Overview: Scope of Assessment

The EPA's 1996 National Air Toxics Screening Level Assessment, targeted for completion by December 2000, will help to characterize the potential health risks associated with inhalation exposures to the 33 hazardous air pollutants (HAPs) identified as priority pollutants in EPA's Integrated Urban Air Toxics Strategy. These 33 toxic air pollutants are a subset of EPA's list of 188 toxic air pollutants. The EPA plans to complete national screening-level air toxics assessments every 3 years. The next such assessment will focus on 1999 air toxics emissions. The 1996 assessment includes the following four major steps:

1. Compiling a 1996 national emissions inventory of air toxics emissions from outdoor sources. The types of emissions sources in the inventory include major stationary sources (e.g., large waste incinerators and factories), area and other sources (e.g., dry cleaners, small manufacturers, consumer products), and both onroad and nonroad mobile sources (e.g., cars, trucks, boats).
   
   
2. Estimating 1996 ambient concentrations of air toxics across the contiguous United States (also Puerto Rico and the Virgin Islands) for the 33 urban HAPs plus diesel PM. This is based on a screening-level air dispersion model (the ASPEN model) and the 1996 national air toxics inventory as input to the model. As part of this modeling exercise, EPA will compare estimated ambient concentrations to available ambient air toxics monitoring data to evaluate model performance.
   
   
3. Estimating 1996 population exposures across the contiguous United States (also Puerto Rico and the Virgin Islands) to the 33 air toxics plus diesel PM. This is based on a screening-level inhalation exposure model (HAPEM) and the estimated ambient concentrations (from the ASPEN model) as input to the exposure model. Exposure modeling is an important step in this assessment because it can provide more realistic estimates of actual population exposures to air toxics from outdoor emission sources by accounting for time people spend indoors and in other "microenvironments" (e.g., in vehicles), patterns of movement (e.g., commuting between home and work locations), and activity levels. This exposure modeling is targeted for completion in August 2000.
   
   
4. Characterizing potential public health risks due to inhalation of air toxics. This includes both cancer and noncancer effects, using available information on air toxics health effects, current Agency risk assessment and risk characterization guidelines, and estimated population exposures. This characterization will quantify, as appropriate, potential cumulative risks to public health due to inhalation of air toxics from outdoor emission sources, discuss the uncertainties and limitations of the assessment, and identify other potential risks to public health from air toxics that are beyond the scope of this quantitative assessment. The characterization is now targeted for completion by August 2000.

The assessment approach outlined above is fundamentally based on using screening-level computer models to estimate ambient air toxics concentrations and population exposures nationwide. While such computer models necessarily require simplifying assumptions and introduce significant uncertainties, they are needed to conduct such a large scale assessment since direct measurements of ambient air toxics concentrations are limited, and direct personal exposure measurements are even more limited. Such measurements are available for only a subset of air toxics in relatively few locations and for small study populations. Although EPA is working to expand the number and locations of ambient air toxics monitors and the study of personal exposures, direct measurement of air toxics concentrations is not practical for all air toxics of interest across all areas of the country. Over time, such measurement data can and will be used, however, to evaluate the models so as to better understand some of the uncertainties in such assessments and to improve modeling tools.

In describing what this assessment will include, it is also important to recognize potentially important sources and pathways of risks to public health that are beyond the scope of this quantitative assessment. For example, while EPA recognizes that indoor sources of air toxics emissions likely contribute substantially to the total exposures that people experience for a number of these toxic air pollutants, assessing these indoor sources of exposure cannot be done on a national scale at this time. Further, for subsets of these HAPs (e.g., those that persist and bioaccumulate in the environment), non-inhalation exposures (e.g., eating contaminated fish) may contribute much more to the total risk associated with exposure to these pollutants than do the inhalation exposures that will be addressed in this assessment. These and other important aspects of total population exposures to air toxics will be addressed more fully over time as part of EPA's assessment activities as more comprehensive data and assessment tools become available.

This initial national screening-level assessment is part of an iterative and evolving process to assess and characterize risks from exposures to air toxics, measure progress in meeting goals, and inform future directions for EPA's national air toxics program. While there continue to be significant uncertainties and gaps in methods, models, and data that limit our ability to assess risks to public health and the environment associated with exposures to air toxics, continued research will enable future assessment activities, both at the national screening-level and at more local refined levels, to yield improved assessments of cumulative air toxics risks. An important component of our future National- Scale Air Toxics Assessment activities will be to repeat this type of national screening-level assessment every three years - with the next such assessment focusing on 1999 air toxics data.