Which county in Colorado has the best air quality?

Based on 2024 EPA AQS data, San Juan County had the best air quality in Colorado with a median AQI of 9 and 120 good-air days.

2024 monitoring year Public dataset EPA AQS

Colorado Air Quality Rankings

Compiled by PlainEnviro Editorial, Environmental Data Editorial Team · July 13, 2026

County-level AQI rollup using EPA Air Quality System monitor records. Refreshed as new EPA AQS data lands.

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County-by-county air quality data for Colorado based on EPA AQS annual AQI readings. Data covers 31 monitored counties for 2024.

Colorado (2024): 31 counties monitored. State average median AQI: 41 (national average: 38.8). 69% of monitored days rated "Good." Most unhealthy air: Jefferson County (7 days above AQI 100).

Colorado air quality on this page is drawn from the EPA Air Quality System (AQS), which aggregates daily readings from federal, state, tribal, and local monitors into annual summaries per county. The current dataset covers 31 Colorado counties with at least ten days of valid AQI readings in 2024, out of the state's total monitored counties.

The state-average median AQI across those counties is 41, compared with a national average of 38.8 in the same reporting period. On average, 69% of monitored days fell into the "Good" AQI bucket (0–50). The heaviest burden of unhealthy days lands on Jefferson County (7 days above AQI 100), while San Juan County records the cleanest air with a median AQI of 9.

AQI values translate ambient concentrations of pollutants (ozone, PM2.5, PM10, NO₂, SO₂, CO) onto a single 0–500 scale with six health-based bands. A low median AQI reflects typical daily conditions; peak values matter separately because short-term exposure drives most acute health effects. Counties without monitors are not "clean" - they are simply unmeasured by EPA AQS. For site-specific exposure guidance, check AirNow.gov alongside the county detail links below.

Counties with Data

State Avg Median AQI

National Avg Median AQI

38.8

Avg Good Days %

69%

Which Colorado counties have the worst air quality?

Ranked by median AQI (higher = worse). Click a county for full environmental data.

#	County	Median AQI	Good Days	Good %	Unhealthy Days	Peak AQI
1	Adams	55	147	40%	2	161
2	Denver	55	129	35%	3	172
3	Weld	54	149	41%	2	164
4	Larimer	54	156	43%	2	161
5	Chaffee	51	74	46%	0	112
6	Jefferson	50	192	52%	7	174
7	Park	50	172	53%	0	147
8	Arapahoe	49	196	54%	2	161
9	Boulder	49	212	58%	1	154
10	Clear Creek	49	202	58%	0	136
11	El Paso	49	202	55%	2	166
12	Garfield	49	216	59%	0	133
13	Douglas	48	208	57%	4	166
14	Mesa	48	215	59%	0	136
15	Gilpin	48	240	66%	0	150
16	Gunnison	47	243	66%	0	100
17	La Plata	46	241	66%	1	240
18	Rio Blanco	46	266	73%	0	142
19	Grand	45	133	83%	0	93
20	Montezuma	45	262	72%	0	101
21	Pueblo	44	249	68%	0	143
22	Delta	44	125	87%	0	80
23	Jackson	44	304	84%	0	105
24	Archuleta	32	349	95%	0	80
25	Alamosa	26	325	89%	0	98
26	Prowers	25	321	88%	0	98
27	Pitkin	19	246	96%	0	76
28	San Miguel	19	311	86%	0	97
29	Fremont	13	354	98%	0	59
30	Routt	12	364	100%	0	53
31	San Juan	9	120	99%	0	51

Frequently Asked Questions

What is the air quality like in Colorado?

In 2024, Colorado had an average median AQI of 41 across 31 monitored counties, with 69% of monitored days rated "Good" (AQI 0–50). The national average median AQI is 38.8.

Which county in Colorado has the worst air quality?

Based on 2024 EPA AQS data, Jefferson County had the most unhealthy air days in Colorado - 7 days above AQI 100, with a median AQI of 50.

Explore More

National AQI Rankings → Colorado Full Profile → Colorado Water Quality →

About This Data

Air quality data is from the EPA Air Quality System (AQS), Annual AQI by County dataset. Counties with fewer than 10 monitored days are excluded. "Unhealthy days" counts days with AQI above 100 (Unhealthy for Sensitive Groups or worse). All data is for 2024.

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

Read our methodology - how this data is sourced, computed, and verified.

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Data sourced from the U.S. Environmental Protection Agency (EPA). See our methodology for details. Retrieved and formatted by PlainEnviro Editorial