# Counties in California by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2024

All 52 values come from U.S. Environmental Protection Agency for 2024, so the places are comparable. 6 counties have no value for that period and are not ranked.

- Indicator id: `aqi_unhealthy_sensitive_or_worse_days_share`
- Unit: %
- Definition: Days with a daily Air Quality Index above 100 (unhealthy for sensitive groups, unhealthy, very unhealthy or hazardous) as a percentage of the days of the year with an AQI in a US county. Calculated by Invokely from the day counts in the EPA AirData annual summaries; the counts are stored with each value.
- Source: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts (https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual)
- Retrieved: 2026-10-12
- Citation: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
- Web page: https://invokely.ai/rankings/unhealthy-air-quality-days-share/california-counties

| Rank | Place | % |
|---:|---|---:|
| 1 | Riverside County | 40.2 |
| 2 | San Bernardino County | 39.3 |
| 3 | Los Angeles County | 36.3 |
| 4 | Tulare County | 30.3 |
| 5 | Kern County | 25.7 |
| 6 | Fresno County | 21.6 |
| 7 | San Diego County | 14.2 |
| 8 | Imperial County | 13.7 |
| 9 | Stanislaus County | 10.4 |
| 10 | Kings County | 9.6 |
| 10 | Merced County | 9.6 |
| 12 | Inyo County | 8.2 |
| 13 | Placer County | 7.4 |
| 14 | Madera County | 6.8 |
| 15 | Orange County | 4.4 |
| 15 | Plumas County | 4.4 |
| 17 | Sacramento County | 4.1 |
| 17 | Ventura County | 4.1 |
| 19 | San Joaquin County | 3 |
| 20 | San Luis Obispo County | 2.7 |
| 21 | Calaveras County | 2.2 |
| 21 | El Dorado County | 2.2 |
| 23 | Amador County | 1.7 |
| 24 | Mariposa County | 1.6 |
| 25 | Sutter County | 1.4 |
| 26 | Shasta County | 1.1 |
| 26 | Tuolumne County | 1.1 |
| 28 | Del Norte County | 0.7 |
| 29 | Butte County | 0.5 |
| 29 | Nevada County | 0.5 |
| 29 | Santa Barbara County | 0.5 |
| 29 | Tehama County | 0.5 |
| 33 | Alameda County | 0.3 |
| 33 | San Benito County | 0.3 |
| 33 | Santa Clara County | 0.3 |
| 33 | Solano County | 0.3 |
| 33 | Trinity County | 0.3 |
| 33 | Yolo County | 0.3 |
| 39 | Colusa County | 0 |
| 39 | Contra Costa County | 0 |
| 39 | Glenn County | 0 |
| 39 | Humboldt County | 0 |
| 39 | Lake County | 0 |
| 39 | Marin County | 0 |
| 39 | Mendocino County | 0 |
| 39 | Mono County | 0 |
| 39 | Monterey County | 0 |
| 39 | San Francisco County | 0 |
| 39 | San Mateo County | 0 |
| 39 | Santa Cruz County | 0 |
| 39 | Siskiyou County | 0 |
| 39 | Sonoma County | 0 |