This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 July 15–17. Over the peak impact period of July 13–18, 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 through July 21, 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 June 28, the country had burned more than 1.2 million hectares less than its 10-year average for the date. Then the North American Heat Dome settled over much of the continent. By July 21, 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 10-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.
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