Skies in North American cities from Toronto to New York have turned an apocalyptic orange as massive wildfires in Canada send plumes of smoke thousands of miles across the continent. In July 2025, more than 800 wildfires were burning in Canada, with nearly 200 in Ontario alone, destroying nearly 5 million acres and forcing evacuations. The smoke has blanketed cities, making Detroit, Toronto and Minneapolis rank as the worst air quality in the world, according to IQAir. This isn't just a local problem – wildfire smoke can travel huge distances and linger for days, posing serious health risks to millions.
Wildfire smoke is a mixture of tiny particles and gases, including PM2.5 (particles smaller than 2.5 micrometres) and nitrogen dioxide. These particles can penetrate deep into the lungs, causing inflammation and aggravating conditions like asthma, heart disease and kidney problems. "When they get into our system they get all the way down into our lungs and cause inflammation," said Jim McDonald, New York State Department of Health commissioner. The smoke can also affect the brain and has been linked to increased cardiac deaths after major fire events.
“Wildfire smoke from Canada is causing hazardous air quality across North America. This explainer looks at why it travels so far and what it means for health.”
The reason smoke can travel so far is down to weather patterns. In Canada, sustained hot weather – caused by a "heat dome", an area of high pressure that traps warm air – has drawn moisture out of soils and plants, creating tinderbox conditions. This hot, unstable air can also generate storms that produce strong winds, spreading both fires and smoke. Winds have carried the smoke south-east from Ontario across to Toronto, New York State and Boston. Smoke from wildfires can remain in the atmosphere for days and travel hundreds to thousands of miles, threatening air quality far from the flames.
While UK readers may not be directly affected by North American smoke, the issue is a stark reminder of how climate change is making wildfires more frequent and intense. A new analysis of wildfire data over the past three decades shows that the number of acres burned each year is increasing. Loretta Mickley, an atmospheric chemist at Harvard University, told USA Today that about half the smoke generated from wildfires between 1997 and 2020 could be linked to climate change, and that proportion has risen in recent years. As global temperatures rise due to burning fossil fuels, extreme heat and drought become more common, setting the stage for larger, more erratic fires.
Q: How does wildfire smoke travel so far? Smoke can be carried thousands of miles by prevailing wind patterns. In this case, a heat dome over northern Ontario created high pressure that trapped warm air and smoke, while strong winds from storms pushed the smoke south-east across Canada and the US. The particles can stay aloft for days, affecting air quality in distant cities.
Q: What health problems can wildfire smoke cause? The primary danger comes from fine particles called PM2.5, which can enter the lungs and bloodstream, causing inflammation. This can worsen respiratory and heart conditions, trigger asthma attacks, and increase the risk of heart attacks and strokes. Even healthy people are advised to avoid strenuous outdoor activity when air quality is hazardous.
Q: Is climate change making wildfires worse? Yes. Climate change increases the frequency and severity of heatwaves and droughts, which dry out vegetation and create fuel for fires. Scientists estimate that around half of wildfire smoke between 1997 and 2020 was attributable to climate change, with the share growing in recent years. Warmer temperatures also make heat domes more likely, trapping smoke and prolonging poor air quality.
As long as global temperatures continue to rise, wildfire seasons are expected to become longer and more destructive. This means more smoke events like the current one, affecting not just fire-prone regions but also major cities far downwind. For now, residents in affected areas are advised to stay indoors, use air purifiers and wear N95 masks if they must go outside.