Data / Share of days with good air quality (US EPA AQI) / Stillwater County

Share of days with good air quality (US EPA AQI) in Stillwater County

Share of days with good air quality (US EPA AQI) in Stillwater County was 97.5% in 2002, according to EPA. That is down 2.5 pts from 2001.

200297.5%
Change on 2001-2.5 pts
Series high1001990
Series low89.71988

Next release: Tuesday, 13 July 2027, estimated by Invokely from past publication dates (low confidence), not announced by the publisher. Release calendar

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Share of days with good air quality (US EPA AQI) in Stillwater County, 1988 to 2002

87.59092.59597.51001988199019921994199620021988: 89.7 %1989: 98 %1990: 100 %1991: 100 %1992: 100 %1993: 100 %1994: 98.9 %1995: 100 %1996: 100 %1997: 100 %2001: 100 %2002: 97.5 %85909510019881992199620021988: 89.7 %1989: 98 %1990: 100 %1991: 100 %1992: 100 %1993: 100 %1994: 98.9 %1995: 100 %1996: 100 %1997: 100 %2001: 100 %2002: 97.5 %
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12, published 2026-07-13.
All 12 values
Period%
200297.5
2001100
1997100
1996100
1995100
199498.9
1993100
1992100
1991100
1990100
198998
198889.7

Stillwater County compared with nearby places

Big Horn County: 95.3 %Cascade County: 96.5 %Fergus County: 100 %Flathead County: 83 %Gallatin County: 93.4 %Glacier County: 88.1 %Lake County: 86.6 %Lewis and Clark County: 89.4 %Lincoln County: 75.4 %Madison County: 98.4 %Missoula County: 83.6 %Musselshell County: 100 %Powell County: 99.1 %Ravalli County: 91.6 %Roosevelt County: 94.7 %Rosebud County: 86.3 %Sanders County: 82.8 %Sheridan County: 99.2 %Silver Bow County: 94.2 %Stillwater County: 97.5 %Sweet Grass County: 100 %Yellowstone County: 94 %
  • Under 86
  • 86 to 92
  • 92 to 95
  • 95 to 99
  • 99 and over
  • No data
Share of days with good air quality (US EPA AQI), 2002, %. Stillwater County is outlined. Same source and period for every area shown. Source: U.S. Environmental Protection Agency. Boundaries: U.S. Census Bureau cartographic boundary files.

Rankings

Definition

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.

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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 (Share of days with good air quality (US EPA AQI), Stillwater County, 1988 to 2002) [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, Share of days with good air quality (US EPA AQI), Stillwater County, 1988 to 2002. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/en/good-air-quality-days-share/stillwater-county-montana. 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. Share of days with good air quality (US EPA AQI), Stillwater County, 1988 to 2002. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/en/good-air-quality-days-share/stillwater-county-montana.

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, Share of days with good air quality (US EPA AQI), Stillwater County, 1988 to 2002 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/en/good-air-quality-days-share/stillwater-county-montana (Accessed: 12 October 2026).

BibTeX
@misc{environmental_share_of_days_with_2002,
  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: Share of days with good air quality (US EPA AQI), Stillwater County, 1988 to 2002}},
  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/en/good-air-quality-days-share/stillwater-county-montana. 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: Share of days with good air quality (US EPA AQI), Stillwater County, 1988 to 2002
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/en/good-air-quality-days-share/stillwater-county-montana
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/en/good-air-quality-days-share/stillwater-county-montana.
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

What is the share of days with good air quality (US EPA AQI) in Stillwater County?

Share of days with good air quality (US EPA AQI) in Stillwater County was 97.5% in 2002, according to EPA. That is down 2.5 pts from 2001.

Where does this figure come from?

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 by Invokely on 2026-10-12.

What was the highest share of days with good air quality (US EPA AQI) in Stillwater County?

100% in 1990, the highest value since 1988 in this series.

Related statistics for Stillwater County

All statistics for Stillwater County | Stillwater County in figures