Canada Wildfires and U.S. Air Quality: What to Know and How Long the Smoke Will Last

As Canada grapples with one of the worst wildfire seasons in decades, heavy smoke from Quebec was wafting into the American Midwest on Tuesday, a day after NASA said it had crossed the Atlantic Ocean all the way to Europe.

Canada has been struggling to fight an extraordinary outbreak of wildfires across the country that has forced tens of thousands of people from their homes, deepened concerns about global warming and, this month, sent choking smoke billowing down the East Coast of the United States, from New York City, past Washington, and as far west as Minnesota.

How has the smoke affected air quality in the American Midwest?

On Tuesday, Canada’s wildfires were worsening air quality in Chicago, Detroit and Milwaukee, among other places. In Chicago, the Air Quality Index reached 209 by noon on Tuesday, the worst reading of any major city in the world for the day, according to IQAir, a Swiss air-quality technology company. (The index climbed higher in Chicago on Wednesday morning to 216, though the air quality was worse in Detroit, where the index hit 306.)

In Green Bay, Wis., the index was 175; in Grand Rapids, Mich., it soared to 255. Any reading above 100 on the index is a warning to people with respiratory conditions to take precautions.

Such figures are rare in the United States; an index above 200 is considered “very unhealthy” for everyone, and one above 300 is labeled “hazardous.” In early June, there were readings above 400 on the East Coast.

How long will the smoke linger this time?

On Tuesday, a storm system located just northeast of the Great Lakes was producing a counterclockwise wind, channeling the smoke produced by wildfires in Canada south into the Midwest. As the system spins its way eastward, the smoke is also likely to shift to the east. However, the storm system was expected to exit the region rather quickly.

Nevertheless, hazy and smoky skies will likely continue to be a regular occurrence in much of the United States this summer.

How many fires are burning in Canada?

On Tuesday, the Canadian Interagency Forest Fire Centre reported 485 active fires burning across the country, of which 257 were out of control.

As of June 7, blazes this year in Canada had already scorched more than 9.8 million acres of forest — more than 10 times the acreage that had burned by around this time last year, officials say.

How far has Canadian wildfire smoke spread?

The blazes in Canada are so intense that NASA said Monday that satellite imagery showed smoke from northern Quebec had crossed all the way to Europe, darkening skies in southwestern Europe, drifting over parts of northern Portugal, Spain and France.

How are the wildfires affecting air quality — and summer fun?

In Canada, Environment Canada warned Tuesday that air quality would deteriorate overnight in Toronto, Canada’s largest city and financial capital. Poor air quality has also buffeted Montreal, where the sun has appeared in recent days as a lurid red dot.

The poor air quality led to the cancellation last weekend of an Ironman triathlon race in Mont Tremblant, Quebec, while summer recreation has been dampened by the closure of some pools and beaches in parts of Quebec and Ontario.

What role does global warming play?

Climate research suggests that heat and drought associated with global warming are major reasons behind the number of fires and their intensity.

Canada has the world’s largest intact forest ecosystem, and many parts of the country have recently experienced drought and high heat. That can make trees vulnerable to fire and can dry out dead grass, pine needles and any other material on the forest floor that can act as kindling.

Judson Jones in New York contributed reporting.