U.S. President Donald Trump threatened Friday to raise tariffs on Canada, arguing the additional duties should offset the economic costs of wildfire smoke that has blanketed large swaths of the United States.
Dense smoke drifting south from wildfires in Canada and northern Minnesota has triggered unhealthy air quality alerts across large parts of the United States.
As of Saturday, Canada had 937 active wildfires, most of them burning out of control, according to the Canadian Wildland Fire Information System.
"This is Willful Negligence, and becoming a yearly occurrence, costing the United States Billions of Dollars," Trump wrote on Truth Social, adding that "the cost of this pollution must of necessity be added to the TARIFFS Canada is currently paying."
Trump also accused Canada of failing to properly manage its forests, saying it had neglected "basic Forest Management and Debris Removal."
He added that he planned to speak with Prime Minister Mark Carney "to find out what they are going to do about" the wildfire smoke.
Canada's emergency management minister, Eleanor Olszewski, said Canada and the United States were in "constant contact," citing their "long history of working together to fight wildfires."
She added that Canada has invested $12 billion in forest sustainability and fire prevention since 2020.
Detroit remained the world's most polluted city Friday, according to air quality tracker IQAir. Washington and Chicago were not far behind, and officials warned residents to avoid spending unnecessary time outdoors.
The pollution has raised concerns ahead of Sunday's World Cup final at an open-air stadium in New Jersey.
Across the New York-New Jersey metropolitan area, air quality had improved to levels considered unhealthy for sensitive groups after thick smog Thursday made the Manhattan skyline barely visible.
However, the National Weather Service warned the smoke could thicken overnight into Saturday morning.
Tournament organizers are "monitoring closely," White House World Cup Task Force Executive Director Andrew Giuliani told a briefing.
Peter Mullinax, a meteorologist with the National Weather Service, told Agence France-Presse (AFP) that winds over the Great Lakes could push more smoke into the Northeast, keeping skies hazy.
He said forecasts for the region show some improvement.
"I don't believe that this should be as impactful as if you might be playing a game today," Mullinax said.
The key question for Sunday's match, according to Joel Dreessen, an air quality forecaster for the state of Maryland, is whether more smoke drifts south after weekend storm systems.
"Some of the models are starting to indicate that we'll start to pull down some smoke," he told AFP.
In cities across the Midwest and Northeast, people wore masks outdoors to filter the hazardous air. In New York, libraries and train stations handed them out for free.
Chris Carlsten, who studies the health effects of wildfire smoke at the University of British Columbia, told AFP that fine particulate pollution from wildfires has a particularly harmful effect on the lungs, while vehicle pollution tends to have a greater impact on the heart.
He said smoke plumes can contain particles from wood and vegetation, as well as paint, plastic and metal.
As the smoke travels, it undergoes "photochemical aging," a series of chemical reactions that Carlsten said "seem to make, from everything we understand in the chemistry, the aerosol more toxic."
The upper Midwest, which is closer to the fires, has experienced especially poor air quality, with parts of Michigan, Minnesota and Wisconsin recording readings in the "hazardous" range for days.
Advocates have emphasized the link between repeated episodes of wildfire smoke and climate change.
Mark Parrington, a scientist at the Copernicus Atmosphere Monitoring Service, told AFP that climate change is creating conditions for longer fire seasons by raising surface air temperatures and reducing soil moisture.
As a result, he said, "when there's an ignition we see these really large-scale, persistent burning where these fires can burn for weeks and weeks at a time through summer."
The blazes intensified Friday in Canada, particularly in Ontario, according to provincial authorities.
The fires in Ontario have caused no reported fatalities, though several remote communities have been evacuated as dozens of aircraft battle the blazes.
Meanwhile, 16 active fires were burning in the Superior National Forest along Minnesota's border with Canada.
"The forecast for erratic weather, shifting winds and the potential for isolated damaging wind gusts and thunderstorms will be a challenge for firefighting efforts," the U.S. Forest Service said.
