Science and Judgment in Risk Assessment (1994) / Chapter Skim
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Executive Summary
Executive Summary
Pages 1-15
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, 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 to humans when exposures are small.
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If it were reliable, quantitative risk assessment could improve the ability of decision-makers and to some extent the public to discriminate between important and trivial threats and improve their ability to set priorities, evaluate tradeoffs among pollutants, and allocate public resources accordingly. In short, it could improve regulatory decisions that affect public health and the nation's economy.
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REGULATION OF HAZARDOUS AIR POLLUTANTS Before the enactment of the Clean Air Act Amendments of 1990 (1990 Amendments) , Section 112 of the Clean Air Act required that the Environmental Protection Agency (EPA)
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directs the EPA to arrange for the National Academy of Sciences to: · Review the methods used by EPA to determine the carcinogenic risk associated with exposure to hazardous air pollutants from sources subject to Section 112; · Include in its review evaluations of the methods used for estimating the carcinogenic potency of hazardous air pollutants and for estimating human exposures to these air pollutants; · Evaluate, to the extent practicable, risk-assessment methods for noncancer health effects for which safe thresholds might not exist. The Academy's report must be considered by EPA in revising its present risk assessment guidelines.
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The result should be a process that supports the risk-management decisions required by the Clean Air Act and that provides appropriate incentives for further research to reduce important uncertainties on the extent of health risks. In 1986, EPA issued risk-assessment guidelines that were generally consistent with the Red Book recommendations.
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Better data and increased understanding of biological mechanisms should enable risk assessments that are less dependent on conservative default assumptions and more accurate as predictions of human risk.
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EPA's practice appears to be to allow departure from a default option in a specific case when it ascertains that there is a consensus among knowledgeable scientists that the available scientific evidence justifies departure from the default option. The agency relies on its Scientific Advisory Board and other expert bodies to determine when such a consensus exists.
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These representations of airborne transport processes are approximations. EPA focuses primarily on stationary outdoor emission sources of hazardous air pollutants.
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· EPA should develop guidelines for the amount and quality of emission information required for particular risk assessments and for estimating and reporting uncertainty in emission estimates, e.g., the predictive accuracy and uncertainty associated with each use of the HEM for exposure assessment. · EPA should evaluate the Gaussian-plume models under realistic conditions of acceptable distances (based on population characteristics)
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, industry, and academe. · In addition to stationary sources of hazardous air pollutants, EPA should consider mobile and indoor sources; the latter might be even more important than outdoor sources.
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· EPA should assess risks to infants and children whenever it appears that their risks might be greater than those of adults. Uncertainty There are numerous gaps in scientific knowledge regarding hazardous air pollutants.
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Recommendations · EPA should conduct formal uncertainty analyses, which can show where additional research might resolve major uncertainties and where it might not. · EPA should consider in its risk assessments the limits of scientific knowledge, the remaining uncertainties, and the desire to identify errors of either overestimation or underestimation.
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· In the analysis of animal bioassay data on the occurrence of multiple tumor types, the cancer potencies should be estimated for each relevant tumor type that is related to exposure, and the individual potencies should be summed for those tumors. · Quantitative uncertainty characterizations conducted by EPA should appropriately reflect the difference between uncertainty and interindividual variability.
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An iterative approach would start with relatively inexpensive screening techniques such as a simple, conservative transport model and then for chemicals suspected of exceeding de minimis risk move on to more resourceintensive levels of data-gathering, model construction, and model application. To guard against serious underestimations of risk, screening techniques must err on the side of caution when there is uncertainty about model assumptions or parameter values.
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Risk assessment is a set of tools, not an end in itself. The limited resources available should be spent to generate information that helps risk managers to choose the best possible course of action among the available options.
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