Wildfire smoke from hundreds of blazes in Canada has spread across large parts of the United States, triggering health alerts and sparking a new political dispute between Washington and Ottawa.
US President Donald Trump has blamed Canada for the pollution, threatening to add the economic cost of the smoke to existing tariffs on Canadian imports.
In a post on Truth Social on Friday, President Donald Trump said Canada should be held accountable for failing to properly maintain its forests, which he claimed allowed wildfire smoke to spread into the United States.
"We are holding Canada responsible for the fact that they are not properly maintaining their Forests ... and the United States is being unnecessarily invaded by filthy, polluted, and unhealthy air," Trump wrote.
Calling the situation "totally unacceptable," Trump described it as "Willful Negligence" and said it had become a yearly occurrence that costs the United States billions of dollars.
He added that the "incalculable cost" of dealing with the pollution should be added to the tariffs Canada is already paying on exports to the United States.
Trump also said he planned to call Canadian Prime Minister Mark Carney to discuss what Canada intended to do about the wildfire crisis.
Smoke blankets major US cities
Heavy smoke from hundreds of active Canadian wildfires spread across a large stretch of the United States from the Midwest to the Northeast on Thursday and Friday.
Authorities issued air quality warnings, advising residents in affected areas to remain indoors because of unhealthy pollution levels.
The smoke also darkened skies over Toronto before moving south into the United States.
Canada responds to Trump's criticism
Canada's Minister of Emergency Management and Community Resilience, Eleanor Olszewski, rejected suggestions that the country had failed to address the wildfire crisis.
She said the Canadian government has invested C$12 billion (about US$8.56 billion) in forest sustainability, wildfire prevention and emergency preparedness since 2020.
Olszewski also highlighted the longstanding partnership between Canada and the United States in fighting cross-border wildfires.
"At this time, our first priority is protecting Canadians and keeping communities safe," she said in a statement.
Prime Minister Mark Carney's office did not immediately respond to Trump's latest comments.
However, Carney said on Thursday that the United States could do more to tackle climate change, which has contributed to prolonged droughts and rising global temperatures that fuel larger wildfires.
Trump and Carney are expected to meet during the FIFA World Cup final in New Jersey on Sunday.
Experts point to rising temperatures
Wildfire experts say climate change is making Canada's fire seasons longer and more destructive.
Mike Flannigan, a professor of wildland fire at Thompson Rivers University in British Columbia, said warming temperatures are drying forests and increasing the frequency of extreme fire conditions.
"As our climate warms, we're seeing more extreme weather, and we're going to see more fire," Flannigan said.
Hundreds of fires continue across Canada
Government data showed that as of Thursday morning, Canada was battling 858 active wildfires, including 111 classified as out of control. Most of the fires are concentrated in the central provinces of Manitoba, Saskatchewan and Ontario.
In Ontario alone, around 650,000 acres (2,630 square kilometres) have burned so far this year, compared with 600,000 acres during the same period last year.
Many of the fires are burning in remote northwestern Ontario, where air travel is often the only way to reach affected communities.
Thousands of residents have already been forced to evacuate.
First Nation community devastated
The Namaygoosisagagun First Nation, also known as Collins First Nation in northwestern Ontario, was largely destroyed by the wildfire.
Residents escaped by boat before being transported to emergency shelters in Thunder Bay.
"There was nothing remaining. So as you can imagine, the membership is totally distraught, upset, overwhelmed, lost," said Matthew Hoppe, the community's incident commander.
Thunder Bay, a city of around 110,000 people on the northern shore of Lake Superior, has reached full capacity while housing wildfire evacuees from across northwestern Ontario.
Mayor Ken Boshcoff said emergency shelters in the city are filled as more displaced residents continue arriving.
Ontario expands firefighting resources
Ontario Premier Doug Ford announced on Friday that the province will purchase 11 new firefighting aircraft to strengthen its response to rapidly spreading wildfires.
Ford also rejected criticism from US politicians who argued Ontario's firefighting efforts were insufficient. The wildfire crisis is not limited to Canada.
According to the National Interagency Fire Center, the United States has also experienced an above-average wildfire season, with 3.7 million acres burned so far in 2026 compared with the 10-year average of 2.7 million acres.
Air quality reaches hazardous levels
Air pollution caused by the Canadian wildfire smoke pushed several North American cities among the world's worst for air quality this week, according to IQAir's Air Quality Index.
Detroit recorded the highest pollution levels among major cities at 11pm local time on July 15.
Chicago, New York City, Toronto and Minneapolis also experienced very unhealthy or hazardous air quality during the week.
Health experts warn that wildfire smoke is significantly more toxic than normal urban air pollution.
Studies have linked prolonged exposure to wildfire smoke with increased risks of heart attacks, strokes, cancer, pregnancy complications and weakened immune systems.
Smoke could affect World Cup weekend
The poor air quality comes just days before the FIFA World Cup final in New Jersey, where more than 80,000 spectators are expected to attend Sunday's match between Spain and Argentina.
Forecasters currently expect rainfall before the final, which could help clear the smoke and improve air quality before kickoff.
Shortly after returning to office in 2025, Trump imposed tariffs on several major Canadian imports, contributing to increasingly strained relations between Washington and Ottawa.
His latest threat to increase tariffs over wildfire smoke adds another point of tension as both countries continue dealing with one of North America's most destructive wildfire seasons in recent years.
