Bad Air Days in Custer County, Montana: 3 a Year
3 bad air days a year
About the middle of monitored US counties. Ranked 324 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 September and May and July. 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 worst days on file
The worst recent year was 2023, with 10 bad days on 364 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-05-17 | 260 | PM2.5 |
| 2023-09-05 | 188 | PM2.5 |
| 2023-07-16 | 171 | PM2.5 |
| 2023-07-15 | 167 | PM2.5 |
| 2023-09-04 | 167 | PM2.5 |
An ordinary day in Custer
Across 1,362 monitored days from 2022 to 2025, the air in Custer County rated Good on 86% of them, more often than the 72% typical of monitored counties, and Moderate on 13%. The median day sat at AQI 39; one day in ten went above 54.
The pollutant setting the AQI on an ordinary day here:
- ozone70%958 days
- fine particles (PM2.5)28%384 days
- nitrogen dioxide1%16 days
Every recent year on file
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 273 | 238 | 34 | 1 | 0 | 40 | 127 |
| 2024 | 361 | 328 | 31 | 2 | 2 | 36 | 153 |
| 2023 | 364 | 288 | 66 | 10 | 6 | 39 | 260 |
| 2022 | 364 | 318 | 46 | 0 | 0 | 40 | 80 |
*2025 may be a partial year in EPA's file.
Elsewhere in Montana
- 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.