New York (AFP) - Dense wildfire smoke billowing down from Canada and northern Minnesota set off unhealthy air quality alerts across the United States on Friday, triggering concern over the weekend’s World Cup final outside New York.

US President Donald Trump lashed out at Canada – where authorities said more than 200 fires were burning out of control on Friday – calling the pollution “totally unacceptable.”

Detroit, in the US Midwest, remained the most polluted city in the world, according to tracker IQAir. Washington and Chicago weren’t far behind, and officials warned against spending unnecessary time outside.

In New York and neighboring New Jersey, where the World Cup football final will be played on Sunday in an open stadium, the metro area was experiencing air that could be unhealthy for sensitive groups, an improvement after smog on Thursday made the Manhattan skyline barely visible.

But the National Weather Service (NWS) warned the smoke may 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 for NWS, told AFP that winds over the Great Lakes could push more smoke into the Northeast, which could keep skies hazy.

He said forecasts for that region do expect some improvement.

“I don’t believe that this should be as impactful as if you might be playing a game today,” Mullinax said.

The issue for Sunday’s game, said Joel Dreessen, an air quality forecaster for the state of Maryland, is whether more smoke spills 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.

Trump said he would call Canadian Prime Minister Mark Carney “to find out what they are going to do about” the smoke.

“The United States is being unnecessarily invaded by filthy, polluted, and unhealthy air, the quality of which is dangerous, and totally unacceptable!” Trump wrote on Truth Social.

- Traveling toxicity -

In cities across the Midwest and Northeast, people wore masks outdoors to filter out the dangerous air. In New York, libraries and train stations were handing them out for free.

Chris Carlsten, who studies the health impacts of fire smoke at the University of British Columbia, told AFP that the fine pollution particles from wildfires are particularly impactful on the lungs, whereas vehicle pollution skews slightly towards heart impacts.

He said plumes can be filled with wood and vegetation debris but also paint, plastic or metal.

And as smoke plumes travel, they undergo “photochemical aging,” a series of reactions that Carlsten said “seem to make, from everything we understand in the chemistry, the aerosol more toxic.”

The upper Midwest that is closer to the fires has faced especially bad air, with parts of Michigan, Minnesota and Wisconsin all recording air quality readings deep into the “hazardous” range for days.

- Climate change link -

Advocates have stressed the connection between repeated episodes of wildfire smoke and climate change.

Mark Parrington, a scientist at the Copernicus Atmosphere Monitoring Service, told AFP that climate change was providing conditions for a longer fire season, with higher surface air temperatures and lower soil moisture.

So, he explained, “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 were worsening on Friday in Canada, where more than 200 fires were burning out of control, especially in Ontario, according to authorities there.

Fires in Ontario have not caused any casualties, and several remote communities have been evacuated, as dozens of aircraft battle the blazes.

Meanwhile 16 active fires were burning in the Superior National Forest, on 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 US forest service said.