Toxic air pollutants, or air toxics, are those pollutants that may cause cancer or other serious health effects, such as reproductive problems or birth defects. Air toxics may also cause other adverse environmental and ecological effects. Examples of toxic air pollutants include benzene, found in gasoline; perchloroethylene, emitted from some dry cleaning facilities; and methylene chloride, used as a solvent by a number of industries. Most air toxics originate from man-made sources, including mobile sources (e.g., cars, trucks, construction equipment) and stationary sources (e.g., factories, refineries, power plants), as well as indoor sources (e.g., some building materials and cleaning solvents). Some air toxics are also released from natural sources, such as volcanic eruptions and forest fires.
EPA is working with its regulatory partners to build on existing monitoring sites to create a national monitoring network for a number of toxic air pollutants. The goal is to ensure that those compounds posing the greatest risk are measured.
EPA also compiles an air toxics inventory as part of the National Emissions Inventory (NEI), formerly the National Toxics Inventory, to estimate and track national emissions trends for the 188 toxic air pollutants regulated under the Clean Air Act. In the NEI, EPA divides emissions into four types: (1) major (large industrial) sources; (2) area and other sources, which include small industrial sources like dry cleaners and gasoline stations, as well as natural sources like wildfires; (3) on-road mobile sources, and (4) non-road mobile sources like aircraft, locomotives, and construction equipment.
Of the 188 Hazardous Air Pollutants, EPA identified 21 Mobile Source Air Toxics (MSAT), and further designated six (6) as priority MSAT. These are: Acetaldeyde, Acrolein, Benzene, 1,3-Butadiene, Formaldehyde, and Diesel Particulate Matter and Diesel Exhaust Organic Gases (DPM + DEOG).
The following chart shows the national contribution by emission source for the 6 priority MSAT. This chart is meant to show the percentage of emissions from on-road vehicles as compared to other sources. Pollutants such as DPM and benzene are primarily on-road related, while acrolein is largely an area source related pollutant.
Sources: U.S. Environmental Protection Agency, 1999 National Emissions Inventory,
Web site: ftp://ftp.epa.gov/pub/EmisInventory/finalnei99ver3/haps/summaries/ from http://www.epa.gov/ttn/chief/net/1999inventory.html, 28 June 2005.