# Cook County, Illinois annual air quality

In the 2025 EPA annual record, Cook County had a median AQI of 53 (Moderate) and 40.8% of 365 monitored days were rated Good.

## Key figures

- Monitored days: 365
- Good days: 149 (40.8%)
- Moderate days: 191
- Unhealthy for Sensitive Groups days: 20
- Unhealthy-or-worse days: 5
- Median AQI: 53 (Moderate)
- 90th-percentile AQI: 84
- Maximum AQI: 168 (Unhealthy)
- Main pollutant-day driver: PM2.5

The Good-day percentage uses monitored days as its denominator; unmonitored calendar days are not estimated. This record clears the 100-day comparison floor.

## Annual history

- 2020: median AQI 58; 23.5% Good (86 of 366 monitored days).
- 2021: median AQI 59; 23.8% Good (87 of 365 monitored days).
- 2022: median AQI 54; 32.1% Good (117 of 365 monitored days).
- 2023: median AQI 57; 28.5% Good (104 of 365 monitored days).
- 2024: median AQI 54; 35.8% Good (131 of 366 monitored days).
- 2025: median AQI 53; 40.8% Good (149 of 365 monitored days).

## Source and scope

- Source: U.S. EPA Air Quality System annual summary for 2025: https://aqs.epa.gov/aqsweb/airdata/download_files.html
- PlainAirData data refresh: 2026-08-23
- Scope: annual monitor records, not current conditions or an air-quality forecast.
- Methodology: https://plainairdata.com/methodology

## Related resources

- Full HTML page: https://plainairdata.com/county/illinois-cook
- County CSV row: https://plainairdata.com/county/illinois-cook.csv
- State overview: https://plainairdata.com/state/illinois