FARGO — Thick haze that shrouded the Red River Valley resulted in unhealthy air quality — including a brief spell of very unhealthy air Wednesday, July 21 — when smoke from Canadian wildfires lingered.

The arrival of the smoky Canadian air, which took a roundabout route in reaching the Red River Valley after first swooping through the Twin Cities, delivered some of the worst air quality in recent memory.

AirNow, a service of the Environmental Protection Agency, reported that as of 9 a.m. on Wednesday, Fargo-Moorhead had an air quality index of 161, which is unhealthy to breathe.

Earlier Wednesday morning, the air quality index in Fargo-Moorhead had reached 195 — a level considered very unhealthy by AirNow. By 3 p.m. Wednesday, however, the air quality index had improved to 99, moderate air quality.

AirNow quality index readings for Grand Forks were not available Wednesday afternoon, but the BreezoMeter app indicated that Grand Forks had moderate air quality with an index of 69.

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The bout of unusually poor air quality was the result of the smoky Canadian air coinciding with a temperature inversion in which cooler air near the surface helped trap the haze, said Stormtracker chief meteorologist John Wheeler.

The plunge in air quality began in the wee hours, Wednesday, as Wheeler was walking home after a late broadcast. He could see particles swirling in the air in the glow of street lights.

“That’s the worst I can recall,” Wheeler said, adding that it doesn’t mean that was the worst the area has experienced. “It was terrible.”

At the time, the visibility in Grand Forks had dropped to one mile. “That’s very thick smoke,” Wheeler said.

On Wednesday morning, AirNow’s national air quality map showed the area surrounding Fargo-Moorhead and the Red River Valley to have some of the worst measured air quality in the nation.

Air quality conditions, which were improving as the afternoon progressed Wednesday, thanks to a strong southerly breeze, should continue to get better in the short term, Wheeler said.

“It’s clearing up hour by hour and it should be much better for a day or two,” he said.

The Minnesota Pollution Control Agency issued an air quality alert for northern, central and southeast Minnesota. The alert is active through 6 a.m. Thursday. Affected areas include Moorhead, East Grand Forks, Bemidji, Brainerd, St. Cloud and the Twin Cities.

“Smoke levels will gradually improve and particulate is forecast to drop below Orange” — unhealthy for sensitive groups — “in all locations by Thursday morning,” according to the MPCA air quality forecast. On Thursday “southerly flow will continue to clear out the smoke. Some smoke/haze will linger, but air quality indexes should improve to the moderate range, the MPCA predicted.

The many wildfires throughout the American West and Canada will continue to burn, so weather conditions will periodically bring the smoky and hazy air to the Red River Valley, Wheeler said.

The valley is most affected by wildfires raging in Manitoba and Ontario north of Lake Winnipeg, he said. Generally, the smoke that reaches the valley from the West stays in the upper atmosphere and produces a milky sky but doesn’t cause air pollution near the ground, Wheeler said.

Jim Semerad, director of air quality for the North Dakota Department of Environmental Quality, said the smoky air drifting into the state from the West has been enough to diminish the state’s air quality ratings from the American Lung Association.

“It’s really had an increasing impact,” he said, referring to fine particles from wildfire smoke. Although areas of North Dakota can experience periods of unhealthy air quality, those spells are brief, Semerad said.

“If this was an industrial source, we would have an enforcement action available,” he said, something not possible for wildfires hundreds of miles away. “The important story is generally our air is very, very good.”

The periodic poor air quality affects people differently. Some, including those with heart and lung conditions, are much more sensitive than others, Semerad said.

“The message here is the wildfires can have a major impact on our air quality, but it’s almost always on a sporadic basis,” he said.