The findings are presented in “The July 2026 Wildfires in Canada and Their Transboundary Smoke Problem,” an Incident Brief that combines satellite-derived fire perimeters, ground-based air-quality observations and gridded population data. The researchers assessed wildfire activity across Canada through 21 July and estimated human exposure in both Canada and the United States during the mid-July smoke episode. Their use of “person-days” captures not only how many people encountered polluted air, but also how long the exposure lasted: one person breathing unhealthy air for five days represents five person-days, as does five people exposed for one day. The measure provides a clearer picture of the cumulative burden than a single daily population estimate.

Canada’s fire season initially appeared relatively quiet. By 28 June, the country had burned more than 1.2 million hectares below its 10-year average for that date. The situation changed rapidly after a major North American heat dome settled over much of the continent. By 21 July, the national fire area had moved from a substantial deficit to a surplus of approximately 367,000 hectares. In total, 3,915 fires had burned nearly 3.04 million hectares, making 2026 the third-largest year-to-date fire season at that point, behind only 2023 and 2025. The abrupt reversal demonstrates how a short period of extreme heat and drying can overwhelm seasonal averages and transform wildfire conditions across a vast area.

Ontario was the main driver of that shift. The province recorded 565 fires covering more than 728,000 hectares, approximately three and a half times its 10-year average. Although Ontario’s annual precipitation was close to normal, weather conditions in the southwest told a different story. Rainfall during the preceding two months was more than two standard deviations below normal in parts of the region, while temperatures averaged more than 2°C above the historical norm. Those combined stresses rapidly reduced moisture in vegetation and soils. In forest ecosystems, low fuel moisture allows flames to spread more efficiently, increases the likelihood of intense burning and promotes the formation of powerful smoke plumes capable of travelling far beyond the fire zone.

The composition and structure of Canada’s forests amplified the event. Dense, carbon-rich vegetation can release enormous quantities of fine particulate matter when it burns, especially particles smaller than 2.5 micrometres in diameter, commonly known as PM2.5. These particles are small enough to penetrate deep into the lungs and, in some cases, enter the bloodstream. Smoke exposure can aggravate asthma and chronic obstructive pulmonary disease, increase stress on the cardiovascular system and raise risks for vulnerable groups, including children, older adults and people with heart or lung conditions. Because smoke can remain suspended and be transported hundreds or even thousands of kilometres, the health consequences are not confined to communities near the flames.

Smoke from Ontario moved across southern Ontario and parts of Quebec before crossing into the United States. Detroit, Minneapolis, Milwaukee, Chicago, New York City and Washington, D.C., all experienced deteriorating air quality during the episode. Authorities in numerous states advised residents to remain indoors, reduce strenuous outdoor activity and use additional precautions when exposure could not be avoided. Some municipalities cancelled public events or moved them indoors. For many urban residents, the crisis arrived not as a visible wall of flame but as an orange or hazy sky, the smell of burning forests and air-quality alerts issued hundreds of kilometres from the nearest active fire.

The event also exposed the political sensitivity of transboundary air pollution. Canada and the United States have a long history of cooperation in firefighting, emergency management and environmental monitoring, yet climate-driven smoke can strain relationships even between close partners. Pollution generated in one country can rapidly affect public health, transportation, schools, workplaces and outdoor economies in another. The Incident Brief argues that fire response and public-health planning must therefore be coordinated across borders, with compatible warning systems, shared data and contingency plans for prolonged smoke episodes. Treating wildfire smoke as a local problem is increasingly out of step with the physical behaviour of the atmosphere.

The most direct displacement occurred in Indigenous communities located near active fires. More than 3,500 First Nations residents were displaced nationally by mid-July, while as many as 11 Ontario First Nations faced evacuation orders or other wildfire-related disruptions. Approximately 1,800 people left remote communities in northwestern Ontario, many of which can be reached only by air or water. Ontario requested federal assistance, including support from the Canadian Armed Forces, to help reach and evacuate isolated areas. In Namaygoosisagagun First Nation, residents escaped by boat as an approaching fire destroyed the community, while Whitewater Lake First Nation was also affected. The fires disrupted transportation and economic activity as well, temporarily halting Canadian National Railway operations and isolating communities dependent on critical supply routes.

The researchers say the response must extend beyond emergency evacuations. National and provincial authorities should strengthen evacuation preparedness, expand shelter capacity in remote and Indigenous communities and establish clean-air shelters for residents who cannot adequately filter indoor air. Public-health guidance should be specific, timely and accessible, explaining when people should reduce outdoor activity, how to improve indoor air quality and which symptoms require medical attention. Investments in early fire detection, satellite monitoring and fire-resistant infrastructure could reduce losses in high-risk regions, while coordinated cross-border communication would help residents understand the risks before smoke becomes severe. Climate mitigation remains central: warming conditions are drying landscapes, increasing fire intensity and lengthening the period in which large fires can develop.

“Climate change has not only increased the frequency of wildfires, especially in the mesic vegetation types, but also made them more intense,” said 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.” Kaveh Madani, Director of UNU-INWEH, described the problem as inherently shared, noting that smoke generated in northern Ontario can be inhaled in a Chicago schoolyard a day later. The July episode, the institute’s third Incident Brief on North American fires following assessments of the January 2025 Los Angeles fires and the May–June 2025 Central Canada fires, offers a warning for a hotter future: cities that have learned to prepare for heat must now plan for smoke, while the communities facing evacuation first require sustained investment and protection.

Subject of Research: Canada’s July 2026 wildfires, transboundary wildfire smoke, air-quality exposure, public health, Indigenous-community evacuations and climate-driven wildfire risk.

