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

La Plata County had the highest highest daily Air Quality Index (US EPA) of 33 counties in 2018, at 243. Fremont County had the lowest, at 36. The median was 133. Source: the U.S. Environmental Protection Agency.

Highest243La Plata County

Lowest36Fremont County

Median13333 counties

Map of counties, 2018

Under 98
98 to 110
110 to 130
130 to 150
150 to 160
160 and over
No data

index, 2018. 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, 2018

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

Full ranking

Rank	Counties	2018 index	Change on 2017
1	La Plata County	243	+131
2	Alamosa County	235	+182
3	Park County	174	+62
4	Larimer County	169	+15
5	Clear Creek County	164	+24
6	Chaffee County	161	+117
7	Boulder County	159	+10
8	Arapahoe County	156	+37
8	Douglas County	156	0
8	Gunnison County	156	+16
11	Garfield County	151	+43
11	Grand County	151	+87
11	Jefferson County	151	+15
14	Denver County	150	+22
14	Mesa County	150	+49
16	Teller County	133	n/a
16	Weld County	133	-22
18	Adams County	130	-2
19	Montrose County	122	+48
20	El Paso County	119	+3
21	Montezuma County	115	+10
22	Moffat County	108	+24
22	Rio Blanco County	108	+11
24	Prowers County	103	+36
25	Pueblo County	101	-7
25	San Miguel County	101	+14
27	San Juan County	100	+51
28	Archuleta County	97	+4
29	Delta County	85	+33
30	Jackson County	64	-46
31	Pitkin County	53	+5
32	Routt County	51	-11
33	Fremont County	36	-8

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. 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: 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), 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: highest daily AQI, Counties in Colorado by highest daily Air Quality Index (US EPA), 2018. 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=2018. 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), 2018. 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=2018.

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), 2018 [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=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: highest daily AQI: Counties in Colorado by highest daily Air Quality Index (US EPA), 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/highest-daily-air-quality-index/colorado-counties?period=2018. 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), 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/highest-daily-air-quality-index/colorado-counties?period=2018
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/colorado-counties?period=2018.
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)?

La Plata County, with 243 in 2018, according to the U.S. Environmental Protection Agency.

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

Fremont County, with 36 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: highest daily AQI. Retrieved 2026-10-12. 7 counties have no value for 2018 and are not ranked.