Smoke from wildfires burning across northwestern Ontario blanketed Toronto and large parts of the northeastern United States this week, driving Toronto's air quality to the worst of any major city on Earth and triggering health warnings across a broad swath of North America. IQAir, a Swiss air quality monitoring company, ranked Toronto — Canada's largest city — at the top of its global pollution index, surpassing Kinshasa in the Democratic Republic of Congo and New Delhi, India. Environment Canada issued an Air Quality Health Index reading of 10+, its highest category, classified as "very high risk," with hazardous conditions forecast to persist through at least Thursday night.

More than 100 wildfires are burning across northern Ontario alone, with some 835 active fires recorded nationwide, covering 1.9 million hectares so far. The fires are concentrated in sparsely populated regions, but their smoke has spread across an enormous area. Mandatory evacuations were ordered for several First Nations communities, including Namaygoosisagagun First Nation, whose residents reported having only minutes to flee across Collins Lake by boat. The community of Collins, Ontario, was reported to have burned to the ground. Striking video of a Canadian National railway train surrounded by walls of fire near Armstrong, Ontario, circulated widely on social media; the train crew was safely evacuated and rail operations in the area were suspended. In Minnesota, rangers worked urgently to extract an estimated 6,000 to 10,000 campers from the Boundary Waters Canoe Area Wilderness — a remote, 1.1-million-acre (445,000-hectare) area accessible primarily by canoe — where roughly 17 lightning-caused fires had been spreading for more than a week.

Smoke drifted southeast across the border, affecting Michigan, Wisconsin, Minnesota, Pennsylvania, New York, Connecticut, Massachusetts, Maine and New Hampshire. New York City officials issued air quality alerts urging residents to limit strenuous outdoor activity, and the National Weather Service warned the smoke could linger through the end of the week. Columbia University researcher Dan Westervelt cautioned that the worst conditions for New York City and New England had likely not yet arrived. The most intense smoke was forecast to reach as far south as Washington, D.C. Toronto also recorded a heat index of 37.3°C in its downtown core — shattering a three-decade record — compounding the respiratory risks.

Experts warn that the simultaneous combination of extreme heat and hazardous smoke represents a serious and growing public health challenge. Fine particulate matter — the primary health concern in wildfire smoke — can bypass the body's natural defences, penetrate deep into the lungs and enter the bloodstream, worsening respiratory and cardiovascular conditions. Children, older adults, pregnant people, and those with pre-existing conditions such as asthma or heart disease face the highest risk. Health authorities advised residents to stay indoors, run air purifiers or air conditioning, and wear N95 masks if outdoor exposure was unavoidable. Greg Evans, a professor of chemical engineering at the University of Toronto, noted that the city had been hit simultaneously by heat and smoke, adding: "I expect that this will occur more frequently over the coming decades, so cities and residents need to prepare for this in the future." Although Canada's 2025 wildfire season has been less severe than the record-breaking years of 2023 and 2024, scientists point to drought, record-low snowpack and warming temperatures as factors making such events increasingly common.