Wildfire smoke: the invisible killer behind 1.5 million deaths

When the sky above Toronto turned a sickly orange in the middle of the day, Dr Erin O'Connor knew this was no ordinary weather event. She stood outside Toronto General Hospital, where air filters shielded patients from the worst of the fumes, and breathed in a smell so thick it was like standing by a campfire. As the city's emergency department chief at University Health Network, she saw a sudden surge of patients with allergies and shortness of breath. "It really hit you in the face as soon as you walked outside," she said. "As the days progressed, it was like that everywhere."

A health emergency hiding in the smoke

The smoke that choked Toronto in late June came from wildfires raging more than a thousand kilometres away in northern Ontario's boreal forests. For O'Connor, it was an unprecedented moment: "I had never looked outside and had the sky seem orange in the middle of the day," she said. Yet this is hardly an isolated event. In recent weeks, vast tracts of vegetation have burned near major European cities, and North America's fire season has already blanketed Chicago and New York City in hazardous air. Last month, Toronto briefly held the unenviable title of the worst air quality of any major metropolis in the world—a distinction that Portland, Oregon, claimed this week.

The health consequences are staggering. A 2024 study in The Lancet estimated that smoke from landscape fires kills 1.5 million people each year, a death toll greater than floods, flames, storms, and heat waves combined. The same study highlighted that even people far from the flames are at risk. Canada's devastating 2023 wildfires triggered 82,000 premature deaths, according to research in Nature, with a third of that burden falling on the United States and another quarter on Europe. In March, scientists calculated that around 10% of the UK's fine particulate pollution comes from biomass burned abroad.

Understanding the deadly reach of fine particles

Scientists are only beginning to untangle the complex chain of events that makes wildfire smoke so lethal. The smoke is a toxic blend of gases that inflame the airways and fine particulates called PM2.5, which are small enough to slip past the lungs' natural defences and enter the bloodstream. From there, they can travel to nearly every organ, triggering heart attacks, strokes, and respiratory failure. The damage accumulates with each exposure, leaving scars on lung tissue that make future episodes more dangerous.

"It's not just a respiratory issue," says O'Connor. "The particles become a systemic problem, affecting the whole body." That systemic impact is why emergency departments see spikes in cardiac and neurological complaints during severe smoke events, not just asthma and bronchitis.

Unanswered questions in a warming world

As climate breakdown makes fire weather more severe, the open research questions are growing increasingly urgent. What if hotter infernos push pollution to even more extreme levels? What if smoke hangs around for days or weeks? What if a severe episode strikes before a person has recovered from the last bout of smoke? The current research has few answers, making it difficult to predict exactly how brutal the fallout from this fire season will be.

For O'Connor, the crisis is now both personal and professional. As the air worsened, she rushed to pull her son from summer camp. "Certainly, we have had other wildfire smoke, but in more of a transient way," she said. "This felt different." The difference, scientists say, is a reminder that the smoke knows no borders—and that the health burden of a burning planet will only grow unless the world confronts the root cause.