Smoke: keep one room clean
Shut the windows and run an air purifier sized for the room.
Wildfire smoke guide →Air quality in California, county by county
52 California counties have enough EPA monitor data to count bad-air days. Weighted by population, they averaged about 53 days a year from 2021 to 2025 when the air was unhealthy for sensitive groups or worse.
CDC's model, which estimates every county from monitors and satellites, gives the average California county, weighted by population, about 11 smoky days (fine particles over 35.4 µg/m³, mostly wildfire smoke) and about 43 high-ozone days a year at its worst spot (2018 to 2022). See every county on the map.
Every California county with monitors
Most bad-air days first. 46 have a full report. Compare with the worst and the cleanest counties in the US.
| County | Bad-air days a year | |
|---|---|---|
| San Bernardino CountySan BernardinoOzone | 129.6 | |
| Riverside CountyRiversideOzone | 126.4 | |
| Los Angeles CountyLos AngelesOzone | 101.8 | |
| Tulare CountyVisaliaOzone | 96.8 | |
| Kern CountyBakersfieldOzone | 95.8 | |
| Fresno CountyFresnoOzone | 64.6 | |
| Imperial CountyEl CentroOzone and particles | 43.4 | |
| Kings CountyHanfordOzone and particles | 35.0 | |
| San Diego CountySan DiegoOzone | 34.2 | |
| Plumas CountySmokeozone not measured | 30.0 | |
| Inyo CountyBishopOzone and particles | 27.8 | |
| Stanislaus CountyModestoOzone and particles | 26.8 | |
| Placer CountyRosevilleOzone | 21.8 | |
| Merced CountyMercedOzone | 19.2 | |
| Madera CountyMaderaOzone | 18.2 | |
| Sacramento CountySacramentoOzone and particles | 17.4 | |
| Mariposa CountySummer smoke | 16.8 | |
| Nevada CountyTruckeeOzone | 15.0 | |
| Trinity CountySmokeozone not measured | 14.6 | |
| Mono CountyParticlesozone not measured | 13.2 | |
| Orange CountyAnaheimOzone | 12.8 | |
| Siskiyou CountySummer smoke | 11.8 | |
| Ventura CountyOxnardOzone | 11.2 | |
| El Dorado CountySouth Lake TahoeOzone | 10.0 | |
| Shasta CountyReddingOzone and particles | 9.8 | |
| Tehama CountyRed BluffOzone and particles | 9.6 | |
| San Joaquin CountyStocktonWinter particles | 9.0 | |
| Alameda CountyOaklandOzone and particles | 7.2 | |
| Butte CountyChicoSummer smoke | 7.2 | |
| Sutter CountyYuba CityOzone and particles | 7.0 | |
| San Luis Obispo CountySan Luis ObispoOzone | 6.8 | |
| Santa Clara CountySan JoseSummer smoke | 4.2 | |
| Calaveras CountyOzone and particles | 3.8 | |
| Colusa CountySummer smoke | 3.4 | |
| Glenn CountySummer smoke | 3.2 | |
| Mendocino CountyUkiahSummer smoke | 3.0 | |
| Contra Costa CountyConcordSummer smoke | 2.2 | |
| Amador CountyOzoneparticles not measured | 2.2 | |
| Humboldt CountyEurekaSummer smoke | 2.2 | |
| Yolo CountyDavisSummer smoke | 2.2 | |
| Del Norte CountyCrescent CitySmokeozone not measured | 2.0 | |
| Tuolumne CountySonoraOzoneparticles not measured | 2.0 | |
| Solano CountyVallejoSummer smoke | 1.0 | |
| Santa Barbara CountySanta MariaOzone and particles | 0.6 | |
| San Benito CountyHollisterRarely bad | 0.4 | |
| Sonoma CountySanta RosaRarely bad | 0.4 | |
| San Francisco CountyRarely bad | 0.2 | |
| Lake CountyClearlakeRarely bad | 0.2 | |
| Monterey CountySalinasRarely bad | 0.2 | |
| Santa Cruz CountySanta CruzRarely bad | 0.2 | |
| Marin CountySan RafaelRarely bad | 0.0 | |
| San Mateo CountySan MateoRarely bad | 0.0 | |
| No California county with a monitor matches “”. Counties without a monitor aren't listed; a neighbor's numbers are a fair guide, or find it on the map. | ||
Counties without a monitor aren't listed. Air doesn't stop at the county line, so a neighbor's numbers are a fair guide.
What helps on a bad-air day
Check AirNow for today's reading and tomorrow's forecast, then match the step to the pollutant.
Upgrade the furnace filter
MERV 13 catches far more smoke than thin fiberglass filters.
Furnace filters →Questions people ask about air quality in California
Which county in California has the worst air?
Among California counties with EPA monitors, San Bernardino County had the most bad-air days from 2021 to 2025: about 130 bad-air days a year with an AQI over 100. That's about 1 day in 3.
How often is the air bad in California?
Weighted by population, California counties with monitors averaged about 53 bad-air days a year from 2021 to 2025 with an AQI over 100, when children, older adults and people with asthma should cut back outdoors. That's about 1 day in 7.
Which county in California has the cleanest air?
2 of the 52 counties with enough monitor data had no bad-air days at all from 2021 to 2025, including San Mateo County and Marin County. Counties without monitors aren't ranked.
Where can I check today's air quality in California?
AirNow, run by the EPA and its partners, shows current readings and forecasts by ZIP code, and its fire and smoke map adds smoke plumes and low-cost sensors.
Sources: EPA AirData county AQI summaries, 2021 to 2025; CDC Environmental Public Health Tracking Network modeled PM2.5 and ozone, 2018 to 2022. Updated November 8, 2026.