Key: NOx = oxides of nitrogen; PM-10 and PM-2.5 = airborne particulates of less than 10 microns or 2.5 microns, respectively; CO = carbon monoxide; VOC = volatile organic compounds.

Note: Transportation emissions include all onroad mobile sources and thefollowing nonroad mobile sources: recreational vehicles and boats, airportservice equipment, aircraft, commercial marine vessels, and railroads. Othernonroad sources, such as lawnmowers and farming equipment, are not included. Lead estimates include onroad mobile sources only.

Source: US EPA, Office of Air Quality Planning and Standards, National Air Pollutant Trends http://www.epa.gov/ttn/chief/trends/index.html as of September 2001.