Bad Air Days in Clark County, Illinois: 1 a Year
1 bad air day a year
Fewer bad air days than most monitored US counties. Ranked 680 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 May. Averaged over 2016 to 2025.
What to buy, and what not to
- Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
- On the odd bad day, check AirNow and keep windows shut.
The worst days on file
In 2000 to 2009 this county averaged 10 bad days a year; in 2016 to 2025, 1. The air has improved. The worst recent year was 2023, with 5 bad days on 358 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-06-02 | 136 | Ozone |
| 2023-05-24 | 126 | Ozone |
| 2023-06-06 | 119 | Ozone |
| 2023-06-03 | 115 | Ozone |
| 2016-06-09 | 101 | Ozone |
The air on a normal day
Across 3,244 monitored days from 2016 to 2025, the air in Clark County rated Good on 93% of them, more often than the 72% typical of monitored counties, and Moderate on 7%. The median day sat at AQI 34; one day in ten went above 48.
The pollutant setting the AQI on an ordinary day here:
- ozone100%3,244 days
Clark County by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 355 | 333 | 22 | 0 | 0 | 33 | 87 |
| 2024 | 363 | 332 | 31 | 0 | 0 | 36 | 90 |
| 2023 | 358 | 320 | 33 | 5 | 0 | 36 | 136 |
| 2022 | 312 | 296 | 15 | 1 | 0 | 34 | 101 |
| 2021 | 363 | 344 | 19 | 0 | 0 | 34 | 84 |
| 2020 | 363 | 347 | 16 | 0 | 0 | 31 | 97 |
| 2019 | 327 | 314 | 13 | 0 | 0 | 34 | 74 |
| 2018 | 359 | 337 | 21 | 1 | 0 | 32 | 101 |
| 2017 | 241 | 211 | 29 | 1 | 0 | 39 | 101 |
| 2016 | 203 | 175 | 27 | 1 | 0 | 38 | 101 |
| 2015 | 210 | 182 | 28 | 0 | 0 | 39 | 87 |
| 2014 | 211 | 185 | 26 | 0 | 0 | 39 | 90 |
| 2013 | 207 | 190 | 17 | 0 | 0 | 38 | 80 |
| 2012 | 152 | 107 | 39 | 6 | 0 | 43 | 129 |
| 2011 | 176 | 151 | 24 | 1 | 0 | 41 | 108 |
| 2010 | 210 | 176 | 34 | 0 | 0 | 40 | 90 |
*2025 may be a partial year in EPA's file.
Nearby counties on file
- Cook County18/yr
- Lake County9/yr
- Madison County8/yr
- Kane County7/yr
- McHenry County7/yr
- DuPage County5/yr
- Jersey County5/yr
- Peoria 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.