Skip to main content

Currently Skimming:

Appendix N-1: The Case for
Pages 601-628

The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.


From page 601...
... This Appendix was written by one member of our committee, who was asked to represent the viewpoint of those members of the committee who believe that EPA should choose and refine its default assumptions by continually evaluating them against two equally important standards: whether the assumption is scientifically plausible, and whether it is "conservative" and thus tends to safeguard public health in the face of scientific uncertainty. Indeed, these three themes of plausibility, uncertainty, and conservatism form most of the framework for the last six chapters of the CAPRA report, as reflected in the "crosscutting" chapters on model evaluation, uncertainty and variability, and on implementing an iterative risk assessment/management strategy.
From page 602...
... First, a useful definition of conservatism should help clarify it in the face of the disparate charges leveled against it. Conservatism is, foremost, one of several ways to generate risk estimates that allow risk management decisions to be made under conditions of uncertainty and variability.
From page 603...
... Assuming that society decides how it wishes to balance lives and dollars, conservatism only affects the decision at the margin, by deliberately preferring, from among the inevitable errors that uncertainty creates, to favor those errors which lead to relatively more dollars spent for the lives saved than those which lead to relatively fewer lives saved for the dollars spent. Some would call this an orientation disposed to being "better safe than sorry" or a tendency towards "prudence," characterizations we do not dispute or shrink from.
From page 604...
... We stress that our version of "plausible conservatism" in risk assessment does not allow EPA to adopt unreasonable assumptions or rely upon biased parameter values, and we believe that the entire committee's consensus recommendations in Chapters 9 and 10 will help combat this tendency, if it exists, and help shed light, rather than heat, on the question of whether EPA's risk estimates are more conservative than they are intended to be.
From page 605...
... When uncertainty is reduced to minimal levels, the conservative estimate and the central tendencies will become so similar that the distinction be 2For two reasons, we believe it is logically consistent to espouse a principle of "plausible conservatism" with regard to model uncertainty and not explicitly recommend the same response to variability: (1) as a pragmatic matter, we believe scientists have more that they alone can contribute to a discussion of how to choose among competing scientific theories than they have to contribute to a discussion of what kind of individuals EPA should try to protect; and (2)
From page 606...
... that the presumption that society should approach uncertain risks with a desire to be "better safe than sorry" has engendered so much skepticism. After all, perhaps it should instead be incumbent upon opponents of conservative defaults to defend their position that EPA ought to ignore or dilute plausible scientific theories that, if true, would mean that risks need to be addressed concertedly.
From page 607...
... What matters is, do Congress and the public view risk management as a social endeavor that should strive both for scientific truth and for the prudent avoidance of unnecessary public health risks, and therefore do not view risk assessment as purely an exercise in coming as close to the "right answer" as possible? If this is so, then the competing proposal offered in Appendix N-2 espouses an unscientific value judgment, and one that also is unresponsive to social realities.
From page 608...
... " There are two different mathematical aspects of risk assessment under uncertainty that mitigate in favor of a conservative approach to selection of default options. Both factors tend to make risk estimates generated from conservative models less conservative than they might appear at first glance, and thus tip the balance further in favor of such models as minimally necessary to support prudent decisions.
From page 609...
... Programs. The policy of preventive action in the face of scientific uncertainty has long been part of the Clean Air Act, as well as most of the other enabling legislation of EPA.
From page 610...
... By declaring that defaults would be chosen to be both scientifically supportable and health-protective, and that scientists would have to examine alternative models by these two criteria, EPA could help ensure that science will assume the leading role in defining evolving risk assessment methodology. Some have asserted that it shows disrespect for science to posit any standard for departure from defaults other than one that simply requires EPA to adopt "new and better science at the earliest possible time." But surely there is a generally inverse relationship between the amount of knowledgeable controversy over a new theory and the likely "staying power" and reliability of such "new science." At the extremes, EPA could either change its defaults over and over again with each new individual voice it hears complaining that a default is passe, or never change a default until absolute scientific unanimity had congealed and remained unshakable for some number of years.
From page 611...
... even if EPA decided to use central-tendency risk estimates for standard-setting or other purposes, it would first have to explore the conservative end of the spectrum in order to have any clear idea where the expected value of the uncertain risk (as discussed above, the correct central-tendency estimate for a nsk-neutral decision) actually falls.
From page 612...
... As discussed below, advocates of "best estimates" frequently fail to consider how difficult, error-prone, and value-laden the search for such desirable end points can be. Since CAPRA has been asked to suggest improvements in the methodology by which EPA assesses risks from exposures to hazardous air pollutants, it is also incumbent upon us at least to remark on the purpose of such risk estimates.
From page 613...
... As an example, let us assume that scientists who believe benign rodent tumors can be surrogates for malignant tumors would admit that the opposite conclusion is also plausible, and vice versa. Then, from this "plausible set," EPA should adopt (or should reaffirms as a generic default that model or assumption which tends to yield risk estimates more conservative than the other plausible choices.
From page 614...
