Data / Rankings / Share of days with good air quality (US EPA AQI) / Counties in Colorado

Counties in Colorado by share of days with good air quality (US EPA AQI)

Fremont County had the highest share of days with good air quality (US EPA AQI) of 31 counties in 2020, at 100%. Denver County had the lowest, at 37.4%. The median was 72.1%. Source: EPA.

Keep it in a collection in your library.
Highest100Fremont County
Lowest37.4Denver County
Median72.131 counties

Map of counties, 2020

Adams County: 52.5 %Alamosa County: 94.1 %Arapahoe County: 66.4 %Archuleta County: 82.7 %Boulder County: 47.3 %Chaffee County: 53.6 %Clear Creek County: 58.6 %Delta County: 85.5 %Denver County: 37.4 %Douglas County: 61.2 %El Paso County: 65.8 %Fremont County: 100 %Garfield County: 63.4 %Gilpin County: 77.9 %Grand County: 76.6 %Gunnison County: 80.6 %Jackson County: 90.1 %Jefferson County: 63.1 %La Plata County: 74.6 %Larimer County: 53.8 %Mesa County: 71.6 %Montezuma County: 82.2 %Park County: 54.5 %Pitkin County: 96.7 %Prowers County: 93.4 %Pueblo County: 87.5 %Rio Blanco County: 72.1 %Routt County: 98.6 %San Juan County: 96.6 %San Miguel County: 62.8 %Weld County: 45.4 %
  • Under 54
  • 54 to 63
  • 63 to 72
  • 72 to 82
  • 82 to 93
  • 93 and over
  • No data
%, 2020. Hatched areas have no value for this period. Source: Invokely calculation with data from EPA. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2020

