1970 = 1.0; 1990 = 1.0 for PM-2.5 and ammonia
Carbon monoxide (CO)
Volatile organic compounds (VOC)
Particulate matter 2.5 microns in diameter or smaller (PM-2.5)
Nitrogen oxides (NOx)
Particulate matter 10 microns in diameter or smaller (PM-10)
SOURCE: U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National Air Pollutant Emission Trends: 19001998, EPA 454/R-00-002 (Research Triangle Park, NC:March 2000),also available at http://www.epa.gov/ttn/chief/trends/, as of May 18, 2001.