Air quality history by county in Illinois
EPA reported a daily Air Quality Index for 33 countys in Illinois in at least one year from 1980 to 2025. In 2025, 23 countys had fuller monitoring; across them the median of county median AQI values was 42, about the same as the national figure of 40. Effingham had the highest share of Good days (95%).
Select a county for its year-by-year AQI history, charts and data notes. Click a column heading to sort.
| Years covered | ||||||
|---|---|---|---|---|---|---|
| Adams | 2025 | 244 | 95% | 38 | 49 | 1980–2025 |
| Champaign | 2025 | 365 | 66% | 43 | 66 | 1980–2025 |
| Clark | 2025 | 355 | 94% | 33 | 48 | 2001–2025 |
| Coles | 1997 | 59 | 100% | 18 | 32 | 1988–1997 |
| Cook | 2025 | 365 | 41% | 53 | 84 | 1980–2025 |
| Crawford | 1981 | 120 | 85% | 32 | 71 | 1980–1981 |
| DuPage | 2025 | 365 | 64% | 42 | 67 | 1980–2025 |
| Effingham | 2025 | 242 | 95% | 35 | 46 | 1981–2025 |
| Hamilton | 2025 | 361 | 68% | 42 | 61 | 1997–2025 |
| Jackson | 2010 | 44 | 100% | 20 | 30 | 1989–2010 |
| Jersey | 2025 | 358 | 72% | 42 | 67 | 1988–2025 |
| Jo Daviess | 2025 | 341 | 95% | 33 | 46 | 1994–2025 |
| Kane | 2025 | 365 | 62% | 45 | 73 | 1980–2025 |
| Kankakee | 1985 | 349 | 89% | 27 | 58 | 1981–1986 |
| La Salle | 2010 | 365 | 64% | 42 | 63 | 1980–2010 |
| Lake | 2025 | 231 | 85% | 39 | 61 | 1980–2025 |
| Livingston | 1992 | 217 | 81% | 38 | 80 | 1988–1992 |
| Logan | 1992 | 214 | 80% | 38 | 71 | 1990–1992 |
| Macon | 2025 | 365 | 65% | 44 | 64 | 1980–2025 |
| Macoupin | 2025 | 364 | 92% | 34 | 49 | 1980–2025 |
| Madison | 2025 | 365 | 52% | 49 | 70 | 1980–2025 |
| McHenry | 2025 | 364 | 68% | 42 | 71 | 1980–2025 |
| McLean | 2025 | 365 | 66% | 44 | 67 | 2000–2025 |
| Monroe | 1989 | 355 | 80% | 36 | 74 | 1980–1990 |
| Peoria | 2025 | 357 | 60% | 45 | 66 | 1980–2025 |
| Randolph | 2025 | 364 | 68% | 43 | 61 | 1989–2025 |
| Rock Island | 2025 | 365 | 67% | 41 | 61 | 1980–2025 |
| Saint Clair | 2025 | 365 | 58% | 47 | 63 | 1980–2025 |
| Sangamon | 2025 | 365 | 70% | 41 | 61 | 1980–2025 |
| Tazewell | 1998 | 60 | 98% | 22 | 37 | 1991–1998 |
| Will | 2025 | 365 | 62% | 44 | 71 | 1980–2025 |
| Williamson | 1986 | 323 | 77% | 36 | 77 | 1980–1986 |
| Winnebago | 2025 | 355 | 69% | 40 | 67 | 1980–2025 |
Latest year is the most recent year with at least 120 monitored days, or the most recent year available if none qualifies. Counties without EPA AQI monitoring do not appear.
Source: U.S. EPA AirData, “Annual AQI by County” files for 1980–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem