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

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

Denver County had the highest median Air Quality Index (US EPA) of 31 counties in 2015, at 54. San Juan County had the lowest, at 8. The median was 44. Source: the U.S. Environmental Protection Agency.

Keep it in a collection in your library.
Highest54Denver County
Lowest8San Juan County
Median4431 counties

Map of counties, 2015

Adams County: 45 indexAlamosa County: 18 indexArapahoe County: 45 indexArchuleta County: 17 indexBoulder County: 44 indexChaffee County: 47 indexClear Creek County: 48 indexDelta County: 16 indexDenver County: 54 indexDouglas County: 44 indexEl Paso County: 44 indexFremont County: 11 indexGarfield County: 45 indexGrand County: 38 indexGunnison County: 44 indexJackson County: 38 indexJefferson County: 45 indexLa Plata County: 43 indexLarimer County: 51 indexMesa County: 48 indexMoffat County: 36 indexMontezuma County: 44 indexPark County: 45 indexPitkin County: 12 indexProwers County: 14 indexPueblo County: 25 indexRio Blanco County: 46 indexRoutt County: 13 indexSan Juan County: 8 indexSan Miguel County: 44 indexWeld County: 47 index
  • Under 16
  • 16 to 38
  • 38 to 44
  • 44 to 45
  • 45 to 47
  • 47 and over
  • No data
index, 2015. 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, 2015

Denver County: 54 indexDenver County54Larimer County: 51 indexLarimer County51Clear Creek County: 48 indexClear Creek County48Mesa County: 48 indexMesa County48Chaffee County: 47 indexChaffee County47Weld County: 47 indexWeld County47Rio Blanco County: 46 indexRio Blanco County46Adams County: 45 indexAdams County45Arapahoe County: 45 indexArapahoe County45Garfield County: 45 indexGarfield County45Jefferson County: 45 indexJefferson County45Park County: 45 indexPark County45Boulder County: 44 indexBoulder County44Douglas County: 44 indexDouglas County44El Paso County: 44 indexEl Paso County44Gunnison County: 44 indexGunnison County44Montezuma County: 44 indexMontezuma County44San Miguel County: 44 indexSan Miguel County44La Plata County: 43 indexLa Plata County43Grand County: 38 indexGrand County38Jackson County: 38 indexJackson County38Moffat County: 36 indexMoffat County36Pueblo County: 25 indexPueblo County25Alamosa County: 18 indexAlamosa County18Archuleta County: 17 indexArchuleta County17Delta County: 16 indexDelta County16Prowers County: 14 indexProwers County14Routt County: 13 indexRoutt County13Pitkin County: 12 indexPitkin County12Fremont County: 11 indexFremont County11San Juan County: 8 indexSan Juan County8Denver County: 54 indexDenver County54Larimer County: 51 indexLarimer County51Clear Creek County: 48 indexClear Creek County48Mesa County: 48 indexMesa County48Chaffee County: 47 indexChaffee County47Weld County: 47 indexWeld County47Rio Blanco County: 46 indexRio Blanco County46Adams County: 45 indexAdams County45Arapahoe County: 45 indexArapahoe County45Garfield County: 45 indexGarfield County45Jefferson County: 45 indexJefferson County45Park County: 45 indexPark County45Boulder County: 44 indexBoulder County44Douglas County: 44 indexDouglas County44El Paso County: 44 indexEl Paso County44Gunnison County: 44 indexGunnison County44Montezuma County: 44 indexMontezuma County44San Miguel County: 44 indexSan Miguel County44La Plata County: 43 indexLa Plata County43Grand County: 38 indexGrand County38Jackson County: 38 indexJackson County38Moffat County: 36 indexMoffat County36Pueblo County: 25 indexPueblo County25Alamosa County: 18 indexAlamosa County18Archuleta County: 17 indexArchuleta County17Delta County: 16 indexDelta County16Prowers County: 14 indexProwers County14Routt County: 13 indexRoutt County13Pitkin County: 12 indexPitkin County12Fremont County: 11 indexFremont County11San Juan County: 8 indexSan Juan County8
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12.

Full ranking

RankCounties2015
index		Change on 2014
1Denver County54+1
2Larimer County51+1
3Clear Creek County48+1
3Mesa County480
5Chaffee County470
5Weld County47+1
7Rio Blanco County46+1
8Adams County45-3
8Arapahoe County450
8Garfield County45+1
8Jefferson County45-1
8Park County45-2
13Boulder County44-1
13Douglas County440
13El Paso County44-1
13Gunnison County440
13Montezuma County44-1
13San Miguel County44-1
19La Plata County43-1
20Grand County38n/a
20Jackson County38-2
22Moffat County36-7
23Pueblo County25-3
24Alamosa County18-1
25Archuleta County170
26Delta County16-5
27Prowers County14-5
28Routt County13-2
29Pitkin County120
30Fremont County11-4
31San Juan County8-2

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. 9 counties with data in other periods have no value for 2015 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), 2015) [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), 2015. 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=2015. 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), 2015. 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=2015.

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), 2015 [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=2015 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_colorado_by_2015,
  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), 2015}},
  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=2015. 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), 2015
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=2015
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/colorado-counties?period=2015.
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)?

Denver County, with 54 in 2015, according to the U.S. Environmental Protection Agency.

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

San Juan County, with 8 in 2015.

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. 9 counties have no value for 2015 and are not ranked.

Median Air Quality Index (US EPA): other rankings

Counties in Colorado: more rankings