Air Quality Rankings for Wisconsin (2026)

Wisconsin has 27 cities tracked by EPA air-quality monitors, with a state-wide 5-year median AQI of 41 — roughly matching the national average of AQI 41. Vilas, Wisconsin ranks #1 with the cleanest air (AQI 34, Grade B), while Milwaukee, Wisconsin sits at the bottom (AQI 49, Grade C). 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.

Wisconsin is bucking the national trend of broad improvement: 23 of 27 monitored cities show measurably worse air over the past decade, more than the 1 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 17 of 27 Wisconsin cities is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days and is the leading air quality concern across much of the Sun Belt and California. Other Wisconsin cities report Fine Particulate Matter (PM2.5) (10) as their dominant concern.

The fastest-improving city in Wisconsin is Fond du Lac, Wisconsin, with median AQI falling by 0.1 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 Eau Claire, Wisconsin, where median AQI is rising by 0.8 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.