Massive wildfires raging in northwestern Ontario, Canada, have sent clouds of smoke across large parts of the province over the past day, making Toronto the major city with the worst air quality in the world and causing pollution in the northeastern United States as well. Authorities in both countries issued health warnings and urged residents to avoid prolonged and strenuous outdoor activity. Across Canada, 857 active fires are currently burning, including 23 new fires that broke out Thursday.

Although the major fires are burning in sparsely populated areas hundreds of kilometers from Toronto and do not directly threaten cities, the smoke rising from them has spread over a wide area and is causing damage in its own right. Environment Canada reported overnight that Toronto’s air quality index stood at 10+, a level defined as “very high risk,” and forecasts said the dangerous conditions could last until Thursday night, or Friday morning Israel time.

Smoke from the fires was also felt over the past day in large parts of the northern United States, including Michigan, Minnesota and New York — just days before neighboring New Jersey is set to host the World Cup final between Spain and Argentina. Authorities in New York warned of air pollution and urged residents to reduce strenuous outdoor activity. The U.S. National Weather Service said wildfire smoke could remain in the region through the weekend, which in the United States includes Sunday.

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“We likely haven’t seen the worst of it yet in New York, and it appears the same is true in the Great Lakes region, upstate New York and New England,” said Dan Westervelt, an associate professor at Columbia University’s Lamont institute.

Swiss air quality technology company IQAir ranked Toronto on Wednesday evening and overnight as the city with the worst air quality in the world among major cities, worse than India’s capital, Delhi, and Kinshasa in the Democratic Republic of Congo. By Thursday afternoon, Toronto had fallen to second place in the grim ranking, while Detroit, in neighboring Michigan, rose to first.

Because of the fires and poor air quality, the Toronto municipality was forced to cancel FIFA’s fan festival and the public viewing event for Wednesday night’s World Cup semifinal between England and Argentina at Nathan Phillips Square. In the New York area, the skies also appeared hazy Thursday, a little more than three days before more than 80,000 people are expected to attend the World Cup final in nearby New Jersey and another 50,000 are set to watch it in Central Park in Manhattan.

New York Gov. Kathy Hochul wrote on social media that smoke and haze from the Canadian wildfires were creating unhealthy air conditions across the state, and urged people, especially those with medical conditions, to take extra precautions. New York City Mayor Zohran Mamdani issued a similar warning. In the coming days, winds are expected to continue carrying smoke into northern U.S. states, raising concerns that polluting particles could drift as far as New Jersey, where the World Cup final is scheduled to take place.

Although the fires in Canada are not burning in densely populated areas, several tense incidents have been reported. In one video that went viral on social media, a Canadian National train was seen surrounded by flames near Armstrong, Ontario. Workers aboard the train filmed themselves requesting urgent rescue, with one heard saying: “We’re surrounded, it could catch up with us, it’s getting a little scary. We need to hurry.” The workers were eventually rescued safely, and no injuries were reported. Service on the rail line was suspended.

Most of the fires currently burning in Canada are in the provinces of Manitoba, Saskatchewan and Ontario. Toronto resident Paula Oreskovich said she noticed the haze and smelled smoke when she went outside in the morning. She said the poor air quality was concerning, especially because wildfire smoke has become a recurring feature of recent summers in the region.

“I think you’d have to be foolish not to be concerned about climate change. It is definitely here, it is definitely happening, and it is happening on a global scale,” Oreskovich said.