Air Quality in Illinois

Illinois earns an average Air Quality Grade of C, with a 5-year median AQI of 45 across 23 monitored areas — 4 points above the national average of 41. The grade combines four signals — 5-year median AQI, 10-year trend direction, count of unhealthy days per year, and dominant pollutant — into a single A-F score. Illinois's 23 monitored areas collectively logged 578 days at "Unhealthy for Sensitive Groups" or worse over the last five years.

Illinois is bucking the national trend of broad improvement: 18 of 23 monitored areas are showing measurably worse air over the past decade, more than the 2 that are improving. Across the western U.S. that pattern usually traces back to expanding wildfire smoke exposure; elsewhere it can reflect rising local emissions from population or freight growth.

The dominant pollutant across 15 of 23 Illinois areas is Fine Particulate Matter (PM2.5). PM2.5 (fine particulate matter) is most often driven by combustion sources — vehicle exhaust, industrial emissions, residential wood burning, and increasingly wildfire smoke. It penetrates deep into lung tissue and the bloodstream and is the air pollutant most strongly linked to long-term health impacts. Other monitored areas in the state report Ground-Level Ozone (8) as their dominant pollutant.

Within Illinois, the gap between best and worst is meaningful: Clark, Illinois tops the state with a Grade B and 5-year median AQI of 34, while Winnebago, Illinois sits at the bottom with a Grade D and 5-year median AQI of 48. Local terrain, prevailing winds, and proximity to industrial or wildfire emission sources drive most of that within-state variation.

Macoupin, Illinois is the fastest-improving area in Illinois, with median AQI falling by 0.7 points per year over the EPA reporting period. Steady improvement at that pace usually reflects fleet turnover (older diesels retiring), upwind power-plant retirements, and tighter local emissions controls.