Wildfire Smoke: The Silent, Rising Death Toll

Wildfire smoke has become a severe global health burden. According to a 2024 study published in The Lancet, smoke from landscape fires kills 1.5 million people each year—more than the combined deaths from floods, fires, storms, and heatwaves. The smoke is laden with toxic gases that inflame airways and ultrafine particles that can penetrate deep into the lungs and, via the bloodstream, affect every organ in the body.

Scientists are racing to unravel the complex relationship between smoke exposure and mortality. A study published in Nature last year found that smoke from Canada's 2023 wildfires caused 82,000 premature deaths, with adverse winds pushing one-third of that mortality burden to the United States and a quarter to Europe. Researchers also found that around 10% of fine particulate matter (PM2.5) in the UK originates from biomass burned abroad.

This year's fire season is showing worrying signs. In Canada, wildfires in Ontario and the Northwest Territories turned Toronto's sky an apocalyptic orange and made its air quality the worst among major cities last month. Dr. Erin O'Connor, who runs the emergency department at University Health Network in Toronto, noted a marked rise in patients with allergy symptoms and breathing difficulties.

In Europe, although major blazes near Madrid and Bordeaux were extinguished, smoke was mostly blown away from populated areas. However, daily fine particulate concentrations remained elevated across southern Europe, reaching 342 micrograms per cubic meter (µg/m³) in Bordeaux on July 24 and 486 µg/m³ in Limoges the following day.

North America was less fortunate. Winds blowing south and southeast from the forests of Ontario and the Northwest Territories exposed at least 36 million people to PM2.5 levels deemed "very unhealthy" (above 125.5 µg/m³) on July 16. At least 10 million people breathed air exceeding 250 µg/m³—levels that New Delhi saw no more than eight times last year.

In a quiet suburb of Duluth, Minnesota, Jae Jeon and Dan McClelland, who grow flowers for weddings, watched thick smoke envelop their property. "It felt like working in a war zone. When I went outside, I couldn't breathe and had to wear a mask, but it was still painful. Within minutes, I'd get a headache," said Jeon. Analysis shows that about 12,000 people in western Duluth experienced average PM2.5 levels up to 541 µg/m³—higher than anywhere else in North America.

Climate change made last month's fire weather at least twice as likely in Canada and France, and 20 times more likely in Spain. Scientists note that compared to other PM2.5 sources like factories or vehicles, wildfire smoke contains more organic compounds that trigger inflammation and higher concentrations of ultrafine particles that embed deep in the lungs. A 2021 study in Nature Communications found that PM2.5 from wildfire smoke is up to 10 times more harmful in the short term.

Experts advise that when wildfire smoke reaches severe levels, people should wear masks, keep windows closed, and check on vulnerable neighbors, especially elderly individuals with chronic conditions living alone. They also recommend checking air quality monitors in the morning and moving activities indoors when pollution spikes.

A tricky dilemma is that keeping indoor air clean can conflict with staying cool, as heatwaves and wildfires often coincide. "Staying inside with windows closed can be awful without air conditioning in the heat," said Cathryn Tonne, an environmental epidemiologist at the Barcelona Institute for Global Health.

On preventing wildfires, scientists argue that Canada's blazes have little to do with land management—most burned area is boreal forest in uninhabited regions—but are mainly driven by rising temperatures from burning fossil fuels. In Europe, however, land-use changes, such as abandoned rural areas overgrown with dry vegetation, also make fires spread more easily. Spanish wildfire researchers have warned that the world has "entered the era of fires that cannot be extinguished."