# Counties in Illinois by share of days with good air quality (US EPA AQI), 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_good_days_share`
- Unit: %
- Definition: Days with a daily Air Quality Index of 0 to 50 (good) as a percentage of the days of the year with an AQI in a US county. Calculated by Invokely from the day counts in the EPA AirData annual summaries; both counts are stored with each value.
- Source: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts (https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual)
- Retrieved: 2026-10-12
- Citation: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
- Web page: https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties

| Rank | Place | % |
|---:|---|---:|
| 1 | Effingham County | 95.5 |
| 2 | Jo Daviess County | 95.3 |
| 3 | Adams County | 94.7 |
| 4 | Clark County | 93.8 |
| 5 | Macoupin County | 91.8 |
| 6 | Lake County | 84.8 |
| 7 | Jersey County | 72.3 |
| 8 | Sangamon County | 70.1 |
| 9 | Winnebago County | 68.7 |
| 10 | Randolph County | 68.1 |
| 11 | Hamilton County | 67.9 |
| 11 | McHenry County | 67.9 |
| 13 | Rock Island County | 66.8 |
| 14 | McLean County | 66 |
| 15 | Champaign County | 65.8 |
| 16 | Macon County | 64.9 |
| 17 | DuPage County | 63.8 |
| 18 | Kane County | 61.9 |
| 19 | Will County | 61.6 |
| 20 | Peoria County | 59.9 |
| 21 | St. Clair County | 58.4 |
| 22 | Madison County | 52.3 |
| 23 | Cook County | 40.8 |