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ble to every carcinogen, its adoption provides at least a provisional answer to a vexing scientific question, namely whether people exposed to low concentrations of substances that are known to be carcinogenic at high concentrations are at some risk of cancer associated with the exposure. That view has been prominent since the 1950s and has guided much decision-making. For example, the ''Delaney clause" of the Food Additive Amendments of 1958 stipulated that no additive that was found to be carcinogenic could be allowed in the food supply, on the grounds that it was not possible to specify a safe human exposure to such an agent. The policies that have flowed from regulations like the Delaney clause involve, where possible, absolute prohibition of exposures to carcinogens, but more commonly, reductions of exposures to the "lowest technically feasible level."

A qualitative response to the question of carcinogenic risk is still viewed by many scientists to be the best that can now be offered, even in the face of impressive scientific advances in understanding chemical carcinogenesis. Nonetheless, it is increasingly recognized that division of the binary division of the world of chemicals into carcinogens and non-carcinogens is overly simplistic and does not provide an adequate basis for regulatory decision-making. Beginning in the 1960s and coming to full force in the 1970s, some scientists have attempted to offer more useful, quantitative information about the risks of low exposures to carcinogens. Quantitative risk assessment is attractive because, at least ideally, it allows decision-makers and the public to discriminate between important and trivial threats (thus going beyond qualitative findings that there is some risk, however small).

The results of risk assessments are important in influencing important regulatory decisions that affect both the nation's economy and public health. They influence decision-makers as they attempt to balance the view that emission of hazardous air pollutants should be minimized or even eliminated, versus the view that meeting stringent control standards might cause other problems unacceptable to society. Accurate risk assessments are also needed to determine whether public health protection is adequate.

Charge To The Committee

The charge to the committee comes from Section 112(o) of the Clean Air Act, as added by the Clean Air Act Amendments of 1990, which requires EPA to enter into a contract with the National Research Council (NRC). NRC created the Committee on Risk Assessment of Hazardous Air Pollutants in the Board on Environmental Studies and Toxicology. Its charge is summarized as follows: