DENVER — Wildfire smoke blanketing much of the Western and Northwestern U.S. is setting off health alarms, but firefighters say it’s helping them by containing temperatures and boosting humidity.

“It’s holding down our afternoon high temperatures by nearly 10 degrees,” said Bryan Henry, assistant manager of prediction services for the National Interagency Fire Center, which coordinates wildfire-fighting.

The lower temperatures keep humidity higher, Henry said. The vegetation absorbs the moisture — grasses more quickly than trees — and makes it less flammable.

Moist air also helps firefighters because it’s more stable than dry air, Henry said. Moist air tends to rise more slowly than dry air does when it warms up.

“It’s a shame when it gets where the smoke is literally so bad it actually helps you,” he said.

Any help is welcome amid a fire season on track to be at least the third-worst in a decade. Crews were trying to control 82 major fires in 10 Western states on Friday, up from 76 fires in nine states the day before, the interagency fire center said.

The fires were burning on about 2,300 square miles (6,000 square kilometers). Montana had 26 large fires, Oregon 18 and California 14.

Wildfire smoke has clouded the region since last weekend, when a high-pressure system moved in, said Bill Wojcik, a National Weather Service meteorologist in Boise, Idaho.

The high-pressure system held the smoke against the ground and spun it slowly in a clockwise direction, Wojcik said. No rain fell to cleanse the air, and no winds blew to move the smoke out.

“It didn’t really have a chance to move anywhere, just circulate around that big high,” Wojcik said.

In the daytime, the smoke reflected some solar radiation back into the atmosphere, the same way clouds do, and that kept temperatures down, Wojcik said. But unlike clouds, smoke does not trap heat against the ground at night, instead letting it radiate away.

The smoke is starting to clear as the high pressure moves east and a low-pressure system moves in from the west, Wojcik said. Winds from the south are beginning to flush the smoke out of Idaho and eastern Oregon with cleaner air, he said.

The health effects are still being added up, and state and local officials say they have mostly anecdotal reports so far.

“People are getting sick. That’s the whole long and short of it,” said Sarah Coefield, an air quality specialist at the Missoula City-County Health Department in western Montana.

Wildfire smoke is especially dangerous to people with chronic heart and lung problems, said Julie Fox, an environmental epidemiologist with the Washington State Department of Health.

The smoke is a combination of several toxins, and its tiny particles can be inhaled deeply into the lungs, she said.

“We do expect to see increases in hospitalizations when the air quality is this bad,” Fox said.


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