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quantities (such as the dose-scaling factor) that are highly uncertain. For screening-level analyses, the EPA (1992d) proposal to adopt a new interspecies dose-equivalence factor is inconsistent with the 1986 guideline stipulation that risk estimated under the guidelines represents a "plausible upper bound" on increased cancer risk, and it is inconsistent with the corresponding stipulation that "upper-bound" or health-conservative assumptions are to be used at each point in cancer-potency assessment that involves substantial scientific uncertainty.

For a screening-level or semiquantitative approach in which component uncertainties associated with predicted upper-bound cancer risk are not aggregated in a rigorous quantitative fashion, the EPA guidelines, to determine upper-bound cancer risk, should require the use of an upper-bound (i.e., reasonably health-conservative), rather than a "best," interspecies dose-scaling factor consistent with the best available scientific information.

Uncertainty versus Variability

A distinction between uncertainty (i.e., degree of potential error) and interindividual variability (i.e., population heterogeneity) is generally required if the resulting quantitative risk characterization is to be optimally useful for regulatory purposes, particularly insofar as risk characterizations are treated quantitatively.

The distinction between uncertainty and individual variability ought to be maintained rigorously at the level of separate risk-assessment components (e.g., ambient concentration, uptake, and potency) as well as at the level of an integrated risk characterization.


1. For example, in the 1980s the Consumer Products Safety Commission (CPSC) had to issue a standard regarding how close together manufacturers had to place the vertical slats in cribs used by infants, with the aim of minimizing the number of accidental strangulations nationwide. Presumably, there was virtually no uncertainty about the diameter of an average infant's head, but there was significant variability in distinguishing different infants from each other. CPSC thus had to make a decision about which estimation of head size to peg the standard to—an "average" estimate, a "reasonable worst case," the smallest (i.e., most conservative) plausible value, etc. We suggest that it is not apropos to use the phrase "better safe than sorry" to apply to this kind of reasoning, because uncertainty is not at work here. Rather, deciding whether to be conservative in the face of variability rests on a policy judgment about how far to extend the attempt to provide safety.

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