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

The threat follows complaints by US lawmakers over the wildfires and Ontario’s premier Doug Ford asking the US to send support to fight the fires, rather than complain.

“The United States is being unnecessarily invaded by filthy, polluted, and unhealthy air,” Trump said, threatening to impose new levies over Canada’s “willful negligence”.

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

More than 190 of those blazes are burning in Ontario, some out of control.

Trump said in his post to Truth Social that would call Canadian Prime Minister Mark Carney to demand an explanation over his country’s “willful negligence,” accusing the country of “not properly maintaining” their forests and brush.

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

According to the Canadian Wildland Fire Information System, nearly 3 million hectares of land in Canada has already been destroyed by the wildfires.

The impacts have been far-reaching, with a thick blanket 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 worst in the world, Swiss air quality tracker IQAir said, external, followed by the Midwestern city of Chicago, Washington DC, and New York in seventh place.

In the open letter addressed to Canadian officials, US lawmakers John James, John Moolenaar, Jack Bergman and Lisa McClain said their “patience has run out”.

“We are done accepting apologies in place of action,” they said, warning the US could explore direct involvement in cross-border wildfire protection and firefighting if Canada failed to act.

“We were told last year that this would be treated with urgency. It was not,” they claimed, adding that instead “American lungs are paying the price for Canadian inaction, year after year”.

They said that issues such as “chronic under-investment in forest thinning, fuel reduction, and prescribed burns, along with inadequate enforcement against arson”, had not been addressed “adequately enough”.

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

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

Once smoke reaches the atmosphere, it travels wherever the winds take it – and smoke from major US 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 wildfire risk in some areas, particularly near communities, it cannot prevent fires across an ecosystem of this scale.

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

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