How does wildfire smoke affect lung cancer risk? And to what extent does it compare with factors like passive smoking?

When wildfire smoke turned the sky red in the San Francisco Bay Area in the summer of 2020, Carrie Nando, a physician and researcher at Stanford University, immediately thought of the most vulnerable people. During those months of August, September and October, he watched air quality systematically reach dangerous levels for those not wearing a mask. Nadeau then told the open group that being outside and breathing that air was like smoking seven cigarettes a day. But now he believes the consequences are likely to be even worse. “Cigarettes at least have filters,” says Nando, director of the Sean N. Parker Allergy and Asthma Research Center at Stanford University.

While cigarette smoke, even a passive smoker, has been shown to cause lung cancer, the same cannot be said for wildfire smoke. Several studies published in recent years have found a link between people exposed to wildfire smoke and lung cancer, but none of them has proven a causal relationship.

“We know little about the long-term health effects of wildfires,” says Scott Weihendahl, assistant professor of epidemiology, biostatistics and occupational health at McGill University in Montreal. Even in the short term, particulate matter pollution from wildfires, including small amounts of ash, dust and soot, can exacerbate heart problems, reduce lung function and worsen asthma, experts know. Thus, wildfire smoke can affect health in the same way as diesel exhaust or cigarette smoke.

What’s included

Fire smoke can also contain heavy metals such as lead and arsenic, as well as hazardous chemicals such as benzene and formaldehyde gas, which are present in cigarette smoke and can cause cancer.

According to the Centers for Disease Control and Prevention, for example, exposure to secondhand smoke at work or at home can increase your risk of lung cancer by 20-30%. Calculating health risks from wildfires, however, is much more difficult.

What the smoke contains and what the potential health hazard is depends partly on what is burned in the fire. Smoke from burning trees and vegetation is a different hazard than smoke from burning houses, cars, electronics, or warehouses.

But as fires intensify due to climate change, grow in size and spread faster, researchers have recently begun to focus on people exposed to smoke and fire over an extended period of time. UC Davis experts monitor survivors of the 2018 wildfire in Butte County. And at McGill University, Weihendahl was part of a team that analyzed almost two decades of Canadian medical records to better understand the health effects of wildfires, driven in part by record-breaking fire years in Ontario and British Columbia.

The study

A study by Weihental, published in The Lancet in May, found that those who lived within 50 kilometers of a wildfire in the past decade had about a 5 percent higher risk of developing lung cancer and a 10 percent higher risk of brain tumors, according to compared to those who lived further.

To date, the most complete data on the relationship between lung cancer and wildfire smoke comes from studies of firefighters. In a study published in 2019, Kathleen Navarro, who studies workplace environmental safety for firefighters at the National Institute for Occupational Safety and Health, along with her colleagues, estimated that seasonal firefighters who work on the front lines for at least seven weeks a year during five to 25 years of age had an 8-26% increased risk of dying from lung cancer as a result of secondhand smoke exposure. They calculated that firefighters who work twice as long each year have a 13-43% increased risk of dying from lung cancer over the same period.

Even without evidence that wildfires cause lung cancer in the population, Nando says there is plenty of evidence to look for safer protection policies and take preventive safety measures. And when smoke is noticeable, “we must evacuate. We shouldn’t just stay in the area and wait.”

* Ms. Molly Peterson is an investigative journalist based in Los Angeles, specializing in the interplay of climate, natural disasters and public health.