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

Glacier County had the highest share of days with good air quality (US EPA AQI) of 22 counties in 1994, at 100%. Flathead County had the lowest, at 74.8%. The median was 91.1%. Source: EPA.

Highest100Glacier County

Lowest74.8Flathead County

Median91.122 counties

Map of counties, 1994

Under 87
87 to 90
90 to 92
92 to 98
98 and over
No data

%, 1994. 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, 1994

%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

Rank	Counties	1994 (%)	Change on 1993
1	Glacier County	100	n/a
1	Roosevelt County	100	0 pts
3	Stillwater County	98.9	-1.1 pts
4	Cascade County	98.1	+2.2 pts
5	Broadwater County	97.9	+1.1 pts
6	Madison County	96.6	-0.6 pts
7	Phillips County	95.6	-4.4 pts
8	Yellowstone County	93.2	+2.8 pts
9	Jefferson County	92.3	-5.4 pts
10	Park County	92	-4.7 pts
11	Ravalli County	91.5	n/a
12	Silver Bow County	90.6	+1.5 pts
13	Lewis and Clark County	90.1	+6.5 pts
14	Lake County	89.2	-4 pts
15	Rosebud County	88.8	-6 pts
16	Fergus County	88.6	-9.1 pts
17	Big Horn County	87.5	-2.7 pts
18	Gallatin County	87.2	+11.8 pts
19	Sanders County	85.7	+1.7 pts
20	Lincoln County	84.1	+15.8 pts
21	Missoula County	83.6	+10 pts
22	Flathead County	74.8	-7.4 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 1994 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), 1994) [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), 1994. 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=1994. 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), 1994. 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=1994.

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), 1994 [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=1994 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_montana_by_1994,
  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), 1994}},
  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=1994. 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), 1994
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=1994
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/montana-counties?period=1994.
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)?

Glacier County, with 100% in 1994, according to EPA.

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

Flathead County, with 74.8% in 1994.

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 1994 and are not ranked.