Quantitative Risk Assessment Since the Red Book: Where Have We Come and Where Should We Be Going?

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Abstract
Of the three “pillars” of risk assessment identified in the Red Book—hazard identification, exposure assessment and dose-response assessment—the latter is by far the most difficult. In the 20 years since the publication of the Red Book, progress in dose-response assessment has not kept pace with those in the other elements of risk assessment. Today dose-response assessment is being performed in a manner very similar to the way it was being performed 20 years ago, with dissimilar methods being applied to chemicals considered to operate by “linear” and “non-linear” (or “threshold”) mechanisms, and often rancorous debate over which mechanism applies to a particular chemical. These dissimilar methods have produced inequities in health protection and skewed public perceptions regarding risk. What levels of risk exist at exposure levels of concern in environmental settings remains essentially a trans-science question: one that can be stated in scientific terms, but cannot be answered by science. In light of this state of affairs, suggestions are made herein for an approach to dose-response assessment that would treat chemicals in a more consistent manner and would better reflect the true state of scientific knowledge regarding low-dose risks. The method involves applying quantitative methods at higher doses where quantification is more reliable, and then employing safety factors based upon scientific judgment to establish exposure guidelines. In addition to the traditional uncertainties addressed by application of safety factors, such as animal-to-human extrapolation and intra-human variation, safety factors should also take into account mechanistic information and the severity of the health effect.

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