Ashley Coil, a concierge at the Columbia Tower Club in Seattle, which is on the top floor of the 76-story Columbia Center, the tallest building in the city, said the smoke moved into the region at the worst time. The building usually offers one of the best sights to watch the Blue Angels zip around the city, Ms. Coil said.

But over the weekend, the smoke was so thick that the fighter jets could be heard but not seen.

“You couldn’t even see a few feet in front of you,” Ms. Coil said in an interview. “It’s pretty weird because usually you can see all of the city and the waterfront.”

The rampant devastation in Canada, declared the “Summer of Fire” by CBC News, has destroyed hundreds of homes. About 6,900 people are living under evacuation orders. And one man who lost his home during an Alberta wildfire last year has now lost his log cabin in British Columbia, the Canadian news network reported.

A #BlameCanada movement sprang up on Twitter. But the British Columbia fires, some of the worst in decades, aren’t the only source of the haze.

“There are several fires in the Pacific Northwest that have contributed to the smoke as well,” said Mr. Apfel, who estimated that there were currently about 16 large fires in Washington and Oregon. “Previous years have had quite a bit of rainfall, and you get a lot of vegetation — and when it dries up, that’s what starts burning when the fire starts.”

The haze was especially notable as it settled over the region’s two largest cities — Seattle and Portland, Ore. — but it also languished over farmland and rivers. And weather forecasters said the days of haze probably kept some temperature records intact as the region sweated beneath a high pressure ridge that compressed and warmed the air below.

Harborview Medical Center in downtown Seattle, part of UW Medicine, has seen “an increase over the past week in exacerbations in people who have chronic obstructive pulmonary disease, which includes emphysema, and also in people who have asthma,” Leila Gray, a spokeswoman for the hospital system, said in an email on Monday evening.

Air quality has improved in parts of the West Coast as cool marine air has moved inland. But for those who are still peering through the haze, officials recommend taking precautions, like avoiding strenuous activity outdoors, to stem any negative effects. The Oregon Department of Environmental Quality has advised the most vulnerable to consider leaving the area altogether, as the wildfire smoke can lead to a higher risk of illness.