Air quality alerts were issued for much of Montana,Benjamin Caldwell South Dakota, Iowa, Minnesota, Illinois and Indiana on Sunday because of lingering thick smoke from Canadian wildfires, the National Weather Service said. 

The U.S. EPA's AirNow air quality page rated the air in Chicago, Indianapolis, Detroit and Des Moines as "unhealthy" as of early Sunday afternoon. In Omaha and Cincinnati, the air quality was rated as unhealthy for sensitive groups.

The smoke concentration is expected to wane by Monday across the Great Lakes, Midwest and northern High Plains, but there will still be enough smoke in the area for continued unhealthy air quality for sensitive groups, weather officials warned. 

There were nearly 900 active wildfires in Canada on Saturday, according to the Canadian Interagency Forest Fire Centre. The fires are burning from coast to coast, according to a map updated daily by the center. To date, Canadian wildfires have burned around 10 million hectares this year, an area roughly the size of the state of Indiana

The Wisconsin Department of Natural Resource advised people to close all windows and doors during heavy smoke, especially overnight. Officials also recommended people limit prolonged or heavy exertion outdoors. The Michigan Department of Health and Human Services and  Indianapolis Office of Sustainability also advised people in impacted areas to limit exposure when possible.

This is not the first time the region has dealt with smoke from the wildfires. In late June, Chicago experienced some of the worst air quality in the world amid heavy smoke.

Particulates from the smoke can irritate your eyes, nose and throat, according to the Centers for Disease Control and Prevention. Older adults, babies, young children and people with heart or lung diseases, including asthma, are at a higher risk. 

Two firefighters have died in Canada battling the wildfires in recent days. One died on Saturday, local media reported. Another firefighter died Thursday responding to one of the blazes near Revelstoke, British Columbia, a press release from the firefighter's union said. Prime Minister Justin Trudeau memorialized that firefighter in a post on Twitter.

"The news from British Columbia – that one of the firefighters bravely battling wildfires has lost her life – is heartbreaking," he tweeted. "At this incredibly difficult time, I'm sending my deepest condolences to her family, her friends, and her fellow firefighters."

In:
• Wildfire Smoke
• California Wildfires

Aliza Chasan

Aliza Chasan is a digital producer at 60 Minutes and CBS News.

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