As thick layers of smoke from hundreds of Canadian wildfires descended this week on a large swath of North America, including Northeast Ohio, millions of people were urged to stay indoors, run HEPA filters and venture outside only with a high-quality face mask. Wildfire smoke, a seasonal hazard in parts of the U.S., had virtually overnight become an everyone problem.
"It's a risk to our health," says Christine Wiedinmyer, the associate director for science at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder. "It's everything from respiratory impacts like asthma to cardiovascular outcomes. There's been evidence that women who are exposed to wildfire smoke during pregnancy have statistically lower birth weights."
In many North American cities, some level of air pollution is routine, whether from fossil fuels, vehicle emissions, natural gas used for heating or fumes from chemical production. But wildfire smoke is particularly bad for humans, and that has to do with both size of the particles involved and what those particles are made of.
"Wildfire smoke, especially smoke that has traveled a long way, is very small," says Luke Montrose, an assistant professor and environmental toxicologist at Colorado State. "That's what gives it the ability to be transient."
Because the particles in wildfire smoke are so small — much, much smaller than the smallest grain of sand — they have little trouble making it past the guardrails our body has set up to keep pollutants out. Smoke particles can get past our nose hairs, and the mucus membranes that line our upper respiratory tract. (We get phlegm from breathing in smoke explicitly because our body's mucus is trapping particles so they can be coughed up and out).
The very smallest particles, known as PM2.5, can even make their way past the mucus membrane and into the lower respiratory tract. That airway's job is "to transfer oxygen across the lung blood barrier," said Montrose, making this kind of pollution particularly devastating for people who already have underlying lung conditions like asthma or COPD. Even people with healthy lungs effectively get less oxygen — and those impacts aren't always indicated by an overt symptom like coughing.
"Folks who maybe aren't sensitive typically may have other symptoms like lethargy," said Montrose. "They may just feel off or groggy or lack energy. And that can be attributed to that lack of oxygen getting to the body."
This oxygen deficit is also why people dealing with poor air quality are discouraged from exercising, particularly outdoors. More activity means heavier and faster breathing, which brings more particles into the body and pushes them deeper into the lungs — ironically, inhibiting the body's ability to take in oxygen when it needs it most. The impacts can be long-lasting, too. One study from 2020 looked at a community in Montana that was exposed to wildfire smoke for more than a month; a year later, residents were still suffering from decreased lung function.
Wildfire smoke also contains thousands of compounds, some of them potentially toxic, such as volatile organic compounds, hydrocarbons and nitrogen oxides. A 2022 study published in the journal The Lancet Planetary Health that looked at the effects of wildfire smoke on Canadians found that people who lived within roughly 30 miles of a wildfire had a 4.9% increased risk of lung cancer and a 10% increased risk of brain tumors than unexposed populations.
Nor do the hazards of wildfire smoke dissipate as it travels — smoke released into the atmosphere becomes "aged" and more toxic over time. A 2020 study found that smoke samples taken over five hours after their release from a fire were twice as toxic as when they were first released; after aging further in a lab, they were four times as toxic.
Smoke that has traveled "has had time to interact with the chemicals in the air, it's had time to interact with the sun," Montrose says. "The caveat to that is, the smoke at the source is going to be so much more concentrated."
This week's fires are just the beginning of what could be a smoke-filled summer in North America — and a new normal of sorts thanks to climate change. "It's only getting progressively worse, like in terms of the severity of the fires, the length of a fire season, and the amount of smoke that's been put up into the air," says Wiedinmyer at the University of Colorado Boulder. "But there are ways to protect yourself. Stay indoors, run your AC when there's a smoky event, wear a mask outside (and) limit your exercise to prevent inhaling these particles for the long term."