Montana Air Quality Rankings
County-level AQI rollup using EPA Air Quality System monitor records. Refreshed as new EPA AQS data lands.
County-by-county air quality data for Montana based on EPA AQS annual AQI readings. Data covers 25 monitored counties for 2024.
Montana (2024): 25 counties monitored. State average median AQI: 27 (national average: 38.8). 81% of monitored days rated "Good." Most unhealthy air: Ravalli County (6 days above AQI 100).
Montana air quality on this page is drawn from the EPA Air Quality System (AQS), which aggregates daily readings from federal, state, tribal, and local monitors into annual summaries per county. The current dataset covers 25 Montana counties with at least ten days of valid AQI readings in 2024, out of the state's total monitored counties.
The state-average median AQI across those counties is 27, compared with a national average of 38.8 in the same reporting period. On average, 81% of monitored days fell into the "Good" AQI bucket (0–50). The heaviest burden of unhealthy days lands on Ravalli County (6 days above AQI 100), while Rosebud County records the cleanest air with a median AQI of 4.
AQI values translate ambient concentrations of pollutants (ozone, PM2.5, PM10, NO₂, SO₂, CO) onto a single 0–500 scale with six health-based bands. A low median AQI reflects typical daily conditions; peak values matter separately because short-term exposure drives most acute health effects. Counties without monitors are not "clean" - they are simply unmeasured by EPA AQS. For site-specific exposure guidance, check AirNow.gov alongside the county detail links below.
Which Montana counties have the worst air quality?
Ranked by median AQI (higher = worse). Click a county for full environmental data.
| # | County | Median AQI | Unhealthy Days |
|---|---|---|---|
| 1 | Lincoln | 49 | 0 |
| 2 | Cascade | 46 | 0 |
| 3 | Lewis And Clark | 46 | 1 |
| 4 | Powder River | 44 | 6 |
| 5 | Missoula | 43 | 1 |
| 6 | Flathead | 42 | 0 |
| 7 | Fergus | 40 | 1 |
| 8 | Yellowstone | 39 | 0 |
| 9 | Richland | 36 | 0 |
| 10 | Custer | 36 | 2 |
| 11 | Silver Bow | 31 | 2 |
| 12 | Ravalli | 24 | 6 |
| 13 | Glacier | 23 | 0 |
| 14 | Sanders | 21 | 0 |
| 15 | Phillips | 21 | 0 |
| 16 | Gallatin | 18 | 0 |
| 17 | Sheridan | 17 | 0 |
| 18 | Hill | 15 | 0 |
| 19 | Dawson | 14 | 0 |
| 20 | Roosevelt | 13 | 0 |
| 21 | Teton | 13 | 0 |
| 22 | Beaverhead | 12 | 2 |
| 23 | Powell | 12 | 0 |
| 24 | Lake | 9 | 0 |
| 25 | Rosebud | 4 | 0 |
Frequently Asked Questions
What is the air quality like in Montana?
In 2024, Montana had an average median AQI of 27 across 25 monitored counties, with 81% of monitored days rated "Good" (AQI 0–50). The national average median AQI is 38.8.
Which county in Montana has the worst air quality?
Based on 2024 EPA AQS data, Ravalli County had the most unhealthy air days in Montana - 6 days above AQI 100, with a median AQI of 24.
About This Data
Air quality data is from the EPA Air Quality System (AQS), Annual AQI by County dataset. Counties with fewer than 10 monitored days are excluded. "Unhealthy days" counts days with AQI above 100 (Unhealthy for Sensitive Groups or worse). All data is for 2024.
Source: U.S. Environmental Protection Agency, Air Quality System (AQS) U.S. Environmental Protection Agency, Air Quality System (AQS)
Read our methodology - how this data is sourced, computed, and verified.