U.S. Counties with the Largest Number of Unhealthy Air Quality Days in 2004

Number of UNHEALTHY AIR DAYS in Descending Order:

Number of UNHEALTHY AIR DAYS FOR SENSITIVE GROUPS in Descending Order:

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.