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including the gathering of exposure and toxicity data. Public confidence that risk managers are addressing real concerns, as opposed to going through a process perfunctorily, is critical to the future of risk assessment as an activity capable of improving the quality of life. Risk managers need to articulate clearly from the beginning who is to be protected from what, when and where, and at what cost (including how much effort and funds are to be expended to collect appropriate data), so that risk assessors can provide relevant information.

Implications For Priority-Setting

It is not necessary, nor would it be cost-effective, to collect all the data needed for a complete health-hazard assessment on all the 189 chemicals (or mixtures) listed in CAAA-90. It is important, however, that the entire list be examined to identify chemicals that are potentially hazardous and that the later full-scale evaluation of each chemical selected for further scrutiny proceed as effectively as possible. An overall strategy is essential for setting priorities among the steps in the information-gathering process and for determining the extent of assessment needed.

Because risk is a function of exposure, as well as toxicity, determining both that a chemical is of low toxicity to all humans and that all humans have only small exposures to it would lead to an overall low priority for a full-scale risk assessment. Obviously, assigning a high priority to both would lead to an overall high priority for such assessment and argue for collection of a complete data set in all categories of exposure and toxicity. There will be various intermediate levels between low and high overall priority.

In the absence of pertinent human data, toxicological evaluation should begin with the simplest, most rapid, and most economical tests and proceed to more complex, time-consuming, and more expensive tests only as warranted by the initial steps. Similarly, emission, transport, and exposure data might be used to rank chemicals for testing, from those with relatively large exposure potential down to those with a very low likelihood of significant exposure, either for the population at large or for any substantial subset of the population. What is "substantial" in this context will of course depend on concurrent assessments of toxicity. Ordering can then be based on an evaluation of a relatively modest or limited data set.

To assess whether there is a potential for exposure, and to gauge the magnitude and duration of exposure, one needs to know:

1.

Is the chemical emitted into the air?

2.

Is the chemical stable enough to be transported from its source to a population?

If the chemical is not emitted or is so unstable that it breaks down into innocuous products before reaching a population, no further data need be col-



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