Scientists race to untangle complex links between exposure and mortality as fire weather grows more prevalent
A thousand kilometres from the infernos gorging on northern Ontario’s boreal forests, Erin O’Connor watched the sky above Canada’s biggest city turn an “almost post-apocalyptic” orange as it filled with light-scattering particles of soot and ash. Stepping outside the halls of Toronto General hospital, where air filters were keeping patients safe from the worst of the fumes, she was struck by a smell so strong it felt like standing by a campfire.
“It really hit you in the face as soon as you walked outside,” said O’Connor, who runs University Health Network’s emergency department and saw a rise in patients presenting with allergies and shortness of breath. “As the days progressed, it was like that everywhere. It seemed even inside you could smell that burning.”
Far more than the flames that have engulfed southern Europe and North America in recent weeks, it is the smoke that dominates the astonishing death toll from combusting nature. Thick with toxic gases that inflame the airways and tiny particulates that breach the lungs and ride the bloodstream to hit every organ in the body, fumes from landscape fires kill 1.5 million people each year, a study in the Lancet found in 2024 – more than floods, flames, storms and heat combined.
So damaging are the lung-scarring effects of charred plant matter that people far from the fires may still perish. The plumes spewed by wildfires in Canada in 2023 caused 82,000 early deaths, a study in Nature found last year, with unfavourable winds pushing a third of the mortality burden into the US and another quarter into Europe. About 10% of the UK’s fine particulate pollution (PM2.5) comes from biomass burned abroad, researchers found in March.
Yet while scientists agree the health burden of wildfire smoke is jaw droppingly large, they are only beginning to untangle the complex links between exposure and mortality. The open research questions are growing more pressing as climate breakdown makes fire weather more severe. What if hotter infernos cause pollution to peak at even more extreme levels? What if bad air lasts several days or weeks? What if a severe episode hits before a person has fully recovered from the last bout of smoke?
The lack of answers in the existing research makes it hard to get a handle on how brutal the fallout from this furious fire season will be. Early signs are troubling, with vast tracts of vegetation torched close to big European cities and, after a late but explosive start to the North American fire season, smoke choking major population centres such as Chicago and New York City. In Toronto, which briefly had the worst air quality of any major metropolis in the world last month – a title claimed by Portland in the US on Wednesday – O’Connor said she could not remember the pollution ever having been so bad, with such a sudden onset that took so long to clear up.
“I had never looked outside and had the sky seem orange in the middle of the day,” said O’Connor, who amid treating patients and fielding media requests rushed to pull her son from summer camp. “Certainly, we have had other wildfire smoke, but in more of a transient way.”
Perhaps surprisingly, the death toll from a fire season can depend on the whims of the wind as much as the size of the fire. Wildfires in France spewed record amounts of pollution in the first half of the year and in Spain emissions have so far fallen just shy of the worst it has known. But even though monster infernos were halted alarmingly close to Madrid and Bordeaux, the smoke was mostly blown away from where people live. Daily particulate pollution levels still worsened across southern Europe and peaked at a staggering 342 micrograms of PM2.5 per cubic metre (μg/m³) in Bordeaux on 24 July – and 486 μg/m³ in Limoges, 180km (112 miles) away, the following day – but other big cities in southern Europe were spared the extremes that doctors fear.
North America has not been so lucky. As winds blew south and south-east from scarcely populated forests in Ontario and the Northwest Territories, at least 36 million people across the continent were breathing air fouled by “very unhealthy” concentrations of PM2.5 that exceeded 125.5μg/m³ on 16 July, preliminary analysis conducted for the Guardian shows. At least 10 million people were exposed to more than 250μg/m³, according to the estimates, a level of daily pollution that smog-choked New Delhi saw no more than eight times last year.
Scientists say the hit to air quality in some big cities is reminiscent of the megafires in 2023 that came with vast death tolls. “As soon as we have massive population centres exposed, we’re going to see those increases,” said Stephanie Cleland, a wildfire health scientist at Simon Fraser University, who considered only people living within 25km of a measuring station in her calculations for the Guardian and said the figures are therefore conservative. “I would be concerned by the fact that it’s only been three years since this last happened and now we’re having it again.”
In a quiet suburb of Duluth, in north-east Minnesota, the worst of the smoke was seen, smelled and tasted on 16 July. Jae Jeon and Dan McClelland, a married couple who grow flowers for weddings and subscription deliveries, were looking forward to opening the windows after days baking in oppressive heat. Instead, they woke up to a fog that they soon realised was made up of something much worse. By late morning, their local hardware store had sold out of masks and air purifiers. People resorted to attaching furnace filters to box fans instead.
The McClellands were among 12,000 people in west Duluth exposed to an average of 541μg/m³ of fine particulate pollution, the analysis shows, higher than anywhere else in North America. They sent their sole farmworker home early after she complained of a headache but, with two weddings coming up on the weekend and little chance of cancellation, they still had plenty of flowers to pick and arrange. The couple got to work harvesting roses, yarrows, clematises and sweet peas, surrounded by a dystopian haze the likes of which they had never experienced.
“It felt like working in a war zone,” said Jae Jeon. “When I went outside, I couldn’t breathe, so I had to wear a mask, but it was still painful. After just a couple of minutes you started to get a headache.”
Climate breakdown made last month’s fire weather twice as likely in Canada and France, and 20 times more likely in Spain, recent rapid analyses have found, the result of planet-heating emissions that have made it easier for vegetation to dry out. As plants and trees combust, they spew a host of harmful particles that also trigger damaging inflammatory responses as the body seeks to purge foreign objects.