Bad Air Days in Missoula County, Montana: 18 a Year
18 bad air days a year
Among the worst tenth of monitored US counties. Ranked 37 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly particles: smoke and dust (100% particle days).
Which months to watch
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in August and September and January. Averaged over 2016 to 2025.
What to buy, and what not to
- 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.
- Upgrade the furnace or AC filter to MERV 13 so the whole house is filtered, not just one room.
The record for Missoula
In 2000 to 2009 this county averaged 7 bad days a year; in 2016 to 2025, 18. It has got worse. The worst recent year was 2017, with 73 bad days on 365 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2017-09-06 | 1130 | PM2.5 |
| 2017-09-07 | 939 | PM2.5 |
| 2017-09-05 | 855 | PM2.5 |
| 2017-08-30 | 719 | PM2.5 |
| 2017-09-08 | 707 | PM2.5 |
An ordinary day in Missoula
Across 3,652 monitored days from 2016 to 2025, the air in Missoula County rated Good on 43% of them, less often than the 72% typical of monitored counties, and Moderate on 52%. The median day sat at AQI 53; one day in ten went above 90.
The pollutant setting the AQI on an ordinary day here:
- fine particles (PM2.5)80%2,924 days
- ozone20%727 days
Missoula County by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 365 | 298 | 67 | 0 | 0 | 38 | 98 |
| 2024 | 366 | 237 | 125 | 4 | 1 | 43 | 153 |
| 2023 | 364 | 156 | 200 | 8 | 0 | 53 | 134 |
| 2022 | 365 | 149 | 201 | 15 | 4 | 54 | 190 |
| 2021 | 365 | 122 | 213 | 30 | 6 | 56 | 162 |
| 2020 | 366 | 155 | 192 | 19 | 6 | 54 | 185 |
| 2019 | 365 | 173 | 188 | 4 | 1 | 52 | 153 |
| 2018 | 365 | 146 | 215 | 4 | 1 | 54 | 154 |
| 2017 | 365 | 52 | 240 | 73 | 49 | 65 | 1130 |
| 2016 | 366 | 72 | 269 | 25 | 0 | 58 | 139 |
| 2015 | 365 | 126 | 216 | 23 | 9 | 56 | 209 |
| 2014 | 365 | 173 | 174 | 18 | 2 | 52 | 159 |
| 2013 | 365 | 94 | 247 | 24 | 6 | 61 | 168 |
| 2012 | 366 | 54 | 265 | 47 | 15 | 65 | 188 |
| 2011 | 365 | 71 | 237 | 57 | 8 | 62 | 160 |
| 2010 | 365 | 84 | 234 | 47 | 7 | 61 | 192 |
*2025 may be a partial year in EPA's file.
Montana compared
- 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
- Lake Countythin data
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.