Counties in Colorado by highest daily Air Quality Index (US EPA)

Douglas County had the highest highest daily Air Quality Index (US EPA) of 32 counties in 2017, at 156. Fremont County had the lowest, at 44. The median was 107. Source: the U.S. Environmental Protection Agency.

Highest156Douglas County

Lowest44Fremont County

Median10732 counties

Map of counties, 2017

Under 54
54 to 85
85 to 110
110 to 140
140 and over
No data

index, 2017. Hatched areas have no value for this period. Source: EPA, EPA AirData annual AQI by county: highest daily AQI. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2017

index. Source: EPA, EPA AirData annual AQI by county: highest daily AQI. Retrieved 2026-10-12.

Full ranking

Rank	Counties	2017 index	Change on 2016
1	Douglas County	156	+5
2	Weld County	155	+26
3	Larimer County	154	+3
4	Boulder County	149	+52
5	Clear Creek County	140	-14
5	Gunnison County	140	+53
7	Jefferson County	136	-23
8	Adams County	132	+27
9	Denver County	128	+16
10	Arapahoe County	119	0
11	El Paso County	116	-10
12	La Plata County	112	-10
12	Park County	112	-21
14	Jackson County	110	+57
15	Garfield County	108	-18
15	Pueblo County	108	+44
17	Montezuma County	105	+8
18	Mesa County	101	-1
19	Rio Blanco County	97	+7
20	Archuleta County	93	+11
21	San Miguel County	87	-18
22	Moffat County	84	+17
23	Montrose County	74	-10
24	Prowers County	67	-48
25	Grand County	64	-16
26	Routt County	62	+2
27	Alamosa County	53	-218
28	Delta County	52	-12
29	San Juan County	49	+18
30	Pitkin County	48	+4
31	Chaffee County	44	-61
31	Fremont County	44	-6

Other periods

About this ranking

Highest daily Air Quality Index (AQI) value recorded in a US county during the year, from the EPA AirData annual summaries. Values above 500 are beyond the AQI scale and are published as EPA computes them.

A single day decides the value, so one wildfire or dust event can set it. Dust from the dry Owens Lake bed puts Inyo and Mono counties in California in the thousands. 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: highest 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: highest daily AQI (Counties in Colorado by highest daily Air Quality Index (US EPA), 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: highest daily AQI, Counties in Colorado by highest daily Air Quality Index (US EPA), 2017. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/colorado-counties?period=2017. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: highest daily AQI. Counties in Colorado by highest daily Air Quality Index (US EPA), 2017. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/colorado-counties?period=2017.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: highest daily AQI, Counties in Colorado by highest daily Air Quality Index (US EPA), 2017 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/highest-daily-air-quality-index/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: highest daily AQI: Counties in Colorado by highest daily Air Quality Index (US EPA), 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/highest-daily-air-quality-index/colorado-counties?period=2017. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest 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: highest daily AQI: Counties in Colorado by highest daily Air Quality Index (US EPA), 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/highest-daily-air-quality-index/colorado-counties?period=2017
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/colorado-counties?period=2017.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest 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 highest daily Air Quality Index (US EPA) using official data, and cite the source.

rank_places {"indicator":"aqi_max","level":"province","within":"Colorado","limit":10}
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Questions

Which county has the highest highest daily Air Quality Index (US EPA)?

Douglas County, with 156 in 2017, according to the U.S. Environmental Protection Agency.

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

Fremont County, with 44 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: highest daily AQI. Retrieved 2026-10-12. 8 counties have no value for 2017 and are not ranked.