Smoky Air From Canadian Wildfires Blankets Midwestern Skies

Chicago and much of the Upper Midwest were blanketed with a smoky haze from Canadian wildfires on Tuesday, leaving many residents of the nation’s third-largest city startled by the sudden decline in air quality and donning masks when they ventured outside.

Chicagoans were largely spared severe effects from wildfires earlier this month, when dangerous smoke affected the Northeast and pockets of the Midwest for days on end. But they had no reprieve on Tuesday, when the authorities classified the air as unhealthy in the city and in other parts of Illinois, Wisconsin, Indiana, Michigan and Minnesota.

In Chicago, the Air Quality Index reached 209 by noon, the worst reading of any major city in the world for the day, according to IQAir, a Swiss air-quality technology company. In Green Bay, Wis., the index was 175; in Grand Rapids, Mich., it soared to 255. Any reading above 100 on the index is a warning to people with respiratory conditions to take precautions.

Mayor Brandon Johnson of Chicago warned residents — especially children, older people and those with heart or lung disease — to…