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

Lincoln County had the highest highest daily Air Quality Index (US EPA) of 22 counties in 2002, at 157. Musselshell County had the lowest, at 9. The median was 73. Source: the U.S. Environmental Protection Agency.

Highest157Lincoln County

Lowest9Musselshell County

Median7322 counties

Map of counties, 2002

Under 52
52 to 65
65 to 84
84 to 94
94 and over
No data

index, 2002. 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, 2002

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

Full ranking

Rank	Counties	2002 index	Change on 2001
1	Lincoln County	157	+23
2	Rosebud County	137	+33
3	Lewis and Clark County	125	+1
4	Lake County	99	-57
5	Missoula County	94	-38
6	Gallatin County	93	-8
7	Flathead County	91	-48
7	Silver Bow County	91	-14
9	Ravalli County	85	-32
10	Glacier County	82	+21
11	Sanders County	76	-44
12	Cascade County	69	-27
13	Yellowstone County	67	-19
14	Big Horn County	64	n/a
15	Powell County	57	-4
15	Roosevelt County	57	+3
17	Sheridan County	54	-4
18	Stillwater County	52	+33
19	Madison County	51	+30
20	Fergus County	39	-15
21	Sweet Grass County	20	-2
22	Musselshell County	9	n/a

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. 14 counties with data in other periods have no value for 2002 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 Montana by highest daily Air Quality Index (US EPA), 2002) [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 Montana by highest daily Air Quality Index (US EPA), 2002. 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/montana-counties?period=2002. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: highest daily AQI. Counties in Montana by highest daily Air Quality Index (US EPA), 2002. 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/montana-counties?period=2002.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: highest daily AQI, Counties in Montana by highest daily Air Quality Index (US EPA), 2002 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/highest-daily-air-quality-index/montana-counties?period=2002 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_montana_by_2002,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: highest daily AQI: Counties in Montana by highest daily Air Quality Index (US EPA), 2002}},
  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/montana-counties?period=2002. 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 Montana by highest daily Air Quality Index (US EPA), 2002
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/montana-counties?period=2002
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/montana-counties?period=2002.
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":"Montana","limit":10}
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Questions

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

Lincoln County, with 157 in 2002, according to the U.S. Environmental Protection Agency.

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

Musselshell County, with 9 in 2002.

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. 14 counties have no value for 2002 and are not ranked.