Smoke from hundreds of wildfires burning across Canada has engulfed major US cities, triggering air quality alerts for more than 100 million people from the Great Lakes to the Mid-Atlantic and casting uncertainty over the World Cup final scheduled for Sunday in New Jersey. Detroit, Chicago, Washington, and New York all recorded hazardous pollution levels on Friday, with Detroit’s air quality index exceeding 600 at one point, according to monitoring platform IQAir. US authorities urged residents to stay indoors, and New York City began distributing free KN95 masks.

Health officials across the affected states warned that fine particulate matter from the smoke can penetrate deep into the lungs and bloodstream, exacerbating respiratory and cardiovascular conditions. Medical sources described the exposure as equivalent to smoking several cigarettes over a day outdoors. The conditions forced the postponement of a Major League Soccer match in Chicago and prompted Major League Baseball to reschedule games. In New Jersey, where Spain and Argentina are due to meet at the open-air MetLife Stadium, air quality on Thursday reached levels considered “very unhealthy” for all groups, though it improved slightly on Friday.

Forecasters offered conflicting assessments of the risk to the final. The National Weather Service said a cold front and rain expected on Saturday could clear much of the smoke, predicting moderate air quality by Sunday afternoon. However, scientists at the European Union’s Copernicus Atmosphere Monitoring Service cautioned that renewed fire activity could push fresh plumes into the region after the rain, leaving conditions uncertain. FIFA and White House officials held informal discussions about the situation, but no contingency plans for postponement have been announced, according to a White House official.

Viewed from Washington, the crisis acquired a diplomatic dimension when President Donald Trump accused Canada of “willful negligence” in forest management and threatened to add the cost of the pollution to existing tariffs on Canadian goods. Canadian Prime Minister Mark Carney, while not directly responding to the tariff threat, said both countries shared a responsibility to address climate change. The exchange came as Canadian authorities reported more than 200 fires burning out of control, primarily in Ontario, and as Republican lawmakers in Congress called for sanctions against Ottawa.

As of Friday evening, air quality alerts remained in effect across 19 states and the District of Columbia. Canadian officials said nearly 900 fires were active nationwide, with no immediate containment expected for the largest blazes. The World Cup final is still scheduled to proceed, but organisers said they were monitoring the situation closely.