Wisconsin remains under an air quality alert as wildfire smoke continues to affect the state, though conditions are expected to improve as shifting winds help mix out the smoke. Earlier, the alert was in effect statewide until noon.

Air quality levels remained poor across much of the Mid-Atlantic early Saturday after smoke from massive wildfires in Canada billowed into the United States and spread across much of the country.

Detroit’s air quality was among the worst in the world for major cities, as a lingering high pressure system trapped smoke from dozens of fires in Canada and northern Minnesota, said Steven Freitag, a...