State air quality · 2025

Colorado Air Quality Ranking

Across 30 monitored counties, the typical county in Colorado posts a median AQI of 40 (Good) and 71.7% of monitored days were rated Good, from EPA AQS.

40
Avg median AQI
71.7%
Good-air days
30
Counties monitored
25%
Cleaner than US states

Colorado's typical county median AQI is 40 (Good) across 30 monitored counties

According to the U.S. EPA Air Quality System, Colorado monitors 30 counties with a day-weighted Good-day share of 71.7%. Cleanest: Routt County (93.7% Good). Most polluted: Weld County (44.7%). These are annual summaries, not live AirNow readings.

Data source EPA Air Quality System (AQS)

Cleaner than US states
Colorado's day-weighted Good-day share of 71.7% places it cleaner than 25% of US states.
In-state spread
Within Colorado, San Juan County leads at 98.9% Good days while Weld County sits at 44.7% across 30 monitored counties.
2024→2025 release
Colorado's Good-day share moved +4.6 points vs the 2024 EPA annual release.
Typical median AQI
Typical median AQI: 40

Verify with AirNow → · EPA AQS →

EPA state monitoring brief

AQS state key COLORADO

Colorado · 2025 EPA AQS · 30 counties · med AQI 40 · 71.7% Good · Routt→Weld spread

  • GOOD 40 Median AQI · Good
  • SHARE 71.7% Good-day share
  • COUNT 30 Counties monitored
  • UNH 3 Unhealthy+ days
  • RANK 25% Cleaner than states
  • O3 63.5% Main pollutant share

Good-day share peers

Indiana71.8%Colorado (this)71.7%South Carolina71.3%Arkansas71.2%
Good-day share neighbourhood · nearest ABS peers vs Colorado

Colorado's 71.7% Good-day share is cleaner than 25% of monitored US states; Routt and Weld County bracket the in-state range. Open San Juan County →

According to EPA AQS annual summaries on PlainAirData, Colorado ranks #39 of 52 by day-weighted good-day share (71.7%) and #28 by mean county median AQI (39.5). Hawaii is #1 of 52 by day-weighted good-day share (93.8% of monitored days) at mean county median-AQI #4 (27.3) ≠ Alaska #1 lowest mean county median AQI (20.3) at good-day share #8 (84.4%).

Data updated August 2026

What changed, what drives it, and how thick is the sample

Three derived views WPR-style word count does not substitute for: release-over-release movement from county annual rows, the EPA main-pollutant day mix, and how many counties sit below the 100-day ranking floor.

Release change

71.7%

Good-day share in 2025, up 4.6 percentage points vs 2024. County-mean median AQI 39, 1.5 points lower (cleaner) than 2024.

Pollutant mix

Ozone

Main driver on 63.5% of pollutant-assigned days.

  • Ozone 63.5%
  • PM2.5 30.5%
  • PM10 4.9%
  • NO2 1.1%

Monitoring coverage

320

Average monitored days per county in the latest release. 2 of 30 counties sit below the 100-day floor used for cleanest/most-polluted claims.

Counties Monitored

30

Good Days %

71.7%

Avg Median AQI

40

Total Unhealthy Days

3

Best Air Quality

Routt County

Worst Air Quality

Weld County

Where Colorado sits nationally

COUNT-derived placement among 52 US states with at least 365 monitored days in the latest county release. Colorado's day-weighted Good-day share is 71.7% - cleaner than 25% of that comparison set.

Colorado Good-day share vs. every US state

Day-weighted share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release

71.7% 25th percentile higher than 25% of 52 US states

0.0%–5.0%: 0 US states (0%). Below this entry. 5.0%–10.0%: 0 US states (0%). Below this entry. 10.0%–15.0%: 0 US states (0%). Below this entry. 15.0%–20.0%: 0 US states (0%). Below this entry. 20.0%–25.0%: 0 US states (0%). Below this entry. 25.0%–30.0%: 0 US states (0%). Below this entry. 30.0%–35.0%: 0 US states (0%). Below this entry. 35.0%–40.0%: 0 US states (0%). Below this entry. 40.0%–45.0%: 0 US states (0%). Below this entry. 45.0%–50.0%: 0 US states (0%). Below this entry. 50.0%–55.0%: 0 US states (0%). Below this entry. 55.0%–60.0%: 0 US states (0%). Below this entry. 60.0%–65.0%: 2 US states (4%). Below this entry. 65.0%–70.0%: 9 US states (17%). Below this entry. 70.0%–75.0%: 11 US states (21%). This entry sits in this band. 75.0%–80.0%: 11 US states (21%). Above this entry. 80.0%–85.0%: 13 US states (25%). Above this entry. 85.0%–90.0%: 5 US states (10%). Above this entry. 90.0%–95.0%: 1 US states (2%). Above this entry. 95.0%–100.0%: 0 US states (0%). Above this entry. 100.0%–105.0%: 0 US states (0%). Above this entry. This state 0.0% 100% every floor-qualified US state, bucketed by value

Each bar is a band; taller bars hold more US states. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.

Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025

Which counties in Colorado have the cleanest and most polluted air?

Cleanest counties in Colorado

Share of monitored days rated Good (AQI 0–50)

  1. 1 San Juan County 98.9%
  2. 2 Jackson County 98.9%
  3. 3 Routt County 93.7%
  4. 4 Fremont County 93.4%
  5. 5 Prowers County 92.6%
  6. 6 Pitkin County 92.3%
  7. 7 Alamosa County 91.5%
  8. 8 Delta County 85.7%
  9. 9 San Miguel County 83%
  10. 10 Archuleta County 83%
  11. 11 Grand County 79.8%
  12. 12 Gilpin County 75.8%

Good-day share · EPA Air Quality System

County Good % Median AQI
San Juan 98.9% 12
Jackson 98.9% 9
Routt 93.7% 22
Fremont 93.4% 23
Prowers 92.6% 27
Pitkin 92.3% 19
Alamosa 91.5% 25
Delta 85.7% 29
San Miguel 83.0% 36
Archuleta 83.0% 34
Grand 79.8% 44
Gilpin 75.8% 46
Montezuma 75.1% 45
Pueblo 73.4% 45
Rio Blanco 72.7% 46
Gunnison 72.5% 46
Garfield 69.0% 47
El Paso 68.2% 46
Clear Creek 67.9% 48
Douglas 67.7% 46
Chaffee 66.0% 48
Mesa 65.2% 46
Park 63.0% 48
La Plata 60.8% 48
Jefferson 58.9% 48
Boulder 58.4% 48
Arapahoe 58.1% 48
Adams 55.1% 49
Larimer 50.7% 50
Denver 50.4% 50
Weld 44.7% 52

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025. See our methodology for how AQI and median figures are computed.

Coverage: 30 counties in Colorado with active monitoring stations. Unmonitored counties are not represented.

Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files, aggregated to the state level · 2020–2025

Every figure on PlainAirData is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.

Informational only, not professional advice. Data from EPA AQS. See our disclaimer.