Bad Air Days in McHenry County, Illinois: 7 a Year
7 bad air days a year
More bad air days than three quarters of monitored counties. Ranked 152 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly ozone: summer smog (15% 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 and May. 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 McHenry
In 2000 to 2009 this county averaged 9 bad days a year; in 2016 to 2025, 7. The air has improved. The worst recent year was 2023, with 21 bad days on 365 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-06-27 | 230 | PM2.5 |
| 2023-06-28 | 196 | PM2.5 |
| 2023-06-29 | 168 | PM2.5 |
| 2025-07-31 | 165 | PM2.5 |
| 2023-06-24 | 151 | Ozone |
An ordinary day in McHenry
Across 3,578 monitored days from 2016 to 2025, the air in McHenry County rated Good on 59% of them, less often than the 72% typical of monitored counties, and Moderate on 39%. The median day sat at AQI 46; one day in ten went above 67.
The pollutant setting the AQI on an ordinary day here:
- fine particles (PM2.5)62%2,222 days
- ozone38%1,356 days
McHenry County by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 364 | 247 | 105 | 12 | 1 | 42 | 165 |
| 2024 | 366 | 245 | 120 | 1 | 0 | 43 | 108 |
| 2023 | 365 | 191 | 153 | 21 | 5 | 49 | 230 |
| 2022 | 365 | 199 | 163 | 3 | 0 | 47 | 126 |
| 2021 | 363 | 187 | 174 | 2 | 0 | 49 | 105 |
| 2020 | 334 | 192 | 133 | 9 | 0 | 47 | 147 |
| 2019 | 340 | 161 | 177 | 2 | 0 | 52 | 126 |
| 2018 | 362 | 213 | 141 | 8 | 0 | 46 | 122 |
| 2017 | 357 | 226 | 128 | 3 | 0 | 44 | 133 |
| 2016 | 362 | 241 | 115 | 6 | 0 | 44 | 129 |
| 2015 | 364 | 173 | 189 | 2 | 0 | 51 | 103 |
| 2014 | 364 | 248 | 113 | 3 | 0 | 42 | 136 |
| 2013 | 361 | 238 | 122 | 1 | 0 | 43 | 119 |
| 2012 | 363 | 187 | 167 | 9 | 1 | 49 | 177 |
| 2011 | 363 | 135 | 223 | 5 | 0 | 54 | 129 |
| 2010 | 362 | 225 | 136 | 1 | 0 | 44 | 115 |
*2025 may be a partial year in EPA's file.
Illinois compared
- Cook County18/yr
- Lake County9/yr
- Madison County8/yr
- Kane County7/yr
- DuPage County5/yr
- Jersey County5/yr
- Peoria County4/yr
- Winnebago County4/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.