Data / Rankings / Median Air Quality Index (US EPA) / Counties in Colorado

Counties in Colorado by median Air Quality Index (US EPA), 2025

Weld County had the highest median Air Quality Index (US EPA) of 30 counties in 2025, at 52. Jackson County had the lowest, at 9. The median was 46. Source: the U.S. Environmental Protection Agency.

Keep it in a collection in your library.
Highest52Weld County
Lowest9Jackson County
Median4630 counties

Map of counties, 2025

Adams County: 49 indexAlamosa County: 25 indexArapahoe County: 48 indexArchuleta County: 34 indexBoulder County: 48 indexChaffee County: 48 indexClear Creek County: 48 indexDelta County: 29 indexDenver County: 50 indexDouglas County: 46 indexEl Paso County: 46 indexFremont County: 23 indexGarfield County: 47 indexGilpin County: 46 indexGrand County: 44 indexGunnison County: 46 indexJackson County: 9 indexJefferson County: 48 indexLa Plata County: 48 indexLarimer County: 50 indexMesa County: 46 indexMontezuma County: 45 indexPark County: 48 indexPitkin County: 19 indexProwers County: 27 indexPueblo County: 45 indexRoutt County: 22 indexSan Juan County: 12 indexSan Miguel County: 36 indexWeld County: 52 index
  • Under 25
  • 25 to 41
  • 41 to 46
  • 46 to 48
  • 48 and over
  • No data
index, 2025. Hatched areas have no value for this period. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2025

Weld County: 52 indexWeld County52Denver County: 50 indexDenver County50Larimer County: 50 indexLarimer County50Adams County: 49 indexAdams County49Arapahoe County: 48 indexArapahoe County48Boulder County: 48 indexBoulder County48Chaffee County: 48 indexChaffee County48Clear Creek County: 48 indexClear Creek County48Jefferson County: 48 indexJefferson County48La Plata County: 48 indexLa Plata County48Park County: 48 indexPark County48Garfield County: 47 indexGarfield County47Douglas County: 46 indexDouglas County46El Paso County: 46 indexEl Paso County46Gilpin County: 46 indexGilpin County46Gunnison County: 46 indexGunnison County46Mesa County: 46 indexMesa County46Montezuma County: 45 indexMontezuma County45Pueblo County: 45 indexPueblo County45Grand County: 44 indexGrand County44San Miguel County: 36 indexSan Miguel County36Archuleta County: 34 indexArchuleta County34Delta County: 29 indexDelta County29Prowers County: 27 indexProwers County27Alamosa County: 25 indexAlamosa County25Fremont County: 23 indexFremont County23Routt County: 22 indexRoutt County22Pitkin County: 19 indexPitkin County19San Juan County: 12 indexSan Juan County12Jackson County: 9 indexJackson County9Weld County: 52 indexWeld County52Denver County: 50 indexDenver County50Larimer County: 50 indexLarimer County50Adams County: 49 indexAdams County49Arapahoe County: 48 indexArapahoe County48Boulder County: 48 indexBoulder County48Chaffee County: 48 indexChaffee County48Clear Creek County: 48 indexClear Creek County48Jefferson County: 48 indexJefferson County48La Plata County: 48 indexLa Plata County48Park County: 48 indexPark County48Garfield County: 47 indexGarfield County47Douglas County: 46 indexDouglas County46El Paso County: 46 indexEl Paso County46Gilpin County: 46 indexGilpin County46Gunnison County: 46 indexGunnison County46Mesa County: 46 indexMesa County46Montezuma County: 45 indexMontezuma County45Pueblo County: 45 indexPueblo County45Grand County: 44 indexGrand County44San Miguel County: 36 indexSan Miguel County36Archuleta County: 34 indexArchuleta County34Delta County: 29 indexDelta County29Prowers County: 27 indexProwers County27Alamosa County: 25 indexAlamosa County25Fremont County: 23 indexFremont County23Routt County: 22 indexRoutt County22Pitkin County: 19 indexPitkin County19San Juan County: 12 indexSan Juan County12Jackson County: 9 indexJackson County9
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12.

