Bad Air Days in St. Louis City, Missouri: 7 a Year
7 bad air days a year
More bad air days than three quarters of monitored counties. Ranked 138 of 996 monitored counties, where 1 is worst.
What makes them bad: a mix of particles and ozone (45% 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 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.
- 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, 7. The air has improved. The worst recent year was 2023, with 16 bad days on 365 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2025-03-14 | 738 | PM10 |
| 2025-03-15 | 343 | PM10 |
| 2021-07-28 | 187 | Ozone |
| 2018-06-06 | 182 | Ozone |
| 2023-06-06 | 172 | Ozone |
An ordinary day in St. Louis City
Across 3,653 monitored days from 2016 to 2025, the air in St. Louis City rated Good on 36% of them, less often than the 72% typical of monitored counties, and Moderate on 63%. The median day sat at AQI 54; one day in ten went above 73.
The pollutant setting the AQI on an ordinary day here:
- fine particles (PM2.5)73%2,659 days
- ozone21%757 days
- coarse dust (PM10)5%192 days
- nitrogen dioxide1%45 days
Every recent year on file
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 365 | 125 | 232 | 8 | 2 | 55 | 738 |
| 2024 | 366 | 137 | 219 | 10 | 1 | 54 | 156 |
| 2023 | 365 | 89 | 260 | 16 | 2 | 58 | 172 |
| 2022 | 365 | 127 | 233 | 5 | 0 | 55 | 117 |
| 2021 | 365 | 114 | 244 | 7 | 1 | 55 | 187 |
| 2020 | 366 | 137 | 224 | 5 | 1 | 54 | 156 |
| 2019 | 365 | 135 | 225 | 5 | 0 | 54 | 129 |
| 2018 | 365 | 129 | 226 | 10 | 1 | 55 | 182 |
| 2017 | 365 | 165 | 198 | 2 | 0 | 52 | 115 |
| 2016 | 366 | 140 | 223 | 3 | 1 | 53 | 162 |
| 2015 | 365 | 122 | 237 | 6 | 2 | 55 | 155 |
| 2014 | 365 | 111 | 247 | 7 | 0 | 56 | 148 |
| 2013 | 365 | 108 | 255 | 2 | 0 | 57 | 133 |
| 2012 | 366 | 75 | 259 | 32 | 5 | 61 | 174 |
| 2011 | 365 | 73 | 271 | 21 | 2 | 61 | 179 |
| 2010 | 365 | 66 | 293 | 6 | 1 | 61 | 162 |
*2025 may be a partial year in EPA's file.
Elsewhere in Missouri
- Saint Charles County9/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.