Index: 1990 = 1.0
Key: R = revised, PM-10 = airborne particulates of less than 10 microns, PM-2.5 = airborne particulates of less than 2.5 microns.
Notes: The indexes are calculated using data on emissions from highway vehicles only. Particle matters include PM without condensibles.
Source: U.S. Environmental Protection Agency, Clearinghouse for Inventories and Emissions Factors (CHIEF), Current Emission Trends Summaries, available at http://www.epa.gov/ttn/chief/trends/index.html as of October 2008.