Increased Cancer Risk Tied to Wildfires Amid Worsening Impacts of Climate Change

New study finds people living within 20 to 50 kilometers of a wildfire face increased risk of lung and brain cancer.

A firefighter tends to a backfire set to battle the Windy Fire on September 26, 2021 south of California Hot Springs, California. Subject is covered in thick smoke from fire.
Photo: David McNew / Stringer / Getty Images

Fact checked on May 23, 2022 by Vivianna Shields, a journalist and fact-checker with experience in health and wellness publishing.

As climate change worsens and wildfires become a far more common occurrence around the world, a new study has found that long-term exposure to such fires may increase the risk of developing certain types of cancer.

The study, published in The Lancet, found that people living within 20 to 50 kilometers of a wildfire were at increased risk of having lung and brain cancer.

Wildfires, the study explained, emit a complex mixture of harmful pollutants into the environment, which have well known effects on outdoor air quality and can contaminate water, soil and terrestrial environments, and even indoor environments. Additionally, many of the pollutants created by such fires are known human carcinogens–including polycyclic aromatic hydrocarbons, benzene, formaldehyde, phenols, and heavy metals.

"If we're really going to understand the overall health impacts of these events, we have to consider what the long-term health effects are as well," Scott Weichenthal, an associate professor in the department of epidemiology, biostatistics and occupational health at McGill University and the study's senior author, told Health. "Our results suggest that an increase in cancer risk might be one of those effects."

As the world grapples with the worsening impacts of climate change-related natural disasters, here's a closer look at the health ramifications of wildfires.

Wildfires and Cancer Risk

Most studies surrounding the health effects of wildfires to date have focused on short-term damage to the lungs and heart, which often contribute to conditions like asthma and heart disease. However, little is known about the long-term health effects of wildfires, including their potential contribution to cancer risks.

To help understand the more far reaching ramifications of such natural disasters, researchers conducted a population-based observation study, which included analyses of more than 2 million people who were followed for a median of 20 years. To do this, researchers used data from the 1996 Canadian Census Health and Environment Cohort, a group of individuals whose cancer incidence and death rates were monitored between 1996 and 2015.

The researchers specifically tracked whether individuals exposed to wildfires developed lung and brain cancer, as well as three types of blood cancer–non-Hodgkin's lymphoma, multiple myeloma, and leukemia.

To measure wildfire exposure, the researchers looked for people who lived within either 20 or 50 kilometers of any wildfires in the past three, five and 10 years. Because most wildfires in Canada tend to happen in the same areas each year, many of the individuals who were exposed once, had some level of chronic exposure.

The study found that those living within 50 kilometers of any wildfires in the past 10 years were at a 10% greater risk of developing brain cancer and a 4.9% higher risk of developing lung cancer than those who were not chronically exposed. The research is one of the first supporting a link between wildfire exposure and cancer risk.

"We've looked at the kinds of chemicals that are known to be released from fires," said Weichenthal. "Based on that list of chemicals, we identified cancer types that had been related to those kinds of chemicals."

Compared to people who were unexposed, every group the researchers measured had an increased risk of developing both brain and lung cancer. Depending on how far from the fire the person lived and how recently they were exposed, people were 3.6% to 4.9% more likely to develop lung cancer and 6.4% to 10% more likely to develop brain cancer than unexposed individuals. There were no associations between wildfire exposure and any type of blood cancer the researchers tracked.

Ramifications of Study Findings

While the results are indeed concerning, researchers say there are many things individuals can do to protect themselves from exposure to air pollution from wildfires.

"A simple mask can be very, very helpful with filtering out these particles," Loren Wold, a professor of physiology and cell biology at The Ohio State University College of Medicine, who was not involved in the study, told Health.

"A typical N95 mask will filter out a lot of the fine particulates," explained Wold, referring to particles released during wildfires and by other sources of pollution which are less than 2.5 micrometers in size. Using high-efficiency particulate air (HEPA) filters in your home can also help filter out these particles, also called PM2.5s, from indoor air, Wold said.

It's also important to pay attention to any wildfire alerts in your area, Éric Lavigne, a research scientist at Health Canada and adjunct professor at the University of Ottawa, told Health.

"Public health officials usually will have alert systems going on and making sure that there are messages that are sent to the population," Lavigne said. Many alerts include instructions to stay inside whenever possible, to avoid physical activity, and to stay in a cooled indoor place. "Those are the types of things that I would say to someone on an individual level basis," said Lavigne.

However, Lavigne also said that these increased risks are relatively slight for individual people, especially compared to other risk factors like smoking. But wildfire exposure affects so many people around the world that even a small cancer risk is cause for concern, he said.

"Even though those risks are small, they can affect populations of millions of people," said Lavigne. This is especially true, noted the researchers, as wildfires increase in frequency and severity due to climate change.

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What's Next?

Though this study is the first of its kind examining wildfire exposure and cancer risk, it also raises many very important questions.

To begin with, it's not clear why the researchers did not find an increased risk of developing any of the blood cancers that were examined as part of the study. It could be that an association truly does not exist, said Wold, or that an even more long-term study would be needed in order to see an association.

Lavigne said it is also possible that there were not enough cases of these cancers in the study to identify a correlation.

In addition, the researchers did not see a correlation between particularly high exposure to wildfires and a more greatly increased cancer risk. This doesn't necessarily mean that more severe wildfires wouldn't increase cancer risk even more, cautioned Weichenthal. It could just be that the methods used in this study, where risk groups were determined by the size of the nearby area burned, didn't effectively capture the effect of more severe wildfires.

"There are probably improvements in how we think about assigning exposures to forest fires," Weichenthal said.

Another limitation of the current study is that it didn't examine which pollutants from the wildfires people were exposed to, said Lavigne. Future research should look at which substances might be behind these results that were uncovered, added Lavigne.

Still, Wold said the study, of which he was not a part, was "exceptionally well-done" and provides important insight into the long-term effects of wildfire exposure.

"I think we worry a lot about the acute effects, the very short-term effects," Wold said. "Thinking about the long-term implications of this is very, very striking."

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