Full ranking

RankCounties2025
index		Change on 2024
1Weld County52-2
2Denver County50-5
2Larimer County50-4
4Adams County49-6
5Arapahoe County48-1
5Boulder County48-1
5Chaffee County48-3
5Clear Creek County48-1
5Jefferson County48-2
5La Plata County48+2
5Park County48-2
12Garfield County47-2
13Douglas County46-2
13El Paso County46-3
13Gilpin County46-2
13Gunnison County46-1
13Mesa County46-2
18Montezuma County450
18Pueblo County45+1
20Grand County44-1
21San Miguel County36+17
22Archuleta County34+2
23Delta County29n/a
24Prowers County27+2
25Alamosa County25-1
26Fremont County23+10
27Routt County22+10
28Pitkin County190
29San Juan County12+3
30Jackson County9-35

Other periods

About this ranking

Median of the daily Air Quality Index (AQI) values of a US county over the days of the year with an AQI, from the EPA AirData annual summaries.

More on how it is measured
  • The daily county AQI is the highest AQI among the monitors and pollutants reported that day (ozone, PM2.5, PM10, carbon monoxide and nitrogen dioxide).
  • Lower is cleaner: 0 to 50 is good, 51 to 100 moderate and above 100 unhealthy for sensitive groups.

United States counties with air quality monitors only, about 1,000 of some 3,100 in recent years; a county without monitors has no value. The number of days with an AQI differs between counties because some monitors run only in the ozone season.

Every value in the ranking comes from the same publisher and the same period, so the counties are compared on one definition. 10 counties with data in other periods have no value for 2025 and are left out rather than filled from another source. Equal values share a rank.

How to cite

U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.

Citation styles

The publisher is the author. Invokely is named only as the place the figures were read, where a style asks for it.

APA 7

U.S. Environmental Protection Agency. (2026). EPA AirData annual AQI by county: median daily AQI (Counties in Colorado by median Air Quality Index (US EPA), 2025) [Data set]. Retrieved October 12, 2026, from https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual

MLA 9

EPA AirData annual AQI by county: median daily AQI, Counties in Colorado by median Air Quality Index (US EPA), 2025. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/median-air-quality-index/colorado-counties. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: median daily AQI. Counties in Colorado by median Air Quality Index (US EPA), 2025. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/median-air-quality-index/colorado-counties.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: median daily AQI, Counties in Colorado by median Air Quality Index (US EPA), 2025 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/median-air-quality-index/colorado-counties (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_colorado_by_2025,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: median daily AQI: Counties in Colorado by median Air Quality Index (US EPA), 2025}},
  year = {2026},
  howpublished = {Data set},
  url = {https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual},
  urldate = {2026-10-12},
  note = {Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/colorado-counties. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download\_files.html\#Annual. Retrieved 2026-10-12.},
}
RIS
TY  - DATA
AU  - U.S. Environmental Protection Agency
TI  - EPA AirData annual AQI by county: median daily AQI: Counties in Colorado by median Air Quality Index (US EPA), 2025
PY  - 2026
DA  - 2026/07/13
PB  - U.S. Environmental Protection Agency
DB  - Invokely
UR  - https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual
L2  - https://invokely.ai/rankings/median-air-quality-index/colorado-counties
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/colorado-counties.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER  -
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Rank Counties in Colorado by median Air Quality Index (US EPA) using official data, and cite the source.
rank_places {"indicator":"aqi_median","level":"province","within":"Colorado","limit":10}
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Questions

Which county has the highest median Air Quality Index (US EPA)?

Weld County, with 52 in 2025, according to the U.S. Environmental Protection Agency.

Which county has the lowest median Air Quality Index (US EPA)?

Jackson County, with 9 in 2025.

Where does this ranking come from?

Every value comes from one source and one period, so the places are comparable: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12. 10 counties have no value for 2025 and are not ranked.

Median Air Quality Index (US EPA): other rankings

Counties in Colorado: more rankings