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Part I Current Approaches to Risk Assessment: 2 Risk Assessment and its Social and Regulatory Contexts
Pages 23-42

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From page 23...
... , and the intake of the pollutant by people who might suffer adverse health effects either soon after exposure or later. Scientists and engineers take four basic interrelated steps to evaluate the potential health impact on people who are exposed to a hazardous air pollutant: emission characterization, exposure assessment, toxicity assessment, and risk characterization.
From page 24...
... , 1 I . Cancer and =' other late developing diseases FIGURE I-2 Relationships in assessing human health risks of exposure to hazardous air pollutants.
From page 25...
... This chapter lists the major criticisms of risk assessment and the ways in which its results have been used, thus providing the justification for the selection of issues discussed in the succeeding chapters. GENERAL CONCEPTS This section briefly discusses some basic definitions and concepts concerning human-health risk assessment, its content, and its relationships to research and to decision-making.
From page 26...
... This step is common to qualitative and quantitative risk assessment. Step 2: Dose-Response Assessment entails a further evaluation of the conditions under which the toxic properties of a chemical might be manifested in exposed people, with particular emphasis on the quantitative relation between the dose and the toxic response.
From page 27...
... Not every risk assessment encompasses all four steps. Risk assessment sometimes consists only of a hazard assessment designed to evaluate the potential of a substance to cause human health effects.
From page 28...
... As amended in l99O, Section 112 of the Clean Air Act gives quantitative risk-assessment results a secondary but still important role relative to technology-based controls. What Is a Default Option?
From page 29...
... Threshold doses were recognized to vary widely among chemicals, but as long as human exposures were limited to subthreshold doses, no injury to health would be expected. The threshold hypothesis thus involved rejection of the simplistic view that the world is divided into toxic and nontoxic substances and acceptance of the principle that, for all chemicals, there were ranges of exposure that were toxic and ranges that were not.
From page 30...
... Experts in occupational health tended to rely heavily on observations of short-term toxicity in highly exposed workers and established acceptable exposure limits (the most prominent of which were the so-called threshold limit values, TLVs, first published by the American Conference of Governmental Industrial Hygienists in the 1950s) that were below the exposures that produced observable toxic effects.
From page 31...
... Although it has since been modified in several minor ways, the basic procedure for setting limits on human exposures to chemicals in air, water, and food persists to this day. The threshold hypothesis has been criticized as inadequate to account for some toxic effects, and it has not been accepted by regulators as applicable to carcinogens, but it remains a cornerstone of other regulatory and public-health risk assessments.
From page 32...
... at first rejected the use of risk quantification as it mounted a major effort during the late 1970s to regulate occupational carcinogens, because it believed that the statute under which it operated did not permit the use of risk assessment. But a Supreme Court decision regarding the agency's efforts to establish a permissible exposure limit for benzene caused OSHA to incorporate risk quantification (see below)
From page 33...
... In particular, NRC's charge involved a close examination of the possibility that risk assessment might be conducted by a separate, centralized scientific body that would serve all the relevant agencies. It was proposed that such an arrangement might reduce the influence of policy-makers on the conduct of risk assessment, so that there would be minimal opportunities for the results of risk assessments to be manipulated to meet predetermined policy objectives.
From page 34...
... Alone among federal agencies, EPA adopted a set of guidelines for carcinogen risk assessment in 1986, as recommended by NRC. The EPA guidelines specify default options, note the distinction between risk assessment and risk management, and otherwise meet NRC's and OSTP's recommendations.
From page 35...
... . The agency also extended the uses of risk assessment to decisions regarding pesticide residues in food, carcinogenic contaminants of drinking-water supplies, industrial emissions of carcinogens to surface waters, and industrial chemicals subject to regulation under the Toxic Substances Control Act (TSCA)
From page 36...
... for the most exposed person would constitute acceptable risk and that the margin of safety should reduce the risk for the greatest possible number of persons to an individual lifetime excess risk no higher than one in 1 million (10-64. Such factors as incidence (e.g., the number of possible new cases of a disease in a population)
From page 37...
... In the first, technology-based emissions standards will be imposed. Each major source (defined, generally, as a stationary source having the potential to emit 10 tons per year of a single hazardous air pollutant or 25 tons per year of a combination of hazardous air pollutants)
From page 38...
... establishes a list of 189 hazardous air pollutants and requires that EPA add a substance to the list on a determination, either on its own accord or in response to a petition, that the substance is "known to cause or may reasonably be anticipated to cause adverse effects to human health or adverse environmental effects." This standard represents a reaffirmation of the Ethyl decision (discussed later) that EPA may regulate in the face of scientific uncertainty about a substance's effects.
From page 39...
... Operators of sources at which a listed substance is present in more than a threshold quantity must prepare a risk-management plan to prevent accidental releases. NONCANCER RISK ASSOCIATED WITH HAZARDOUS AIR POLLUTANTS The current EPA approach to risk assessment for noncancer hazards posed by hazardous air pollutants, refined in several ways, is conceptually similar to the traditional approach to threshold agents described earlier.
From page 40...
... That scrutiny has led to frequent and sharp criticisms of the methods used for assessing risk and of ways in which the results of risk assessment have been used to guide decision-making. The criticisms have not been directed solely at the use of risk assessment in regulation of hazardous air pollutants, but rather cover a range of uses.
From page 41...
... The too-rigid adherence to the preselected default options also impedes research, because there is little likelihood that novel data will be incorporated into EPA risk assessments.
From page 42...
... NOTE 1. The chemicals listed as hazardous air pollutants under the National Standards for Hazardous Air Pollutants (NESHAP)


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