Chicago Air Quality in May
Across 2019–2025, Chicago's May air quality had a median AQI of 58 (Moderate) over 217 monitored days in the Chicago-Naperville-Elgin, IL-IN-WI area. 4.6% of May days (10 of 217) reached Unhealthy for Sensitive Groups or worse, and fine particulate matter (PM2.5) set the daily AQI on 55.8% of days. The cleanest May on record here was 2025 (median AQI 51, 0 days above 100) and the worst was 2023 (median 77, 7 days above 100, peak AQI 140).
- Good: 26.7%
- Moderate: 68.7%
- Unhealthy for Sensitive Groups: 4.6%
- Unhealthy or worse: 0.0%
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How is the air in Chicago in May?
Moderate on a typical day: median AQI 58 across 2019–2025, with 4.6% of May days above AQI 100. Fine particulate matter (PM2.5) is the main driver.
- ✓26.7% of May days were Good (AQI 0–50), 68.7% Moderate (51–100).
- ✓10 days of 217 reached AQI 101+; the May peak was 140.
- ✓Best May: 2025; worst May: 2023.
Frequently asked questions
What is the air quality like in Chicago in May?▾
Across 2019–2025, the median daily AQI in Chicago in May was 58 (Moderate). 26.7% of days were Good, 68.7% Moderate, and 4.6% reached Unhealthy for Sensitive Groups or worse, based on 217 monitored days in EPA's Air Quality System.
Is May a good time for outdoor exercise in Chicago?▾
A typical May day in Chicago is Moderate (median AQI 58): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 22 of May days (4.6%) 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 May air quality problems?▾
Fine particulate matter (PM2.5) set the daily AQI on 55.8% of May days in Chicago. The highest May AQI recorded in 2019–2025 was 140 (Unhealthy for Sensitive Groups), in 2023.
Chicago May air quality, year by year
Chicago May 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 |
|---|---|---|---|---|---|
| 2019 | 31 | 59 | 0 | 84 | PM2.5 |
| 2020 | 31 | 61 | 0 | 90 | PM2.5 |
| 2021 | 31 | 58 | 1 | 101 | PM2.5 |
| 2022 | 31 | 54 | 2 | 108 | PM2.5 |
| 2023worst | 31 | 77 | 7 | 140 | O3 |
| 2024 | 31 | 57 | 0 | 97 | O3 |
| 2025cleanest | 31 | 51 | 0 | 97 | PM2.5 |
What May air means in Chicago
A typical May day in Chicago is Moderate (median AQI 58): outdoor exercise is acceptable for most people, but unusually sensitive individuals should watch for symptoms, and about 1 day in 22 of May days (4.6%) reached AQI 101+ — the level at which EPA advises people with asthma, children and older adults to shorten hard outdoor exertion.
May's median AQI of 58 is in line with Chicago's whole-year median of 60, so this period is neither a clean nor a dirty stretch for the area. Nationally, the median May AQI across every EPA metro area (104,064 metro-days, 2019–2025) is 44, so Chicago runs above the national May norm; 4.6% of its May days topped AQI 100 versus 1.3% nationally (above average).
May is part of Chicago's spring season (March, April, May); the spring page pools all three months.
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 month 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.