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
