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Overview: The 4 Steps

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.
The national-scale assessment includes the following 4 major steps for assessing air toxics across the contiguous United States (also Puerto Rico and the Virgin Islands) for 33 air pollutants (32 of the 188 air toxics listed in the Clean Air Act plus diesel particulate matter).
  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, wildfires), and both onroad and nonroad mobile sources (e.g., cars, trucks, boats). EPA made some modifications to the 1996 National Toxics Inventory to prepare the emissions for computer modeling. For more information about these emission modifications see limitations.


  2. Estimating 1996 ambient concentrations based on the 1996 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 1996 population exposures based on a screening-level inhalation exposure model (HAPEM4) 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 1996 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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