ST. PAUL — A haze of wildfire smoke originating in Canada blanketed much of Minnesota Wednesday, July 21, spurring state and federal regulators to issue safety advisories and tips.

Smoke and particulate matter carried by northerly winds from fires in Manitoba and Ontario are affecting air quality in northern, central and southeast Minnesota. The smoke is expected to persist to some degree until early Thursday, July 22.

By late Wednesday, July 21, air quality in the state was poorest in the Twin Cities metro area.

"By (Thursday) afternoon, we expect that all the heavy smoke, at least, will be out of the state," said David Brown, of the Minnesota Pollution Control Agency's Risk Evaluation & Air Modeling Unit.

Smoke from Canada crossed the border into Minnesota at a time when the state is dealing with conditions of its own that are ripe for wildfires. Much of the state is under a drought watch.

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Air quality was worse overall on Tuesday, July 21, according to Brown, and was poorest in the Red River Valley in particular. A heavy smoke hanging over the Brainerd and Bemidji areas moved west to the valley overnight.

Photos of the smoky, hazy week in Minnesota:

By late Wednesday, a swath of Minnesota spanning its southwest and northwest corners was still affected by a haze that the MPCA rated as "unhealthy for sensitive groups." The agency had earlier cautioned children and older adults as well as for those with asthma, chronic obstructive pulmonary disease, heart disease or high blood pressure to limit their time outdoors.

That fires elsewhere are affecting Minnesotans is not unusual, according to Jesse Berman, a University of Minnesota environmental epidemiologist and assistant professor.

"It only makes a difference in those particles getting pushed down to the surface, and then it can be harmful for human health, especially in terms of respiratory and cardiovascular related issues," Berman said in an interview. "And that's what we're seeing right now."

ALERT: Air quality alert in effect for much of Minnesota through Thursday

According to Brown, this particular air quality event set records. Particulate matter in the air was so dense over the Red Lake Indian Reservation and surrounding area on Tuesday, he said, that the MPCA recorded a one-hour concentration there of "397 micrograms per cubic meter."

"And for comparison, the health standard over a 24-hour period is 35 1/2," he said.

Aside from those with underlying medical conditions, Berman said even "a healthy young adult might experience a tickle in the back of the throat" or feel as though they cannot catch their breath, "and that's going to be from the effects of air pollution exposure."

He said to window fans should be avoided in light of current conditions but that consumer-grade air conditioner units have the filtering capacity for safe use.

Regularly updated air quality information is posted regularly at the pollution control agency website. Conditions in Minnesota and elsewhere in the U.S. can also be monitored online at the U.S. Environmental Protection Agency and U.S. Forest Service's AirNow web tool.

"The updated map provides additional tools to help communities near the front lines better understand their risks from wildfire smoke and the actions they can take to protect their health during wildfire events," EPA Administrator Michael S. Regan said of the tool in a recent news release.

Air travel in the Upper Midwest, meanwhile, does not appear to be impacted by the air quality event. A Federal Aviation Administration webpage showed airports in the region reporting arrival and departure delays of 15 minutes or less by late Wednesday.