Wildfire smoke blowing into the U.S. from Canada triggered air quality alerts in the Midwest on Wednesday and Thursday after the fires brought hazy conditions to a swath of the East Coast last week.

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Officials with the National Weather Service warned Thursday that parts of southern Minnesota and all of Wisconsin could see air quality deemed unhealthy or unhealthy for sensitive groups.

“In these areas, people with heart or lung disease, older adults, and children should avoid prolonged or heavy exertion, while everyone else should reduce prolonged or heavy exertion,” according to NWS officials in Wisconsin.

Officials with the Minnesota Pollution Control Agency on Wednesday said the Twin Cities saw what might be the highest air quality index number in the area since recording began in 1980. Higher air quality index numbers indicate greater pollution in the air.

The Minnesota Pollution Control Agency noted that the numbers fell short of topping the worst air quality index numbers seen in the state.

On Wednesday, the North Dakota Department of Environmental Quality warned of ongoing smoky conditions across the northcentral and eastern parts of the state.

“In general, Environmental Quality advises everyone to use common sense and avoid prolonged exposure outdoors,” officials said in a statement. They added that, with the wildfires still burning and wind patterns changing, “air quality can be variable and unpredictable. Air quality may improve at times or get worse very quickly.”

Officials expect the smoke to clear from the region by Friday afternoon.

The smoky conditions stem from wildfires burning in the southern half of Canada, from British Columbia and part of the Northwest Territories in the west to Quebec in the east, according to the National Oceanic and Atmospheric Administration.

On Thursday, a thicker area of smoke stretched south from the northern and central High Plains to the western Great Lakes, while thinner smoke stretched from the middle Mississippi Valley to the eastern parts of the country. Smoke from the blazes blew as far east as Europe, according to NOAA.