Cumulative Exposure to Air Toxics: Exposure Modeling for Outdoor Sources. A. S. Rosenbaum and M. P. Ligocki, Systems Applications International, 101 Lucas Valley Rd., San Rafael, CA 94903; and D. Atkinson, D. Axelrad, and T. J. Woodruff, U.S. Environmental Protection Agency, 401 M St., SW, Washington, DC 20460
The 1990 Clean Air Act (CAA) designates 189 pollutants as Hazardous Air Pollutants (HAPs), or "air toxics". Various provisions of the CAA require EPA to identify sources of these pollutants, and to assess human exposures. The list of 189 air toxics includes a wide variety of chemicals with an equally wide variety of sources including stationary, mobile, indoor, and natural sources. Some are produced primarily by atmospheric reaction rather than direct emission. Given this diversity, it is not surprising that existing exposure modeling studies for air toxics have focused on limited subsets of pollutants and sources. The purpose of this study is (1) to develop a modeling approach, an associated nationwide HAP emission inventory, and nationwide demographic data bases that will allow for the estimation of cumulative population exposure to listed air toxics from all identified source categories, and the estimation of geographic variability of exposures at the resolution of census tracts, and (2) to implement that approach. This presentation will discuss the estimation of inhalation exposure to HAPs emitted by outdoor sources through modeling with the Assessment System for Population Exposure Nationwide (ASPEN), a new version of the Human Exposure Model (HEM), which includes considerations of population activity patterns and concentrations in indoor and in-vehicle microenvironments, as well as outdoors. Selected summary results will be presented. Key uncertainties and data limitations will be identified. The modeling of exposures to indoor sources will be presented separately.