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.