Wildfire smoke: the hidden health burden scientists are only now measuring

A thousand kilometres from the wildfires consuming northern Ontario's boreal forest, the sky over Toronto filled with soot and light-scattering ash, turning an "almost post-apocalyptic" orange. Erin O'Connor, who runs the emergency department at Toronto General Hospital, described the smell outside as feeling like standing beside a campfire.

While the hospital's air filters kept patients inside protected from the worst of the fumes, O'Connor saw a clear rise in people arriving with shortness of breath and allergy symptoms as the days wore on. "It really hit you in the face as soon as you walked outside," she said.

The experience is becoming increasingly familiar as climate change lengthens wildfire seasons around the world. Yet scientists still do not fully understand exactly how smoke exposure translates into harm for human health.

Wildfire smoke carries a different chemical mixture than ordinary traffic pollution — a blend of burning trees, decaying vegetation and, at times, ash from destroyed homes. That complexity makes it difficult for researchers to apply standard air-pollution models directly to smoke events.

Epidemiologists say the short-term effects are relatively well established: flare-ups increase among people with asthma and chronic obstructive pulmonary disease, and emergency-room visits climb. Data on longer-term, cumulative health effects, however, remains far more limited.

Some research suggests that repeated smoke exposure could also place a measurable burden on the cardiovascular system, though confirming that link firmly will require long-running monitoring studies that track the same populations over years.

Health officials recommend staying indoors, keeping windows closed, and using air filtration where possible on heavy smoke days — advice that can be difficult to follow in practice for people who are unhoused or whose jobs require them to work outdoors.

Researchers also note that smoke can affect general health and wellbeing through indirect routes beyond the lungs, including eye irritation and disrupted sleep during smoke events.

As wildfire seasons grow more frequent and severe, the question of how public health systems should prepare for them is taking on greater urgency; some cities have begun treating smoke advisories with the same seriousness as conventional air-quality alerts.

For scientists, the priority now is measuring the link between smoke exposure and mortality more precisely, in the hope of providing more reliable guidance on which protective measures will save the most lives.