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 stay indoors if possible, or to wear masks if they had to be outside, to avoid the worst effects of the smoky air.

“This summer, cities across North America have seen unhealthy levels of air quality as a result of wildfire smoke, impacting over 20 million people from New York City; Washington, D.C.; Montreal; and today here in Chicago,” Mr. Johnson said in a statement. “As we work to respond to the immediate health concerns in our communities, this concerning episode demonstrates and underscores the harmful impact that the climate crisis is having on our residents, as well as people all over the world.”

Similar levels of smoke, and possibly even thicker concentrations, were expected Tuesday in other cities in the region, including Milwaukee.

“Today would normally be looked at as a beautiful day to go outside,” forecasters with the National Weather Service office in Milwaukee wrote. “But smoke is reducing visibility to only a mile to three miles today.”

Conditions are expected to improve in Chicago and Milwaukee overnight, but the haze is expected to linger on Wednesday.

Officials in New York warned that smoky skies may return to their state on Wednesday. Gov. Kathy Hochul said that air quality could be poor across the western and central parts of the state, and other areas may also be affected.

“Due to incoming winds tonight and ongoing Canadian wildfires, New York City could experience worse air quality tomorrow,” Mayor Eric Adams said on Twitter Tuesday, reminding New Yorkers to take a mask if they leave home on Wednesday.

Many people in Chicago seemed caught off guard by the sudden influx of smoke, since the city is rarely affected by air pollution from wildfires. “Has anyone noticed a weird acrid smell in the neighborhood?” one North Side resident asked on a Nextdoor forum Tuesday morning.

Summer camps scrambled to make new plans for children, keeping them indoors and away from the polluted air. Along the shore of Lake Michigan, in areas that are usually thick with runners, bicyclists and beachgoers during Chicago’s mild summers, the lakefront seemed largely deserted on Tuesday.

The state veterinarian’s office in Michigan issued an alert to people who own animals, reminding the owners that strenuous activities should be avoided — and that even birds can be affected by unhealthy air.

The Minnesota Pollution Control Agency on Tuesday issued its 23rd air quality alert of the year — a record high — as a layer of smoke hovered over much of the state.

David Brown, an air quality meteorologist at the agency, said Minnesota usually issues no more than two or three air quality alerts a year. The previous record was 21 alerts for all of 2021, he said.

Mr. Brown said the wildfire season in Canada usually doesn’t even begin until early July, so Americans in northern states may have to face the threat of poor air quality for weeks to come.

“There are a lot of fires that are getting very large, with the hottest days yet to come,” he said. “These fires will likely continue to grow.”

The latest air quality alert for Minnesota, issued Tuesday morning, was set to expire on Thursday. It covers much of the southern and eastern areas of the state.

Judson Jones and Ernesto Londoño contributed reporting.

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