Air quality deteriorated across much of Montana on Wednesday as smoke from wildfires in Alberta, Canada infiltrated the region overnight.

The Department of Environmental Quality issued an air quality alert for 43 Montana counties, including the Flathead Valley. The smoke is expected to blanket the region through at least Friday, warned Jeff Kitsmiller, a meteorologist at the National Weather Service in Missoula.

Kitsmiller said a shift in the winds pushed the smoke into the U.S. on Wednesday.

“There are a lot of fires up in Canada and that smoke had been building up,” he said. “Then our flow shifted to the north and grabbed all that smoke and sent it right into the Flathead Valley and more.”

The smoke affected a swath from the Washington coast to North Dakota.

“It’s a big wall of smoke coming straight down,” Kitsmiller said.

Air quality in Kalispell was rated as unhealthy early Wednesday morning after smoke concentration levels spiked at around 1 a.m. The worst air quality in the state was reported in the Miles City area, while much of west central Montana escaped the thick haze.

When air quality is rated as unhealthy, health officials recommend that people with respiratory or heart disease, the elderly and children avoid prolonged exertion. Everyone else should limit prolonged exertion.

“People should make sure to limit their time outside, especially when the visibility is lower,” Kitsmiller added.

He said a high pressure system is expected to build over the Northern Rockies by Friday afternoon, which should cut off the smoke and usher in clean air.

Alberta wildfire officials on Wednesday reported 91 active fires across the province. All of the fires were burning north of Calgary and east of the Canadian Rockies.

According to a Canadian Broadcasting Corporation report, around 19,500 Alberta residents had been evacuated from their homes due to the fires.

The National Interagency Fire Center confirmed Wednesday that 10 Hotshot crews and a Northern Rockies IMT had been deployed to Alberta to support the firefighting effort.