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Every three years, USEPA compiles a National-Scale Air Toxics Assessment (NATA) that estimates exposure to over 100 air toxics (hazardous air pollutants, or "HAPs") throughout the country. The assessment has been done for 1996, 1999, 2002 and now 2005. NATA follows the four-step process described below. For more detailed information, go to USEPA's web site for the National Air Toxics Assessment (NATA).


The emission inventory quantifies the type and amount of emissions of HAPs from a wide variety of sources. USEPA divides the sources into four categories:  

  • Point (large manufacturing facilities, utilities, etc.)

  • Area/nonpoint (small industrial facilities, dry cleaners, residential fuel combustion, consumer product use, etc.)

  • On-road mobile (cars, trucks and buses) and

  • Nonroad mobile (construction equipment, agricultural vehicles, etc.).


To predict how pollutants from the sources in the emission inventory would move through the air and be distributed throughout the country, USEPA uses computer-based tools to feed emissions information into dispersion models. For NATA, USEPA uses the Human Exposure Model, AERMOD version, (HEM-AERMOD) to model emissions from point, on-road and nonroad mobile sources. The Assessment System for Population Exposure Nationwide (ASPEN) was used for non-point sources, and Community Multiscale Air Quality (CMAQ) model was used to estimate the secondary formation concentrations of acetaldehyde, acrolein, 1,3 - butadiene and formaldehyde.


For NATA, USEPA takes an extra step to adjust the predicted air concentrations to account for activity patterns and demographies, thus making exposure a composite of different concentrations experienced in various places throughout the year. This was done using a screening-level inhalation exposure model, HAPEM5. The result is an average "exposure concentration" of a pollutant that people at the state, county or census-tract level are estimated to breathe. It can be either higher or lower than the ambient concentration for the corresponding area.


In this step, USEPA considers the risk of both cancer and non-cancer effects from inhalation of these hazardous air pollutants nationwide. They were able to identify the pollutants posing the highest cancer risk nationwide. They also identified one non-carcinogen (acrolein) as posing the greatest relative hazard for health effects other than cancer.


USEPA states that although NATA is not designed to characterize risks sufficiently fo regulatory action, it expects that the results can help to:

  • Identify air toxics of greatest potential concern.
  • Characterize the relative contributions to air toxics concentrations and population exposures of different types of air toxics emissions sources (e.g. major, mobile).
  • Set priorities for the collection of additional air toxics data and research to improve estimates of air toxics concentrations and their potential public health impacts.
  • Track trends over time in modeled ambient concentrations of toxics.
  • Measure progress toward meeting goals for risk reduction from inhalation of ambient air toxics.

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