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

Counties in Montana by median Air Quality Index (US EPA), 2025

Lincoln County had the highest median Air Quality Index (US EPA) of 23 counties in 2025, at 45. Rosebud County had the lowest, at 11. The median was 19. Source: the U.S. Environmental Protection Agency.

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Highest45Lincoln County
Lowest11Rosebud County
Median1923 counties

Map of counties, 2025

Beaverhead County: 13 indexCuster County: 40 indexDawson County: 18 indexFergus County: 41 indexFlathead County: 41 indexGallatin County: 11 indexGlacier County: 23 indexHill County: 14 indexLake County: 11 indexLewis and Clark County: 44 indexLincoln County: 45 indexMissoula County: 38 indexPhillips County: 19 indexPowell County: 13 indexRavalli County: 21 indexRichland County: 40 indexRoosevelt County: 16 indexRosebud County: 11 indexSanders County: 16 indexSheridan County: 18 indexSilver Bow County: 28 indexTeton County: 12 indexYellowstone County: 38 index
  • Under 13
  • 13 to 18
  • 18 to 24
  • 24 to 40
  • 40 and over
  • No data
index, 2025. 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, 2025

Lincoln County: 45 indexLincoln County45Lewis and Clark County: 44 indexLewis and Clark County44Fergus County: 41 indexFergus County41Flathead County: 41 indexFlathead County41Custer County: 40 indexCuster County40Richland County: 40 indexRichland County40Missoula County: 38 indexMissoula County38Yellowstone County: 38 indexYellowstone County38Silver Bow County: 28 indexSilver Bow County28Glacier County: 23 indexGlacier County23Ravalli County: 21 indexRavalli County21Phillips County: 19 indexPhillips County19Dawson County: 18 indexDawson County18Sheridan County: 18 indexSheridan County18Roosevelt County: 16 indexRoosevelt County16Sanders County: 16 indexSanders County16Hill County: 14 indexHill County14Beaverhead County: 13 indexBeaverhead County13Powell County: 13 indexPowell County13Teton County: 12 indexTeton County12Gallatin County: 11 indexGallatin County11Lake County: 11 indexLake County11Rosebud County: 11 indexRosebud County11Lincoln County: 45 indexLincoln County45Lewis and Clark County: 44 indexLewis and Clark County44Fergus County: 41 indexFergus County41Flathead County: 41 indexFlathead County41Custer County: 40 indexCuster County40Richland County: 40 indexRichland County40Missoula County: 38 indexMissoula County38Yellowstone County: 38 indexYellowstone County38Silver Bow County: 28 indexSilver Bow County28Glacier County: 23 indexGlacier County23Ravalli County: 21 indexRavalli County21Phillips County: 19 indexPhillips County19Dawson County: 18 indexDawson County18Sheridan County: 18 indexSheridan County18Roosevelt County: 16 indexRoosevelt County16Sanders County: 16 indexSanders County16Hill County: 14 indexHill County14Beaverhead County: 13 indexBeaverhead County13Powell County: 13 indexPowell County13Teton County: 12 indexTeton County12Gallatin County: 11 indexGallatin County11Lake County: 11 indexLake County11Rosebud County: 11 indexRosebud County11
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12.

Full ranking

RankCounties2025
index		Change on 2024
1Lincoln County45-4
2Lewis and Clark County44-2
3Fergus County41+1
3Flathead County41-1
5Custer County40+4
5Richland County40+4
7Missoula County38-5
7Yellowstone County38-1
9Silver Bow County28-3
10Glacier County230
11Ravalli County21-3
12Phillips County19-2
13Dawson County18+4
13Sheridan County18+1
15Roosevelt County16+3
15Sanders County16-5
17Hill County14-1
18Beaverhead County13+1
18Powell County13+1
20Teton County12-1
21Gallatin County11-7
21Lake County11+2
21Rosebud County11+7

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. 13 counties with data in other periods have no value for 2025 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), 2025) [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), 2025. 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. 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), 2025. 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.

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), 2025 [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 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_montana_by_2025,
  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), 2025}},
  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. 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), 2025
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
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/montana-counties.
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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Rank Counties in Montana by median Air Quality Index (US EPA) using official data, and cite the source.
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Questions

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

Lincoln County, with 45 in 2025, according to the U.S. Environmental Protection Agency.

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

Rosebud County, with 11 in 2025.

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. 13 counties have no value for 2025 and are not ranked.

Median Air Quality Index (US EPA): other rankings

Counties in Montana: more rankings