As wildfires rage across Los Angeles, the smoke billowing into the air carries more than the expected risks of particulate matter and carbon emissions.
Urban wildfires, unlike those in remote forests, burn through large areas of neighbourhoods filled with synthetic materials like plastics, rubber, fiberglass and treated asphalt, releasing a cocktail of hazardous chemicals into the air.
“Wildfire smoke when it’s purely burning biomass is bad enough. But when there are large fires in the wild and urban interface, then you get all these kinds of smoke and toxins that are from structural fires,” said John Balmes, a professor emeritus at the University of California, San Francisco, and the University of California, Berkeley.
“It’s much more toxic. The combination of burning biomass and structural fire smoke… like the Palisades fire, that is blocks and blocks of homes, that is particularly toxic smoke,” he added.
As of Friday, the fires have engulfed iconic Los Angeles neighborhoods, killing 10 people so far and destroying nearly 10,000 structures. The fires have torn through buildings, roads, cars and bridges.
On Friday, air quality in the Los Angeles area remained “unhealthy”‘ due to the multiple wildfires ravaging the region.
But wildfires can travel long distances, and the harmful particles they produce can cross the country and even the continent, Balmes warned.
Here’s what to know about the risks of wildfire smoke when fires hit cities.
What is wildfire smoke made of?
As wildfires rage through forests, grasslands and urban areas, they burn a variety of natural and human-made materials, producing a dense, hazardous smoke.
This smoke is a mixture of gases and fine particles, with some of the most dangerous components being particulate matter (PM2.5), carbon monoxide, carbon dioxide, volatile organic compounds (VOCs), and a range of toxic chemicals.
PM2.5 refers to tiny particles that are less than 2.5 micrometers in diameter—far smaller than the width of a human hair. These particles, though invisible to the naked eye, are extremely dangerous. And because of their tiny size, they can easily penetrate deep into the lungs, and even enter the bloodstream, posing serious health risks.
There is no known safe level of exposure to some of these pollutants, meaning that even low levels of wildfire smoke can negatively impact your health, according to Health Canada.
Why urban wildfires are more toxic
Urban wildfires pose a unique threat because they burn through a mix of synthetic materials, including buildings, vehicles and industrial products (like paint and household cleaners). Unlike wildfires in rural areas, these fires generate smoke filled with toxic chemicals.
“Urban firefighters wear self-contained breathing apparatus (Scott air packs) because the air is so toxic. The smoke from burning synthetic materials, such as plastics and car parts, contains dangerous substances,” Balmes said.
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One of the most hazardous chemicals produced during urban wildfires is phosgene, a toxic gas that used to be used as a chemical weapon during the First World War.
Phosgene is generated when modern plastics burn, and inhaling it can lead to severe lung damage, respiratory failure and even death, according to the U.S. Centers for Disease Control and Prevention.
“There are other toxic compounds in urban wildfire smoke, such as hydrochloric acid, which is produced when synthetic fabrics and materials burn. You also get harmful metals and hydrogen cyanide, which is incredibly toxic. Along with these, formaldehyde and benzene are commonly found in the smoke, and these are carcinogenic,” he said.
While many wildfires, such as those in Canada, have primarily affected rural areas and small communities around them,a 2021 report by the National Research Council of Canada on urban fires predicts that, in the coming decades, wildfires in urban centres will increase both in regions with a long history of fires and in areas that have been less affected in recent decades.
As climate change intensifies and urban areas expand, experts like Balmes warn that larger urban centres will become more susceptible to wildfires, with the tragic Los Angeles fire serving as a prime example.
What are the health risks?
Small particles are some of the most health-damaging air pollution, linked to a range of chronic and deadly health issues, according to the U.S. Environmental Protection Agency.
Studies have linked wildfire smoke with higher rates of heart attacks, strokes, and cardiac arrests, increases in emergency room visits for respiratory conditions and weakened immune systems.
A 2024 U.S. study published in JAMA Network Open, looked at the impact of wildfire smoke from Western Canada (during the 2023 wildfire season) on cardiopulmonary disease in the eastern U.S. The study found a spike in heart and lung illnesses that the authors said was linked to wildfire smoke originating up to 3,380 kilometres away in Canada.
The effects of wildfire exposure can also persist for years.
After Australia’s 2014 Hazelwood coal mine fire, rates of heart disease remained elevated for two and a half years and respiratory illnesses for five years, researchers have reported.
Wildfire smoke exposure in pregnancy has been associated with pregnancy loss, low birth weight and preterm delivery.
A 2022 study published in the American Journal of Obstetrics and Gynecology found a link between wildfire smoke exposure and cellular damage in first and second-trimester placentas.
And a 2022 Canadian study published in The Lancet found that people who lived outside of major cities and within 50 kilometres of a wildfire in the past decade had a 4.9 per cent higher risk of lung cancer and a 10 per cent higher risk of brain tumours.
“Firefighters are at increased risk for cancer because of career exposure,” Balmes said.
“The public can get massively exposed during these fires, but there’s only a cancer risk if there’s continuous exposure, and of course definitely a risk to people with preexisting heart conditions and lung cancer patients.”
He stressed that when it comes to wildfire smoke, the most vulnerable people are those with pre-existing respiratory conditions, the elderly, children, pregnant people and those with weakened immune systems.
How to stay safe
Protecting yourself from wildfire smoke is crucial, especially if you’re in a vulnerable group, Balmes said.
The first step to protecting yourself is to stay indoors. Keep windows and doors closed, and if possible, use an air conditioner or an air purifier with a high-efficiency particulate air (HEPA) filter to maintain clean indoor air, he said.
If you need to go outside, Balmes said to wear a high-quality mask, such as an N95 respirator, to help filter out fine particles in the air. Standard cloth masks are not effective against smoke particles.
He added that if you need to travel by car in wildfire smoke, set your vehicle’s air system to recirculate to minimize exposure.
In an email to Global News on Friday, Global Affairs Canada said Canadians in the area of the California wildfires should stay away from the affected area, particularly if they suffer from respiratory ailments, and always follow the instructions of local emergency services personnel, including any evacuation order, and monitor local media for up-to-date information on the situation.
“I would urge also people to remember that climate change is real,” Balmes added.
“And Canada and the U.S. in particular have lots of forests that are suffering from our relative drought. And so the risk of forest fires we have is high. And so he will have to take personal responsibility for reducing greenhouse gas emissions.”
A NASA study in August 2024 said the extreme forest fires in Canada in 2023 had been stoked by “Canada’s warmest and driest conditions in decades.”
— With files from Reuters