Air Quality Rankings for Colorado (2026)

Cities Tracked

State Avg AQI

Improving

Worsening

How Colorado Compares

Colorado has 32 cities tracked by EPA air-quality monitors, with a state-wide 5-year median AQI of 39 — 2 points cleaner than the national average of AQI 41. San Juan, Colorado ranks #1 with the cleanest air (AQI 9, Grade B), while Denver, Colorado sits at the bottom (AQI 54, Grade C). The rankings below are computed from the EPA Air Quality System (AQS), which aggregates daily AQI readings from federally certified monitors into annual averages. Cities are sorted by 5-year median AQI (lowest = cleanest = #1). The 5-year window smooths out year-to-year volatility from weather and wildfire events.

Colorado is bucking the national trend of broad improvement: 23 of 32 monitored cities show measurably worse air over the past decade, more than the 3 that are improving. Across western states this usually traces back to expanding wildfire smoke exposure; elsewhere it can reflect rising local emissions from population or freight growth.

The dominant pollutant across 23 of 32 Colorado cities is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days and is the leading air quality concern across much of the Sun Belt and California. Other Colorado cities report Coarse Particulate Matter (PM10) (5), Fine Particulate Matter (PM2.5) (4) as their dominant concern.

The fastest-improving city in Colorado is Alamosa, Colorado, with median AQI falling by 0.5 points per year. Steady improvement at that pace usually reflects fleet turnover (older diesels retiring), upwind power-plant retirements, or tighter regional emissions controls.

The city with the steepest decline is Delta, Colorado, where median AQI is rising by 4.1 points per year. Rapid deterioration in a single city usually points to either wildfire-smoke exposure (in the West) or a new local emissions source — a power plant, port, or freight corridor coming online.

Full Colorado Ranking

#	City	5yr Avg AQI	Current AQI	Worst Pollutant	Trend	Grade
1	San Juan, Colorado	9	9	PM2.5	Stable	B
2	Fremont, Colorado	13	12	PM10	Stable	A
3	Pitkin, Colorado	13	13	PM10	Stable	B
4	Routt, Colorado	14	13	PM10	Stable	A
5	Alamosa, Colorado	14	22	PM10	Improving	A
6	Prowers, Colorado	17	19	PM10	Stable	B
7	Jackson, Colorado	21	45	PM2.5	Worsening	B
8	Pueblo, Colorado	27	43	PM2.5	Worsening	B
9	Archuleta, Colorado	39	35	Ozone	Worsening	C
10	Moffat, Colorado	42	42	Ozone	Worsening	C
11	Delta, Colorado	42	44	Ozone	Worsening	C
12	Montezuma, Colorado	44	44	Ozone	Stable	C
13	Mesa, Colorado	45	46	Ozone	Improving	B
14	Gilpin, Colorado	46	47	Ozone	Worsening	C
15	Grand, Colorado	46	46	Ozone	Worsening	C
16	Gunnison, Colorado	46	46	Ozone	Stable	C
17	La Plata, Colorado	46	46	Ozone	Stable	C
18	Arapahoe, Colorado	46	47	Ozone	Stable	C
19	El Paso, Colorado	46	47	Ozone	Stable	C
20	San Miguel, Colorado	47	47	Ozone	Worsening	C
21	Rio Blanco, Colorado	47	47	Ozone	Stable	C
22	Jefferson, Colorado	47	49	Ozone	Stable	D
23	Douglas, Colorado	47	49	Ozone	Worsening	D
24	Clear Creek, Colorado	48	49	Ozone	Stable	C
25	Park, Colorado	49	50	Ozone	Worsening	C
26	Garfield, Colorado	50	54	Ozone	Worsening	C
27	Adams, Colorado	50	51	Ozone	Worsening	C
28	Boulder, Colorado	50	50	Ozone	Worsening	D
29	Larimer, Colorado	51	51	Ozone	Stable	C
30	Chaffee, Colorado	53	58	Ozone	Worsening	C
31	Weld, Colorado	53	55	Ozone	Worsening	D
32	Denver, Colorado	54	54	PM2.5	Stable	C

Air quality data for Colorado is sourced from the EPA Air Quality System (AQS), which monitors outdoor air quality at thousands of stations nationwide.
, EPA Air Quality System

Frequently Asked Questions

San Juan, Colorado has the best air quality in Colorado with a 5-year average AQI of 9 and a Grade B (78/100). Its dominant pollutant is Fine Particulate Matter (PM2.5) and the long-run trend is stable.

Denver, Colorado has the worst air quality in Colorado with a 5-year average AQI of 54 and a Grade C (52/100). Its dominant pollutant is Fine Particulate Matter (PM2.5).

Colorado has 32 cities with EPA air quality monitoring data, covering 2014-2023 of daily AQI measurements aggregated into annual averages.

Colorado's state-wide 5-year median AQI is 39, 2 points cleaner than the national average of AQI 41. Colorado is bucking the national trend of broad improvement: 23 of 32 monitored cities show measurably worse air over the past decade, more than the 3 that are improving. Across western states this usually traces back to expanding wildfire smoke exposure; elsewhere it can reflect rising local emissions from population or freight growth.

Ground-Level Ozone is the dominant pollutant in 23 of 32 monitored Colorado cities. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days and is the leading air quality concern across much of the Sun Belt and California.

Colorado cities log an average of 6 days per year at "Unhealthy for Sensitive Groups" or worse, based on EPA monitor data over the last five years. Across all 32 Colorado cities tracked, that totals 967 unhealthy days over the period.

Sources: EPA Air Quality System (AQS)

Cities ranked by 5-year average AQI (lower is better). Grades factor in average AQI, trend direction, unhealthy days, and dominant pollutant.

Cleanest Cities Nationwide →All Air Quality in Colorado →Worst Air Quality Cities →Methodology →

The this entity category groups every U.S. air quality and pollution monitoring entity sharing this attribute. The list above is the data; the paragraphs below explain what the grouping means against the broader the EPA Air Quality System (AQS) distribution and how to read the relative rankings within the category.

For readers using this category as a starting point, the per-entity detail pages linked from the table above carry the underlying the EPA Air Quality System (AQS) data in full. The category-level view is the filter; the per-entity pages are the actual answer.

Source: EPA Outdoor Air Quality Data, 2026.