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The Four Steps of the Assessment

The 1999 national-scale assessment includes the following four major steps for assessing air toxics across the United States (also Puerto Rico and the Virgin Islands).
  1. Compiling a 1999 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, wildfires), and both onroad and nonroad mobile sources (e.g., cars, trucks, boats). The 1999 National Emissions Inventory is the underlying basis for the emissions information.

  2. Estimating 1999 ambient concentrations based on the 1999 emissions as input to an air dispersion model (the ASPEN model). As part of this modeling exercise, EPA compared estimated ambient concentrations to available ambient air toxics monitoring data to evaluate model performance. Learn more about these comparisons.

  3. Estimating 1999 population exposures based on a screening-level inhalation exposure model (HAPEM5) and the estimated ambient concentrations (from the ASPEN model) as input to the exposure model. Estimating exposure is a key step in determining potential health risk. People move around from one location to another, outside to inside, etc., so exposure isn't the same as concentration at a static site. People also breathe at different rates depending on their activity levels, so the amount of air they take in varies. For these reasons, the average concentration of a pollutant that people breathe (i.e., exposure concentration) may be significantly higher or lower than the concentration at a fixed location (i.e., ambient concentration).

  4. Characterizing 1999 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 EPA risk assessment and risk characterization guidelines, and estimated population exposures.

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