Air Quality Rankings for Illinois (2026)

Illinois has 23 cities tracked by EPA air-quality monitors, with a state-wide 5-year median AQI of 45 — 4 points more polluted than the national average of AQI 41. Macoupin, Illinois ranks #1 with the cleanest air (AQI 33, Grade B), while Cook, Illinois sits at the bottom (AQI 57, Grade D). The rankings below are computed from the EPA Air Quality System (AQS), which aggregates daily AQI readings from federally certified monitors into annual averages. Cities are sorted by 5-year median AQI (lowest = cleanest = #1). The 5-year window smooths out year-to-year volatility from weather and wildfire events.

Illinois is bucking the national trend of broad improvement: 18 of 23 monitored cities show measurably worse air over the past decade, more than the 2 that are improving. Across western states this 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 cities 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 Illinois cities report Ground-Level Ozone (8) as their dominant concern.

The fastest-improving city in Illinois is Macoupin, Illinois, with median AQI falling by 0.7 points per year. Steady improvement at that pace usually reflects fleet turnover (older diesels retiring), upwind power-plant retirements, or tighter regional emissions controls.

The city with the steepest decline is Winnebago, Illinois, where median AQI is rising by 1.9 points per year. Rapid deterioration in a single city usually points to either wildfire-smoke exposure (in the West) or a new local emissions source — a power plant, port, or freight corridor coming online.