Illinois Air Quality
EPA monitor data through 2025 · Updated August 13, 2026 · Methods
In 2025, Illinois recorded 30 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 365 days with monitoring data. Most of those bad days were driven by ozone. Under the AQI rules that applied before 2013, the same air would have scored 18 days. Since 2000, unhealthy days are 58% lower on a consistent definition.
How to read a statewide figure. The Air Quality Index describes one place at one time. A statewide number is the worst monitor anywhere in the state on each day, so it answers “was the air unhealthy somewhere in Illinois?” — not “what did a typical resident breathe?” In a large state with many monitors it will read high even if most of the state was clean. EPA publishes the AQI for metropolitan areas rather than states for exactly this reason. For what people actually breathed, use the metro pages below.
Unhealthy days by year, scored two ways
EPA changed what an AQI of 100 means in 2013 and again in 2015, so a count published before those changes is not comparable to one published after. Each panel below applies a single fixed rulebook to every year, so each is internally consistent. Why this matters.
Scored by today's rules
Scored by the pre-2013 rules
What 2025 looked like
| Air quality category | Days | Share of monitored days |
|---|---|---|
| Good | 98 | 27% |
| Moderate | 237 | 65% |
| Unhealthy for sensitive groups | 22 | 6% |
| Unhealthy or worse | 8 | 2% |
Median AQI 58 · 90th-percentile AQI 93 · worst day 260. Scored under current rules.
What drove the unhealthy days
Of the 30 days above 100 in 2025, 20 were driven by ozone and 9 by fine particles, with 1 from another pollutant. The two behave differently. Ozone peaks on hot sunny afternoons; fine particles now spike mainly with wildfire smoke. In the eastern United States, ozone has stopped responding to emission cuts: nitrogen oxide emissions kept falling after 2014 and ambient ozone barely moved.
The unhealthy days in 2025
Every day the index passed 100, what it reached, and which pollutant drove it. 12 of these 30 would not have counted as unhealthy under the pre-2013 rules.
| Date | Peak AQI | Driven by | Pre-2013 score |
|---|---|---|---|
| Mar 15 | 260 | Coarse particles | 260 |
| Jun 4 | 121 | Fine particles | 107 |
| Jun 5 | 154 | Fine particles | 138 |
| Jun 6 | 119 | Fine particles | 106 |
| Jun 7 | 106 | Fine particles | 94 (not unhealthy) |
| Jun 11 | 122 | Ozone | 104 |
| Jun 12 | 115 | Ozone | 100 (not unhealthy) |
| Jun 16 | 136 | Ozone | 114 |
| Jul 2 | 115 | Ozone | 100 (not unhealthy) |
| Jul 3 | 101 | Ozone | 87 (not unhealthy) |
| Jul 4 | 168 | Fine particles | 159 |
| Jul 14 | 159 | Ozone | 135 |
| Jul 15 | 115 | Ozone | 100 (not unhealthy) |
| Jul 21 | 108 | Fine particles | 96 (not unhealthy) |
| Jul 22 | 119 | Ozone | 101 |
| Jul 29 | 159 | Ozone | 135 |
| Jul 31 | 169 | Fine particles | 160 |
| Aug 1 | 154 | Fine particles | 139 |
| Aug 3 | 117 | Fine particles | 104 |
| Aug 5 | 108 | Ozone | 93 (not unhealthy) |
| Aug 6 | 119 | Ozone | 101 |
| Aug 16 | 129 | Ozone | 109 |
| Sep 10 | 101 | Ozone | 87 (not unhealthy) |
| Sep 12 | 133 | Ozone | 111 |
| Sep 17 | 105 | Ozone | 90 (not unhealthy) |
| Sep 18 | 156 | Ozone | 132 |
| Sep 19 | 129 | Ozone | 109 |
| Oct 1 | 101 | Ozone | 87 (not unhealthy) |
| Oct 2 | 101 | Ozone | 87 (not unhealthy) |
| Oct 3 | 108 | Ozone | 93 (not unhealthy) |
Metro areas in Illinois
Ranked by unhealthy days in their latest year. These are the geographies EPA actually reports the AQI for.
| Metro area | Unhealthy days | Pre-2013 rules | Main driver |
|---|---|---|---|
| Chicago-Naperville-Elgin, IL-IN-WI | 32 | 24 | ozone |
| St. Louis, MO-IL | 15 | 11 | ozone |
| Rockford, IL | 6 | 3 | fine particles |
| Bloomington, IL | 4 | 3 | fine particles |
| Champaign-Urbana, IL | 4 | 2 | ozone and fine particles equally |
| Peoria, IL | 4 | 3 | fine particles |
| Decatur, IL | 3 | 2 | fine particles |
| Springfield, IL | 3 | 1 | fine particles |
| Davenport-Moline-Rock Island, IA-IL | 2 | 2 | fine particles |
| Mount Vernon, IL | 0 | 0 | ozone and fine particles equally |
| Ottawa-Peru, IL | 0 | 0 | ozone and fine particles equally |
| Paducah, KY-IL | 0 | 0 | ozone and fine particles equally |
| Quincy, IL-MO | 0 | 0 | ozone and fine particles equally |
The full series
| Year | Unhealthy days, current rules | Pre-2013 rules | Days monitored |
|---|---|---|---|
| 2000 | 70 | 43 | 366 |
| 2001 | 96 | 59 | 365 |
| 2002 | 86 | 64 | 365 |
| 2003 | 69 | 39 | 365 |
| 2004 | 46 | 23 | 366 |
| 2005 | 85 | 51 | 365 |
| 2006 | 45 | 27 | 365 |
| 2007 | 73 | 55 | 365 |
| 2008 | 47 | 34 | 366 |
| 2009 | 28 | 12 | 365 |
| 2010 | 53 | 31 | 365 |
| 2011 | 44 | 27 | 365 |
| 2012 | 69 | 49 | 366 |
| 2013 | 21 | 12 | 365 |
| 2014 | 25 | 8 | 365 |
| 2015 | 16 | 3 | 365 |
| 2016 | 29 | 16 | 366 |
| 2017 | 29 | 11 | 365 |
| 2018 | 30 | 17 | 365 |
| 2019 | 16 | 6 | 365 |
| 2020 | 27 | 17 | 366 |
| 2021 | 28 | 12 | 365 |
| 2022 | 17 | 8 | 365 |
| 2023 | 45 | 31 | 365 |
| 2024 | 17 | 10 | 366 |
| 2025 | 30 | 18 | 365 |
Sources
- EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
- Download the full dataset for every state and metro.