UN University: Ontario wildfire smoke in mid-July exposed North Americans to unhealthy air for 106 million person-days in just three days
Tens of millions breathed unhealthy air on both sides of the border as Canada's slow fire season intensified within weeks, new analysis finds
Richmond Hill, Ontario, Canada (13 August 2026) – Smoke from Canada's July wildfires exposed people across Canada and the United States to unhealthy, very unhealthy or hazardous air for nearly 106 million person-days between 15 and 17 July. Over the peak impact period of 13 to 18 July, moderate or worse air quality accounted for more than 769 million person-days. For a period, Toronto recorded the poorest air quality of any major city in the world.
"The July 2026 Wildfires in Canada and Their Transboundary Smoke Problem," an Incident Brief published by the United Nations University Institute for Water, Environment and Health (UNU-INWEH) evaluates wildfire activity across Canada up to 21 July 2026 and human exposure to smoke in Canada and the United States during the mid-July peak. The analysis pairs satellite-derived fire perimeters with ground-based air quality readings, overlaid on gridded population maps. Person-days are the number of people exposed multiplied by the number of days they were exposed. It is the third Incident Brief the institute has published on North American fires, after assessments of the January 2025 Los Angeles fires and the May to June 2025 Central Canada fires.
Canada's fire season began slowly. By 28 June the country had burned more than 1.2 million hectares less than its ten-year average for the date. Then the North American Heat Dome settled over much of the continent. By 21 July the deficit had become a surplus of 367,000 hectares. Nationally, 3,915 fires had burned nearly 3.04 million hectares, the third largest total for this point in the year, behind only 2023 and 2025.
Ontario drove the reversal. The province recorded 565 fires burning more than 728,000 hectares, roughly three and a half times its ten-year average. Annual precipitation across Ontario was close to normal, but the southwest of the province had received more than two standard deviations less rain than usual over the previous two months, and temperatures had run more than 2°C above average. The forests dried out fast.
“Climate change has not only increased the frequency of wildfires, especially in the mesic vegetation types, but also made them more intense,” said Dr. Mojtaba Sadegh, Climate and Wildfire Analytics Lead at UNU-INWEH. “When these dense forests burn, they release a huge amount of smoke that impacts millions of people far away from the burn perimeters. There is no easy solution to this problem; it requires a collective commitment to curbing climate change, and even if we act today, the problem will not disappear overnight, but we can make sure it does not get worse.”
Smoke from the Ontario fires travelled hundreds of kilometres. It pushed Toronto's air quality to unhealthy levels and spread across southern Ontario and parts of Quebec, then moved south into the United States, degrading air quality in Detroit, Minneapolis, Milwaukee, Chicago, New York City and Washington, D.C. Officials in numerous states advised residents to stay indoors and limit outdoor exercise, with particular warnings for children, older adults and people with respiratory or cardiovascular conditions. Some cities cancelled public events or moved them inside.
Transboundary smoke also became a source of political friction between Canada and the United States, two countries with a long record of cooperation in emergency management and firefighting. The brief argues that worsening climate-driven wildfire and smoke hazards can strain even close international relationships, and that coordinated firefighting and cross-border public health planning matter more as those hazards intensify.
Closer to the fires, evacuation fell heaviest on Indigenous communities. More than 3,500 First Nations residents were displaced nationally by mid-July, and as many as 11 Ontario First Nations were under evacuation because of wildfire impacts. Around 1,800 residents left remote northwestern communities. Ontario requested federal assistance, including support from the Canadian Armed Forces, to reach places accessible only by air or water. In Namaygoosisagagun First Nation, residents escaped by boat as the approaching fire destroyed the community. Whitewater Lake First Nation was also affected. The fires also disrupted transport and economic activity, halting Canadian National Railway operations for a time.
The brief calls on national and provincial authorities to strengthen evacuation preparedness and shelter capacity in remote and Indigenous communities. It also wants clean-air shelters and clearer public health guidance during smoke events, alongside investment in early detection and in infrastructure that can withstand fire in high-risk regions. Over the longer term, it ties the risk to global temperature increases, particularly across boreal regions where carbon-rich ecosystems matter for climate stability well beyond Canada.
“Smoke does not check passports,” said Professor Kaveh Madani, Director of UNU-INWEH. “What burns in a forest in northern Ontario is breathed in a schoolyard in Chicago a day later. It is a shared problem and it has shared solutions. The communities that are evacuated first and hit hardest need investment. And cities need to plan for smoke the way they have learned to plan for heat.”
Report Information
Teymoor, S., Abatzoglou, J., AghaKouchak, A., Pigeon, J., Madani, K., Matin, M., and Sadegh, M. (2026). The July 2026 Wildfires in Canada and Their Transboundary Smoke Problem. United Nations University Institute for Water, Environment and Health (UNU-INWEH), Richmond Hill, Ontario, Canada. doi: 10.53328/INI26IMS001
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About UNU-INWEH
Marking its 30th anniversary of operation in 2026, the United Nations University Institute for Water, Environment and Health (UNU-INWEH) is one of 13 institutions that make up the United Nations University (UNU), the academic arm of the UN. Known as 'The UN's Think Tank on Water', UNU-INWEH addresses critical water, environmental, and health challenges around the world. Through research, training, capacity development, and knowledge dissemination, the institute contributes to solving pressing global sustainability and human security issues of concern to the UN and its Member States. Headquartered in Richmond Hill, Ontario, UNU-INWEH has been hosted and supported by the Government of Canada since 1996. With a global mandate and extensive partnerships across UN entities, international organizations, and governments, UNU-INWEH operates through its UNU Hubs in Calgary, Hamburg, New York, Lund, and Pretoria, and an international network of affiliates.
Article Title
The July 2026 Wildfires in Canada and Their Transboundary Smoke Problem
Article Publication Date
13-Aug-2026
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