Counties in Colorado by share of days unhealthy for sensitive groups or worse (US EPA AQI)

Jefferson County had the highest share of days unhealthy for sensitive groups or worse (US EPA AQI) of 31 counties in 2021, at 15.1%. San Miguel County had the lowest, at 0%. The median was 1.4%. Source: EPA.

Highest15.1Jefferson County

Lowest0San Miguel County

Median1.431 counties

Map of counties, 2021

Under 0.5
0.5 to 1.4
1.4 to 5.4
5.4 to 9.6
9.6 and over
No data

%, 2021. 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, 2021

%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

Rank	Counties	2021 (%)	Change on 2020
1	Jefferson County	15.1	+7.7 pts
2	Larimer County	13.7	+7.7 pts
3	Douglas County	11.7	+7 pts
4	Weld County	11	+7.4 pts
5	Arapahoe County	10.1	+7.1 pts
6	Boulder County	9.6	+3.9 pts
6	Denver County	9.6	+5 pts
8	El Paso County	7.7	+5.8 pts
9	Clear Creek County	6.7	+1.7 pts
10	Adams County	6.3	+3 pts
11	Chaffee County	5.4	+1.1 pts
12	Gilpin County	5.2	+2.7 pts
13	Park County	5	+2.1 pts
14	Grand County	4.8	+3.7 pts
15	Garfield County	3	+1.6 pts
16	Rio Blanco County	1.4	+0.6 pts
17	La Plata County	1.1	+0.6 pts
18	Alamosa County	0.8	+0.8 pts
19	Prowers County	0.6	-0.5 pts
20	Delta County	0.5	+0.5 pts
20	Mesa County	0.5	-0.3 pts
20	Montezuma County	0.5	0 pts
20	Pueblo County	0.5	+0.5 pts
24	Gunnison County	0.3	-0.6 pts
25	Archuleta County	0	0 pts
25	Fremont County	0	0 pts
25	Jackson County	0	0 pts
25	Pitkin County	0	0 pts
25	Routt County	0	0 pts
25	San Juan County	0	0 pts
25	San Miguel County	0	-0.8 pts

Other periods

About this ranking

Days with a daily Air Quality Index above 100 (unhealthy for sensitive groups, unhealthy, very unhealthy or hazardous) 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; the counts are stored with each value.

Use the share rather than the number of days to compare counties, since the days with an AQI differ. 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 2021 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: days with an AQI above 100 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts (Counties in Colorado by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2021) [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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts, Counties in Colorado by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2021. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/colorado-counties?period=2021. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. Counties in Colorado by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2021. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/colorado-counties?period=2021.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts, Counties in Colorado by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2021 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/unhealthy-air-quality-days-share/colorado-counties?period=2021 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_colorado_by_2021,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts: Counties in Colorado by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2021}},
  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/unhealthy-air-quality-days-share/colorado-counties?period=2021. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts: Counties in Colorado by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2021
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/unhealthy-air-quality-days-share/colorado-counties?period=2021
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/colorado-counties?period=2021.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 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 unhealthy for sensitive groups or worse (US EPA AQI)?

Jefferson County, with 15.1% in 2021, according to EPA.

Which county has the lowest share of days unhealthy for sensitive groups or worse (US EPA AQI)?

San Miguel County, with 0% in 2021.

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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12. 9 counties have no value for 2021 and are not ranked.