Fremont County: 100 %Fremont County100Routt County: 98.6 %Routt County98.6Pitkin County: 96.7 %Pitkin County96.7San Juan County: 96.6 %San Juan County96.6Alamosa County: 94.1 %Alamosa County94.1Prowers County: 93.4 %Prowers County93.4Jackson County: 90.1 %Jackson County90.1Pueblo County: 87.5 %Pueblo County87.5Delta County: 85.5 %Delta County85.5Archuleta County: 82.7 %Archuleta County82.7Montezuma County: 82.2 %Montezuma County82.2Gunnison County: 80.6 %Gunnison County80.6Gilpin County: 77.9 %Gilpin County77.9Grand County: 76.6 %Grand County76.6La Plata County: 74.6 %La Plata County74.6Rio Blanco County: 72.1 %Rio Blanco County72.1Mesa County: 71.6 %Mesa County71.6Arapahoe County: 66.4 %Arapahoe County66.4El Paso County: 65.8 %El Paso County65.8Garfield County: 63.4 %Garfield County63.4Jefferson County: 63.1 %Jefferson County63.1San Miguel County: 62.8 %San Miguel County62.8Douglas County: 61.2 %Douglas County61.2Clear Creek County: 58.6 %Clear Creek County58.6Park County: 54.5 %Park County54.5Larimer County: 53.8 %Larimer County53.8Chaffee County: 53.6 %Chaffee County53.6Adams County: 52.5 %Adams County52.5Boulder County: 47.3 %Boulder County47.3Weld County: 45.4 %Weld County45.4Denver County: 37.4 %Denver County37.4Fremont County: 100 %Fremont County100Routt County: 98.6 %Routt County98.6Pitkin County: 96.7 %Pitkin County96.7San Juan County: 96.6 %San Juan County96.6Alamosa County: 94.1 %Alamosa County94.1Prowers County: 93.4 %Prowers County93.4Jackson County: 90.1 %Jackson County90.1Pueblo County: 87.5 %Pueblo County87.5Delta County: 85.5 %Delta County85.5Archuleta County: 82.7 %Archuleta County82.7Montezuma County: 82.2 %Montezuma County82.2Gunnison County: 80.6 %Gunnison County80.6Gilpin County: 77.9 %Gilpin County77.9Grand County: 76.6 %Grand County76.6La Plata County: 74.6 %La Plata County74.6Rio Blanco County: 72.1 %Rio Blanco County72.1Mesa County: 71.6 %Mesa County71.6Arapahoe County: 66.4 %Arapahoe County66.4El Paso County: 65.8 %El Paso County65.8Garfield County: 63.4 %Garfield County63.4Jefferson County: 63.1 %Jefferson County63.1San Miguel County: 62.8 %San Miguel County62.8Douglas County: 61.2 %Douglas County61.2Clear Creek County: 58.6 %Clear Creek County58.6Park County: 54.5 %Park County54.5Larimer County: 53.8 %Larimer County53.8Chaffee County: 53.6 %Chaffee County53.6Adams County: 52.5 %Adams County52.5Boulder County: 47.3 %Boulder County47.3Weld County: 45.4 %Weld County45.4Denver County: 37.4 %Denver County37.4
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2020 (%)Change on 2019
1Fremont County1000 pts
2Routt County98.6-0.6 pts
3Pitkin County96.7-3.3 pts
4San Juan County96.6-3.4 pts
5Alamosa County94.1-5.9 pts
6Prowers County93.4-5.7 pts
7Jackson County90.1-9.9 pts
8Pueblo County87.5-11.7 pts
9Delta County85.5-7.1 pts
10Archuleta County82.7-8.5 pts
11Montezuma County82.2+5.8 pts
12Gunnison County80.6+7.7 pts
13Gilpin County77.9-3.9 pts
14Grand County76.6-3.9 pts
15La Plata County74.6+1.4 pts
16Rio Blanco County72.1+3.1 pts
17Mesa County71.6+6.2 pts
18Arapahoe County66.4+5.6 pts
19El Paso County65.8-2.4 pts
20Garfield County63.4-4.5 pts
21Jefferson County63.1-2.1 pts
22San Miguel County62.8-15.6 pts
23Douglas County61.2+0.3 pts
24Clear Creek County58.6-3.6 pts
25Park County54.5+3.2 pts
26Larimer County53.8+3.7 pts
27Chaffee County53.6-2 pts
28Adams County52.5+2.4 pts
29Boulder County47.3-5.3 pts
30Weld County45.4+3.8 pts
31Denver County37.4-0.7 pts

Other periods

About this ranking

Days with a daily Air Quality Index of 0 to 50 (good) as a percentage of the days of the year with an AQI in a US county. Calculated by Invokely from the day counts in the EPA AirData annual summaries; both counts are stored with each value.

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 2020 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: good days as a share of days with an AQI, calculated with stored counts. 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: good days as a share of days with an AQI, calculated with stored counts (Counties in Colorado by share of days with good air quality (US EPA AQI), 2020) [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: good days as a share of days with an AQI, calculated with stored counts, Counties in Colorado by share of days with good air quality (US EPA AQI), 2020. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2020. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. Counties in Colorado by share of days with good air quality (US EPA AQI), 2020. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2020.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts, Counties in Colorado by share of days with good air quality (US EPA AQI), 2020 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2020 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_colorado_by_2020,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts: Counties in Colorado by share of days with good air quality (US EPA AQI), 2020}},
  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/good-air-quality-days-share/colorado-counties?period=2020. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. 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: good days as a share of days with an AQI, calculated with stored counts: Counties in Colorado by share of days with good air quality (US EPA AQI), 2020
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/good-air-quality-days-share/colorado-counties?period=2020
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2020.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER  -
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Questions

Which county has the highest share of days with good air quality (US EPA AQI)?

Fremont County, with 100% in 2020, according to EPA.

Which county has the lowest share of days with good air quality (US EPA AQI)?

Denver County, with 37.4% in 2020.

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: good days as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12. 9 counties have no value for 2020 and are not ranked.

Share of days with good air quality (US EPA AQI): other rankings

Counties in Colorado: more rankings