The Inexact Science of Risk Assessment (and Implications for Risk Management)

Abstract

Human health risk estimates are calculated for use in setting standards, cleanup levels for hazardous wastes, or otherwise expressing an exposure level that is believed to be safe or associated with some risk. Most risk estimates are calculated to be protective of human health, rather than predictive of actual toxicity. For example, cancer potency factors calculated by the U.S. EPA are presented as the 95% upper confidence limit on the dose-response curve, rather than the maximum likelihood estimate. EPA goes on to say that risk assessors believe the actual cancer risk to be somewhere below this upper confidence limit, and that it could be as low as zero. This is an acknowledgm ment of the uncertainty inherent in the process of cancer risk assessment, which is a function of both cross-species and high-to-low dose extrapolation. Similarly, for noncancer risk assessments, U.S. EPA (1995) calculates Reference Doses (RfDs) and Reference Concentrations (RfCs) , which are defined as: ...an estimate (with uncertainty spanningperhaps an order ofmagnitude) ofa daily or continuous exposure... The values generated by the process of risk assessment are imprecise; such imprecision is clearly articulated by the definitions provided. Somewhere between the steps of risk assessment and risk management, however, the concept of risk estimates as inherently imprecise has been lost. This is probably due to a number of reasons, one of which is likely because the risk manager has to communicate with a public that wants to know with some certainty and precision what the risks from exposure to hazardous substances actually are (and in rather succinct terms), rather than hearing the risks described more appropriately as scientific judgments that are, by their very