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

Air Quality, Los Angeles

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


Also see: Annual Air Quality Days by Year - Los Angeles County


As measured over 365/366 days in 2021. Counties ordered by CUMULATIVE UNHEALTHY AIR DAYS in descending order.

County Number of Days When Air Quality Was:
Good Moderate Unhealthy for Sensitive Groups Unhealthy Very Unhealthy Hazardous Cumulative Unhealthy Days
Maricopa County, AZ 15 125 91 61 73 0 225
San Bernardino County, CA 42 178 72 61 11 1 145
Riverside County, CA 35 194 89 44 2 1 136
Tulare County, CA 78 153 93 39 1 1 134
Kern County, CA 65 172 98 28 1 1 128
County Number of Days When Air Quality Was:
Good Moderate Unhealthy for Sensitive Groups Unhealthy Very Unhealthy or Hazardous Hazardous Cumulative Unhealthy Days
Los Angeles County, CA 40 228 70 26 1 0 97
Plumas County, CA 123 158 37 26 14 7 84
Fresno County, CA 102 182 64 16 1 0 81
Jefferson County, CO 222 88 44 11 0 0 55
Nevada County, CA 221 91 34 15 4 0 53
County Number of Days When Air Quality Was:
Good Moderate Unhealthy for Sensitive Groups Unhealthy Very Unhealthy or Hazardous Hazardous Cumulative Unhealthy Days
Kings County, CA 113 201 40 10 0 1 51
Klamath County, OR 195 120 19 29 2 0 50
Placer County, CA 165 152 30 11 5 2 48
Salt Lake County, UT 174 144 38 9 0 0 47
Stanislaus County, CA 136 182 42 5 0 0 47
County Number of Days When Air Quality Was:
Good Moderate Unhealthy for Sensitive Groups Unhealthy Very Unhealthy or Hazardous Hazardous Cumulative Unhealthy Days
Trinity County, CA 218 49 4 23 7 12 46
Pinal County, AZ 80 240 44 1 0 0 45
Inyo County, CA 131 191 25 13 2 3 43
Douglas County, CO 215 101 34 8 0 0 42

Good Quality Air Days - Number of days in the year when the Air Quality Index (AQI) value is 0 through 50. Air quality is considered satisfactory, and air pollution poses little or no risk.
Moderate Quality Air Days - Number of days in the year when the Air Quality Index (AQI) value is 51 through 100. Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.
Unhealthy for Sensitive Groups - Number of days in the year when the Air Quality Index (AQI) value is 100 through 150. Members of sensitive groups may experience health effects. The general public is not likely to be affected.
Unhealthy Air Days - Number of days in the year when the Air Quality Index (AQI) value is 151 through 150. Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.
Very Unhealthy Air Days - Number of days in the year when the Air Quality Index (AQI) value is 201 or higher.Health alert: everyone may experience more serious health effects. When the AQI exceeds 300, the air situation is considered hazardous, meriting a health emergency. The entire population is more likely to be affected.

Source: U.S. Environmental Protection Agency