Wisconsin air quality today

Wisconsin’s ozone problem is shaped like a ribbon. On warm days with a southerly wind, emissions out of the Chicago area travel north over Lake Michigan; the cold water holds the air above it stable and shallow, so the mixture reacts in sunlight instead of dispersing upward. In the afternoon the lake breeze pushes that layer back ashore, and a monitor within a few miles of the water records a number that a monitor an hour inland never sees.

The regulatory map follows the same geometry. All of Milwaukee and Ozaukee counties, plus parts of Washington, Waukesha, Racine, Sheboygan and Kenosha, are nonattainment for ozone under the 2015 standard, on boundaries that stop a short way inland rather than following county lines. Which classification applies has been unsettled since: EPA’s reclassification of the area from moderate to serious took effect in January 2025, the Seventh Circuit stayed that action in September 2025, and EPA granted the Department of Natural Resources’ petition to reconsider the rule in July 2026. Moderate requirements have applied throughout.

For reading any Wisconsin number, distance from the lake matters more than distance from a city. A shoreline reading at Sheboygan or Kenosha and an inland reading at Madison or Wausau on the same July afternoon describe two different air masses, and there is no page here for most of the places in either group.