- Increases in extreme wildfires in many parts of the world are fueling concerns over the impacts of smoke on human health, with global warming, forest management and land-use change the primary drivers of increasing wildfires.
- Similar to other types of air pollution, wildfire smoke is associated with a wide range of short-term health impacts, especially for vulnerable groups such as the elderly and people with preexisting medical conditions.
- Wildfire smoke is increasingly adding to the cumulative air pollution burden experienced by modern industrial societies — even in remote or rural areas.
- Emerging evidence also suggests wildfire smoke may have long-term health impacts and contribute to increased risk of chronic conditions.
When the Parker Lake wildfire erupted near Fort Nelson in British Columbia, Canada on May 10, it marked the start of what many fear could be yet another devastating Canadian fire season.
With an ongoing multiyear drought, forests across western Canada are tinder dry and primed to burn. By February, hundreds of “zombie fires” that had lain dormant underground over winter started to smolder. In April, British Columbia’s spring snowpack was just 63% of normal, the lowest ever recorded. Then, in mid-May, there was a week of hot and windy days — and a spark set the forest outside Fort Nelson alight. Strong winds drove the flames toward the community, advancing 8 kilometers (5 miles) in just four hours. By May 12, officials had told nearly 4000 people to evacuate.
As of June 13, about 323,000 hectares (798,000 acres) of forest had already burned this year in British Columbia. That’s three-quarters of the average annual amount over the past 20 years. To the east, in Alberta, similar scenes have unfolded.
This year’s fires have erupted even while memories of last year’s record fire season are still fresh. Last year, 15 million hectares (37 million acres) burned in Canada, an area one-sixth the size of British Columbia and more than twice the previous record. Four percent of Canada’s forested land went up in flames that year alone.
Those fires drove nearly a quarter of a million people from their homes. But the evacuees weren’t the only ones directly affected by Canada’s record-setting fire season.
All last summer, thick columns of smoke billowed up from the flames and were carried by the prevailing winds hundreds and even thousands of kilometers away; the smoke was something you could see, smell, taste and feel. Yellowknife, a city of 20,000 in the Northwest Territories, had poor air quality for 45 days. In New York City, smoke from the Quebec fires turned the sky orange and made early June air quality the worst of any city on Earth.
Fire is, of course, a natural part of many landscapes. But as climate change heats up, wildfires are burning hotter, longer, and in places that don’t usually burn. This year, exceptional fires have already razed vast sections of the Amazon region in Venezuela, Colombia and Brazil, while blazes are also ravaging Mexico.
In a warming world, more fire means more smoke — and that has health experts worried.
Breathing in the world’s torched forests
Outdoor air pollution produced by vehicles, power plants, industry and agriculture is strongly linked to a wide variety of health problems, including respiratory conditions, cardiovascular impacts, negative birth outcomes, lung cancer and more; it causes at least 4 million premature deaths annually worldwide.
But for a long time, scientists weren’t sure if wildfire smoke posed similar risks, says Michael Brauer, a professor at the University of British Columbia’s School of Population and Public Health. There was a general feeling that wildfire smoke, being natural and more sporadic, was unlikely to have a large impact.
However, over the past two decades, as intensified wildfires have become increasingly common — with immense fires scarring Australia, Siberia, the Amazon, the western U.S., Greece, Hawai‘i and of course Canada — researchers scrambled to figure out what all that smoke means for our health.
“Increasingly, it looks just as bad [as other types of air pollution] and in some cases, maybe even worse,” Brauer says.
Wildfire smoke is composed of thousands of chemical compounds and particle sizes. The exact mixture is hard to pin down as it depends on what’s burning, and changes as compounds and particles react in the atmosphere. To get around this variability problem, researchers often use the concentration of very small particulate matter, abbreviated as PM2.5, and measured in micrograms per cubic meter (μg/m3), to represent the entire suite of pollutants.
PM2.5 is defined as any particle less than 2.5 microns in size (about 1/28 as wide as a human hair) and is a characteristic of all types of air pollution, including wildfire smoke.
These particles are dangerous because they’re so small that they evade the body’s defenses and penetrate deep into the lower respiratory tract. That triggers an immune response: inflammation, oxidative stress and other mechanisms. According to current science, it’s this prolonged immune response that inflicts much of the bodily damage. The particles can also enter the bloodstream or cause harm in other ways.
There are some indications that wildfire smoke can even be more harmful than other air pollutants, at least for respiratory health. Because it’s such a complex mix of organic compounds, wildfire smoke may trigger an even greater immune response, says Fay Johnston, a professor at the University of Tasmania in Australia.