Article Title: The July 2026 Wildfires in Canada and Their Transboundary Smoke Problem

News Publication Date: 13 August 2026

Web References: https://unu.edu/publication/july-2026-wildfires-canada-and-their-transboundary-smoke-problem; https://unu.edu/inweh

References: 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. DOI: 10.53328/INI26IMS001

Keywords: Canada wildfires, wildfire smoke, air pollution, air quality, PM2.5, public health, climate change, transboundary pollution, Ontario wildfires, Indigenous communities, evacuations, environmental health, extreme heat, boreal forests, disaster preparedness, environmental policy

The study analyzed approximately 64.5 million pregnancies across 2,842 counties in 48 states. Researchers combined pollution modeling with demographic information, including maternal ethnicity, county income, urban or rural residence, and access to maternity care. Their goal was to measure not only how much PM₂.₅ pregnant people encountered, but also who was most likely to experience pollution linked specifically to wildfires. This population-based approach allowed the team to move beyond maps showing where fires occurred and instead estimate how smoke affected communities and pregnancies across the contiguous United States.

PM₂.₅ refers to airborne particles with a diameter of 2.5 micrometers or smaller. Because these particles are tiny enough to penetrate deep into the lungs and enter the bloodstream, they can affect organs far beyond the respiratory system. During pregnancy, exposure has been associated with outcomes such as low birth weight and preterm birth. The researchers noted that particles generated by fires may be more harmful than the same mass of PM₂.₅ from some other sources because wildfire smoke contains a complex mixture of combustion products, including organic compounds, metals, and other reactive substances.

To separate wildfire pollution from other sources, the team conducted two computer-modeling runs. The first simulated total PM₂.₅ concentrations from human and natural sources, including vehicles, industrial activity, and fires. The second removed all fire-related emissions from the calculation. By comparing the results, the researchers estimated the portion of prenatal PM₂.₅ exposure attributable to wildfires. They also counted the number of days on which modeled concentrations exceeded 35 micrograms per cubic meter, the threshold used by the US Environmental Protection Agency to identify unhealthy daily particle pollution levels.

Across the study period, average daily prenatal exposure to PM₂.₅ from all sources fell by approximately 34%. This decline reflects the impact of cleaner vehicles, industrial controls, reduced emissions from power generation, and other air-quality policies implemented over recent decades. However, when wildfire-related pollution was excluded, the decrease was larger, at about 37%. The difference indicates that smoke from wildfires offset part of the improvement achieved through emissions regulation. In other words, air pollution would have declined more sharply if wildfire contributions had remained constant or diminished rather than increasing.

Wildfires’ share of total prenatal PM₂.₅ exposure more than doubled during the 17-year period, rising from approximately 3.7% in 2003 to 8.2% in 2019. The change was even more consequential when researchers examined unhealthy pollution days rather than average exposure. By the end of the study period, around 65% of prenatal PM₂.₅ exceedance days could be attributed to wildfire smoke. This does not mean that wildfire smoke accounted for 65% of all particle exposure, but it shows that fires increasingly determined when pollution reached levels considered unhealthy.

The increase was particularly pronounced in regions already vulnerable to severe fire seasons. In the northwestern United States, wildfire contributions to prenatal PM₂.₅ rose from 5.1% to 13.3%. Across western states, the proportion increased from roughly 3.7% to 9.7%. The burden was not distributed evenly within these regions. The highest wildfire-related exposures were recorded among low-income communities, rural populations, American Indian and Alaska Native communities, and areas facing shortages of maternity-care services. These overlapping vulnerabilities may limit residents’ ability to avoid smoke, obtain timely medical guidance, or use protective measures such as high-efficiency air filtration.

The researchers emphasized that the findings do not diminish the value of air-quality regulations. The reductions achieved through legislation and pollution-control programs have protected millions of pregnant people and developing fetuses, and current exposure levels would be considerably higher without those measures. Instead, the results suggest that wildfire smoke is becoming a major obstacle to continued progress. As climate conditions contribute to hotter, drier landscapes and longer fire seasons in many regions, smoke can travel hundreds or even thousands of kilometers, exposing communities far from the flames themselves.

Several major smoke events occurred after the study ended, including the extensive western US fires of 2020 and the Canadian wildfire outbreaks of 2023, which sent hazardous smoke across large parts of the eastern United States. The study cannot determine whether wildfire pollution continued to rise after 2019, but the researchers said it is plausible that the proportion of unhealthy prenatal exposure days caused by wildfire smoke has increased. They also acknowledged limitations, including the inability to track whether people moved between counties during pregnancy or whether they reduced exposure with masks, indoor air cleaners, or temporary relocation. Nevertheless, the study provides a detailed national picture of an emerging public-health challenge and offers evidence that wildfire preparedness, air-quality alerts, and targeted maternal-health resources should be directed toward the communities carrying the greatest burden.

Subject of Research: Not applicable

Article Title: Wildfire Contributions to Prenatal PM₂.₅ Exposure in the Contiguous United States, 2003–2019

News Publication Date: 7-Aug-2026

Web References: https://doi.org/10.3389/fenvh.2026.1838836

References: Liang M et al., “Wildfire Contributions to Prenatal PM₂.₅ Exposure in the Contiguous United States, 2003–2019,” Frontiers in Environmental Health, DOI: 10.3389/fenvh.2026.1838836

Keywords: Wildfires, wildfire smoke, prenatal exposure, pregnancy, PM₂.₅, air pollution, maternal health, fetal health, preterm birth, low birth weight, environmental health, climate change, health disparities