to known HAPs and thus are likely to have adverse impacts. If chemicals so identified are also emitted in large amounts and have the potential for relatively high hot-spot exposures, they should be listed by EPA for enhanced monitoring and effects research and perhaps some low level of regulatory oversight. Because of the interrelated nature of environmental media (air, soil, water, and biota [see Box 7-1]), new chemicals suspected of being toxic or causing exposure but with few data on environmental fate and effects should be examined for inclusion on the HAPs list based on their production and use qualities and their likelihood of release into the air.

followed by decision-making on controlling exposures to those compounds that pose the most significant risks, is essential to incorporating the as-yet-unlisted chemicals in future AQM strategies.

Institute a Dynamic Review of Pollutant Classification

EPA, as mandated in the CAA, must undertake a periodic review of the classifications given to pollutants. For example, successful mitigation of some criteria pollutants could logically result in their reclassification as HAPs to address remaining exposure and risk issues, and the proliferation of new technologies and products might require that some HAPs be reclassified as criteria pollutants. As new scientific information becomes available, the tier assigned a given HAP might need to be changed. Especially important is the need to identify and regulate pollutants that pose significant risks to human health and welfare but that are not yet listed as HAPs.

Classifying and setting priorities for air toxicants would be facilitated by the development of benchmark air concentrations. The process for developing such values within EPA is resource intensive and protracted, and benchmark concentrations (for example, a reference air concentration of a pollutant likely to cause a harmful effect in humans) are not available for a number of substances on the HAPs list that have sufficient data for guidance level derivations. A tiered system could be adopted for the development of guidance values. The first tier would be the de novo resource-intensive derivations of guidance values. The second tier would be the adoption of values derived by other EPA programs or federal or state agencies. The third tier would be the development of guidance values by expedited techniques. Different levels of review would apply to each of the tiers.

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