An especially virulent wildfire season in Canada is pouring smoke high into the atmosphere, where the jet stream is pushing the noxious byproducts into the U.S. across the West and the Midwest.
Canadian wildfires have consumed hundreds of thousands of acres and sent dangerous pollutants across the longest border in the world. From the towering Rocky Mountains in Colorado to the 10,000 lakes in Minnesota, air quality alerts have been reported in recent weeks, endangering everyone but especially the young, the elderly and those with lung diseases.
"At one point the entire state was under extreme air alert," Steve Mikkelson, a spokesman for the Minnesota Pollution Control Agency, said Wednesday. "The readings we got on Monday, I had not seen a reading that high before in Minnesota. I don't know if it was a record, but it is very unusual for Minnesota."
Various levels of air quality alerts have been posted in recent days in western and central Washington, Colorado, parts of North Dakota, South Dakota, eastern Nebraska, Iowa, Illinois and Missouri.
Haze lingered Wednesday over much of Colorado, where officials said they hoped coming rains would scrub the atmosphere of small particles of ash and other matter from the Canadian fires. People were urged to limit their outdoor exertion in areas where the visibility was less than five miles.
Residents of Denver and its suburbs awakened to a second day of weirdly dark skies, a soupy mix of smoke and fog cloaking the Front Range and bringing flight delays and warnings from the Colorado State Patrol to drive safely. The smell of smoke lingered.
"When I woke up this morning, all you could see was this huge haze over Denver," said Trevor Adigwe, 23, who lives near downtown. "I was up north and probably couldn't see a mile because of the fog."
His friend Zach Sigwart, 26, said the sky looked "blurry."
"It was so strange," he said. "I didn't even know what it was."
The freakish weather is out of character with Colorado's unofficial motto of having "300 days of sunshine a year."
"I have lived here for 60 years and I can tell you that it's very strange to see fog. I have seen fog maybe five times in my life here," said Christy Frank as she stood outside her office in a drizzle. "The skies were so dark around Interstate 25 this morning that it looked like nighttime. Something strange is happening."
Most of eastern Colorado, including all of the northern and southern Front Range and all of the state's southeast, was under high alert.
The Canadian fires "are hitting us pretty hard," said Christopher Dann, a spokesman for the air pollution division of the Colorado Department of Public Health and Environment.
"Smoke is incredibly dynamic," Dann said. "So far, we have had a multiple-state, regional impact starting from north to south, then shifting east and southerly. But things can change within a day, depending on the wind."
British Columbia, Canada's Pacific Coast province, has been parched by drought in the last three months, prompting a ban on open fires throughout the province and water restrictions in several cities. Saskatchewan province, just over the border from Montana and North Dakota, also has been hit exceptionally hard, according to Canadian officials.
There have been 582 wildfires in Saskatchewan so far this season, compared with 210 in the same period last year, according to the provincial government. Evacuations have been ordered in 54 communities, with more than 13,000 people forced to leave their homes. A dozen structures have been destroyed, but no fatalities have been reported.
Hundreds of soldiers were headed to Saskatchewan to help firefighters.
In British Columbia, 911 fires have been reported this year, and 184 are still burning, according to the provincial government. The active blazes have consumed about 543,600 acres and have been blamed for at least one death, officials say.