Air Pollution Drops 40 Percent in US Metropolitan Areas
|
Number of Days with PSI Values Greater than 100 at Trend Sites
|
|
Metropolitan Area |
1988 |
1997 |
Change |
|
Total: 92 Metropolitan Areas |
5,299 |
3,137 |
-40.8% |
|
NOTES: The Pollutant Standards Index (PSI) integrates information on 5 major pollutants (particulate matter less than 10 microns in diameter, sulfur dioxide, carbon monoxide, ozone, and
nitrogen dioxide) across an entire monitoring network into a single number that represents the worst daily air quality experienced in an urban area. A PSI greater than 100 indicates that
at least 1 criteria pollutant exceeded air quality standards on a given day; therefore, air quality would be in the unhealthful range on that day. In 1997, 4,738 monitoring sites reported air
quality data. Air quality monitoring sites are selected as "trend sites" if they have complete data for at least 8 of the 10 years between 1988 and 1997. This table has been substantially revised
from last year. |
|
Source: Calculated from U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, National Air Quality and Emissions Trends Report, 1997 (Research Triangle Park, NC: 1999), Table A-15. |
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