Uncertainty in risk assessment: exceedence frequencies, acceptable risk, and risk-based decision making

The use of "exceedence frequencies" is proposed as a means of evaluating the acceptability of uncertain environmental risks. A survey was conducted of U.S. Environmental Protection Agency personnel. When respondents were requested to identify preferred "acceptable" exceedence frequencies at four different regulatory risk thresholds (10(-3) to 10(-6), three general response patterns were found: "absolutist" responses in which no level of exceedence was deemed acceptable; "fixed" responses in which respondents felt that the level of exceedence should remain constant at all risk levels; and "outcome sensitive" responses in which preferred exceedence frequencies were inversely related to the risk level. This last group's responses were based on the rationale that the "marginal cost" of risk (in terms of expected mortalities) increases as the risk threshold increases. Although the majority of respondents accepted the concept of an acceptable risk exceedence frequency, the low exceedence frequencies selected as being "acceptable" by survey respondents (generally 5% or less) appear to be overly conservative given the degree of uncertainty inherent in risk assessments. Finally, two regulatory uses of exceedence frequencies are proposed as a tool for streamlined risk ranking.