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U.S. Counties with the Largest Number of Unhealthy Air Quality Days in 1998 and 2008

Number of UNHEALTHY AIR DAYS in Descending Order, 2008:

Number of UNHEALTHY AIR DAYS in Descending Order, 1998:

Good - Percent of days in the year when air quality rated "good" on the PSI scale. Calculated as 100 * (days with PSI values 0
through 50) / (total days with PSI values).

Moderate - Percent of days in the year when air quality rated "moderate" on the PSI scale. Calculated as 100 * (days with PSI
values 51 through 100) / (total days with PSI values).

Unhealthful - Percent of days in the year when air quality rated "unhealthful" (or worse) on the PSI scale. Calculated as 100*
(days with PSI values above 100) / (total days with PSI values).

A PSI (Pollutant Standards Index) value is an approximate indicator of overall air quality, because it takes into account all of
the pollutants measured within a county. Many counties have monitoring stations for some, but not all, of the pollutants. PSI
values reflect all of the available measurements in each county.