Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
DEALING WITH UNCERTAINTY ABOUT RISK IN RISK MANAGEMENT 47 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. about what risk-management steps should be taken. The report clearly describes how science and policy cannot be entirely separated and makes the point that many seemingly scientific issues such as the assumptions made in a risk assessment have direct relevance to management decisions. As seen by the committee that wrote the report, The goal of risk assessment is to describe, as accurately as possible, the possible health consequences of changes in human exposure to a hazardous substance; the need for accuracy implies that the best available scientific knowledge, supplemented as necessary by assumptions that are consistent with science, will be applied [National Research Council, 1983]. The difficulty arises when there is no scientific basis for selection among alternative assumptions. The study committee did not offer a general recommendation for choosing assumptions when this occurs. However, it did note that in such cases it may be appropriate to select the most conservative assumptions (that is, those leading to the highest estimate of risk). The committee chose carcinogenic risks and their assessment to illustrate many points in the report primarily because the estimation of these risks has become more standardized than it has for other risks. Assumptions generally thought to be conservative are routinely used by agencies in evaluating potential carcinogens. For example, conservative risk-assessment assumptions are used by EPA's Carcinogen Assessment Group to estimate a plausible upper bound for risk; the plausible lower bound is taken to be zero risk except where direct human evidence indicates otherwise. These upper-bound risk estimates are based on data from the most sensitive sex, strain, and species of test animal, and for the cancer tumor type (often including benign tumors) and site that maximize the estimated potency. Extrapolation of animal results to humans are calculated from the ratio of surface areas, an approach more conservative than scaling by weight, and extrapolation from high-to low-dose response is based on a dose-response model that exhibits linearity at low doses. The selection of sensitive sex, strain, and species is at times justified on the grounds that humans are genetically diverse, differ widely in health status, and are exposed to many other potentially harmful agents (Anderson, 1983), but these assumptions are generally thought to be conservative in their application to human cancer risk. Is Conservatism Protective? Does reliance on assumptions producing upper-bound risk estimates protect health? The question is analytically tractable. Not surprisingly, its answer depends on what assumptions are made. For some seemingly reasonable analytical assumptions conservatism is protective; for others it is not.
DEALING WITH UNCERTAINTY ABOUT RISK IN RISK MANAGEMENT 48 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. Certainly the perception of many risk analysts is that conservative risk- assessment assumptions are protective. High-risk estimates are associated with stringent standards. An analyst's own sense of responsibility encourages conservatism. Although the social costs of false alarms are acknowledged, to give a incorrect assurance of safety is believed to be far worse. The relative social cost of risk underestimation is taken to outweigh that of overestimation. An analytical case for conservatism in risk assessment is made by Talbot Page (1978) who argues that the appropriate response to uncertain environmental risks is to balance the social costs of false negatives (substances or activities incorrectly thought to be safe) with the costs of false positives (those incorrectly believed to be hazardous). His analysis indicates that the use of this expectation rule is clearly preferable to approaches aimed exclusively at avoiding either type of risk misclassification (that is, false positives or false negatives). Page observed, "Application of this approach requires four pieces of information: the cost of a false negative; the cost of a false positive; and the probability of each." Given the difficulty in ascertaining the probabilities of false positives and negatives, he argues that when the potential adverse effects of an environmental risk are many times greater than the potential benefits, a proper standard of proof of danger under the expected cost minimization criterion may be that there is only "at least a reasonable doubt" that the adverse effect will occur, rather than requiring a greater probability, such as "more likely than not," that the effect will occur. Simple rules of thumb embodied in legal and regulatory institutions may come closer to expected cost minimization than elaborate attempts at quantification [Page, 1978]. The interesting feature of Page's analysis is his lack of aversion to uncertainty; uncertain risks are judged to the extent they can be estimated and characterized. Page proposes a rather stringent rule, that a substance or activity be considered hazardous if there is "at least a reasonable doubt" of safety. This rule derives in part from his analysis that for most environmental risks, the relative social costs of a false negative (leading to a failure to regulate a hazardous substance) greatly exceed the costs of regulating a safe substance. Among the common characteristics of environmental risk, Page lists modest benefits and catastrophic costs. For substances like food-color additives or fluorocarbon propellants, where benefits are easy to forgo or where safer substitutes exist, the "at least a reasonable doubt" rule is appropriate from a cost- benefit viewpoint. But to find that analytical conservatism is generally protective requires three assumptions: (1) that the disparity in social costs between false negatives and false positives is great, (2) that risk-management decisions are