People exposed to wildfire smoke have a higher risk of suffering a stroke, according to research published in the European Heart Journal [1] today (Tuesday).
The researchers estimate that in the US alone, smoke from wildfires may be responsible for around 17,000 strokes each year.
The research was led by Professor Yang Liu from Emory University's Rollins School of Public Health, Atlanta, US. He said: "Wildfires are becoming a major contributor to ambient air pollution. For example, in the Western US, wildfire smoke is now reversing decades of improvement in air quality.
"Stroke is a major cause of disability and death around the world, and it is becoming more common. Traditional risk factors such as hypertension and diabetes do not fully explain this trend. We are interested in understanding how long-term exposure to wildfire smoke affects the risk of having a stroke."
The study included around 25 million people aged over 65 who were living in the US and covered by the federal health insurance programme Medicare. Around 2.9 million of these experienced a stroke between 2007 and 2018.
Researchers used artificial intelligence to estimate each person's exposure to wildfire smoke with a measure called PM₂.₅ (the concentration of fine particulate matter with a diameter of 2.5 micrometres or less). They also looked at exposure to other air pollution, and took account of known stroke risk factors, such as smoking and socio-economic status. This allowed the team to compare people's levels of long-term exposure to wildfire smoke with the risk of stroke.
They found that for every one microgram per cubic meter (1 µg/m³) increase in the average concentration of wildfire smoke, stroke risk increased by 1.3%. The same increase in PM₂.₅ from other sources like traffic and power plants was linked to a 0.7% increase in stroke risk.
Professor Liu said: "This suggests wildfire smoke may be even more harmful to the brain and blood vessels than pollution from other sources. Looking at the US as whole, our results suggest that exposure to wildfire smoke could be responsible for around 17,000 strokes every year in this age group.
"Since most fires start in remote areas, the rural and suburban population are often exposed to a higher level of fire smoke. This is quite different from other emission sources such as traffic that mostly affect the urban population.
"This study does not explain why breathing wildfire smoke may be linked to stroke, but there are several possibilities. For example, this type of smoke contains compounds that can trigger inflammation and oxidative stress in the body. These processes damage blood vessels and promote the formation of blood clots, which are known contributors to stroke. People living near wildfires may experience the stress of having to evacuate their home, which can also disrupt their normal medical care.
"For people living in fire prone areas, it is important to understand that wildfire smoke is not only an immediate breathing hazard, it may also raise long-term stroke risk. Preventative behaviours, like turning on indoor air filtration and limiting outdoor exertion on heavy smoke days, are not just about comfort, they may help prevent a stroke.
"Our results also suggest that there is no apparent safe threshold for smoke exposure. That means even 'moderate' recurring smoke could matter, not just extreme events. Policies that prevent or manage fires; improve community-level clean air shelters; and ensure access to medical care and medications during and after smoke events could lower stroke burden in an aging population."
In an accompanying editorial [2] Dr Kai Chen from the Yale School of Public Health, US, and colleague said: "Wildfire smoke has become an increasingly important source of PM₂.₅ as climate change drives more frequent and intense wildfires in recent years. Wildfire smoke contains complex mixtures of particles and gaseous pollutants that can travel across continental scales, leading to exposures among populations far beyond the areas directly affected by fires. Compared with relatively stable levels of urban background air pollution, wildfire smoke is episodic, highly concentrated, and different in composition, which may lead to distinct pathophysiological effects on the cardiovascular system.
"Epidemiological studies linking wildfire smoke to cardiovascular outcomes are limited compared with the extensive literature on urban air pollution. Most prior research has focused on short-term associations with respiratory and cardiovascular events, whereas the long-term health impact of prolonged, repeated smoke exposures remains understudied. In this issue, Hao and colleagues provide new evidence linking long-term exposure to wildfire smoke PM₂.₅ with incident stroke in the US older adults.
"The study by Hao et al. adds to a growing body of research suggesting long-term health impacts of wildfire smoke. […] Collectively, the findings from these large-scale studies indicate that wildfire smoke PM₂.₅ should be recognised as a long-term health risk factor, not only an acute environmental hazard."
