Bad Air Days in Yellowstone County, Montana: 3 a Year
3 bad air days a year
About the middle of monitored US counties. Ranked 356 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly particles: smoke and dust (97% particle days).
Which months to watch
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in July and August and September. Averaged over 2016 to 2025.
What actually helps here
- Particles (wildfire smoke, dust) cause enough bad days here to matter. A HEPA purifier sized to your bedroom removes them; run it with windows shut on bad days.
The record for Yellowstone
In 2000 to 2009 this county averaged under 1 bad days a year; in 2016 to 2025, 3. It has got worse. The worst recent year was 2021, with 10 bad days on 340 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-09-05 | 191 | PM2.5 |
| 2021-08-15 | 167 | PM2.5 |
| 2020-07-04 | 163 | PM2.5 |
| 2019-07-04 | 159 | PM2.5 |
| 2018-08-19 | 155 | PM2.5 |
An ordinary day in Yellowstone
Across 3,464 monitored days from 2016 to 2025, the air in Yellowstone County rated Good on 80% of them, more often than the 72% typical of monitored counties, and Moderate on 19%. The median day sat at AQI 34; one day in ten went above 57.
The pollutant setting the AQI on an ordinary day here:
- fine particles (PM2.5)87%3,015 days
- ozone13%433 days
Every recent year on file
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 365 | 315 | 50 | 0 | 0 | 38 | 91 |
| 2024 | 357 | 272 | 84 | 1 | 0 | 39 | 105 |
| 2023 | 357 | 306 | 47 | 4 | 1 | 29 | 191 |
| 2022 | 365 | 307 | 56 | 2 | 1 | 32 | 155 |
| 2021 | 340 | 212 | 118 | 10 | 1 | 40 | 167 |
| 2020 | 364 | 304 | 57 | 3 | 1 | 31 | 163 |
| 2019 | 292 | 271 | 20 | 1 | 1 | 31 | 159 |
| 2018 | 345 | 251 | 91 | 3 | 1 | 37 | 155 |
| 2017 | 356 | 272 | 79 | 5 | 0 | 33 | 141 |
| 2016 | 323 | 273 | 49 | 1 | 0 | 31 | 124 |
| 2015 | 325 | 260 | 64 | 1 | 0 | 37 | 122 |
| 2014 | 324 | 247 | 77 | 0 | 0 | 33 | 78 |
| 2013 | 355 | 325 | 30 | 0 | 0 | 19 | 79 |
| 2012 | 362 | 303 | 59 | 0 | 0 | 23 | 95 |
| 2011 | 362 | 319 | 43 | 0 | 0 | 26 | 80 |
| 2010 | 365 | 319 | 46 | 0 | 0 | 26 | 84 |
*2025 may be a partial year in EPA's file.
Montana compared
- Missoula County18/yr
- Ravalli County11/yr
- Lewis and Clark County9/yr
- Lincoln County8/yr
- Silver Bow County7/yr
- Sanders County7/yr
- Powder River County6/yr
- Flathead County6/yr
How this is counted
A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day.