U.S. Counties with Highest Number of Unhealthy Air Quality Days

2008 and 2016

Smoggy versus clean air over Downtown Los Angeles. From The History of the California Environmental Protection Agency, courtesy of the California Environmental Protection Agency.

Number of UNHEALTHY AIR DAYS in Descending Order, 2016:

Number of UNHEALTHY AIR DAYS in Descending Order, 2008:

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.

Source: U.S. Environmental Protection Agency