Much of eastern Wisconsin is breathing unhealthy air on Thursday, as the Wisconsin Department of Natural Resources warns that ground-level ozone will reach harmful levels from Green Bay south to the Illinois line — part of a broader bout of summer smog spreading across the Great Lakes and Ohio Valley.
Where and When the Advisory Applies
The Wisconsin DNR issued an Air Quality Advisory for ozone running from noon until 11 p.m.
Thursday across 21 counties spanning southeast, east-central and northeast Wisconsin — including Milwaukee, Racine, Kenosha, Sheboygan, Fond du Lac, Appleton, Oshkosh and Green Bay.
Most of the region is forecast to reach the “Orange,” or Unhealthy for Sensitive Groups, level. But forecasters warned that conditions could be worse right along the shoreline: “Red,” or Unhealthy for everyone, ozone is possible within the lake breeze from Sheboygan south to Kenosha. A breezy south-southwest flow was expected to keep that pocket of dirtier air pinned near the lakefront rather than pushing it far inland.
The Lake Breeze That Traps the Ozone
The lakefront geography that makes Wisconsin’s shoreline so pleasant in summer is also what concentrates its pollution. On warm, sunny days, a cool lake breeze develops off Lake Michigan and pushes inland, and it can carry a dense ribbon of ozone with it. According to coverage from WKOW, those elevated concentrations on Thursday were expected to stay confined to that narrow lake-breeze zone.
Ground-level ozone forms when sunlight and heat drive a reaction among pollutants such as nitrogen oxides and volatile organic compounds. The lakeshore’s sunny, stagnant warm-season afternoons are the classic trigger.
Pollution That Drifts In From Elsewhere
A frustrating wrinkle for the state is that much of the raw material is not homegrown. The DNR notes that ozone-forming pollutants emitted by upwind states are transported over Lake Michigan, where they react and are then carried onshore by the lake breeze, repeatedly pushing the state’s lakeshore counties out of compliance.
Gail Good, the DNR’s Air Management Program director, has noted that although most of that pollution does not originate in Wisconsin, the state remains “responsible for meeting federal air quality standards regardless of where the pollutants originate.” The Milwaukee and Sheboygan areas were reclassified by the EPA in early 2025 as “serious” ozone nonattainment areas.
How to Protect Yourself
The DNR’s guidance is tailored to risk. Sensitive groups — children, older adults and people with asthma or other lung conditions — should avoid long or intense outdoor activity and shift exercise to the cooler morning hours, when ozone is lower, or move it indoors. People with asthma were urged to keep quick-relief medication handy.
Everyone else was advised to ease up on strenuous outdoor exertion, take more breaks and schedule activity earlier in the day. Residents can track real-time readings and sign up for alerts through the DNR’s air quality portal as the agency reassesses conditions each day.
