BOX 7-3
Example of a Potential Classification Scheme for Hazardous Pollutants

A number of schemes could be used to aid in setting resource priorities for HAPs on the basis of the relative risks posed to human health and welfare by these pollutants. One example would be a system with three tiers, applied on a national basis or, if a more effective allocation of resources is allowed, on a multistate airshed basis.

  • Tier 1 would most likely contain a few HAPs that, because of their diverse sources, ubiquitous presence in the atmosphere, and exceptionally high risk to human health and welfare, merited treatment similar (although not necessarily identical) to the treatment for criteria pollutants (for example, benzene). These HAPs would probably be drawn from the high-priority list identified in EPA’s urban air toxics strategy (EPA 2000b) and be those identified as posing the highest population risk in such assessments as EPA’s national air toxics assessment (EPA 2000b). In a small number of cases, these pollutants might be proposed for formal criteria pollutant status. In most cases, however, the Tier 1 HAPs not assigned NAAQS, as is done for criteria pollutants, might reasonably be incorporated into national monitoring programs and required or recommended for inclusion in an AQMP. As is the case for all HAPs, Tier 1 HAPs would be regulated through nationally mandated emission controls.

  • Tier 2 HAPs, perhaps initially drawn from the remainder of the list of high-priority urban toxics identified by EPA in its urban air toxics strategy, would receive increased resources for monitoring and research, so that the risk they pose to human health and welfare could be assessed more accurately. In addition to nationally mandated emission controls, incentive programs could be implemented to encourage Tier 2 HAP inclusion in multipollutant AQMPs.

  • Tier 3 HAPs, presumably initially drawn from the list of remaining HAPs, would be given the lowest priority for research and monitoring but would still be subject to nationally mandated emission controls.

Beyond these tiers, the committee recommends that a list of potential air toxicants be established and that these toxicants be subject to some minimal level of regulatory review and consideration (see Box 7-4).

Possible regulatory approaches include the development of exposure triggers (for example, emission concentrations, volume of use, or high exposures to some urban populations) for suspicious chemicals with sparse test data; some degree of testing and control would be required when the trigger measure was exceeded. Testing might include a minimal battery of tests, such as an expanded version of the current high-production-volume testing program instituted by EPA and the chemical industry. Inclusion on such a list of potential air toxicants might encourage the development of substitutes for those that exhibit initial indications of toxicity. However, a dynamic review of all pollutants, including those not on the current list,

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