# Counties in Illinois by highest daily 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_max`
- Unit: index
- Definition: Highest daily Air Quality Index (AQI) value recorded in a US county during the year, from the EPA AirData annual summaries. Values above 500 are beyond the AQI scale and are published as EPA computes them.
- Source: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: highest 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: highest daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
- Web page: https://invokely.ai/rankings/highest-daily-air-quality-index/illinois-counties

| Rank | Place | index |
|---:|---|---:|
| 1 | Madison County | 260 |
| 2 | Winnebago County | 169 |
| 3 | Cook County | 168 |
| 4 | Rock Island County | 167 |
| 5 | Kane County | 165 |
| 5 | McHenry County | 165 |
| 7 | Randolph County | 161 |
| 7 | St. Clair County | 161 |
| 9 | DuPage County | 158 |
| 10 | Jersey County | 156 |
| 10 | Will County | 156 |
| 12 | McLean County | 154 |
| 12 | Peoria County | 154 |
| 14 | Sangamon County | 142 |
| 15 | Macon County | 141 |
| 16 | Champaign County | 138 |
| 17 | Lake County | 108 |
| 18 | Macoupin County | 105 |
| 19 | Hamilton County | 98 |
| 20 | Clark County | 87 |
| 20 | Jo Daviess County | 87 |
| 22 | Effingham County | 84 |
| 23 | Adams County | 71 |