Hundreds of wildfires burning across the Canadian province of Ontario have sent vast plumes of toxic smoke southward, blanketing major cities from the Great Lakes to the northeastern United States in a thick, acrid haze. As of Thursday, 858 fires were actively burning across Canada, the large majority out of control, with a dense cluster in northwestern Ontario identified as the primary source of the smoke now choking cities including Toronto, Detroit, Chicago, Minneapolis, Pittsburgh and New York.

Air quality reached hazardous or "very unhealthy" levels across wide swaths of the US Midwest and East Coast, with Detroit briefly ranked by Swiss air quality tracker IQAir as having the worst air quality of any major city in the world, followed by Minneapolis, Chicago and Toronto. A lingering high-pressure system has trapped smoke close to the ground, reducing visibility in some areas to as little as half a mile. "It arrived in force here and it's really pretty extreme levels," said Steven Freitag, a National Weather Service meteorologist in Detroit. New York Governor Kathy Hochul declared the situation a serious public health emergency, warning that smoke was "blanketing" much of the state and making it "very unhealthy to be outdoors". The sky over Manhattan glowed orange and yellow, obscuring iconic landmarks including the Empire State Building and the Statue of Liberty. New York City distributed tens of thousands of KN95 masks at transit hubs and opened hundreds of cooling centres, while schools and parks moved events indoors. Philadelphia health officials urged residents to avoid strenuous outdoor activity, with the city's public health commissioner quipping: "Today is not the day to start your marathon training plan." In Chicago, an outdoor concert by rock group The Black Keys was rescheduled, children's summer camp activities were cancelled, and beaches along popular lakefronts were closed.

The smoke is likely to persist for months. National Weather Service meteorologist Jake Petr warned that even if winds temporarily clear the skies, smoky air could return repeatedly until the fires are extinguished — something that may not happen until snow falls in Canada and northern Minnesota. In Minnesota, authorities closed the Boundary Waters wilderness area along the US-Canada border, and helicopters struggled to reach remaining visitors due to the poor visibility. The Royal Canadian Air Force evacuated 11 American teenagers and four staff members from an Ontario provincial park roughly 280 kilometres north of the Minnesota border. A Columbia University air quality researcher warned that the worst may not yet have arrived for the northeast.

Scientists and health officials stress that wildfire smoke poses serious risks to everyone, not just those with pre-existing conditions. The smoke contains fine particles known as PM2.5 — particles 2.5 microns or smaller in diameter — which can penetrate deep into the lungs, enter the bloodstream and affect multiple organ systems, contributing to respiratory illness, cardiovascular disease and, with prolonged exposure, premature death. A study published this year found that long-term exposure to wildfire smoke particles contributes to an average of 24,100 deaths a year in the United States. Experts recommend staying indoors with windows closed, using air purifiers with clean filters, and wearing well-fitting N95 or KN95 masks when going outside.

The scale and frequency of these fire seasons reflects a longer trend. Wildfires across Canada's vast boreal forest — the broad band of coniferous forest stretching across the country's interior — have grown more frequent and intense since 2015, driven by hotter and drier conditions linked to climate change. "This is associated with some of the extreme climate warming that we've been seeing, and the atmospheric drying of the surface," said Laura Chasmer, a professor of geography and environment at the University of Western Ontario. Republican lawmakers from Michigan have written an open letter to Canadian Prime Minister Mark Carney demanding better fire management, saying their "patience has run out" after three consecutive years of smoke impacts on their constituents. The US Ambassador to Canada took a more measured tone, commending joint firefighting efforts between the two countries.