| Year | |||||||||||||
| Pollutant | 1986 | 1987 | 1988 | 1989 | 1990 | 1991 | 1992 | 1993 | 1994 | 1995 | 1996 | 1997 | 1998 |
| population, in millions | |||||||||||||
| SO2 | 0.90 | 1.60 | 1.70 | 0.10 | 1.40 | 5.20 | 0.00 | 1.40 | 0.04 | 0.00 | 0.20 | 0.10 | 0.00 |
| NO2 | 7.50 | 7.50 | 8.30 | 8.50 | 8.50 | 8.90 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
| CO | 41.40 | 29.40 | 29.50 | 33.60 | 21.70 | 19.90 | 14.30 | 11.60 | 15.30 | 12.00 | 12.70 | 9.10 | 10.20 |
| O3 | 75.00 | 88.60 | 111.90 | 66.70 | 62.90 | 69.70 | 44.60 | 51.30 | 50.20 | 70.80 | 39.30 | 101.60 | 131.40 |
| Pb | 4.50 | 1.70 | 1.60 | 1.60 | 5.30 | 14.70 | 4.70 | 5.50 | 4.40 | 4.80 | 4.10 | 2.40 | 4.30 |
| PM-10 | 41.70 | 21.50 | 25.60 | 27.40 | 18.80 | 21.50 | 25.80 | 9.40 | 13.10 | 24.40 | 7.30 | 9.70 | 9.80 |
| Any NAAQS | na | 101.80 | 121.30 | 84.40 | 47.40 | 86.40 | 53.60 | 59.10 | 62.04 | 79.80 | 46.60 | 107.00 | 133.90 |
Source: U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National Air Quality and Emissions Trends Report, 1998 (EPA, OAQPS, Research Triangle Park, NC, 2000) and earlier trends reports.
Notes: NAAQS = National Ambient Air Quality Standards. PM-10 = particulate matter with a diameter of 10 micrometers or less. Data for ozone and particulate matter are based on the revised NAAQS. Population estimates are intended to provide a relative measure of the extent of the problem for each pollutant in a single year. An individual living in a county that had a measured concentration above the level of the NAAQS may not actually be exposed to unhealthy air.
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