In June 2009, EPA released the results of its 2002 national-scale assessment of air toxics emissions. The purpose of the national-scale assessment is to identify and prioritize air toxics, emission source types and locations which are of greatest potential concern in terms of contributing to population risk. EPA uses the results of these assessments in many ways, including:
- To work with communities in designing their own local-scale assessments,
- To set priorities for improving data in emissions inventories, and
- To help direct priorities for expanding and improving the network of air toxics monitoring.
The national-scale assessment modeled 180 of the 187 Clean Air Act air toxics (PDF) (3pp, 15k) plus diesel particulate matter (diesel PM). The assessment includes four steps that focus on the year 2002:
- Compiling a national emissions inventory of air toxics emissions from outdoor sources
- Estimating ambient concentrations of air toxics across the United States
- Estimating population exposures across the United States
- Characterizing potential public health risk due to inhalation of air toxics including both cancer and noncancer effects
The emissions used in the current assessment are from the 2002 emission inventory which is the most complete and up-to-date available. Working with the states, EPA updates air toxics emission inventories every 3 years. The next national-scale assessment will focus on 2005 emissions and will be available in early 2011.
As part of EPA's National Air Toxics Assessment activities, EPA conducted its first national-scale assessment for the year 1996. That assessment included 33 air pollutants (a subset of 32 air toxics on the Clean Air Act's list of 187 air toxics plus diesel particulate matter (diesel PM). In February of 2006, EPA released the second of its NATA assessments. This assessment was based on emissions from the 1999 National Emission Inventory and included the assessment of 177 hazardous air toxics plus diesel particulate matter.