Bad Air Days in St. Clair County, Michigan: 5 a Year
5 bad air days a year
More bad air days than three quarters of monitored counties. Ranked 195 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly ozone: summer smog (30% particle days).
Which months to watch
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in June and July and May and August. Averaged over 2016 to 2025.
What to buy, and what not to
- 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 record for St. Clair
In 2000 to 2009 this county averaged 21 bad days a year; in 2016 to 2025, 5. The air has improved. The worst recent year was 2023, with 13 bad days on 362 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-06-28 | 197 | PM2.5 |
| 2023-06-27 | 193 | PM2.5 |
| 2023-06-29 | 161 | PM2.5 |
| 2023-06-07 | 156 | PM2.5 |
| 2025-08-04 | 152 | PM2.5 |
Most days, not the worst ones
Across 3,536 monitored days from 2016 to 2025, the air in St. Clair County rated Good on 65% of them, less often than the 72% typical of monitored counties, and Moderate on 33%. The median day sat at AQI 43; one day in ten went above 65.
The pollutant setting the AQI on an ordinary day here:
- fine particles (PM2.5)63%2,216 days
- ozone37%1,320 days
St. Clair County by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 365 | 254 | 100 | 11 | 1 | 39 | 152 |
| 2024 | 360 | 261 | 96 | 3 | 0 | 41 | 134 |
| 2023 | 362 | 186 | 163 | 13 | 4 | 49 | 197 |
| 2022 | 359 | 232 | 125 | 2 | 0 | 43 | 133 |
| 2021 | 365 | 198 | 160 | 7 | 0 | 48 | 140 |
| 2020 | 328 | 247 | 80 | 1 | 0 | 39 | 105 |
| 2019 | 365 | 266 | 96 | 3 | 0 | 41 | 108 |
| 2018 | 310 | 200 | 103 | 7 | 1 | 44 | 151 |
| 2017 | 358 | 232 | 124 | 2 | 0 | 44 | 112 |
| 2016 | 364 | 236 | 124 | 4 | 0 | 43 | 140 |
| 2015 | 363 | 198 | 155 | 10 | 0 | 48 | 126 |
| 2014 | 365 | 187 | 173 | 5 | 0 | 49 | 116 |
| 2013 | 244 | 171 | 67 | 6 | 0 | 40 | 146 |
| 2012 | 243 | 146 | 82 | 15 | 1 | 44 | 202 |
| 2011 | 242 | 168 | 65 | 9 | 1 | 40 | 159 |
| 2010 | 243 | 161 | 71 | 11 | 0 | 42 | 150 |
*2025 may be a partial year in EPA's file.
Michigan compared
- Wayne County11/yr
- Marquette County9/yr
- Muskegon County9/yr
- Allegan County9/yr
- Berrien County8/yr
- Macomb County7/yr
- Cass County6/yr
- Oakland 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, and on why a purifier does not touch ozone in smoke vs ozone.