A well-funded periodic review of air pollutants and their classification as HAPs and criteria pollutants should include an effort to identify new toxicants that pose a threat to human health and welfare. Some newly identified toxicants should be added to the HAPs list, and others should be listed for low-level regulatory oversight, as discussed below.
Identifying New HAPs
To conserve resources in reviewing the numerous unregulated air pollutants for potential placement on the HAPs list, an EPA-based program could be established that relied on existing hazard evaluations from other agencies and institutions, as well as new hazard evaluations for chemicals that have not received prior adequate evaluation. Special attention should be given to the implementation of these evaluation processes to ensure that they do not become too protracted or resource intensive and that output of chemical evaluations is sufficient. Candidates could be screened by emission concentrations, and screening-level exposure analyses could be performed. New hazard evaluations should focus on those air pollutants that have not been assessed adequately by other institutions and that have a current or future potential for large exposures, such as chemical substitutes for listed HAPs. The evidentiary threshold for listing a chemical as a HAP is that it can be identified as reasonably anticipated to cause toxicity. The number of chemicals undergoing traditional toxicity testing in traditional toxicity studies is diminishing as methods for toxicity screening are evolving. Beyond traditional toxicity tests, EPA should explore adding chemicals to the HAPs list with the use of the full range of analyses. Persistent, bioaccumulative toxins released into the air in relatively small volumes may pose substantial toxic risks (Lunder et al. 2004) and are important candidates for evaluation using non-traditional approaches. With regard to the use of hazard evaluations from other institutions, there are several possible sources for use in identifying HAPs candidates. Examples are chemicals required for reporting in the Toxics Release Inventory Program; chemicals classified in categories 2B, 2A, or 1 by the International Agency for Research on Cancer; chemicals identified as reproductive toxicants of concern by the National Toxicology Program’s Center for Evaluation of Reproductive Health Risks and listed on California’s Proposition 65 list of chemicals known to cause cancer or reproductive toxicity; chemicals identified by EPA as known or likely to cause cancer (old B2 category and above); chemicals regulated on the basis of adverse health effects by the Occupational Safety and Health Administration; chemicals identified as toxic by the National Institute for Occupational Safety and Health; and chemicals described as emitted into the air with a toxicological profile published by the Agency for Toxic Substances and Disease Registry. A screening analysis for addition of chemicals to the HAPs list has recently been provided (Lunder et al. 2004).
Identifying Chemicals for Regulatory Oversight
There is a vast array of unregulated emitted substances with sparse or no toxicological data to assess hazard potential adequately, and thus they cannot be placed on the HAPs list. Nevertheless, some attempt should be made to identify those chemicals that have sparse toxicological data but have structural similarities