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)

Chaffee County had the highest share of days with good air quality (US EPA AQI) of 32 counties in 2017, at 100%. Denver County had the lowest, at 40.8%. The median was 76.8%. Source: EPA.

Keep it in a collection in your library.
Highest100Chaffee County
Lowest40.8Denver County
Median76.832 counties

Map of counties, 2017

Adams County: 43.8 %Alamosa County: 99.2 %Arapahoe County: 60.5 %Archuleta County: 86.4 %Boulder County: 65.3 %Chaffee County: 100 %Clear Creek County: 55.1 %Delta County: 99.1 %Denver County: 40.8 %Douglas County: 53.4 %El Paso County: 61.1 %Fremont County: 100 %Garfield County: 65.8 %Grand County: 93.2 %Gunnison County: 75.8 %Jackson County: 98.3 %Jefferson County: 58.6 %La Plata County: 69.9 %Larimer County: 45.2 %Mesa County: 58.4 %Moffat County: 80.8 %Montezuma County: 77.8 %Montrose County: 91.3 %Park County: 64.9 %Pitkin County: 100 %Prowers County: 98.3 %Pueblo County: 90.4 %Rio Blanco County: 64.9 %Routt County: 97.4 %San Juan County: 100 %San Miguel County: 86 %Weld County: 47.4 %
  • Under 56
  • 56 to 65
  • 65 to 77
  • 77 to 91
  • 91 to 99
  • 99 and over
  • No data
%, 2017. 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, 2017

Chaffee County: 100 %Chaffee County100Fremont County: 100 %Fremont County100Pitkin County: 100 %Pitkin County100San Juan County: 100 %San Juan County100Alamosa County: 99.2 %Alamosa County99.2Delta County: 99.1 %Delta County99.1Jackson County: 98.3 %Jackson County98.3Prowers County: 98.3 %Prowers County98.3Routt County: 97.4 %Routt County97.4Grand County: 93.2 %Grand County93.2Montrose County: 91.3 %Montrose County91.3Pueblo County: 90.4 %Pueblo County90.4Archuleta County: 86.4 %Archuleta County86.4San Miguel County: 86 %San Miguel County86Moffat County: 80.8 %Moffat County80.8Montezuma County: 77.8 %Montezuma County77.8Gunnison County: 75.8 %Gunnison County75.8La Plata County: 69.9 %La Plata County69.9Garfield County: 65.8 %Garfield County65.8Boulder County: 65.3 %Boulder County65.3Park County: 64.9 %Park County64.9Rio Blanco County: 64.9 %Rio Blanco County64.9El Paso County: 61.1 %El Paso County61.1Arapahoe County: 60.5 %Arapahoe County60.5Jefferson County: 58.6 %Jefferson County58.6Mesa County: 58.4 %Mesa County58.4Clear Creek County: 55.1 %Clear Creek County55.1Douglas County: 53.4 %Douglas County53.4Weld County: 47.4 %Weld County47.4Larimer County: 45.2 %Larimer County45.2Adams County: 43.8 %Adams County43.8Denver County: 40.8 %Denver County40.8Chaffee County: 100 %Chaffee County100Fremont County: 100 %Fremont County100Pitkin County: 100 %Pitkin County100San Juan County: 100 %San Juan County100Alamosa County: 99.2 %Alamosa County99.2Delta County: 99.1 %Delta County99.1Jackson County: 98.3 %Jackson County98.3Prowers County: 98.3 %Prowers County98.3Routt County: 97.4 %Routt County97.4Grand County: 93.2 %Grand County93.2Montrose County: 91.3 %Montrose County91.3Pueblo County: 90.4 %Pueblo County90.4Archuleta County: 86.4 %Archuleta County86.4San Miguel County: 86 %San Miguel County86Moffat County: 80.8 %Moffat County80.8Montezuma County: 77.8 %Montezuma County77.8Gunnison County: 75.8 %Gunnison County75.8La Plata County: 69.9 %La Plata County69.9Garfield County: 65.8 %Garfield County65.8Boulder County: 65.3 %Boulder County65.3Park County: 64.9 %Park County64.9Rio Blanco County: 64.9 %Rio Blanco County64.9El Paso County: 61.1 %El Paso County61.1Arapahoe County: 60.5 %Arapahoe County60.5Jefferson County: 58.6 %Jefferson County58.6Mesa County: 58.4 %Mesa County58.4Clear Creek County: 55.1 %Clear Creek County55.1Douglas County: 53.4 %Douglas County53.4Weld County: 47.4 %Weld County47.4Larimer County: 45.2 %Larimer County45.2Adams County: 43.8 %Adams County43.8Denver County: 40.8 %Denver County40.8
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2017 (%)Change on 2016
1Chaffee County100+35.8 pts
1Fremont County1000 pts
1Pitkin County1000 pts
1San Juan County1000 pts
5Alamosa County99.2+0.9 pts
6Delta County99.1+0.8 pts
7Jackson County98.3-0.8 pts
7Prowers County98.3+0.3 pts
9Routt County97.40 pts
10Grand County93.2+7 pts
11Montrose County91.3-2 pts
12Pueblo County90.4-4.1 pts
13Archuleta County86.4-12.4 pts
14San Miguel County86+8.9 pts
15Moffat County80.8-10.8 pts
16Montezuma County77.8-0.3 pts
17Gunnison County75.8-10.5 pts
18La Plata County69.9+4.9 pts
19Garfield County65.8-6.1 pts
20Boulder County65.3-22 pts
21Park County64.9-8.9 pts
21Rio Blanco County64.9-9.1 pts
23El Paso County61.1-8 pts
24Arapahoe County60.5-10.8 pts
25Jefferson County58.6-8.3 pts
26Mesa County58.4-8.5 pts
27Clear Creek County55.1-12.4 pts
28Douglas County53.4-15.9 pts
29Weld County47.4-7.5 pts
30Larimer County45.2+0.9 pts
31Adams County43.8-20.7 pts
32Denver County40.8+6.1 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. 8 counties with data in other periods have no value for 2017 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), 2017) [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), 2017. 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=2017. 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), 2017. 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=2017.

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), 2017 [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=2017 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_colorado_by_2017,
  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), 2017}},
  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=2017. 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), 2017
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=2017
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/colorado-counties?period=2017.
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)?

Chaffee County, with 100% in 2017, according to EPA.

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

Denver County, with 40.8% in 2017.

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. 8 counties have no value for 2017 and are not ranked.

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

Counties in Colorado: more rankings