Even a little smoke can be dangerous
For most people, the short-term consequences of a single smoke event will likely be mild. If you go for a walk, your eyes tear up, the throat tickles, and the chest might feel congested.
But for people with underlying health issues, the elderly or other vulnerable groups, the impacts can be worse. Those with asthma might need to reach for an inhaler. For diabetics, blood sugar could go up. For anyone already at risk of heart disease, blood will be more likely to clot. That might even bring on a heart attack or stroke.
The increased risk of hospitalization or death following wildfire smoke exposure is perceptible on a population level. A meta-analysis of existing studies, published in the Annual Review of Medicine, reported a 0.15% increase in overall deaths and a 0.25% increase in respiratory-related hospitalizations for every daily 1μg/m3 of wildfire PM2.5 exposure, as well as some evidence of increases in cardiovascular-related outcomes.
And, surprisingly, the spike in smoke pollution doesn’t need to be very intense to have a measurable impact, says Johnston.
It’s not so much the amount of smoke, but rather the body’s response to it, that causes the damage. And that response kicks in early, then starts to taper off.
When Johnston’s team looked at the health impacts of Australia’s infamous 2019-2020 fires, they counted more than 3,000 hospital admissions for cardiovascular and respiratory disorders and 429 premature deaths from smoke-related causes. Eighty percent of those deaths occurred at mild to moderate levels of smoke exposure, she says, on days when the sky was hazy and levels of PM2.5 were just 50 μg/m3 or less.
“The big public health impacts are actually in the low fluctuations [of smoke], rather than the rare extreme ones,” Johnston explains.
She adds that there are also numerous smaller health impacts not as easily captured in the data. “For every death and heart attack, you’ve got 10 times more people needing to see a GP or needing to go to a pharmacy,” she says. “You’ve got 100 times more people with symptoms missing work, kids [missing school], so the ramifications … are quite big.”
Wildfire smoke also adds to the general air pollution mix of modern life. As fires become more frequent and widespread, they’re having a noticeable impact on air quality in unexpected places. For example, wildfire smoke is reversing or stalling progress on clean air improvements in many U.S. states, according to a 2023 Nature study,
And people living in remote areas aren’t immune from the onslaught of multiple pollutants: Indigenous communities, for example, living downwind of oil sands mining in far northern Alberta, Canada, are already exposed to significant amounts of hydrocarbon pollutants. But in many of these communities, wildfire smoke is now a regular feature of summer.
“Our policy tools fail when it comes to air pollution … from wildfires, at least at this point. And yet [smoke] has a real fingerprint on the air pollution exposures across our country and much of the world,” says Carlos Gould, associate professor at the University of California, San Diego, and one of the Nature study co-authors.
Puzzling impacts of single smoke events
There is emerging evidence that relatively short spikes in air pollution from wildfire smoke might have outsized longer-term impacts.
Long-term health studies are notoriously difficult to do, but in 2014 there was a large coal mine fire in Morwell, Australia, that allowed researchers, including Johnston, to track the long-term effects of this single but intense six-week smoke event. Because coal is carbonized plant matter, the composition, severity, timing and duration of the smoke was similar to what’s found in peat or forest fires.
“We thought [the coal fire] was an episodic event that would precipitate problems in those at risk, and then we’d all go back to normal,” Johnston said. But instead, the impacts persisted for several years.
There was an increased risk of emergency room visits for cardiovascular conditions in the first two and a half years following the fire, and for respiratory conditions in the first five years. When adults and young children were tested three and a half years after the fire, researchers found higher levels of smoke exposure to be associated with decreased lung function, though those impacts were not detected seven and a half years post-fire.
“I didn’t expect [those long-term effects] at all,” Johnston says.
Johnston stresses that the effects were small, and that smoke is just one of the very many risk factors that shape health. But the results of the long-term study, called the Hazelwood Health Study, show that a one-off event can have longer-term measurable impacts, particularly for children.
“These signals are tiny; they don’t affect day-to-day life of a child, but [this study] tells us that it does shape development. That’s why children and pregnant women are in the high-risk groups” when it comes to air pollution, Johnston says.
There’s also emerging evidence that relatively short but intensive wildfire smoke exposures can contribute to increased risk of chronic conditions, which is both surprising and concerning, Brauer says.
For example, a Canadian study found that wildfire smoke exposure led to a slightly increased risk of both brain and lung cancer. Another study documented an association between wildfire smoke and dying from cancer in Brazil. And a 2023 Jama Internal Medicine study found that while all types of PM2.5 air pollution increased risk of dementia, air pollution from wildfires and agriculture were associated with the highest risk.
