Illinois: 23 monitored counties (2025), avg median AQI 42 (Good), 70.0% good-air days, 19 total unhealthy days (0.2%). Best: Effingham County (95.5%). Worst: Cook County (40.8%). Worst single reading: AQI 260. EPA AQS methodology + AQI interpretation →

A statewide average smooths over real differences between counties, so it should be read as a starting point rather than a verdict on any one place. Within almost every state, a handful of urban or wildfire-prone counties carry most of the unhealthy days while rural counties sit near the clean end of the scale, and the gap between the best and worst county is often wider than the gap between two neighboring states. Coverage matters too: the Environmental Protection Agency places monitors where population density, emission sources, or attainment status warrant them, so a state with few monitored counties is described by a thinner sample than one blanketed with stations. The figures here count only the days each monitor actually reported, which keeps lightly monitored and heavily monitored counties on comparable footing but cannot speak for the roughly two thousand U.S. counties that have no monitor at all. For a fuller picture, open the individual county profiles below and compare the primary pollutant, the share of Good days, and the five-year trend for the places that matter to you.