# Counties in Illinois by median Air Quality Index (US EPA), 2025

All 23 values come from U.S. Environmental Protection Agency for 2025, so the places are comparable. 10 counties have no value for that period and are not ranked.

- Indicator id: `aqi_median`
- Unit: index
- Definition: Median of the daily Air Quality Index (AQI) values of a US county over the days of the year with an AQI, from the EPA AirData annual summaries. The daily county AQI is the highest AQI among the monitors and pollutants reported that day (ozone, PM2.5, PM10, carbon monoxide and nitrogen dioxide). Lower is cleaner: 0 to 50 is good, 51 to 100 moderate and above 100 unhealthy for sensitive groups.
- Source: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: median daily AQI (https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual)
- Publisher page: https://invokely.ai/en/publishers/environmental-protection-agency
- Retrieved: 2026-10-12
- Citation: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
- Web page: https://invokely.ai/rankings/median-air-quality-index/illinois-counties

| Rank | Place | index |
|---:|---|---:|
| 1 | Cook County | 53 |
| 2 | Madison County | 49 |
| 3 | St. Clair County | 47 |
| 4 | Kane County | 45 |
| 4 | Peoria County | 45 |
| 6 | Macon County | 44 |
| 6 | McLean County | 44 |
| 6 | Will County | 44 |
| 9 | Champaign County | 43 |
| 9 | Randolph County | 43 |
| 11 | DuPage County | 42 |
| 11 | Hamilton County | 42 |
| 11 | Jersey County | 42 |
| 11 | McHenry County | 42 |
| 15 | Rock Island County | 41 |
| 15 | Sangamon County | 41 |
| 17 | Winnebago County | 40 |
| 18 | Lake County | 39 |
| 19 | Adams County | 38 |
| 20 | Effingham County | 35 |
| 21 | Macoupin County | 34 |
| 22 | Clark County | 33 |
| 22 | Jo Daviess County | 33 |