Data / Rankings / Median Air Quality Index (US EPA) / Counties in Montana

Counties in Montana by median Air Quality Index (US EPA)

Lincoln County had the highest median Air Quality Index (US EPA) of 22 counties in 1992, at 42. Stillwater County had the lowest, at 8. The median was 20. Source: the U.S. Environmental Protection Agency.

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Highest42Lincoln County
Lowest8Stillwater County
Median2022 counties

Map of counties, 1992

Big Horn County: 16 indexBlaine County: 17 indexBroadwater County: 18 indexCascade County: 23 indexDeer Lodge County: 11 indexFergus County: 13 indexFlathead County: 38 indexGallatin County: 32 indexJefferson County: 16 indexLake County: 28 indexLewis and Clark County: 30 indexLincoln County: 42 indexMadison County: 11 indexMissoula County: 41 indexPark County: 10 indexPhillips County: 11 indexRoosevelt County: 12 indexRosebud County: 21 indexSanders County: 26 indexSilver Bow County: 22 indexStillwater County: 8 indexYellowstone County: 23 index
  • Under 11
  • 11 to 16
  • 16 to 23
  • 23 to 30
  • 30 and over
  • No data
index, 1992. Hatched areas have no value for this period. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 1992

Lincoln County: 42 indexLincoln County42Missoula County: 41 indexMissoula County41Flathead County: 38 indexFlathead County38Gallatin County: 32 indexGallatin County32Lewis and Clark County: 30 indexLewis and Clark County30Lake County: 28 indexLake County28Sanders County: 26 indexSanders County26Cascade County: 23 indexCascade County23Yellowstone County: 23 indexYellowstone County23Silver Bow County: 22 indexSilver Bow County22Rosebud County: 21 indexRosebud County21Broadwater County: 18 indexBroadwater County18Blaine County: 17 indexBlaine County17Big Horn County: 16 indexBig Horn County16Jefferson County: 16 indexJefferson County16Fergus County: 13 indexFergus County13Roosevelt County: 12 indexRoosevelt County12Deer Lodge County: 11 indexDeer Lodge County11Madison County: 11 indexMadison County11Phillips County: 11 indexPhillips County11Park County: 10 indexPark County10Stillwater County: 8 indexStillwater County8Lincoln County: 42 indexLincoln County42Missoula County: 41 indexMissoula County41Flathead County: 38 indexFlathead County38Gallatin County: 32 indexGallatin County32Lewis and Clark County: 30 indexLewis and Clark County30Lake County: 28 indexLake County28Sanders County: 26 indexSanders County26Cascade County: 23 indexCascade County23Yellowstone County: 23 indexYellowstone County23Silver Bow County: 22 indexSilver Bow County22Rosebud County: 21 indexRosebud County21Broadwater County: 18 indexBroadwater County18Blaine County: 17 indexBlaine County17Big Horn County: 16 indexBig Horn County16Jefferson County: 16 indexJefferson County16Fergus County: 13 indexFergus County13Roosevelt County: 12 indexRoosevelt County12Deer Lodge County: 11 indexDeer Lodge County11Madison County: 11 indexMadison County11Phillips County: 11 indexPhillips County11Park County: 10 indexPark County10Stillwater County: 8 indexStillwater County8
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12.

Full ranking

RankCounties1992
index		Change on 1991
1Lincoln County42+1
2Missoula County41+1
3Flathead County38-2
4Gallatin County32+4
5Lewis and Clark County30+7
6Lake County28+3
7Sanders County26+2
8Cascade County230
8Yellowstone County23+1
10Silver Bow County22+1
11Rosebud County21-1
12Broadwater County18-5
13Blaine County17+1
14Big Horn County16+10
14Jefferson County16+4
16Fergus County13+1
17Roosevelt County12n/a
18Deer Lodge County11+1
18Madison County11-4
18Phillips County11-3
21Park County10-3
22Stillwater County8+1

Other periods

About this ranking

Median of the daily Air Quality Index (AQI) values of a US county over the days of the year with an AQI, from the EPA AirData annual summaries.

More on how it is measured
  • The daily county AQI is the highest AQI among the monitors and pollutants reported that day (ozone, PM2.5, PM10, carbon monoxide and nitrogen dioxide).
  • Lower is cleaner: 0 to 50 is good, 51 to 100 moderate and above 100 unhealthy for sensitive groups.

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 1992 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: median 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: median daily AQI (Counties in Montana by median Air Quality Index (US EPA), 1992) [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: median daily AQI, Counties in Montana by median Air Quality Index (US EPA), 1992. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/median-air-quality-index/montana-counties?period=1992. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: median daily AQI. Counties in Montana by median Air Quality Index (US EPA), 1992. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/median-air-quality-index/montana-counties?period=1992.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: median daily AQI, Counties in Montana by median Air Quality Index (US EPA), 1992 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/median-air-quality-index/montana-counties?period=1992 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_montana_by_1992,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: median daily AQI: Counties in Montana by median Air Quality Index (US EPA), 1992}},
  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/median-air-quality-index/montana-counties?period=1992. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median 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: median daily AQI: Counties in Montana by median Air Quality Index (US EPA), 1992
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/median-air-quality-index/montana-counties?period=1992
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/montana-counties?period=1992.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER  -
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Questions

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

Lincoln County, with 42 in 1992, according to the U.S. Environmental Protection Agency.

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

Stillwater County, with 8 in 1992.

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: median daily AQI. Retrieved 2026-10-12. 14 counties have no value for 1992 and are not ranked.

Median Air Quality Index (US EPA): other rankings

Counties in Montana: more rankings