Wildfire smoke can irritate the eyes and throat, and lead to coughing, wheezing and headaches. It can also exacerbate respiratory and cardiovascular illnesses, making asthma symptoms worse and increasing the risk of heart attacks and strokes.
But as large wildfires have become regular occurrences in Canada, some scientists warn that repeated exposure to the air pollution they produce could pose long-term health risks, potentially leading to a higher incidence of illnesses such as cancer and dementia.
“It’s not just a short-term exposure anymore, right?” said Scott Weichenthal, an associate professor in the department of epidemiology at McGill University. “If it happens every year, it’s become a long-term, chronic exposure.”
Wildfires have burned through more than one million hectares in Alberta this spring and have forced thousands from their homes. Saskatchewan, Manitoba, B.C. and the Northwest Territories are also battling active fires. The website Firesmoke.ca, maintained by the University of British Columbia’s Weather Forecast Research Team, shows these fires are contributing to a wide ribbon of air pollution, snaking across much of the country.
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In the short-term, this could mean increased hospital visits, especially among people with existing health conditions who have trouble breathing, Dr. Weichenthal said. But in a study last year in The Lancet Planetary Health, he and his team found evidence to suggest long-term exposure may increase the risk of lung and brain cancers.
By examining Canadian historical forest fire data and national census data, they found people who lived near wildfires had a 4.9 per cent relatively higher incidence of lung cancer and a 10 per cent higher incidence of brain tumours than populations that were not exposed to wildfires.
“Essentially, everything that comes off of a cigarette would be coming off of a forest fire,” Dr. Weichenthal said.
Poor air quality can be particularly harmful to children, according to Anna Parenteau, a doctoral student in the department of psychology at the University of California, Davis. Ms. Parenteau is the first author of a study, published last year in the journal New Directions for Child and Adolescent Development, that found that healthy children exposed to higher levels of air pollution from wildfires had elevated levels of markers of inflammation in their blood.
While inflammation is the body’s natural defence against infection and injury, it damages healthy cells, too, and can be toxic when it is chronic.
Ms. Parenteau and her colleagues also found the children had lower cardiac autonomic regulation, which controls heart rate and blood pressure.
Extrapolating from her findings, Ms. Parenteau suggested children could be at risk of developing long-term metabolic and cardiometabolic health problems, including heart attacks and obesity, if they are chronically exposed to air pollution.
Compared with adults, children breathe in more pollutants relative to their body size, she said. And the fact that their bodies and organs are still developing adds to their vulnerability.
Chris Carlsten, head of respiratory medicine at the University of British Columbia, said there is a lot of speculation about the chronic health effects of air pollution from wildfires, but not yet a lot of hard evidence. Part of the issue is that much of this speculation is based on scientific literature on traffic-related pollution, he said.
“That’s sort of both reasonable and also, at the same time, potentially problematic,” said Dr. Carlsten, who is also director of Vancouver Coastal Health’s Legacy for Airway Health program. “It’s reasonable because traffic-related pollution and fire smoke have a lot in common, but they also have a lot that’s not in common.”
Both comprise a mixture of particles and gases. But with traffic-related pollution, gases are generally a greater concern, since they occur at higher concentrations near the source, he said. By contrast, particles are typically a greater concern for those near fire smoke, since their concentrations can be very high close to the fire.
Further complicating matters, the composition of air pollution changes, the further it moves away from the source, he explained. A process called photochemical transformation occurs, whereby sunlight catalyzes reactions that change the chemical mixture of the air.
This means that by the time a smoky plume from wildfires in Alberta reaches Eastern Canada, the concentration of pollutants declines, Dr. Carlsten said. “But the farther it goes, there are some aspects of the mixture that can actually be more harmful.”
Its oxidative potential increases, he said, which means pollution molecules become more reactive. They interact with cells and tissue and cause damage.
Fluctuating weather patterns and the ever-changing composition of air pollution are just some of the hurdles to obtaining concrete answers about the chronic health effects of wildfire smoke. Research is also needed to follow people for years to allow time for any long-term illnesses to develop, said Janet Martin, an associate professor in the department of epidemiology and biostatistics at Western University.
The paradox, however, is that even though any long-term harms won’t be apparent until years into the future, it’s necessary to take measures now to mitigate the risks, Dr. Martin said. She said the frequency of wildfires makes it all the more urgent to reduce other forms of air pollution and adopt greener ways of living and cleaner industries.
“It’s a strange and deceptive dichotomy,” she said. “We have huge risks happening right now, today, and we can’t measure it.”
Nevertheless, she said, existing research does offer clues about the potential dangers ahead. In the journal Neurology, Dr. Martin and her team found a significant link between dementia and air pollution, likely including from wood-burning and wildfires, in the form of small particulate matter, called PM 2.5.
These particles, less than 2.5 microns in diameter, can make their way deep into the lungs and into the blood.
“That’s a foreign pollutant which is entering the bloodstream and able to travel to all parts of the body,” she said, explaining the link to dementia is but one risk. “We’re just not sure yet what that means in terms of other chronic diseases.”