Dense smoke from several large wildfires raging in Canada and Minnesota is expected to blanket large swaths of the Midwest and northeastern United States this week, exposing millions of people to dangerous air pollution.

Minnesota officials issued an air quality alert Tuesday through Friday for areas including the Twin Cities metropolitan area, Alexandria and Two Harbors, with very heavy smoke expected in the northeast corner of the state as large wildfires spread. Air quality levels in Two Harbors, the Grand Portage Tribal Nation and other regions of northeastern Minnesota were expected to reach hazardous levels, making it unsafe for everyone.

Dan Westervelt, an associate professor at Columbia University’s Climate School, said severe drought conditions combined with heat in Canada and the United States have created “a perfect storm for really dry conditions to provide a lot of fuel for these wildfires to burn.” Research shows that rising temperatures due to the burning of coal, oil and gas are making fires more frequent and intense.

People should stay indoors as much as possible to avoid extreme heat, especially when smoke comes in, said Tyler Hasenstein, a meteorologist with the National Weather Service in Chanhassen, Minnesota.

“For those two things to coincide is not good from a health point of view,” he said.

Authorities in Michigan and Wisconsin warned residents Tuesday of air quality problems that could last for days. High levels of fine particles in the air from wildfire smoke can be harmful to the health of sensitive groups, such as children and people with heart or lung disease.

In parts of Maine, residents reported a yellowish-brown color to the sky. “There is a lot of smoke upstream, so expect periods of hazy skies throughout the next day before the cold front moves through,” the National Weather Service in Gray, Maine, said on social media Tuesday.

By Wednesday afternoon, heavy smoke will spread toward the East Coast and Midwest, including parts of coastal New England, northern Pennsylvania, Detroit and Milwaukee, Hasenstein said.

Several counties in western and central New York were under an air quality advisory Wednesday until midnight. The heaviest smoke could extend as far south as Washington, D.C., by midday Thursday, Hasenstein said.

Fine particle pollution from wildfire smoke can cause shortness of breath, cough, dizziness or fatigue and aggravate heart and lung disease and other chronic health problems. Experts suggest reducing or eliminating outdoor activities, wearing an N95 mask if you must be outside, and keeping indoor air cleaner by closing windows and turning on an air purifier or air conditioner. Long-term exposure to fine particles from wildfire smoke is one of the leading causes of premature death.

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