As dozens of wildfires rage across the US and Canada, blackening the skies once again this summer, scientists are revealing even more about how dangerous wildfire smoke is for our health.
Some of the worst fires – including the Durkee fire in Oregon, the Park fire in California and the Jasper fire in Alberta, Canada – have sent smoke billowing for hundreds of miles around them, blanketing cities like Boise and Calgary with poor-quality air.
Wildfire smoke is known to be toxic to the body, but just how toxic is starting to come into focus as new studies emerge. Just this week, a decade-long study involving more than 1 million people in southern California found that exposure to wildfire smoke significantly increases the risk of being diagnosed with dementia by 21%, versus other types of air pollution such as from motor vehicles or factories.
Another recent study, also conducted in California, attributed more than 50,000 premature deaths to wildfire smoke exposure. Smoky skies have been linked to spikes in hospital admissions and ambulance calls for conditions such as asthma, and for people who have cardiovascular issues, the risk of cardiac arrests rises 70% during days with heavy smoke.
Those are just a few examples of how wildfire smoke is dangerous to health. The more research is done, the more we are learning about how different parts of the human body are impacted, says Dr Lisa Patel, a pediatrician and executive director at the Medical Society Consortium on Climate and Health. “We’ve long been concerned,” she says, “but people don’t fully understand this threat, because it’s a new one.”
Acres burned
US wildfires are measured in terms of acres. While the size of a wildfire doesn’t necessarily correlate to its destructive impact, acreage provides a way to understand a fire’s footprint and how quickly it has grown.
There are 2.47 acres in a hectare, and 640 acres in a square mile, but this can be hard to visualise. Here are some easy comparisons: one acre equates to roughly the size of an American football field. London’s Heathrow airport is about 3,000 acres. Manhattan covers roughly 14,600 acres, while Chicago is roughly 150,000 acres, and Los Angeles is roughly 320,000 acres.
Megafire
A megafire is defined by the National Interagency Fire Center as a wildfire that has burned more than 100,000 acres (40,000 hectares). That’s an area about the size of Rhode Island.
Containment level
A wildfire’s containment level indicates how much progress firefighters have made in controlling the fire. Containment is achieved by creating perimeters the fire can’t move across. This is done through methods such as putting fire retardants on the ground, digging trenches, or removing brush and other flammable fuels.
Containment is measured in terms of the percentage of the fire that has been surrounded by these control lines. A wildfire with a low containment level, such as 0% or 5%, is essentially burning out of control. A fire with a high level of containment, such as 90%, isn’t necessarily extinguished but rather has a large protective perimeter and a rate of growth that is under control.
Evacuation orders and warnings
Evacuation warnings and orders are issued by officials when a wildfire is causing imminent danger to people’s life and property. According to the California office of emergency services, an evacuation warning means that it's a good idea to leave an area or get ready to leave soon. An evacuation order means that you should leave the area immediately.
Red flag warning
A red flag warning is a type of forecast issued by the National Weather Service that indicates when weather conditions are likely to spark or spread wildfires. These conditions typically include dryness, low humidity, high winds and heat.
Prescribed burn
A prescribed burn, or a controlled burn, is a fire that is intentionally set under carefully managed conditions in order to improve the health of a landscape. Prescribed burns are carried out by trained experts such as members of the US forest service and indigenous fire practitioners. Prescribed burns help remove flammable vegetation and reduce the risk of larger, more catastrophic blazes, among other benefits.
Prescribed burning was once a common tool among Native American tribes who used “good fire” to improve the land, but was limited for much of the last century by a US government approach based on fire suppression. In recent years, US land managers have returned to embracing the benefits of prescribed burns, and now conduct thousands across the country every year.
Patel, who is also a hospital-based pediatrician, points to the research around wildfires and birth outcomes. “We have pretty good evidence now that it increases low birthweight for infants, and then there is emerging evidence that wildfire smoke increases stillbirth as well,” she says. Exposure to wildfire smoke also increases the risk of preterm birth by 3.4%.
Patel points out that fine pollutants have been studied for years, and the regular kind of fossil fuel-created particulate matter pollution still drives 8.1 million deaths worldwide – more than from tobacco.
But wildfire smoke is even worse. Every part of the body is affected by the tiny particles that enter the circulatory system. There is evidence that pollution from wildfires can move through the placenta and impact a developing fetus’s brain, lungs and liver. “It goes everywhere,” says Patel.
“We’ve known for years that there’s something different about wildfire smoke that makes it more toxic,” Patel says, “And now we’re starting to understand it’s not just respiratory, it’s playing out in other parts of our body as well.”
Researchers are still trying to understand exactly why, but wildfire smoke contains a toxic brew of compounds – including volatile organic compounds, aromatic hydrocarbons and heavy metals. That’s partly due to what is burning: when buildings go up in flames, the smoke will include metals, cleaners and many different types of substances – some that are carcinogenic – that become aerosolized. “All of these things make it worse for our bodies than the fossil fuel pollution that we’re always breathing,” Patel says.
In addition to pregnant people, chronically ill people and the elderly, young children are particularly vulnerable, because their lungs are developing rapidly in the first five years of life, and pound for pound, they are breathing in more smoke than a full-grown adult. A 2021 study estimated that the same level of smoke from wildfires was 10 times more harmful for children’s respiratory health than air pollution from other sources.
How to stay safe
So, given an alarming body of evidence, what’s the best way to stay safe? Patel, who lives in California, says she watches the air-quality index website every day and advises others to make it a habit. She also advises to invest in air purifiers in the home – especially for rooms where you sleep. If you can’t afford one, you can make a DIY version using a box fan and a high-quality filter. Those boxes can reduce air pollution inside the home by 50%-60%.
And if you need to go outside on days with high levels of wildfire smoke, wearing an N95 mask is the best protection, Patel says. “But as we know, the protection is only as good as the fit and only as good as the users willing to wear it. And definitely don’t wear an N95 and go out jogging,” she adds. “The N95 is just meant to keep you safe, but you should still not treat it like a normal day.”
Ultimately, she says, people need to think of these smoky days differently. “We need a new consciousness about this new era of unnatural disasters we’re living in,” says Patel, who is also a member of Science Moms, a group of leading climate scientists and moms who encourage moms to treat the climate crisis like a parenting issue.
“Wildfire days are going to happen again and again. And so we need to change our frame, from thinking that this is going to be a one-off, to understanding that for our kids, it’s more or less going to be the equivalent of a chronic exposure.”
