Air quality in Illinois
EPA monitors report from 23 of Illinois’s 102 counties. That is the honest limit of this page: the other 79 are not measured, not clean. In 2025 the most affected monitored county was Cook County, with 25 days above the moderate category.
23
Counties with monitors
79
Counties without
99
Days above moderate, 2025
Summed across monitored counties
9.9%
Adults with asthma
Population-weighted county estimate
County by county
Monitored Illinois counties
GoodModerateUnhealthy for sensitive groupsUnhealthyVery unhealthyHazardous
| County | 90th %ile AQI | Above moderate | Max AQI | 2025 days by category |
|---|---|---|---|---|
| Cook County | 84 | 25 | 168 | |
| McHenry County | 71 | 12 | 165 | |
| Kane County | 73 | 9 | 165 | |
| DuPage County | 67 | 6 | 158 | |
| Winnebago County | 67 | 6 | 169 | |
| Will County | 71 | 5 | 156 | |
| Champaign County | 66 | 4 | 138 | |
| Jersey County | 67 | 4 | 156 | |
| Madison County | 70 | 4 | 260 | |
| McLean County | 67 | 4 | 154 | |
| Peoria County | 66 | 4 | 154 | |
| Lake County | 61 | 3 | 108 | |
| Macon County | 64 | 3 | 141 | |
| Sangamon County | 61 | 3 | 142 | |
| St. Clair County | 63 | 3 | 161 | |
| Rock Island County | 61 | 2 | 167 | |
| Macoupin County | 49 | 1 | 105 | |
| Randolph County | 61 | 1 | 161 | |
| Adams County | 49 | 0 | 71 | |
| Clark County | 48 | 0 | 87 | |
| Effingham County | 46 | 0 | 84 | |
| Hamilton County | 61 | 0 | 98 | |
| Jo Daviess County | 46 | 0 | 87 |
EPA Air Quality System annual summaries by county, 2025. The 90th percentile AQI describes a day worse than nine out of ten, which is a steadier measure than the annual maximum a single wildfire can set.
Not measured
79 counties without a monitor
No EPA monitor reported from these counties, so there is no air quality record to publish. Their radon zone is known, and radon is measured in the building rather than the air outside it.
- Alexander County
- Bond County
- Boone County
- Brown County
- Bureau County
- Calhoun County
- Carroll County
- Cass County
- Christian County
- Clay County
- Clinton County
- Coles County
- Crawford County
- Cumberland County
- De Witt County
- DeKalb County
- Douglas County
- Edgar County
- Edwards County
- Fayette County
- Ford County
- Franklin County
- Fulton County
- Gallatin County
- Greene County
- Grundy County
- Hancock County
- Hardin County
- Henderson County
- Henry County
- Iroquois County
- Jackson County
- Jasper County
- Jefferson County
- Johnson County
- Kankakee County
- Kendall County
- Knox County
- LaSalle County
- Lawrence County
- Lee County
- Livingston County
- Logan County
- Marion County
- Marshall County
- Mason County
- Massac County
- McDonough County
- Menard County
- Mercer County
- Monroe County
- Montgomery County
- Morgan County
- Moultrie County
- Ogle County
- Perry County
- Piatt County
- Pike County
- Pope County
- Pulaski County
- Putnam County
- Richland County
- Saline County
- Schuyler County
- Scott County
- Shelby County
- Stark County
- Stephenson County
- Tazewell County
- Union County
- Vermilion County
- Wabash County
- Warren County
- Washington County
- Wayne County
- White County
- Whiteside County
- Williamson County
- Woodford County
Other states
Air quality elsewhere
- Alabama
- Alaska
- Arizona
- Arkansas
- California
- Colorado
- Connecticut
- Delaware
- Florida
- Georgia
- Hawaii
- Idaho
- Indiana
- Iowa
- Kansas
- Kentucky
- Louisiana
- Maine
- Maryland
- Massachusetts
- Michigan
- Minnesota
- Mississippi
- Missouri
- Montana
- Nebraska
- Nevada
- New Hampshire
- New Jersey
- New Mexico
- New York
- North Carolina
- North Dakota
- Ohio
- Oklahoma
- Oregon
- Pennsylvania
- Rhode Island
- South Carolina
- South Dakota
- Tennessee
- Texas
- Utah
- Vermont
- Virginia
- Washington
- West Virginia
- Wisconsin
- Wyoming