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1
Introduction

In recent decades, there have been seemingly innumerable reports of health threats from the environment. Myriad announcements about pesticides in food, pollutants in the air, chemical contaminants in drinking water, and hazardous-waste sites have created public concern about the chemical products and byproducts of modern industrial society. Alongside that concern exists skepticism about many of the possible threats to human health. The skepticism has arisen in part because scientists disagree. But it is also apparent that most people want to understand whether and how much their exposures to chemicals threaten their health and well-being.

Many environmental issues that have risen to public prominence involve carcinogens—substances that can contribute to the development of cancer. Sometimes the decision that a substance is a carcinogen is based on evidence from workers exposed to high concentrations in the workplace, but more often it is based on evidence obtained in animals exposed to high concentrations in the laboratory. When such substances are found to occur in the general environment (even in much lower concentrations), efforts are made to determine the exposed population's risk of developing cancer, so that rational decisions can be made about the need for reducing exposure. However, scientists do not have and will not soon have reliable ways to measure carcinogenic risks when exposures are small. In the absence of an ability to measure risk directly, they can offer only indirect and somewhat uncertain estimates.

Some hypotheses about carcinogens are qualitative. For example, biological data suggests that any exposure to a carcinogen may pose some health risk. Although some scientists disagree with that view or believe that it is not applica-



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Page 16 1 Introduction In recent decades, there have been seemingly innumerable reports of health threats from the environment. Myriad announcements about pesticides in food, pollutants in the air, chemical contaminants in drinking water, and hazardous-waste sites have created public concern about the chemical products and byproducts of modern industrial society. Alongside that concern exists skepticism about many of the possible threats to human health. The skepticism has arisen in part because scientists disagree. But it is also apparent that most people want to understand whether and how much their exposures to chemicals threaten their health and well-being. Many environmental issues that have risen to public prominence involve carcinogens—substances that can contribute to the development of cancer. Sometimes the decision that a substance is a carcinogen is based on evidence from workers exposed to high concentrations in the workplace, but more often it is based on evidence obtained in animals exposed to high concentrations in the laboratory. When such substances are found to occur in the general environment (even in much lower concentrations), efforts are made to determine the exposed population's risk of developing cancer, so that rational decisions can be made about the need for reducing exposure. However, scientists do not have and will not soon have reliable ways to measure carcinogenic risks when exposures are small. In the absence of an ability to measure risk directly, they can offer only indirect and somewhat uncertain estimates. Some hypotheses about carcinogens are qualitative. For example, biological data suggests that any exposure to a carcinogen may pose some health risk. Although some scientists disagree with that view or believe that it is not applica-

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Page 17 ble to every carcinogen, its adoption provides at least a provisional answer to a vexing scientific question, namely whether people exposed to low concentrations of substances that are known to be carcinogenic at high concentrations are at some risk of cancer associated with the exposure. That view has been prominent since the 1950s and has guided much decision-making. For example, the ''Delaney clause" of the Food Additive Amendments of 1958 stipulated that no additive that was found to be carcinogenic could be allowed in the food supply, on the grounds that it was not possible to specify a safe human exposure to such an agent. The policies that have flowed from regulations like the Delaney clause involve, where possible, absolute prohibition of exposures to carcinogens, but more commonly, reductions of exposures to the "lowest technically feasible level." A qualitative response to the question of carcinogenic risk is still viewed by many scientists to be the best that can now be offered, even in the face of impressive scientific advances in understanding chemical carcinogenesis. Nonetheless, it is increasingly recognized that division of the binary division of the world of chemicals into carcinogens and non-carcinogens is overly simplistic and does not provide an adequate basis for regulatory decision-making. Beginning in the 1960s and coming to full force in the 1970s, some scientists have attempted to offer more useful, quantitative information about the risks of low exposures to carcinogens. Quantitative risk assessment is attractive because, at least ideally, it allows decision-makers and the public to discriminate between important and trivial threats (thus going beyond qualitative findings that there is some risk, however small). The results of risk assessments are important in influencing important regulatory decisions that affect both the nation's economy and public health. They influence decision-makers as they attempt to balance the view that emission of hazardous air pollutants should be minimized or even eliminated, versus the view that meeting stringent control standards might cause other problems unacceptable to society. Accurate risk assessments are also needed to determine whether public health protection is adequate. Charge To The Committee The charge to the committee comes from Section 112(o) of the Clean Air Act, as added by the Clean Air Act Amendments of 1990, which requires EPA to enter into a contract with the National Research Council (NRC). NRC created the Committee on Risk Assessment of Hazardous Air Pollutants in the Board on Environmental Studies and Toxicology. Its charge is summarized as follows: 1. Review the risk assessment methods used by EPA (Environmental Protection Agency).

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Page 18 2. Evaluate methods used for estimating the carcinogenic potencies of hazardous air pollutants. 3. Evaluate methods used for estimating human exposures to hazardous air pollutants. 4. To the extent practicable, evaluate risk-assessment methods for noncancer health effects for which safe thresholds might not exist. 5. Indicate revisions needed in EPA's risk-assessment guidelines. The specific congressional language is provided in Appendix M. Section 112(o) requires that if EPA decides not to comply with all of the report's recommendations and the Science Advisory Board's views of the report, it must provide a detailed explanation in the Federal Register of the reasons that any of the recommendations in the report are not implemented. In its charge to EPA, Congress assigned NRC the task of evaluating whether EPA's risk-assessment methods express in a scientifically supportable way the risks posed by a substance. We therefore ask whether EPA's methods are consistent with current scientific knowledge. We also ask whether EPA's methods give policy-makers and the public the information they need to make judgments about risk management. Such methods should be logical and consistent and should, in particular, reveal the inevitable uncertainties in the underlying science. We make no judgment regarding the appropriate risk-management decision, e.g., the extent to which society should control hazardous air pollutants. Such decisions ultimately hinge on nonscientific issues; for instance, the extent of risk from hazardous air pollutants that society is willing to accept in return for other benefits. Such issues involve not only science or science-policy judgments, but also matters of value on which scientists cannot purport to have any special insight. Such issues are therefore ultimately the province of policy-makers and the public. It was precisely for this reason, we believe, that Congress specified in the Clean Air Act Amendments of 1990 that this committee is to undertake an investigation of EPA's risk-assessment methods, rather than of the validity of EPA's regulatory decisions. We have therefore refrained from addressing such risk management issues. We do, however, note that risk assessment and risk management are integrally related. As we explain later, Congress has generally directed EPA to be protective of health ("conservative" in the lexicon of public health) in its risk-management decisions. It is therefore essential for us to appraise whether EPA's risk-assessment methods are capable of supporting a policy of protective public-health regulation. In addition, in its charge to EPA, Congress indicated that noncancer effects should be addressed to the extent feasible, but time constraints reduced the committee's ability to focus fully on this issue.

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Page 19 Section 303 of the 1990 Amendments created the Risk Assessment and Management Commission, part of whose charge is to examine risk-management policy issues. Specific subjects that the commission is to address are • The report of the NRC committee. • The use and limitations of risk assessment in establishing emission or effluent standards, ambient standards, exposure standards, acceptable concentrations, tolerances, or the environmental criteria for hazardous substances that present a risk of carcinogenic or other chronic health effects and the suitability of risk assessment for such purposes. • The most appropriate methods for measuring and describing cancer risks or risks of other chronic health effects associated with exposure to hazardous substances. • Methods to reflect uncertainties in measurement and estimation techniques, the existence of synergistic or antagonistic effects among hazardous substances, the accuracy of extrapolating animal-exposure data to human health risks, and the existence of unquantified direct or indirect effects on human health in risk-assessment studies. • Risk-management policy issues, including the use of lifetime cancer risks to the people most exposed, the incidence of cancer, the cost and technical feasibility of exposure-reduction measures, and the use of site-specific actual exposure information in setting emission standards and other limitations applicable to sources of exposure to hazardous substances. • The degree to which it is possible or desirable to develop a consistent risk-assessment method, or a consistent standard of acceptable risk among various federal programs. Besides the Academy's report and the activities of the commission, both EPA and the Surgeon General are to evaluate the methods for evaluating health risks, the significance of residual risks, uncertainties associated with this analysis, and recommend legislative changes. As a result, the committee highlights here some important and controversial subjects in risk assessment and management that it felt were beyond its charge. 1. The use of a specific individual lifetime cancer risk number (e.g., 10-4 or 10-6) as a target for risk regulations. The committee notes that Congress has set a standard for considering regulatory decisions. We note that such a number should be tied to a method and that uncertainty will always surround such estimates. 2. The use of comparative risk analysis for the allocation of resources to minimize health impacts. Congress decides how much of the country's economic and social resources should be spent on reducing threats to public health and how to allocate resources among the many threats present in our daily lives. 3. The relative risk associated with synthetic or industrial byproducts ver-

