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

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

Flathead County had the highest median Air Quality Index (US EPA) of 22 counties in 1997, at 36. Park County had the lowest, at 6. The median was 18. Source: the U.S. Environmental Protection Agency.

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Highest36Flathead County
Lowest6Park County
Median1822 counties

Map of counties, 1997

Big Horn County: 21 indexBroadwater County: 12 indexCascade County: 16 indexFergus County: 9 indexFlathead County: 36 indexGallatin County: 25 indexGlacier County: 31 indexJefferson County: 16 indexLake County: 18 indexLewis and Clark County: 18 indexLincoln County: 25 indexMadison County: 8 indexMissoula County: 28 indexPark County: 6 indexPhillips County: 8 indexRavalli County: 16 indexRoosevelt County: 17 indexRosebud County: 20 indexSanders County: 18 indexSilver Bow County: 19 indexStillwater County: 8 indexYellowstone County: 20 index
  • Under 10
  • 10 to 16
  • 16 to 19
  • 19 to 24
  • 24 and over
  • No data
index, 1997. 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, 1997

Flathead County: 36 indexFlathead County36Glacier County: 31 indexGlacier County31Missoula County: 28 indexMissoula County28Gallatin County: 25 indexGallatin County25Lincoln County: 25 indexLincoln County25Big Horn County: 21 indexBig Horn County21Rosebud County: 20 indexRosebud County20Yellowstone County: 20 indexYellowstone County20Silver Bow County: 19 indexSilver Bow County19Lake County: 18 indexLake County18Lewis and Clark County: 18 indexLewis and Clark County18Sanders County: 18 indexSanders County18Roosevelt County: 17 indexRoosevelt County17Cascade County: 16 indexCascade County16Jefferson County: 16 indexJefferson County16Ravalli County: 16 indexRavalli County16Broadwater County: 12 indexBroadwater County12Fergus County: 9 indexFergus County9Madison County: 8 indexMadison County8Phillips County: 8 indexPhillips County8Stillwater County: 8 indexStillwater County8Park County: 6 indexPark County6Flathead County: 36 indexFlathead County36Glacier County: 31 indexGlacier County31Missoula County: 28 indexMissoula County28Gallatin County: 25 indexGallatin County25Lincoln County: 25 indexLincoln County25Big Horn County: 21 indexBig Horn County21Rosebud County: 20 indexRosebud County20Yellowstone County: 20 indexYellowstone County20Silver Bow County: 19 indexSilver Bow County19Lake County: 18 indexLake County18Lewis and Clark County: 18 indexLewis and Clark County18Sanders County: 18 indexSanders County18Roosevelt County: 17 indexRoosevelt County17Cascade County: 16 indexCascade County16Jefferson County: 16 indexJefferson County16Ravalli County: 16 indexRavalli County16Broadwater County: 12 indexBroadwater County12Fergus County: 9 indexFergus County9Madison County: 8 indexMadison County8Phillips County: 8 indexPhillips County8Stillwater County: 8 indexStillwater County8Park County: 6 indexPark County6
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12.

Full ranking

RankCounties1997
index		Change on 1996
1Flathead County36-5
2Glacier County31+2
3Missoula County28-2
4Gallatin County25-4
4Lincoln County25+1
6Big Horn County210
7Rosebud County20+4
7Yellowstone County200
9Silver Bow County190
10Lake County180
10Lewis and Clark County180
10Sanders County18-1
13Roosevelt County170
14Cascade County16-2
14Jefferson County16-1
14Ravalli County16-1
17Broadwater County12-2
18Fergus County9+2
19Madison County8-3
19Phillips County8+1
19Stillwater County8-1
22Park County6-5

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 1997 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), 1997) [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), 1997. 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=1997. 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), 1997. 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=1997.

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), 1997 [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=1997 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_montana_by_1997,
  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), 1997}},
  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=1997. 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), 1997
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=1997
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/montana-counties?period=1997.
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)?

Flathead County, with 36 in 1997, according to the U.S. Environmental Protection Agency.

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

Park County, with 6 in 1997.

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 1997 and are not ranked.

Median Air Quality Index (US EPA): other rankings

Counties in Montana: more rankings