Heavy smoke from Canadian and Alaskan wildfires descended on much of Minnesota on Monday, causing some of the worst air-quality levels in almost a decade, according to the Minnesota Pollution Control Agency.

The dense haze hung over a large swath of Minnesota, prompting the MPCA to warn that the air was unhealthy for everyone in those areas.

The air quality improved by mid-evening for most of the state.

On an air-quality scale that ranges from 0 to 200, some parts of Minnesota, including the Twin Cities, pushed into the mid-170s and 180s, said Steve Mikkelson, a spokesman for the MPCA.

"That's unusually high," Mikkelson said, pointing out that he hasn't seen readings this high for more than a decade. "We've had alerts in the 'unhealthy for everyone' range of the scale before, but not this high."

A map showing some of the readings was not accurate at times on Monday because equipment used to monitor air quality in Detroit Lakes and St. Cloud wasn't functioning correctly, he said. So the good air portrayed on the map in those areas may not have been so good.

The MPCA issued some air quality alerts late last week, Mikkelson said. But the rains on Sunday helped clear the air. "Then another weather front brought a new wave of this problem. … We're at the mercy of the way the winds blow."

The massive plume of smoke that hovered over Minnesota on Monday was due in part to the location of the fires in Canada and Alaska and a storm system that pushed air downward.

In Saskatchewan alone, more than 110 forest fires are burning because of dry, hot weather there and an unusually thin snowpack from last winter, according to news reports.

"The same storm that dumped 2 to 7 inches of rain in central and southeastern Minnesota also helped to pull some of the smoke down to the surface," said meteorologist Paul Douglas.

"Usually this plume of smoke is a few miles above the ground. It was a little disconcerting to see visibility down to a half mile to a mile because of smoke. It looked like LA on a bad day."

At some points during the day on Monday, air quality in the Twin Cities was as bad or worse than in such cities as Los Angeles, Beijing and Sao Paolo, Brazil, where air quality is notoriously poor at times.

Douglas said rain can sometimes wash pollutants out of the air, and the strong downward motion of the air can cause skies to clear.

"We should have seen blue sky, but we saw haze and smoke from the northern fires. … It's unusual to be smelling smoke from fires that far away."

The MPCA on Monday warned people to postpone exercising outdoors when the air quality pushes into "unhealthy for everyone" range. And if people have to work outdoors, they should take frequent breaks, Mikkelson said.

"We urge people to stay inside as much as you can," he said. "Take it easy, and if you start feeling the effects, take it seriously. Try to get indoors and rest up until you feel better."

People more sensitive to air quality problems — the elderly, the very young and those with cardiovascular or respiratory diseases — could experience health effects sooner and more seriously when pollution gets to the levels it reached on Monday. For that group, the air is unhealthy when the index ranges from 101 to 150.

The smoke-filled air also grounded some pilots at some of the state's smaller airports and delayed flights at Minneapolis-St. Paul International Airport, where only two of the three runways were in operation, said airport spokesman Patrick Hogan.

All planes were directed to the two runways that are lit for low visibility, he said. Planes also had to be spaced out more between landings and takeoffs because of the limited visibility, Hogan said. This caused delays that ranged from 15 to 30 minutes until about 8 p.m. Monday, he said.

The smoky air should clear a bit on Tuesday and maybe into Wednesday, according to Tony Zaleski, meteorologist at the National Weather Service. But it could return later, he said.