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

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

Chaffee County had the highest median Air Quality Index (US EPA) of 30 counties in 2023, at 58. San Juan County had the lowest, at 9. The median was 47. Source: the U.S. Environmental Protection Agency.

Keep it in a collection in your library.
Highest58Chaffee County
Lowest9San Juan County
Median4730 counties

Map of counties, 2023

Adams County: 51 indexAlamosa County: 22 indexArapahoe County: 47 indexArchuleta County: 35 indexBoulder County: 50 indexChaffee County: 58 indexClear Creek County: 49 indexDenver County: 54 indexDouglas County: 49 indexEl Paso County: 47 indexFremont County: 12 indexGarfield County: 54 indexGilpin County: 47 indexGrand County: 46 indexGunnison County: 46 indexJackson County: 45 indexJefferson County: 49 indexLa Plata County: 46 indexLarimer County: 51 indexMesa County: 46 indexMontezuma County: 44 indexPark County: 50 indexPitkin County: 13 indexProwers County: 19 indexPueblo County: 43 indexRio Blanco County: 47 indexRoutt County: 13 indexSan Juan County: 9 indexSan Miguel County: 47 indexWeld County: 55 index
  • Under 22
  • 22 to 46
  • 46 to 47
  • 47 to 49
  • 49 to 51
  • 51 and over
  • No data
index, 2023. 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, 2023

Chaffee County: 58 indexChaffee County58Weld County: 55 indexWeld County55Denver County: 54 indexDenver County54Garfield County: 54 indexGarfield County54Adams County: 51 indexAdams County51Larimer County: 51 indexLarimer County51Boulder County: 50 indexBoulder County50Park County: 50 indexPark County50Clear Creek County: 49 indexClear Creek County49Douglas County: 49 indexDouglas County49Jefferson County: 49 indexJefferson County49Arapahoe County: 47 indexArapahoe County47El Paso County: 47 indexEl Paso County47Gilpin County: 47 indexGilpin County47Rio Blanco County: 47 indexRio Blanco County47San Miguel County: 47 indexSan Miguel County47Grand County: 46 indexGrand County46Gunnison County: 46 indexGunnison County46La Plata County: 46 indexLa Plata County46Mesa County: 46 indexMesa County46Jackson County: 45 indexJackson County45Montezuma County: 44 indexMontezuma County44Pueblo County: 43 indexPueblo County43Archuleta County: 35 indexArchuleta County35Alamosa County: 22 indexAlamosa County22Prowers County: 19 indexProwers County19Pitkin County: 13 indexPitkin County13Routt County: 13 indexRoutt County13Fremont County: 12 indexFremont County12San Juan County: 9 indexSan Juan County9Chaffee County: 58 indexChaffee County58Weld County: 55 indexWeld County55Denver County: 54 indexDenver County54Garfield County: 54 indexGarfield County54Adams County: 51 indexAdams County51Larimer County: 51 indexLarimer County51Boulder County: 50 indexBoulder County50Park County: 50 indexPark County50Clear Creek County: 49 indexClear Creek County49Douglas County: 49 indexDouglas County49Jefferson County: 49 indexJefferson County49Arapahoe County: 47 indexArapahoe County47El Paso County: 47 indexEl Paso County47Gilpin County: 47 indexGilpin County47Rio Blanco County: 47 indexRio Blanco County47San Miguel County: 47 indexSan Miguel County47Grand County: 46 indexGrand County46Gunnison County: 46 indexGunnison County46La Plata County: 46 indexLa Plata County46Mesa County: 46 indexMesa County46Jackson County: 45 indexJackson County45Montezuma County: 44 indexMontezuma County44Pueblo County: 43 indexPueblo County43Archuleta County: 35 indexArchuleta County35Alamosa County: 22 indexAlamosa County22Prowers County: 19 indexProwers County19Pitkin County: 13 indexPitkin County13Routt County: 13 indexRoutt County13Fremont County: 12 indexFremont County12San Juan County: 9 indexSan Juan County9
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12.

Full ranking

RankCounties2023
index		Change on 2022
1Chaffee County58+9
2Weld County55+4
3Denver County54+2
3Garfield County54+5
5Adams County510
5Larimer County510
7Boulder County50+3
7Park County50+2
9Clear Creek County49+2
9Douglas County49+3
9Jefferson County49+2
12Arapahoe County47+1
12El Paso County47+1
12Gilpin County47+2
12Rio Blanco County470
12San Miguel County47+2
17Grand County46+2
17Gunnison County460
17La Plata County460
17Mesa County46+2
21Jackson County45+5
22Montezuma County440
23Pueblo County43+22
24Archuleta County35-7
25Alamosa County22+11
26Prowers County19+1
27Pitkin County13-1
27Routt County130
29Fremont County120
30San Juan County9-1

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 2023 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), 2023) [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), 2023. 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?period=2023. 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), 2023. 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?period=2023.

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), 2023 [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?period=2023 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_colorado_by_2023,
  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), 2023}},
  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?period=2023. 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), 2023
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?period=2023
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/colorado-counties?period=2023.
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)?

Chaffee County, with 58 in 2023, according to the U.S. Environmental Protection Agency.

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

San Juan County, with 9 in 2023.

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 2023 and are not ranked.

Median Air Quality Index (US EPA): other rankings

Counties in Colorado: more rankings