Cumulative Exposures of Air Toxics: Exposure Modeling for Indoor Sources. M. P. Ligocki, A. S. Rosenbaum, and P.S. Stiefer, Systems Applications International, 101 Lucas Valley Rd., San Rafael, CA 94903; and D. Axelrad, T. J. Woodruff, and D. Atkinson, 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, assess human exposures, and promulgate regulations to control emissions of air toxics. 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. The purpose of EPA's Cumulative Exposure study is to characterize nationwide cumulative chronic inhalation and non-inhalation exposures to air toxics from both indoor and outdoor sources. As part of this study, Systems Applications International has developed an Assessment System for Population Exposure Nationwide (ASPEN), capable of estimating annual-average ambient concentrations and exposures at the census tract level. Our approach for characterizing exposures due to indoor sources revolves around the development of indoor concentration "increments" for each source category and for two indoor microenvironments, and combining these concentration increments with time spent in each microenvironment by each demographic group. Indoor sources addressed include: volatilization from water, environmental tobacco smoke (ETS), consumer products, formaldehyde from building materials, and radon. Selected results will be presented and compared to exposures arising from outdoor sources. Key uncertainties and data limitations will also be identified.