It's been an early start to the wildfire season in Canada, with more than one million hectares burned in Alberta alone and thousands of people forced to evacuate their homes across Western Canada and in parts of the Maritimes.
Wildfire smoke has also been affecting air quality for millions of people in this country, causing short-term health effects such as burning eyes, sore throat, cough and headache.
If you have an underlying respiratory or cardiovascular health condition, you are more at risk.
"I always talk about people with chronic respiratory diseases like asthma and COPD as the canaries in the coal mine of wildfire smoke," Sarah Henderson, an epidemiologist who researches wildfire smoke, told Dr. Brian Goldman, host of CBC's The Dose.
"They are very sensitive to it and more sensitive to wildfire smoke than to other kinds of air pollution," said Henderson, the scientific director of environmental health services at the B.C. Centre for Disease Control.
As Canadians learn to deal with the immediate health effects of wildfire smoke — from running indoor air purifiers to adjusting outdoor plans — researchers are gradually learning more about the long-term impacts smoke exposure could have on the body.
It's an important area of research, they say, because more of us are going to be exposed to smoke in the coming years — and in increasing amounts — as wildfires increase in intensity and size.
"What we're seeing is communities having really big exposures in one summer, or exposures that last for weeks or months, summer after summer," said Henderson.
"We just don't have a lot of evidence yet about what the health impacts of those longer-term patterns are going to be."
What makes wildfire smoke harmful?
The chemical makeup of wildfire smoke is complex, said Henderson.
"It depends on what's burning, it depends on how hot it's burning, and it depends on what the weather conditions are around the fire. So the mix of gasses and particles can be really different depending on those conditions," she said.
One of the known dangers of wildfire smoke is particulate matter, measured in micrometres, or microns. Any particles smaller than 2.5 microns — known as PM2.5 — can infiltrate the lungs and cross the blood-brain barrier.
But what exactly is in that PM2.5 depends on what materials are burning: vegetation or human-made structures and fuels.
Smoke from fires that burn human-made materials is going to be more complex, said Henderson.
"There's a lot of material when we're talking about communities or houses that are burning, and so it's a very thick type of smoke," she said.
Wildfire smoke in the short term
When PM2.5 gets into your lungs, your body's immune system kicks into gear, similar to the way it would treat a virus or bacteria, said Henderson.
But those tiny smoke particles can't be killed the way viruses or bacteria can be, so your immune system just keeps working.
That can bring on systemic inflammation in the body, which can lead over time to chronic diseases such as heart disease, said Dr. Courtney Howard, an ER physician in Yellowknife who researches wildfires and health.
Studies have shown people with underlying heart problems have an increased risk for heart attack and other cardiovascular issues after being around wildfire smoke.
Along with people who have respiratory or cardiovascular conditions, young children and pregnant women are some of the most vulnerable to the effects of wildfire smoke, said Howard.
"Little ones breathe faster per unit of body weight than people who are bigger, and so they end up with a larger exposure to the air pollution than adults at a similar level of air pollution," Howard said.
Very young infants are of particular concern, said Henderson, because their lungs are extremely sensitive and still growing.
If that growth is disrupted by wildfire smoke, it could mean lung issues throughout their lives, she said.
WATCH / Why Nova Scotia is having a tough start to wildfire season
Two wildfires covering more than 7,000 hectares of land have forced more than 15,000 people from their homes in Nova Scotia. Andrew Chang talks to Environment Canada's David Phillips about how the fires got out of control so quickly, and how this could be just the beginning of a challenging season.
Henderson also pointed to studies that have been done on the effects of wildfire smoke on developing fetuses.
"There's not really enough to feel super confident about the results, but what we see is that the infants are a slightly lower birth weight," she said.
There is also evidence around a higher risk of preterm birth for people with gestational diabetes exposed to wildfire smoke, and some evidence around birth defects, she added.
Smoke linked to increasing cancer rates
There are studies linking higher rates of cancer with exposure to wildfire smoke, including a 2022 study in the Lancet that looked at population data in Canada over 20 years.
It found that long-term exposure to wildfire smoke might increase the risk of lung cancer and brain tumours, but the study's authors said further work is needed.
Though it's not related to a wildfire, Henderson pointed to research from Australia around exposure to smoke from a coal mine fire in 2014 that affected a nearby community for weeks.
"What they do see is some higher rates of cancer in that community that they were able to detect within five years of that fire occurring," Henderson said.
More research needed on mental health impacts
Researchers say the mental health impacts need to be studied more.
"There's not a lot of literature yet on the mental health impacts of wildfires. What we have shows that it does have an adverse impact on our mental health," said Howard.
And PM2.5 could have direct impacts on the brain itself, said Henderson, who did a recent study on cognition during smoky periods using data from Lumosity, a brain-training platform.
"What we saw is that when it was smoky, there were these small decrements to people's cognitive performance," said Henderson.
Study suggests link between smoke and flu season
A study in 2020 in Environment International used hospital data to look at rates of influenza following the wildfire season in the state of Montana, which typically lasts from July to September.
The study found that after a bad wildfire season, rates of influenza in Montana were three to five times higher than normal, said lead author Erin Landguth, associate professor in the school of public and community health sciences at the University of Montana.
Landguth said she was shocked by her findings about just how long after the fire season there were higher rates of flu, given that most previous air pollution research had focused on short-term health impacts in the days following wildfire smoke exposure.
She cautioned that her study results can't be extrapolated to other states or countries, and nor should they be used to draw conclusions about what is happening in the body due to wildfire smoke.
Landguth and her team are currently running models for all of the western U.S. for influenza correlated with wildfires, building on the Montana research.
"It's really important that we understand and we can extrapolate the results in a meaningful way," said Landguth.