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 33 counties in 2018, at 10.1%. San Juan County had the lowest, at 0%. The median was 1.2%. Source: EPA.

Highest10.1Jefferson County

Lowest0San Juan County

Median1.233 counties

Map of counties, 2018

Under 0.4
0.4 to 1.2
1.2 to 3
3 to 6.8
6.8 and over
No data

%, 2018. 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, 2018

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

Full ranking

Rank	Counties	2018 (%)	Change on 2017
1	Jefferson County	10.1	+2.4 pts
2	Clear Creek County	9.6	+8.3 pts
3	Boulder County	8	+4.9 pts
4	Larimer County	7.7	+4.1 pts
5	Teller County	7.5	n/a
6	Douglas County	7.4	+2.7 pts
7	Park County	5.6	+4.5 pts
8	La Plata County	5.5	+4.4 pts
9	Garfield County	4.4	+4.1 pts
10	Arapahoe County	4.1	+2.2 pts
11	Chaffee County	3.7	+3.7 pts
12	Denver County	2.7	+2.2 pts
12	Weld County	2.7	+0.5 pts
14	El Paso County	2.2	+1.4 pts
15	Gunnison County	1.4	+1.1 pts
15	Mesa County	1.4	+1.1 pts
17	Grand County	1.2	+1.2 pts
18	Montezuma County	1.1	+0.8 pts
19	Adams County	0.8	+0.3 pts
19	Pueblo County	0.8	-0.8 pts
19	San Miguel County	0.8	+0.8 pts
22	Rio Blanco County	0.5	+0.5 pts
23	Alamosa County	0.3	+0.3 pts
23	Moffat County	0.3	+0.3 pts
23	Montrose County	0.3	+0.3 pts
23	Prowers County	0.3	+0.3 pts
27	Archuleta County	0	0 pts
27	Delta County	0	0 pts
27	Fremont County	0	0 pts
27	Jackson County	0	-0.9 pts
27	Pitkin County	0	0 pts
27	Routt County	0	0 pts
27	San Juan County	0	0 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. 7 counties with data in other periods have no value for 2018 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), 2018) [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), 2018. 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=2018. 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), 2018. 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=2018.

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), 2018 [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=2018 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_colorado_by_2018,
  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), 2018}},
  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=2018. 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), 2018
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=2018
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/colorado-counties?period=2018.
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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Rank Counties in Colorado by share of days unhealthy for sensitive groups or worse (US EPA AQI) using official data, and cite the source.

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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 10.1% in 2018, according to EPA.

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

San Juan County, with 0% in 2018.

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. 7 counties have no value for 2018 and are not ranked.