major factors driving the site risks (e.g., substances, pathways, and pathway combinations); • the major factors reducing the certainty in the results and the significance of these uncertainties (e.g., adding risks over several substances and pathways); • exposed population characteristics; and • comparison with site-specific health studies, when available. SOURCE: Adapted from EPA, 1989a. sufficient accuracy to support decisionmaking" (EPA, 1989b). One chapter of the document, however, provides a detailed analysis of uncertainties in the calculated risks that was undertaken by EPA's Office of Radiation Programs for four selected exposure sites, such as a uranium-mill tailings pile in Washington and an elemental-phosphorus plant in Idaho. The stated reason for the uncertainty analysis was that "quantitative uncertainty analysis can provide results that indicate the likelihood of realizing different risk levels across the range of uncertainty. This type of information is very useful for incorporating acceptable and reasonable confidence levels into decisions" (EPA, 1989b). The EPA uncertainty analysis for radionuclide risks focused on "parameter uncertainty," because it was felt that other sources of uncertainty involving alternative or additional exposure pathways and risk-model structures were "not readily amenable to explicit analysis" (EPA, 1989b). Parameter uncertainties were first modeled as particular probability distributions for each parameter involved in four key components of the radionuclide risk assessments: source terms, atmospheric-dispersion factors, environmental-transport and radionuclide-uptake factors, and risk-conversion (that is, radionuclide-potency) factors. All the distributions pertaining to exposure-related factors were intended to model uncertainty in factor values characteristic of a maximally exposed person. All the distributions pertaining to uptake-related factors were intended to model uncertainty in factor values characteristic of an average individual, except in a set of separate corresponding analyses in which census-based interindividual variabili-

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Page 75 ty in home-residence time was incorporated into the analysis, where it was computationally treated as an uncertain parameter. Monte Carlo methods were used to propagate uncertainty within contamination-uptake-risk models for calculating radionuclide-specific, increased lifetime risks of fatal cancer to an otherwise typical person who is maximally exposed over a lifetime (70 years) or over some shorter period sampled randomly from the distribution used to characterize home-residence time. The resulting characterization obtained for uncertainty in estimated total increased fatal-cancer risk associated with potential maximal exposure to all radionuclides for an exposure scenario involving a uranium-mill tailings pile is shown in Figure 5-1. The horizontal axis in that figure represents increased risk multiplied by 3.5 × 10-6, which is the geometric mean of the distribution (shown as the solid curve) of risk to an individual maximally exposed for 70 years. (Normalization to the geometric mean value was done simply because all the risk distributions obtained were very close to lognormal.) The vertical axis in Figure 5-1 represents cumulative probability expressed as a percentage, that is, the probability that the true (but certain) risk is less than or equal to a given, corresponding particular risk value shown on the horizontal axis. The solid horizontal line in the figure corresponds to cumulative probability equal to 50%. The dashed curve in the figure represents estimated risk accounting for less-than-lifetime home residence. In commenting on the substantial difference between the solid and dashed curves for the four types of exposure scenarios considered in this uncertainty analysis, EPA concluded that "it is clear … that many moves are to nearby locations," that "we do not believe that including a factor for exposure duration improves the assessment of maximum individual risk," and that "improper application of such a factor can easily lead to erroneous conclusions regarding uncertainties in the risk assessment" (EPA, 1989b). FIGURE 5-1 Uncertainty in estimated total increased fatal-cancer risk associated with potential maximal exposure to all radionuclides for an exposure scenario involving a uranium-mill tailings pile. SOURCE: Adapted from EPA, 1989b.

