Toward less misleading comparisons of uncertain risks: the example of aflatoxin and alar.

Critics of comparative risk assessment (CRA), the increasingly common practice of juxtaposing disparate risks for the purpose of declaring which one is the "larger" or the "more important," have long focused their concern on the difficulties in accommodating the qualitative differences among risks. To be sure, people may disagree vehemently about whether "larger" necessarily implies "more serious," but the attention to this aspect of CRA presupposes that science can in fact discern which of two risks has the larger statistical magnitude. This assumption, encouraged by the indiscriminate calculation of risk ratios using arbitrary point estimates, is often incorrect: the fact that environmental and health risks differ in unknown quantitative respects is at least as important a caution to CRA as the fact that risks differ in known qualitative ways. To show how misleading CRA can be when uncertainty is ignored, this article revisits the claim that aflatoxin contamination of peanut butter was "18 times worse" than Alar contamination of apple juice. Using Monte Carlo simulation, the number 18 is shown to lie within a distribution of plausible risk ratios that ranges from nearly 400:1 in favor of aflatoxin to nearly 40:1 in the opposite direction. The analysis also shows that the "best estimates" of the relative risk of aflatoxin to Alar are much closer to 1:1 than to 18:1. The implications of these findings for risk communication and individual and societal decision-making are discussed, with an eye toward improving the general practice of CRA while acknowledging that its outputs are uncertain, rather than abandoning it for the wrong reasons.