Bad Air Days in Saint Charles County, Missouri: 9 a Year
9 bad air days a year
Among the worst tenth of monitored US counties. Ranked 92 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly ozone: summer smog (0% particle days).
The bad-air season here
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in June and September and May and August. Averaged over 2016 to 2025.
Is a purifier worth it here?
- Ozone drives many bad days here. Purifiers and filters do not remove ozone, so do not buy one for that. Ozone peaks on hot afternoons: exercise outside in the morning.
The worst days on file
In 2000 to 2009 this county averaged 29 bad days a year; in 2016 to 2025, 9. The air has improved. The worst recent year was 2023, with 15 bad days on 245 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2025-09-18 | 164 | Ozone |
| 2017-06-02 | 159 | Ozone |
| 2016-06-13 | 151 | Ozone |
| 2019-08-05 | 151 | Ozone |
| 2018-06-06 | 147 | Ozone |
The air on a normal day
Across 2,502 monitored days from 2016 to 2025, the air in Saint Charles County rated Good on 78% of them, more often than the 72% typical of monitored counties, and Moderate on 19%. The median day sat at AQI 42; one day in ten went above 68.
The pollutant setting the AQI on an ordinary day here:
- ozone98%2,441 days
- nitrogen dioxide2%61 days
Every recent year on file
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 309 | 248 | 57 | 4 | 1 | 40 | 164 |
| 2024 | 265 | 194 | 68 | 3 | 0 | 44 | 129 |
| 2023 | 245 | 156 | 74 | 15 | 0 | 45 | 147 |
| 2022 | 245 | 206 | 32 | 7 | 0 | 42 | 129 |
| 2021 | 245 | 209 | 34 | 2 | 0 | 41 | 119 |
| 2020 | 245 | 216 | 25 | 4 | 0 | 39 | 112 |
| 2019 | 244 | 202 | 38 | 4 | 1 | 42 | 151 |
| 2018 | 245 | 190 | 47 | 8 | 0 | 42 | 147 |
| 2017 | 245 | 185 | 56 | 4 | 1 | 44 | 159 |
| 2016 | 214 | 154 | 48 | 12 | 1 | 44 | 151 |
| 2015 | 214 | 174 | 37 | 3 | 0 | 42 | 126 |
| 2014 | 214 | 166 | 40 | 8 | 0 | 43 | 126 |
| 2013 | 214 | 152 | 57 | 5 | 1 | 44 | 156 |
| 2012 | 214 | 121 | 61 | 32 | 6 | 49 | 185 |
| 2011 | 245 | 161 | 59 | 25 | 3 | 44 | 169 |
| 2010 | 304 | 222 | 54 | 28 | 3 | 40 | 164 |
*2025 may be a partial year in EPA's file.
Nearby counties on file
- St. Louis City7/yr
- Clay County6/yr
- Saint Louis County5/yr
- Jefferson County4/yr
- Lincoln County4/yr
- Clinton County3/yr
- Sainte Genevieve County3/yr
- Perry County2/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, and on why a purifier does not touch ozone in smoke vs ozone.