Chicago Air Quality in Fall
Across 2019–2025, Chicago's fall (September–November) air quality had a median AQI of 56 (Moderate) over 637 monitored days in the Chicago-Naperville-Elgin, IL-IN-WI area. 2.7% of fall days (17 of 637) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 71.9% of days. The cleanest fall on record here was 2019 (median AQI 58, 0 days above 100) and the worst was 2025 (median 55, 5 days above 100, peak AQI 147).
- Good: 25.9%
- Moderate: 71.4%
- Unhealthy for Sensitive Groups: 2.5%
- Unhealthy or worse: 0.2%
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How is the air in Chicago in fall?
Moderate on a typical day: median AQI 56 across 2019–2025, with 2.7% of fall days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓25.9% of fall days were Good (AQI 0–50), 71.4% Moderate (51–100).
- ✓17 days of 637 reached AQI 101+; the fall peak was 202.
- ✓Cleanest month: November; worst month: September.
Frequently asked questions
What is the air quality like in Chicago in fall?▾
Across 2019–2025, the median daily AQI in Chicago in fall was 56 (Moderate). 25.9% of days were Good, 71.4% Moderate, and 2.7% reached Unhealthy for Sensitive Groups or worse, based on 637 monitored days in EPA's Air Quality System.
Is fall a good time for outdoor exercise in Chicago?▾
A typical fall day in Chicago is Moderate (median AQI 56): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 37 of fall days (2.7%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Which pollutant causes Chicago's fall air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 71.9% of fall days in Chicago. The highest fall AQI recorded in 2019–2025 was 202 (Very Unhealthy), in 2025.
Chicago fall air quality, year by year
Chicago fall air quality by year (Chicago-Naperville-Elgin, IL-IN-WI, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019cleanest | 91 | 58 | 0 | 93 | PM2.5 |
| 2020 | 91 | 56 | 1 | 103 | PM2.5 |
| 2021 | 91 | 58 | 4 | 117 | PM2.5 |
| 2022 | 91 | 56 | 2 | 202 | PM2.5 |
| 2023 | 91 | 56 | 1 | 101 | PM2.5 |
| 2024 | 91 | 55 | 4 | 115 | PM2.5 |
| 2025worst | 91 | 55 | 5 | 147 | PM2.5 |
What fall air means in Chicago
A typical fall day in Chicago is Moderate (median AQI 56): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 37 of fall days (2.7%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
Fall's median AQI of 56 is below Chicago's whole-year median of 60, making it one of the area's cleaner periods. Nationally, the median Fall AQI across every EPA metro area (296,275 metro-days, 2019–2025) is 40, so Chicago runs above the national Fall norm; 2.7% of its Fall days topped AQI 100 versus 1.5% nationally (above average).
Within the season, November is the cleanest month (median AQI 57, 1.5% of days above 100) and September the worst (median 57, 4.8% above 100).
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Chicago
The latest EPA AirNow observation puts Chicago's AQI at 46 (PM2.5). The figures above are multi-year season statistics, not today's reading.
View current Chicago air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Chicago-Naperville-Elgin, IL-IN-WI), 2019–2025. Daily AQI per CBSA is the highest AQI reported by any monitor in the area that day; the defining parameter is the pollutant that set it. Month and season statistics pool every such day in the period across all years listed. Retrieved 2026-09-09.