EPA's National-Scale Air Toxics Assessment focuses on estimating exposures to 41 air toxics, including 33 identified as priority pollutants in EPA's Integrated Urban Air Toxics Strategy. The assessment also includes diesel particulate matter, an indicator of diesel exhaust, and 7 other volatile organic compounds.
The scope of NATA (in terms of number of hazardous air pollutants covered) is considerably smaller than earlier EPA efforts to characterize the health risks of toxic air pollution. There are 188 Hazardous Air Pollutants (HAPs) listed under Section 112 of the Clean Air Act. EPA's Cumulative Exposure Project (which Scorecard relied on for its hazardous air pollution reports until June 2001) covered 148 HAPs. EPA reduced the number of HAPs addressed to 41 in order to focus on the highest risk substances in ambient air and to improve the emissions inventories and modeling methods used to estimate risks.
EXPOSURE MODELING METHODOLOGY
To develop nationwide estimates of the annual average ambient concentrations of HAPs, EPA uses the Assessment System for Population Exposure Nationwide (ASPEN) model. The scope of this national modeling effort is the contiguous United States, Puerto Rico and the Virgin Islands. Modeling was not conducted for Alaska and Hawaii. The ASPEN model simulates the impacts of atmospheric processes (winds, temperature, atmospheric stability, etc.) on pollutants after they are emitted by a variety of source categories. The output of this air dispersion model is an estimate of the annual average ambient concentration of each hazardous air pollutant at the centroid of each census tract within the geographic scope of the assessment. ASPEN model results for the 1990 census tracts were spatially reapportioned to create exposure estimates for 2000 census tracts, in order to make use of the most current population data.
SOURCES OF EMISSIONS DATA FOR EXPOSURE MODELING
To provide emissions data for ASPEN, EPA compiled a 1996 national emissions inventory of air toxics emissions from outdoor sources. The types of emissions sources in the inventory include major stationary point sources (e.g., large waste incinerators and factories), area and other sources (e.g., dry cleaners, small manufacturers, wildfires), and both onroad and nonroad mobile sources (e.g., cars, trucks, boats). The 1996 National Toxics Inventory is the underlying basis for all emissions estimates used in NATA (with the exception of diesel particulate matter). EPA compiled the 1996 NTI using various sources of data. The five primary sources of 1996 NTI data are: (1) state and local emission inventories developed by air pollution control agencies, (2) existing databases related to EPA's Maximum Achievable Control Technology programs to reduce HAP emissions, (3) Toxic Release Inventory data, (4) emissions estimated by using mobile source methodology developed by experts in EPA's Office of Transportation and Air Quality, and (5) area source emission estimates generated using emission factors and activity data. Much of the state/local, TRI and MACT emissions data has been generated by the sources themselves. Extensive documentation is available for the NTI.
NTI is a composite of emissions estimates generated by state and local regulatory agencies, industry, and EPA. Because the estimates originated from a variety of sources and estimation methods, as well as for differing purposes, they will in turn vary in quality, included pollutants, level of detail and geographic coverage. However this compilation of emissions estimates represents the best available information to date.
INCORPORATION OF BACKGROUND CONCENTRATIONS
For 12 air toxics, estimated outdoor concentrations also included a "background" portion attributable to long-range transport, re-suspension of historical emissions, and natural sources. Background concentrations were based on measurements taken at clean air locations remote from known emissions sources. For these 12 compounds, the background values were treated as a constant across all census tracts and added to the modeled concentration estimates from area, mobile and point sources.
EPA EFFORTS TO MODEL HUMAN POPULATION EXPOSURES
The NATA project is using ASPEN ambient concentration estimates as an input into a sophisticated model that is capable of assessing average long-term inhalation exposures to the general population. The Hazardous Air Pollutant Exposure Model (HAPEM) uses the general exposure modeling approach of tracking representatives of specified demographic groups as they move among indoor and outdoor microenvironments and between geographic locations performing various activities. Estimated pollutant concentrations in each microenvironment visited are combined into the time-weighted average exposure concentration experienced by people.
HAPEM exposure estimates are likely to provide a more accurate picture of actual human exposures than ASPEN ambient concentration estimates, because they take into account microenvironments and activity patterns that determine the dose of toxicants people receive from air pollution. Preliminary HAPEM results indicate that people are generally exposed to average concentrations that are about 70% of ASPEN's estimated outdoor ambient concentrations. However, the EPA's Science Advisory Board has identified a number of major improvements needed in the HAPEM model before it can adequately reflect the full range of interindividual variability in air toxics exposures. HAPEM exposure data are not currently available from EPA.
Frequently Asked Questions about EPA's National-Scale Air Toxics Assessment