Deaths linked to wildfire smoke in Canada have exploded by about 275 per cent since the early 2000s according to a major global report that tracks connections between health and climate change.

The report, published Tuesday in The Lancet, found that an average of approximately 4,000 deaths were associated with wildfire smoke in Canada each year between 2021-25. That's compared with an average of only 1,050 annually in 2003-12.

Ian Hamilton, the chair in planetary health at the George Institute for Global Health at Imperial College in London, co-authored the study, which comes a decade since the prestigious medical journal began tracking the health impacts of climate change.

He says it looked at how many additional people died during periods of wildfire smoke compared with historical records, rather than count case by case.

"For any individual, it might not be that [wildfire smoke] was the trigger," he said. "But on average across the population, we're seeing more death associated with those periods."

Hamilton says the year range used for wildfire smoke-associated deaths, 2003-12, is meant to represent a historical baseline. The scientists chose different sets of dates as the baselines of different phenomena.

According to the report, Canada lost approximately 5.2 million hectares of tree cover in 2024, 148 per cent more than in 2016 — and wildfires accounted for 86 per cent of the loss. That meant an increase in wildfire smoke, and the loss of natural cooling systems like shade that can provide some relief on Canada's hottest days.

Tragic consequences The study looked at other heat-related risks. For example, it says the number of hours people in Canada were exposed to potentially dangerous heat during physical activity — for example, going on brisk walks on hot days –- increased by 36 hours annually in 2016-25, compared with during the 1990s.

Dr. Courtney Howard, chair of the Global Climate and Health Alliance and the president-elect of the Canadian Medical Association, says that shift can be particularly dangerous for young athletes.

"Young people are almost so strong, especially athletes, that they push through heat and heat related exhaustion," she said. "And unfortunately that can actually lead to mortality." Howard says that the reality of climate change is steadily getting worse.

"We're not at a new normal," she said. "There's no flat line currently on the emissions graph, and therefore there is no flat line in terms of the heat."

Dr. Anna Gunz, a pediatrician in London, Ont., says that she's seen the consequences of heat-related illness first hand, recalling a young patient who went into cardiac arrest while playing outside and then died. When tests showed the patient's asthma had been the cause, Gunz says his mother's immediate response was "Oh, it was really humid out today." "It sends chills down my spine," Gunz said, to hear a parent relate their child's death to the weather.

Not just an exercise The report also says 80 per cent of Canada's heatwave days in 2025 were directly attributable to fossil fuel-driven climate change. Hamilton says the numbers are not just a modelling exercise.

It looks back and asks, "Under the same conditions, what were we experiencing 10, 15, 25, 50 years ago?" he said. "And what are we experiencing now?" The difference, he says, is that we have a lot more carbon dioxide in our atmosphere.

Canadians should prepare for a 5 C hotter future, report warns Wildfires are reversing Canada's progress on improving air quality Howard says Canada needs to invest in adapting to the changing climate, pointing to the growing need for air conditioning in many regions, and the need to upgrade health facilities to cope with wildfire smoke.

"We need to know that this is going to keep getting worse year on year and year, unless we decrease greenhouse gas emissions," she said. "Decreasing greenhouse gas emissions is the most important adaptation."