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

Prowers County had the highest highest daily Air Quality Index (US EPA) of 30 counties in 2014, at 247. San Juan County had the lowest, at 37. The median was 101. Source: the U.S. Environmental Protection Agency.

Highest247Prowers County

Lowest37San Juan County

Median10130 counties

Map of counties, 2014

Under 75
75 to 90
90 to 100
100 to 120
120 to 130
130 and over
No data

index, 2014. 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, 2014

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

Full ranking

Rank	Counties	2014 index	Change on 2013
1	Prowers County	247	-938
2	Denver County	152	+19
3	Clear Creek County	150	-6
4	Jefferson County	140	-29
4	Larimer County	140	-24
6	Boulder County	126	-25
6	Weld County	126	-7
8	Adams County	124	-16
8	Alamosa County	124	-22
8	Douglas County	124	-27
11	Arapahoe County	122	-28
12	Park County	115	+14
13	Pueblo County	110	+18
14	Garfield County	108	+7
15	La Plata County	101	-192
15	Mesa County	101	-16
17	Chaffee County	100	-26
18	Montezuma County	93	-35
18	San Miguel County	93	-63
20	El Paso County	90	-50
20	Gunnison County	90	-27
20	Moffat County	90	0
23	Rio Blanco County	87	-114
24	Delta County	77	+22
24	Jackson County	77	-35
26	Routt County	65	+1
27	Archuleta County	51	-120
27	Fremont County	51	-27
29	Pitkin County	44	-53
30	San Juan County	37	-61

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. 10 counties with data in other periods have no value for 2014 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), 2014) [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), 2014. 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=2014. 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), 2014. 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=2014.

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), 2014 [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=2014 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_colorado_by_2014,
  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), 2014}},
  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=2014. 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), 2014
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=2014
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/colorado-counties?period=2014.
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_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)?

Prowers County, with 247 in 2014, according to the U.S. Environmental Protection Agency.

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

San Juan County, with 37 in 2014.

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. 10 counties have no value for 2014 and are not ranked.