... the empirical observation that the LMS model gives more conservative results than other plausible models.9 Step 2 Armed with this set of scientifically supportable and health-protective models, EPA should then strive to amass and communicate information about the uncertainty and variability in the parameters that drive these models.~° The uncertainty distributions that result from such analyses will permit the risk manager to openly choose a level of conservatism concordant with the particular statutory, regulatory, and economic framework, confident that regardless of the level of conservatism chosen, the risk estimate will reflect an underlying scientific structure that is both plausible and designed to avoid the gross underestimation of risk. In Chapters 9 and 11, the committee supports this notion that the level of conservatism should be chosen quantitatively with reference to parameter uncertainty and variability, but qualitatively with reference to model uncertainty (i.e., under this proposal, models would be chosen to represent the "conservative end of the spectrum of plausible models"~.
From page 615...
... calibrating its risk estimates against available "reality checks," such as the upper confidence limit on human carcinogenic potency one can sometimes derive in the absence of positive epidemiologic data (Tollefson et al., 1990; Goodman and Wilson, 1991) or physical or observational constraints on the emissions estimates used or the ambient concentration estimates generated by the exposure models used; and (2)
From page 616...
... The "plausible conservatism" standard recommended here acknowledges that simplicity in risk assessment is useful for certain risk management purposes but is not an end in itself. Thus, the actual default model for certain atmospheric transport calculations might well be a more complex version of a simpler and more conservative screening model (e.g., 1lIn special circumstances, a new scientific consensus may emerge that a model or assumption that is more conservative than the default is clearly plausible, either as a general rule or for specific chemicals or exposure scenarios.
From page 617...
... For example, the problem that conservatism can lead to incorrect risk comparisons and pr~onty-setting decisions can be remedied in part by striving to make the "level of conservatism" explicit and roughly constant across assessments, and by generating additional estimates of central tendency (perhaps even derived via subjective weights applied to different basic biological theories) for use in ranking exercises only.~3 Similarly, there is a legitimate concern that the policy of conservatism can stifle research if EPA is perceived as uninterested in 12The only important caveat to this principle, which would apply to the transport model example as well as the PBPK example, is that with the addition of new model parameters (e.g., partition coefficients and rate constants in the PBPK case)
From page 618...
... On one side, those convinced that EPA's procedures yield estimates far above the true values of risk can cite numerous examples where individual assumptions seem to each contribute more and more conservatism (Nichols and Zeckhauser, 1988; OMB, 1990; Hazardous Waste Cleanup Project, 1993~. Others believe the evidence shows that current procedures embody a mix of conservative, neutral, and anti-conservative assumptions, and that the limited observational "reality checks" available suggest that existing exposure, potency, and risk estimates are in fact not markedly conservative (Allen et al., 1988; Bailar et al., 1988; Goodman and Wilson, 1991; Finley and Paustenbach, in press; Cullen, in press)
From page 619...
... are only as stringent as they desire. But in the narrow area of model uncertainty, this proposal deems it unwise to encourage risk managers to guess at what a protective decision would be, by censoring information about models which, although conservative, are still deemed by experts to be plausibly true.
From page 620...
... A related example in the exposure assessment arena is discussed in Chapter 10. A CTE of approximately 7 years of exposure to a typical stationary source of toxic air pollutants is indeed based on much more data (in this case, data on the variation in the number of years a person stays at one residence before moving)
From page 621...
... In the specific context of default options, this proposal remains that EPA should retain its "plausible conservative" default until scientific consensus emerges that the alternative model supplants the default at the conservative end of the plausible set of model choices. Case 3: "More science" means introducing more data-intensive models without considering uncertainty or variability in the parameters that drive them.
From page 622...
... As this Appendix shows, both principles rely in part on science and decision theory, and both embody specific sets of value judgments. Unfortunately, some of the critics of "plausible conservatism" have shone a spotlight on the values inherent in that position while ignoring the value judgments inherent in the alternatives.
From page 623...
... And whatever one thinks about the advisability of a clear attempt to separate risk assessment from risk management, this discussion has shown that a "plausible conservatism" orientation is no more violative of that boundary than a central tendency orientation would or could be. Furthermore, both "plausible conservatism" and the "best science" alternative leave vast room for risk managers to exercise their rightful discretion, particularly in the selection of decision alternatives and the integration of information external to the risk assessment (e.g., cost and efficiency estimates, public concerns)
From page 624...
... Some of the ambiguity stems from the lack of responsiveness to important issues raised in this Appendix. For example, Appendix N-2 asserts that "risk managers should not be restricted by value judgments made during risk assessment," but nowhere does it explain how this vision could be realized, in light of the assertions herein that a vague call for "full use of scientific information" must either impose a set of value judgments of its own or else restrict risk assessors to presenting every conceivable interpretation of every model, data set, and observation.~4 Similarly, the statement that "risk characterizations must be as accurate as possible," and the implicit equating of accuracy with the amount of data amassed, responds neither to the assertion that accuracy may not be the most appropriate response to uncertainty nor to the four examples in Appendix N-1 showing that "more science" may lead to less accuracy as well as substitute risk-neutral or risk-prone value judgments for risk-averse ones.
From page 625...
... Their stated objection is that it will "freeze risk characterizations at the level determined by USA substantial amount of uncertainty may be contributed by the parameters that drive risk models, even before interindividual variability is taken into account. For example, even if one specifies that the linearized multistage model must be used, the uncertainty in cancer potency due only to random sampling error in the typical bioassay can span five orders of magnitude at a 90 percent confidence level (Guess et al., 1977)
From page 626...
... 12:333-337. Hazardous Waste Cleanup Project.
From page 627...
... 1990. Comparison of the cancer risk of methylene chloride predicted from animal bioassay data with the epidemiologic evidence.


This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More information on Chapter Skim is available.