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A high-pressure weather system setting up over western Montana is likely to lead to stagnant air until at least Tuesday, accompanied by worsening air quality. That's according to the Montana Department of Environment Quality and the Missoula City-County Health Department.

Sarah Coefield, an air quality specialist with the Health Department, said it will be important for Missoula County residents to limit their use of wood-fired heat.

"This means using an alternative means of heat if one is available," she explained. "If wood is your sole source of heat, it is important to burn as cleanly as possible. Only burn seasoned, dry wood and make sure your fire is hot. A hotter fire is cleaner fire. Hotter fires have more complete combustion and therefore produce less smoke."   

In a constrained mountain valley, high pressure systems often mean particulates are trapped in the valley's lowest portions where humans breathe.

According to the Montana DEQ, a strong ridge of high pressure is expected to build over Montana starting Friday night.

"This will bring very light winds both at the surface and aloft. The light winds, combined with clear skies, will promote the formation of strong overnight inversions across western Montana," said Kristen Martin, an air quality meteorologist with the Montana DEQ, in an email. "Beginning Saturday morning, we will likely see widespread moderate air quality in the valleys. Some areas may see hourly levels reach unhealthy for sensitive groups."

She went on to say many valleys will struggle to lift out from under these inversions, due to poor vertical air movement associated with the stable air mass.

"This will be most prevalent in northwest Montana, although Missoula, Helena and Butte may also struggle," Martin explained. "This will lead to worsening air quality as the weekend progresses and we move into next week. We may see widespread unhealthy for sensitive groups air quality in the valleys by Sunday or Monday morning, with hourly readings possibly reaching unhealthy levels."

In larger valleys in southwest Montana, such as near Dillon and Bozeman, air quality should reach good levels in the afternoons as inversions lift.

Martin said it looks like Missoula may see some relief on Tuesday or Wednesday next week when a more active weather pattern tries to move in. However, she said history shows that these types of high-pressure patterns can be stubborn to break.

Coefield said the Missoula City-County Health Department Air Pollution Control Program will monitor the air and issue Air Alerts or Air Warnings as needed.

Missoula County residents can check the current air quality and air alert status by calling the Air Quality Hotline: 258-3600 or visiting

"Even in the absence of an alert, we ask that all Missoula County residents be mindful of the stagnant air and strive to limit emissions from wood burning fireplaces and wood stoves," Coefield said. "You can also help protect our airshed by using sustainable transportation."

The American Lung Association's annual "State of the Air" report in 2017 listed the city of Missoula along with Ravalli and Lincoln counties among the worst areas in the country for the number of days with small particle pollution that makes breathing the air unhealthy for at least some residents.

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