Counties in Montana by highest daily Air Quality Index (US EPA)

Custer County had the highest highest daily Air Quality Index (US EPA) of 23 counties in 2023, at 260. Gallatin County had the lowest, at 98. The median was 165. Source: the U.S. Environmental Protection Agency.

Highest260Custer County

Lowest98Gallatin County

Median16523 counties

Map of counties, 2023

Under 130
130 to 160
160 to 170
170 to 190
190 and over
No data

index, 2023. Hatched areas have no value for this period. Source: EPA, EPA AirData annual AQI by county: highest daily AQI. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2023

index. Source: EPA, EPA AirData annual AQI by county: highest daily AQI. Retrieved 2026-10-12.

Full ranking

Rank	Counties	2023 index	Change on 2022
1	Custer County	260	+180
2	Richland County	219	+140
3	Hill County	215	n/a
4	Phillips County	199	+110
5	Powder River County	195	+87
6	Yellowstone County	191	+36
7	Lincoln County	189	+15
8	Fergus County	185	+63
9	Sanders County	181	-7
10	Cascade County	170	+42
11	Glacier County	166	n/a
12	Lewis and Clark County	165	-2
13	Roosevelt County	161	+97
14	Flathead County	160	+8
14	Sheridan County	160	+94
16	Lake County	157	+37
17	Missoula County	134	-56
18	Rosebud County	132	+96
19	Beaverhead County	124	n/a
19	Silver Bow County	124	-38
21	Powell County	115	+11
22	Ravalli County	110	-153
23	Gallatin County	98	-50

Other periods

About this ranking

Highest daily Air Quality Index (AQI) value recorded in a US county during the year, from the EPA AirData annual summaries. Values above 500 are beyond the AQI scale and are published as EPA computes them.

A single day decides the value, so one wildfire or dust event can set it. Dust from the dry Owens Lake bed puts Inyo and Mono counties in California in the thousands. 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 2023 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: highest 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: highest daily AQI (Counties in Montana by highest daily Air Quality Index (US EPA), 2023) [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: highest daily AQI, Counties in Montana by highest daily Air Quality Index (US EPA), 2023. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/montana-counties?period=2023. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: highest daily AQI. Counties in Montana by highest daily Air Quality Index (US EPA), 2023. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/montana-counties?period=2023.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: highest daily AQI, Counties in Montana by highest daily Air Quality Index (US EPA), 2023 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/highest-daily-air-quality-index/montana-counties?period=2023 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_montana_by_2023,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: highest daily AQI: Counties in Montana by highest daily Air Quality Index (US EPA), 2023}},
  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/highest-daily-air-quality-index/montana-counties?period=2023. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest 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: highest daily AQI: Counties in Montana by highest daily Air Quality Index (US EPA), 2023
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/highest-daily-air-quality-index/montana-counties?period=2023
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/montana-counties?period=2023.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER  -

Embed this chart

Paste this code into a web page or a blog post. The chart shows the latest figures as they are published, with the source and a link back to this page.

Embed code

<iframe src="https://invokely.ai/embed/rankings/highest-daily-air-quality-index/montana-counties?period=2023" width="760" height="500" title="Counties in Montana by highest daily Air Quality Index (US EPA), 2023, chart from Invokely" loading="lazy" style="border:0;width:100%;max-width:760px" referrerpolicy="strict-origin-when-cross-origin"></iframe>

Sites that read oEmbed links, such as WordPress, can show the chart when you paste the link to this page.

Use this data in your AI agent

Invokely serves this figure to AI assistants through MCP and to code through a REST API, with the source and citation attached. Ask your assistant:

Rank Counties in Montana by highest daily Air Quality Index (US EPA) using official data, and cite the source.

rank_places {"indicator":"aqi_max","level":"province","within":"Montana","limit":10}
GET https://invokely.ai/api/v1/rank?indicator=aqi_max&level=province&within=Montana&limit=10

Connect Claude, ChatGPT or Cursor in one step. The trial answers 25 calls a day without an account.

Questions

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

Custer County, with 260 in 2023, according to the U.S. Environmental Protection Agency.

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

Gallatin County, with 98 in 2023.

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: highest daily AQI. Retrieved 2026-10-12. 13 counties have no value for 2023 and are not ranked.