Being exposed to wildfire smoke over a long period of time could increase the risk of developing dementia even more than exposure to other sources of air pollution, according to new research.
In the study, published last week in the journal JAMA Neurology, researchers looked at a cohort of more than 1.2 million people over the age of 60 living in Southern California between 2008 and 2019. They tracked dementia incidence among the cohort and compared it to the average concentration of fine particulate matter, also known as PM2.5, in the air due to wildfires in the region.
They found that when the three-year average concentration of wildfire PM2.5 went up by just one microgram per cubic metre of air, there was an associated 18 per cent increase in the odds of a dementia diagnosis.
The same increase in concentration of non-wildfire PM2.5 was associated with only a three per cent increase in the odds of a dementia diagnosis.
Although previous research has connected long-term exposure to air pollution with a greater risk of dementia, less research has been done into the effect of specific air pollution sources.
“They’re seeing a stronger effect of PM2.5 from wildfires than from other sources,” said Scott Weichenthal, an associate professor in the department of epidemiology, biostatistics and occupational health at McGill University in Montreal. He was not involved in the study.
“That’s important because we need to start paying more attention to the kinds of particles that are in the air, not just how much mass is in the air, because the chemical composition also plays a really important role in the health effects.”
How smoke can reach the brain
Fine particulate matter are tiny particles in the air found in smoke and fumes that measure less than 2.5 micrometres and can travel deep into the lungs when inhaled.
Wildfire smoke can contain a dangerous cocktail of chemicals from burned residences, vehicles and plastics, Weichenthal said. And unlike smog, which is a steady source of air pollution, wildfire smoke brings massive spikes of PM2.5 concentration in the air — which can affect not only the lungs but the entire body.
There might be fewer wildfires than in recent decades, but they’re getting worse.
When the body encounters foreign particles, it mounts a response, sending the lungs into a “heightened state of inflammation” that can spill over into other organs through inflammatory cells in the bloodstream, said Michael Brauer, a professor at the school of population and public health at the University of British Columbia in Vancouver.
This is the leading theory for how long-term exposure to air pollution can impact the brain, he told CBC News, although scientists aren’t yet certain about the mechanisms.
Brauer, who was not part of the study but has previously examined the connection between PM2.5 and dementia in his research, said air pollution strongly impacts the cardiovascular system, potentially affecting blood flow to the brain as well.
How to minimize exposure
Many Canadians live in areas where wildfires are simply a reality of life. So how can you protect yourself?
Air filtration systems, such as HEPA filters, go a long way in cutting down long-term exposure, Weichenthal said. On days when the air quality is poor during wildfire season, wearing a KN95 or N95 mask will also minimize risk when outdoors.
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But last week’s study found that the connection between long-term exposure to wildfire smoke and dementia was stronger for those living in high-poverty areas — where people may not be able to afford air filters, access air quality warnings or stay home from work to avoid being outdoors on bad smoke days.
It’s a sign that society needs to shift toward more climate infrastructure, Brauer said.
“For a fraction of the cost that it takes to treat somebody in [the] emergency room, you pay for a portable air cleaner in their home, or we outfit every school with the air filtration…. And in fact, over the long term, we’ll probably save money, just because of … the extreme cost of actually treating people.”
When researchers looked more closely at the data, they also found that the sharp increase in the risk of developing dementia associated with wildfire PM 2.5 mostly impacted those between the ages of 60 and 75.
This could be because younger seniors are more likely to spend time outdoors and experience more long-term exposure to wildfire smoke, researchers suggested.
Those older than 75 who haven’t yet developed dementia may also have been “least susceptible” to developing dementia at all, as the disease doesn’t impact all seniors, Weichenthal said.
Wildfire health risks only increasing
The health impacts of wildfire smoke are a growing concern as climate change spurs extreme weather to new heights. Wildfires in Canada have grown in size and intensity, with a clear increase in the area burned decade over decade within the last 50 years. In 2023, Canada experienced its most destructive wildfire season ever recorded, with more than 15 million hectares of land burned.
It’s the rise in the baseline exposure over time that is most concerning, Brauer said, as those long-term health impacts will keep adding up.
A new report says heavy wildfire smoke made Canada’s 2023 air quality worse than the U.S. for the first time, and scientists warn it could happen again this year.
When he was younger, Brauer said, Vancouver used to experience about one day of serious smoke from wildfires every 10 years, “and then it was sort of once every three years, and now it’s like at least a week every single summer.”
“Think about a kid that has grown up now. They’re 10 years old. They’ve had this every single year of their life, and they’re going to have that for another 50 or 60 years.”
With wildfires only projected to get worse over time, scientists are working to keep up. McGill University launched the McGill Centre for Climate Change and Health in November to try to tackle the problems that climate change will bring.
“This sort of frequency and intensity of forest fires is probably going to be something that we’re going to be dealing with for our lifetimes,” said Weichenthal, who is an associate director of the centre. “So we need to be thinking about what we can do to adapt and take actions to reduce our exposures, recognizing that we’re probably not going to be able to prevent them.”
