Q: How does wildfire smoke affect the risk of lung cancer? And how does this compare to things like secondhand cigarette smoke?
When wildfire smoke turned the skies of the San Francisco Bay Area red in the summer of 2020, Dr. Kari Nadeau, a physician and scientist at Stanford University, thought about the people who were most vulnerable. She worried about the workers at local wineries who raced to protect their harvest; and the children who lived near refineries and breathed in pollutants every day.
During that August, September and October, she watched the air quality routinely reach unhealthy levels for anyone without a mask. At the time, Dr. Nadeau said in a public panel that being outside and breathing that air was similar to smoking seven cigarettes a day.
But now, she said she believes that the health effects of breathing heavy wildfire smoke is probably worse. “Cigarettes at least have filters,” said Dr. Nadeau, who directs the Sean N. Parker Center for Allergy and Asthma Research at Stanford University.
While cigarette smoke, even secondhand, is proven to cause lung cancer, wildfire smoke is not. Some recent, limited studies published in the last few years have found correlations between people exposed to wildfire smoke and lung cancer. But none have proved causation, the scientists who performed those studies have said, and much more research is needed.
“We don’t know a lot about the long-term health effects of forest fires,” said Scott Weichenthal, an assistant professor in the department of epidemiology, biostatistics and occupational health at McGill University in Montreal. Until recently, fires have been studied as one-off disasters, he said, and we don’t understand how heavy, sometimes recurring short-term exposures to smoke can affect people’s health down the road.
Experts do know that, even in the short term, particle pollution from wildfires — including tiny bits of ash, dust and soot — can worsen heart problems, reduce lung function and aggravate asthma. In this way, wildfire smoke can affect health in similar ways as diesel exhaust or smoke from cigarettes.
Wildfire smoke can also include heavy metals like lead and arsenic, and hazardous chemicals like benzene and formaldehyde gas, all of which are present in cigarette smoke and can cause cancer.
“There’s enough pieces of evidence that we should not look the other way,” Dr. Nadeau said.
To understand how the air you breathe can influence lung cancer risk, scientists say it’s essential to understand what harmful things are in the air, how much of it is present and how long you’re exposed to it.
According to the Centers for Disease Control and Prevention, for instance, exposure to secondhand cigarette smoke, at work or at home, can raise a nonsmoker’s lung cancer risk by 20 to 30 percent.
Calculating the health risks from wildfires, though, is much more challenging. What the smoke contains, and the potential health risks it may bring, will in part depend on what the fire consumed. Smoke from burning trees and vegetation, for example, will present different dangers than that from burning houses, cars, electronics or tool sheds.
Wildfire smoke is also temperamental; it literally blows away with the wind. The harmful substances fires carry can be fleeting and hard to characterize, Dr. Weichenthal said. And it can be challenging to measure the extent to which people are exposed.
But as wildfires intensify because of climate change, growing larger and spreading faster, researchers have recently begun focusing on people exposed to smoke and fires over extended periods of time. Experts from the University of California, Davis, for instance, are following survivors of the 2018 Camp Fire in Butte County, Calif. And at McGill University, Dr. Weichenthal was part of a team that analyzed about two decades’ worth of Canadian public health records to better understand the health consequences of wildfires, motivated in part by record fire years in Ontario and British Columbia.
“It shouldn’t be shocking to us that we would see some sort of elevated cancer risk in these places,” he said. “We know that the chemicals that are being released are carcinogenic.”
Dr. Weichenthal’s study, which was published in The Lancet in May, found that those who lived within about 30 miles of a wildfire in the last decade were about 5 percent more likely to develop lung cancer, and 10 percent more likely to develop brain tumors, than people living farther away.
While the study had some limitations, Dr. Weichenthal said, these findings are “important because so many people may be exposed.”
To date, the best evidence we have that links lung cancer to wildfire smoke comes from studies of firefighters. At the peak of fire season, tens of thousands of them work long shifts, day after day, often without masks.
In a study published in 2019, Kathleen Navarro, who researches environmental workplace safety issues for firefighters at the National Institute for Occupational Safety and Health, estimated with her colleagues that short-season firefighters, who work on the front lines for at least seven weeks each year for five to 25 years, would increase their risk of dying from lung cancer by 8 to 26 percent as a result of smoke exposure. Firefighters who work twice that length of time each year, they calculated, would have a 13 to 43 percent increased risk of dying from lung cancer during the same period.
“But there’s still a lot of unknowns about what happens cumulatively, over every season,” said Dr. Navarro, who worked as a Hotshot firefighter in Oregon in 2019. She noted that the federal government is working to track and analyze cancer trends and risk factors more closely among firefighters in the United States. A national registry for firefighter health will open this fall.
Even without proof that wildfires cause lung cancer among the public, though, Dr. Nadeau said there’s plenty of evidence to seek more protective policies and to take safety precautions.
“We should let this be a catalyst for us to be even more prepared to adapt for wildfires, and climate change,” she said. And when smoke is noticeable, “You should evacuate. You shouldn’t just stay around and wait. Smoke itself is a hazard to avoid.”
Molly Peterson is an investigative journalist based in Los Angeles who focuses on the intersections of climate, catastrophe and public health.