Smoke: keep one room clean
Shut the windows and run an air purifier sized for the room.
Wildfire smoke guide →Air quality in Colorado, county by county
24 Colorado counties have enough EPA monitor data to count bad-air days. Weighted by population, they averaged about 15 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 Colorado county, weighted by population, about 2 smoky days (fine particles over 35.4 µg/m³, mostly wildfire smoke) and about 14 high-ozone days a year at its worst spot (2018 to 2022). See every county on the map.
Every Colorado county with monitors
Most bad-air days first. 18 have a full report. Compare with the worst and the cleanest counties in the US.
| County | Bad-air days a year | |
|---|---|---|
| Jefferson CountyLakewoodOzoneparticles not measured | 30.8 | |
| Douglas CountyCastle RockOzone | 21.4 | |
| Larimer CountyFort CollinsOzone | 19.8 | |
| Denver CountyOzone | 16.6 | |
| Weld CountyGreeleyOzone | 16.4 | |
| Arapahoe CountyAuroraOzone | 15.2 | |
| Adams CountyThorntonOzone | 13.0 | |
| Boulder CountyBoulderOzone | 12.2 | |
| Clear Creek CountyOzoneparticles not measured | 9.4 | |
| El Paso CountyColorado SpringsOzone | 9.2 | |
| Gilpin CountyOzoneparticles not measured | 8.0 | |
| Park CountyOzoneparticles not measured | 7.0 | |
| Garfield CountyRifleOzone | 4.4 | |
| Rio Blanco CountyOzone | 4.0 | |
| La Plata CountyDurangoOzone and particles | 2.8 | |
| Pueblo CountyPuebloOzone | 2.8 | |
| Mesa CountyGrand JunctionOzone | 2.0 | |
| Archuleta CountyOzone and particles | 1.4 | |
| Prowers CountyDustozone not measured | 1.4 | |
| Jackson CountyOzone | 1.3 | |
| Gunnison CountyOzoneparticles not measured | 0.8 | |
| Montezuma CountyOzone | 0.8 | |
| San Miguel CountyRarely bad | 0.2 | |
| Routt CountySteamboat SpringsRarely badozone not measured | 0.0 | |
| No Colorado 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 Colorado
Which county in Colorado has the worst air?
Among Colorado counties with EPA monitors, Jefferson County had the most bad-air days from 2021 to 2025: about 31 bad-air days a year with an AQI over 100. That's about 1 day in 12.
How often is the air bad in Colorado?
Weighted by population, Colorado counties with monitors averaged about 15 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 24.
Which county in Colorado has the cleanest air?
Of the 24 counties with enough monitor data, Routt County had the fewest, with no bad-air days at all from 2021 to 2025. Counties without monitors aren't ranked.
Where can I check today's air quality in Colorado?
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.