US President Donald Trump has threatened to impose new tariffs on Canada after hundreds of wildfires left much of the northern United States choked by a blanket of smoke.

The threat comes after complaints from US lawmakers about the wildfires and Ontario Premier Doug Ford calling on the US to send support to fight the fires, rather than complain.

“America is being unnecessarily invaded by dirty, polluted, unhealthy air,” Trump said, threatening to impose new taxes over Canada’s “willful negligence.”

As of Friday, there were about 888 active fires in Canada, according to the Canadian Wildfire Information System., external – with the majority out of control.

More than 190 such fires are burning in Ontario, some of them out of control.

Trump said in his Truth Social post that he would call Canadian Prime Minister Mark Carney to demand an explanation for his country’s “deliberate neglect,” accusing the country of “failing to properly maintain” its forests and bushlands.

Canada has not responded to the threat, but Carney previously noted that it was the responsibility of both countries to fight climate change.

According to the Canadian Wildfire Information System, almost 3 million hectares of land in Canada have already been destroyed by wildfires.

The impacts have been far-reaching, with a thick layer of smoke spreading across US states, from Minnesota and Michigan to Pennsylvania, Ohio and New York.

“Hazardous” air quality alerts have been issued across much of the region, leading to the cancellation of many outdoor events.

As of Friday, air quality in Detroit was the worst in the world, according to Swiss air quality tracker IQAir., externalfollowed by the Midwestern city of Chicago, Washington DC and New York in seventh place.

In the open letter to Canadian officials, U.S. lawmakers John James, John Moolenaar, Jack Bergman and Lisa McClain said they “ran out of patience.”

“We are done accepting apologies instead of action,” they said, warning that the United States could explore direct involvement in protecting and extinguishing transboundary wildfires if Canada did not act.

“Last year we were told this would be treated urgently. It was not,” they said, adding that instead, “American lungs are paying the price for Canadian inaction, year after year.”

They said issues such as “chronic underinvestment in forest thinning, fuel reduction and prescribed burning, along with inadequate enforcement against arson”, had not been “sufficiently” addressed.

But scientists BBC Verify spoke to say the picture is more complicated.

“The climate doesn’t care about international borders,” says Dr. Patrick James of the University of Toronto.

Once smoke reaches the atmosphere, it travels wherever the winds take it, and smoke from major U.S. wildfires has also affected Canada in recent years.

Experts also say many of the current fires are burning in Canada’s vast, remote forests, where fires can be difficult to detect or contain before they become too large.

While better forest management can reduce the risk of wildfires in some areas, particularly near communities, it cannot prevent fires in an ecosystem of this scale.

Scientists also say increasingly severe wildfire seasons are being driven in part by climate change, which is creating warmer, drier conditions that allow fires to spread more easily.

“Climate change is a global problem and it would be inaccurate to suggest that Canada alone caused or could have prevented these wildfires,” says Dr. Anabela Bonada of the University of Waterloo.