Brauer says researchers are just starting to explore what these early studies indicate in terms of long-term impacts.
Wildfire smoke tends to be episodic. It blows in and air pollution levels shoot up for days or weeks, but then it stops or moves elsewhere. It could be that the spike in air pollution somehow overwhelms the body’s defense systems or happens during a critical period of development such as during pregnancy, Brauer says, though more research is needed.
“I would say that the jury’s definitely still out on that,” he notes. “And that’s … an interesting kind of problem we didn’t have 10 years ago, because we didn’t have these repeated kinds of [smoke] events.”
Living in a world on fire
Canada’s jaw-dropping wildfire season last summer wasn’t just about the amount of land that burned. It was also about how long the season lasted, the intensity of the fires, and the way they burned unabated through the night.
Globally, 2023 was the hottest year on record, with Canada warming twice as fast as the rest of the world. Driving the intense fire season was what ecologists call extreme fire weather: hot, dry days that sucked moisture from the forest, winds that whipped up flames, and rain that just didn’t come.
“A window into the future,” says Piyush Jain, a research scientist at Natural Resources Canada, the government department overseeing forests, energy and natural resources.
Analysis by the World Resources Institute shows that tree cover loss due to wildfires has doubled over the past 20 years, and a 2022 U.N. Environment Programme report estimates that the likelihood of extreme wildfires will increase by up to 30% by 2050, depending on greenhouse gas emissions.
The reasons are complex. For one, climate change is increasing extreme fire weather almost everywhere in the world, priming landscapes to burn. In many places, there’s an increase in lightning ignitions, nighttime fire activity, and the length of the fire season. This climate signal is especially evident in places like Canada’s boreal forests, says Stefan Doerr, professor of wildlands fire science at Swansea University in the U.K. That’s partly because the north is warming much faster than the rest of the world.
But it takes more than hot, dry days for fires to start. In many places, the human imprint also has a big influence on when and where fires occur, Doerr explains. Forestry practices, land-use change and deforestation are changing fuel loads (the amount of flammable material) and sources of ignitions.
In some places, for example, aggressive long-term fire suppression and the prohibition of traditional Indigenous burning practices have left forests fuel heavy, leading to larger, more intense fires. In others, replacement of natural forests with tree plantations makes forested areas more flammable. Likewise, people are building homes in fire-prone areas, within what’s called the urban-wildlands interface, which increases the chances of human-caused accidental ignitions.
A problem for the Global North and South
The human footprint in fire is especially evident in tropical rainforests. These ecosystems aren’t adapted to wildfires, and didn’t naturally burn in the past. But deforestation and degradation in the Amazon has opened up much of the forest, drying it out and making it more vulnerable to fire, says Ane Alencar, science director at Brazil’s Amazon Environmental Research Institute (IPAM).
In hot, dry years, which are now more numerous, those secondary forest patches in the Amazon become like tinder, with even small land-clearing fires easily getting out of control and spreading. Often, too, fires are set intentionally by landowners to expand their fields, or by land grabbers who burn the standing trees, illegally lay claim to the now valuable cleared property, then sell it to ranchers as pasture.
Overall, tropical forest loss due to wildfires grew by 5% per year from 2001 to 2022, according to a recent analysis by the World Resources Institute. And, of course, every time a wildfire burns, carbon is released into the atmosphere, intensifying climate change, creating a wicked cycle.
Landscape fires — a term that includes wildfires, agricultural burning, peat fires and other types of vegetation fires — exposed 2.18 billion people to at least one day of substantial air pollution per year from 2010-2019, according to a 2023 Nature study. A separate 2022 Atmospheric Environment study found that landscape fires expose 44 million people globally to unhealthy air quality, causing more than 650,000 premature deaths each year.
Lower-income countries bear most of this burden, though that harm isn’t well documented, as the research remains concentrated in higher-income countries.
“The vast majority [of health problems due to smoke] is borne in tropical savannas and in tropical deforestation fires, in low-income countries who get seasonally bad pollution every year and have done for decades,” Johnston says. “But until climate change began driving awful [smoke] episodes, particularly for those of us in higher-income countries, there wasn’t a lot of attention to the problem.
“So we are now waking up to what our colleagues around the world have been living with for a long time and that’s driving more research, but there … still is a gap … that needs addressing.”
Banner image: A member of a Quebequois fire crew assisting on a fire in the Plumas National Forest in northern California in 2020. Numerous countries have international agreements in place to share firefighting resources. Canada received assistance from 16 countries during the 2023 fire season. Image by Region 5 Photography via Wikimedia Commons (CC BY 2.0).
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