Data / Rankings / Share of days with good air quality (US EPA AQI) / Counties in Montana

Counties in Montana by share of days with good air quality (US EPA AQI)

Fergus County had the highest share of days with good air quality (US EPA AQI) of 22 counties in 1996, at 100%. Flathead County had the lowest, at 80.3%. The median was 96.9%. Source: EPA.

Keep it in a collection in your library.
Highest100Fergus County
Lowest80.3Flathead County
Median96.922 counties

Map of counties, 1996

Big Horn County: 81 %Broadwater County: 97.5 %Cascade County: 98.8 %Fergus County: 100 %Flathead County: 80.3 %Gallatin County: 93.5 %Glacier County: 97.6 %Jefferson County: 98 %Lake County: 95.3 %Lewis and Clark County: 96.6 %Lincoln County: 89.3 %Madison County: 100 %Missoula County: 91.3 %Park County: 98.9 %Phillips County: 100 %Ravalli County: 97.1 %Roosevelt County: 98.1 %Rosebud County: 96.2 %Sanders County: 90 %Silver Bow County: 96.6 %Stillwater County: 100 %Yellowstone County: 94.3 %
  • Under 92
  • 92 to 96
  • 96 to 98
  • 98 to 99
  • 99 and over
  • No data
%, 1996. Hatched areas have no value for this period. Source: Invokely calculation with data from EPA. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 1996

Fergus County: 100 %Fergus County100Madison County: 100 %Madison County100Phillips County: 100 %Phillips County100Stillwater County: 100 %Stillwater County100Park County: 98.9 %Park County98.9Cascade County: 98.8 %Cascade County98.8Roosevelt County: 98.1 %Roosevelt County98.1Jefferson County: 98 %Jefferson County98Glacier County: 97.6 %Glacier County97.6Broadwater County: 97.5 %Broadwater County97.5Ravalli County: 97.1 %Ravalli County97.1Lewis and Clark County: 96.6 %Lewis and Clark County96.6Silver Bow County: 96.6 %Silver Bow County96.6Rosebud County: 96.2 %Rosebud County96.2Lake County: 95.3 %Lake County95.3Yellowstone County: 94.3 %Yellowstone County94.3Gallatin County: 93.5 %Gallatin County93.5Missoula County: 91.3 %Missoula County91.3Sanders County: 90 %Sanders County90Lincoln County: 89.3 %Lincoln County89.3Big Horn County: 81 %Big Horn County81Flathead County: 80.3 %Flathead County80.3Fergus County: 100 %Fergus County100Madison County: 100 %Madison County100Phillips County: 100 %Phillips County100Stillwater County: 100 %Stillwater County100Park County: 98.9 %Park County98.9Cascade County: 98.8 %Cascade County98.8Roosevelt County: 98.1 %Roosevelt County98.1Jefferson County: 98 %Jefferson County98Glacier County: 97.6 %Glacier County97.6Broadwater County: 97.5 %Broadwater County97.5Ravalli County: 97.1 %Ravalli County97.1Lewis and Clark County: 96.6 %Lewis and Clark County96.6Silver Bow County: 96.6 %Silver Bow County96.6Rosebud County: 96.2 %Rosebud County96.2Lake County: 95.3 %Lake County95.3Yellowstone County: 94.3 %Yellowstone County94.3Gallatin County: 93.5 %Gallatin County93.5Missoula County: 91.3 %Missoula County91.3Sanders County: 90 %Sanders County90Lincoln County: 89.3 %Lincoln County89.3Big Horn County: 81 %Big Horn County81Flathead County: 80.3 %Flathead County80.3
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties1996 (%)Change on 1995
1Fergus County1000 pts
1Madison County1000 pts
1Phillips County1000 pts
1Stillwater County1000 pts
5Park County98.9+1.2 pts
6Cascade County98.8+2 pts
7Roosevelt County98.1-1.9 pts
8Jefferson County98+1.3 pts
9Glacier County97.6-2.4 pts
10Broadwater County97.5-1.2 pts
11Ravalli County97.1-0.5 pts
12Lewis and Clark County96.6+2.5 pts
12Silver Bow County96.6+0.5 pts
14Rosebud County96.2+2.8 pts
15Lake County95.3+1.9 pts
16Yellowstone County94.3-1.3 pts
17Gallatin County93.5+7.4 pts
18Missoula County91.3+2 pts
19Sanders County90-0.7 pts
20Lincoln County89.3-2.1 pts
21Big Horn County81-12.3 pts
22Flathead County80.3-3.8 pts

Other periods

About this ranking

Days with a daily Air Quality Index of 0 to 50 (good) as a percentage of the days of the year with an AQI in a US county. Calculated by Invokely from the day counts in the EPA AirData annual summaries; both counts are stored with each value.

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 1996 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: good days as a share of days with an AQI, calculated with stored counts. 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: good days as a share of days with an AQI, calculated with stored counts (Counties in Montana by share of days with good air quality (US EPA AQI), 1996) [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: good days as a share of days with an AQI, calculated with stored counts, Counties in Montana by share of days with good air quality (US EPA AQI), 1996. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/good-air-quality-days-share/montana-counties?period=1996. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. Counties in Montana by share of days with good air quality (US EPA AQI), 1996. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/good-air-quality-days-share/montana-counties?period=1996.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts, Counties in Montana by share of days with good air quality (US EPA AQI), 1996 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/good-air-quality-days-share/montana-counties?period=1996 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_montana_by_1996,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts: Counties in Montana by share of days with good air quality (US EPA AQI), 1996}},
  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/good-air-quality-days-share/montana-counties?period=1996. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. 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: good days as a share of days with an AQI, calculated with stored counts: Counties in Montana by share of days with good air quality (US EPA AQI), 1996
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/good-air-quality-days-share/montana-counties?period=1996
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/montana-counties?period=1996.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. 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 share of days with good air quality (US EPA AQI)?

Fergus County, with 100% in 1996, according to EPA.

Which county has the lowest share of days with good air quality (US EPA AQI)?

Flathead County, with 80.3% in 1996.

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: good days as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12. 14 counties have no value for 1996 and are not ranked.

Share of days with good air quality (US EPA AQI): other rankings

Counties in Montana: more rankings