Where the air is worst in the US
In San Bernardino County, California, about 1 day in 3 was a bad-air day from 2021 to 2025. 17 counties averaged 30 or more a year, a month or more of days when children, older adults and people with asthma should cut back outdoors.
Every county
Most bad-air days a year first, 2021 to 2025.
| # | County | Bad-air days a year |
|---|---|---|
| 1 | San Bernardino County, CASan BernardinoOzone | 130 |
| 2 | Riverside County, CARiversideOzone | 126 |
| 3 | Los Angeles County, CALos AngelesOzone | 102 |
| 4 | Maricopa County, AZPhoenixOzone | 100 |
| 5 | Tulare County, CAVisaliaOzone | 97 |
| 6 | Kern County, CABakersfieldOzone | 96 |
| 7 | Fresno County, CAFresnoOzone | 65 |
| 8 | Pinal County, AZMaricopaDust | 48 |
| 9 | Imperial County, CAEl CentroOzone and particles | 43 |
| 10 | Harris County, TXHoustonOzone | 37 |
| 11 | El Paso County, TXEl PasoOzone and particles | 37 |
| 12 | Kings County, CAHanfordOzone and particles | 35 |
| 13 | San Diego County, CASan DiegoOzone | 34 |
| 14 | Fairbanks North Star Borough, AKFairbanksWinter particles | 32 |
| 15 | Doña Ana County, NMLas CrucesOzone and particles | 31 |
| 16 | Clark County, NVLas VegasOzone | 30 |
| 17 | Salt Lake County, UTSalt Lake CityOzone | 30 |
| 18 | Inyo County, CABishopOzone and particles | 28 |
| 19 | Eddy County, NMCarlsbadOzone | 27 |
| 20 | Stanislaus County, CAModestoOzone and particles | 27 |
| 21 | Tarrant County, TXFort WorthOzone | 25 |
| 22 | Denton County, TXDentonOzone | 24 |
| 23 | Placer County, CARosevilleOzone | 22 |
| 24 | Bernalillo County, NMOzone and particles | 22 |
| 25 | Douglas County, COCastle RockOzone | 21 |
Counties of 250,000 people or more
Most bad-air days a year first, 2021 to 2025.
| # | County | Bad-air days a year |
|---|---|---|
| 1 | San Bernardino County, CASan BernardinoOzone | 130 |
| 2 | Riverside County, CARiversideOzone | 126 |
| 3 | Los Angeles County, CALos AngelesOzone | 102 |
| 4 | Maricopa County, AZPhoenixOzone | 100 |
| 5 | Tulare County, CAVisaliaOzone | 97 |
| 6 | Kern County, CABakersfieldOzone | 96 |
| 7 | Fresno County, CAFresnoOzone | 65 |
| 8 | Pinal County, AZMaricopaDust | 48 |
| 9 | Harris County, TXHoustonOzone | 37 |
| 10 | El Paso County, TXEl PasoOzone and particles | 37 |
| 11 | San Diego County, CASan DiegoOzone | 34 |
| 12 | Clark County, NVLas VegasOzone | 30 |
| 13 | Salt Lake County, UTSalt Lake CityOzone | 30 |
| 14 | Stanislaus County, CAModestoOzone and particles | 27 |
| 15 | Tarrant County, TXFort WorthOzone | 25 |
| 16 | Denton County, TXDentonOzone | 24 |
| 17 | Placer County, CARosevilleOzone | 22 |
| 18 | Bernalillo County, NMOzone and particles | 22 |
| 19 | Douglas County, COCastle RockOzone | 21 |
| 20 | Fairfield County, CTOzone | 21 |
| 21 | Cook County, ILChicagoOzone | 20 |
| 22 | Larimer County, COFort CollinsOzone | 20 |
| 23 | Lane County, OREugeneSummer smoke | 19 |
| 24 | Merced County, CAMercedOzone | 19 |
| 25 | Sacramento County, CASacramentoOzone and particles | 17 |
Counties of 1 million people or more
Most bad-air days a year first, 2021 to 2025.
| # | County | Bad-air days a year |
|---|---|---|
| 1 | San Bernardino County, CASan BernardinoOzone | 130 |
| 2 | Riverside County, CARiversideOzone | 126 |
| 3 | Los Angeles County, CALos AngelesOzone | 102 |
| 4 | Maricopa County, AZPhoenixOzone | 100 |
| 5 | Fresno County, CAFresnoOzone | 65 |
| 6 | Harris County, TXHoustonOzone | 37 |
| 7 | San Diego County, CASan DiegoOzone | 34 |
| 8 | Clark County, NVLas VegasOzone | 30 |
| 9 | Salt Lake County, UTSalt Lake CityOzone | 30 |
| 10 | Tarrant County, TXFort WorthOzone | 25 |
| 11 | Cook County, ILChicagoOzone | 20 |
| 12 | Sacramento County, CASacramentoOzone and particles | 17 |
| 13 | Dallas County, TXDallasOzone | 17 |
| 14 | Bexar County, TXSan AntonioOzone | 14 |
| 15 | Orange County, CAAnaheimOzone | 13 |
| 16 | Wayne County, MIDetroitOzone and particles | 12 |
| 17 | Pima County, AZTucsonOzone and particles | 11 |
| 18 | Allegheny County, PAPittsburghWinter particles | 10 |
| 19 | Philadelphia County, PAOzone | 9.4 |
| 20 | Suffolk County, NYLindenhurstOzone | 8.6 |
| 21 | Alameda County, CAOaklandOzone and particles | 7.2 |
| 22 | Cuyahoga County, OHClevelandOzone | 7.0 |
| 23 | Hennepin County, MNMinneapolisSummer smoke | 6.8 |
| 24 | King County, WASeattleSummer smoke | 6.6 |
| 25 | Queens County, NYNew York CityOzone | 6.6 |
Every county we rank, all 571 of them
How we rank
Every number comes from the EPA's AirData county summaries for 2021 to 2025. A bad-air day is a day when a county's worst monitor shows an AQI over 100, the point where the EPA says children, older adults and people with asthma or heart or lung disease should cut back outdoors. Days over 150 are unhealthy for everyone.
We rank only the 571 counties whose monitors measure both ozone and fine particles. A county that measures only one can't record the other's bad days. Counties without monitors aren't ranked; their state page and the map have CDC estimates.
The American Lung Association's State of the Air report ranks counties too. It uses 3 years of data and gives more weight to worse days, so its order differs from ours.
See the cleanest air · States ranked · How we count · Download every county's numbers
Questions people ask
Which US county has the worst air quality?
Going by EPA monitors from 2021 to 2025, San Bernardino County, CA had the most bad-air days: 130 bad-air days a year, mostly ozone. On 64 of those days a year the AQI passed 150, unhealthy for everyone.
Which city has the worst air quality in the US?
We rank counties, because the EPA reports air quality by county. The most bad-air days were in San Bernardino County, CA (San Bernardino), Riverside County, CA (Riverside) and Los Angeles County, CA (Los Angeles).
Sources: EPA AirData county AQI summaries, 2021 to 2025. Updated November 8, 2026.