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Air quality in Colorado

EPA monitors report from 31 of Colorado’s 64 counties. That is the honest limit of this page: the other 33 are not measured, not clean. In 2025 the most affected monitored county was Jefferson County, with 13 days above the moderate category.

31
Counties with monitors
33
Counties without
73
Days above moderate, 2025
Summed across monitored counties
10.9%
Adults with asthma
Population-weighted county estimate

County by county

Monitored Colorado counties

GoodModerateUnhealthy for sensitive groupsUnhealthyVery unhealthyHazardous
County90th %ile AQIAbove moderateMax AQI2025 days by category
Jefferson County8413119
Larimer County8411119
Douglas County717122
Denver County746115
Weld County806119
Chaffee County845108
Rio Blanco County674142
Boulder County713119
Arapahoe County712101
Archuleta County612115
La Plata County742112
Park County772105
Prowers County452652
Adams County711112
Delta County531103
Garfield County671131
Gilpin County641101
Gunnison County671115
Montezuma County641101
Pitkin County431154
Pueblo County641101
Alamosa County49076
Clear Creek County670100
El Paso County67097
Fremont County45077
Grand County61077
Jackson County26056
Mesa County64097
Routt County42087
San Juan County29057
San Miguel County60094

EPA Air Quality System annual summaries by county, 2025. The 90th percentile AQI describes a day worse than nine out of ten, which is a steadier measure than the annual maximum a single wildfire can set.

Not measured

33 counties without a monitor

No EPA monitor reported from these counties, so there is no air quality record to publish. Their radon zone is known, and radon is measured in the building rather than the air outside it.

Other states

Air quality elsewhere