Bad Air Days in Lewis and Clark County, Montana: 9 a Year
9 bad air days a year
More bad air days than three quarters of monitored counties. Ranked 99 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly particles: smoke and dust (98% particle days).
When the bad days come in Lewis and Clark
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in September and January and August and July. Averaged over 2016 to 2025.
Is a purifier worth it 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.
- Upgrade the furnace or AC filter to MERV 13 so the whole house is filtered, not just one room.
Better or worse than it was
In 2000 to 2009 this county averaged 4 bad days a year; in 2016 to 2025, 9. It has got worse. The worst recent year was 2017, with 29 bad days on 365 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2017-08-29 | 177 | PM2.5 |
| 2018-08-19 | 169 | PM2.5 |
| 2020-09-15 | 168 | PM2.5 |
| 2022-09-05 | 167 | PM2.5 |
| 2023-05-18 | 165 | PM2.5 |
The air on a normal day
Across 3,653 monitored days from 2016 to 2025, the air in Lewis and Clark County rated Good on 68% of them, less often than the 72% typical of monitored counties, and Moderate on 30%. The median day sat at AQI 44; one day in ten went above 69.
The pollutant setting the AQI on an ordinary day here:
- ozone55%2,017 days
- fine particles (PM2.5)45%1,636 days
Every recent year on file
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 365 | 261 | 104 | 0 | 0 | 44 | 93 |
| 2024 | 366 | 233 | 127 | 6 | 1 | 46 | 160 |
| 2023 | 365 | 240 | 120 | 5 | 2 | 45 | 165 |
| 2022 | 365 | 239 | 119 | 7 | 3 | 45 | 167 |
| 2021 | 365 | 249 | 99 | 17 | 3 | 44 | 156 |
| 2020 | 366 | 277 | 84 | 5 | 4 | 42 | 168 |
| 2019 | 365 | 251 | 112 | 2 | 0 | 43 | 144 |
| 2018 | 365 | 238 | 117 | 10 | 3 | 46 | 169 |
| 2017 | 365 | 209 | 127 | 29 | 12 | 47 | 177 |
| 2016 | 366 | 281 | 78 | 7 | 0 | 42 | 146 |
| 2015 | 365 | 254 | 98 | 13 | 5 | 44 | 178 |
| 2014 | 365 | 268 | 92 | 5 | 0 | 42 | 150 |
| 2013 | 365 | 278 | 87 | 0 | 0 | 40 | 100 |
| 2012 | 366 | 232 | 127 | 7 | 0 | 43 | 148 |
| 2011 | 365 | 247 | 116 | 2 | 0 | 44 | 115 |
| 2010 | 360 | 248 | 98 | 14 | 3 | 33 | 160 |
*2025 may be a partial year in EPA's file.
Elsewhere in Montana
- Missoula County18/yr
- Ravalli County11/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.