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Page 20   sus natural chemicals. A recent study (Gold et al., 1992) contends that natural chemicals make up the vast bulk of chemicals to which humans are exposed, that natural chemicals are not much different from synthetic chemicals in their toxicology, and that about half the natural chemicals tested in chronic studies in rats and mice are carcinogens. The implication is that humans are likely to be exposed to a large background of rodent carcinogens as defined by high-dose testing. Some believe that this has implications for the amount of resources currently devoted to the study and control of synthetic chemicals. However, other studies (e.g., Perera and Bofetta, 1988) question the scientific underpinnings of these conclusions. The issue of the degree to which natural versus synthetic chemicals should be regulated is a policy issue that we cannot address. The scientific aspects of the issue will be discussed in a forthcoming NRC report on the relative risks of natural carcinogens. It is important to note that the present study focuses on airborne hazardous air pollutants and that, although some natural carcinogens are in food and water, there is little evidence of their widespread presence in air. 4. The setting of relative policy priorities regarding the regulation of all sources of hazardous air pollutants. The focus of Section 112 is on stationary sources of hazardous air pollutants; therefore, it was not within the charge of this committee to conduct an analysis of all sources of hazardous air pollutants and recommend which ones should be regulated and which should not. Congress already determined the extent to which it wanted to do that in the 1990 Amendments. Therefore, although the committee points out later in the report the potential impact of indoor versus outdoor pollutants, it is beyond our charge to go further and say whether, when, and how to take action on nonstationary and indoor sources of hazardous air pollutants. 5. The uncertainty in engineering and economic assumptions. There is, of course, uncertainty in the engineering and economic assumptions leading to EPA's estimates of the impact on industry of a regulation mandating specified magnitudes of risk. However, the committee was asked only to address EPA's implementation of risk assessment relative to public health, not the economic consequences of such regulation. 6. The extent to which chemicals should be on or off the list of chemicals in the 1990 Amendments. Although this report discusses how to set priorities for the collection and analysis of chemicals on the list, it is a policy judgment as to whether these chemicals, once ranked, should be included on such a list. (That does not imply that outside review of the list is not appropriate.) 7. The presentation of uncertainty in the context of background risk. Although this committee does discuss the issue of presentation of uncertainty, it was beyond its charge to indicate the extent to which it was appropriate to place the 1990 Amendments or other legislation in the context of all societal risk. Risk communication is complicated and involves such issues as involuntary versus voluntary risks, costs, benefits, and values, both individual and societal.

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Page 21 Conceptual Framework Of The Report This report is aimed at a multidisciplinary audience with different levels of technical understanding. In discussing the many controversial aspects of risk assessment, the committee decided to address three categories of issues: • Background of risk assessment and current practice at EPA. We organize this section (Chapters 2-5) via the old Red Book four-step paradigm. • Specific concerns in risk assessment, such as the use of defaults and extrapolations. For example, is EPA justified in assuming, in the absence of contrary evidence, that the linearized multistage model should be used in determining the dose-response relationship for carcinogens? • Cross-cutting issues that affect all parts of risk assessment. For example, how should uncertainty be handled? How should the accuracy of a model be evaluated? • Implementation issues related to Section 112 of the 1990 Amendments. For example, how should EPA accommodate the tension between the goals of providing stability in its process and staying abreast of changing scientific knowledge? The report addresses each type of issue. Our categorization of the issues reflects the analytical framework used by the committee and influences the structure of its recommendations. Although that might lead to some repetition, the committee feels that a degree of repetition is desirable because of the need to address audiences with different levels of knowledge. The committee attempted to address the specific issues that arise from the uses of risk assessment under Section 112 of the Clean Air Act, which mandates the regulation of hazardous air pollutants. As amended in 1990, Section 112 deemphasizes risk assessment in the initial phase of regulation, in which EPA is to establish "technology-based" standards for categories of sources that emit hazardous air pollutants. Risk assessment's main role will be in the second phase of regulation, in which EPA must determine whether residual risk (the risk presented by the emissions remaining after compliance with technology-based standards) should be further reduced. Risk assessment will also be used in several other ways (e.g., to determine whether an entire source category may be exempted from technology-based standards on the grounds that no source in the category creates more than a one-in-a-million lifetime risk of cancer for the most exposed person). The appendixes to the report include EPA's responses to questions from the committee and some important EPA documents not readily available. Risk assessment is an ever-changing process, and these documents illustrate its status within EPA during the time when the committee is making its recommendations.

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Page 22 Two documents were also prepared by some committee members to reflect the committee's inability to reach consensus on how EPA should choose and refine its "default options" for conducting risk assessments when basic scientific mechanisms are unknown. One view espouses a principle of "plausible conservatism," while the other advocates "making full use of science."