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Page 76 Presentation of Risk Estimates Several methods can be used to display health risk estimates. Some of the terms used most often are listed in Table 5-2. The definitions are from the new 1992 exposure guidelines (EPA, 1992a). Any combination of them can be used to display the risk estimate to either the risk manager or the public. The choice of descriptors is often based on legal mandates. In general, the display includes a table indicating the risk estimated for the exposed population by route of exposure. 1992 Exposure-Assessment Guidelines EPA's 1992 Guidelines for Exposure Assessment shows a clear presentation of hazard-identification, dose-response, and exposure-assessment information that might be useful in future risk assessments. Risk assessors are to examine the judgments made during the process, the constraints of available data, and the state of knowledge. According to EPA, the risk characterization should include (EPA, 1992a) • the qualitative, weight-of-evidence conclusions about the likelihood that the chemical may pose a specific hazard (or hazards) to human health, the nature and severity of the observed effects, and by what route(s) these effects are seen to occur. These judgments affect both the dose-response and exposure assessments. • for noncancer effects, a discussion of the dose-response behavior of the critical effect(s), data such as the shapes and slopes of the dose-response curves for the various other toxic end points, and how this information was used to determine the appropriate dose-response assessment techniques; and • the estimates of the magnitude of the exposure, the route, duration and pattern of the exposure, relevant pharmacokinetics, and the number and characteristics of the population exposed. This information must be compatible with both the hazard identification and dose-response assessments. The risk-characterization summary should highlight the key points of each step of the risk-assessment process. Risk-Characterization Memorandum EPA is in transition on risk characterization. Besides the exposure guidelines described above, the risk-characterization memorandum (Appendix B) provides guidance on risk characterization and uncertainty analysis for EPA risk managers and risk assessors. The memorandum addresses a problem that affects public perception regarding the reliability of EPA's scientific assessments and related regulatory decisions… Significant information is often omitted as the results of the assessment are passed along in the decision-making process. … Often, when risk information is presented to

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Page 77 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 characterizations clarified the scientific basis for EPA decisions, while numbers alone do not give a true picture of the assessment. A statement attached to the memorandum from the Risk Assessment Council, made up of EPA senior managers, emphasized the following principles: • Full Characterization of Risk: A 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. • Comparability and Consistency: confusion as to the comparability of similar looking (but quite different) risks, for example, the risk estimate for an average individual risk relative to the risk estimate for the most exposed individual, have led to misunderstandings about the relative significance of risks and the protectiveness of risk, reduction action. Therefore, several different descriptors of risk as outlined in the newly revised Exposure Assessment Guidelines, should be presented to provide a more complete picture of the risk than available from a single descriptor of risk. • Professional Judgment: There are limits to the degree to which a full characterization of risk may be provided. The degree to which confidence and uncertainty are addressed depends largely on the scope of the assessment and available sources. So decision-makers and the public are not overwhelmed, only the most significant data and uncertainties need be presented. Further, when special circumstances (e. g., lack of data, extremely complex situations, resource limitations, statutory deadlines) preclude an assessment, such circumstances should be explained. In implementing that guidance, EPA staff should: 1. Clearly present risk assessment information separate from any non-scientific risk management considerations. 2. 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, and a statement of confidence in the assessments that identifies all major uncertainties along with comment on their influence on the assessment must be provided. 3. 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) should be presented.

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Page 78 The risk-characterization memorandum goes through each step of risk assessment and outlines the questions to be answered. These are shown in Table 5-1, which suggests several issues that should be addressed to describe the information in each step fully. Communication of Risk Risk communication consists of two parts: communication between the risk assessor and the risk manager and communication between the risk-assessment management team and the public. The risk manager often receives the individual and population risk estimates (generally point estimates but occasionally ranges of these estimates) with only a qualitative description of the uncertainties in each. The general public often receives much less information—only the point estimate or range (without a description of the uncertainty) and the risk manager's decision—although far more is available from published sources or on request. In most regulatory situations, the manager's decision and supporting information are published in the Federal Register. In addition, extensive background documents that discuss the risk analysis in much more depth are often available to the public. The public is generally given an opportunity to comment within 30-60 days on the analysis and resulting decision. EPA may adjust a risk assessment on the basis of public comments.