# Counties in Illinois by share of days unhealthy for sensitive groups or worse (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_unhealthy_sensitive_or_worse_days_share`
- Unit: %
- Definition: Days with a daily Air Quality Index above 100 (unhealthy for sensitive groups, unhealthy, very unhealthy or hazardous) 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; the counts are stored with each value.
- Source: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: days with an AQI above 100 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: days with an AQI above 100 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/unhealthy-air-quality-days-share/illinois-counties

| Rank | Place | % |
|---:|---|---:|
| 1 | Cook County | 6.8 |
| 2 | McHenry County | 3.3 |
| 3 | Kane County | 2.5 |
| 4 | Winnebago County | 1.7 |
| 5 | DuPage County | 1.6 |
| 6 | Will County | 1.4 |
| 7 | Lake County | 1.3 |
| 8 | Champaign County | 1.1 |
| 8 | Jersey County | 1.1 |
| 8 | Madison County | 1.1 |
| 8 | McLean County | 1.1 |
| 8 | Peoria County | 1.1 |
| 13 | Macon County | 0.8 |
| 13 | Sangamon County | 0.8 |
| 13 | St. Clair County | 0.8 |
| 16 | Rock Island County | 0.5 |
| 17 | Macoupin County | 0.3 |
| 17 | Randolph County | 0.3 |
| 19 | Adams County | 0 |
| 19 | Clark County | 0 |
| 19 | Effingham County | 0 |
| 19 | Hamilton County | 0 |
| 19 | Jo Daviess County | 0 |