Proximity to forest fires may influence cancer risk

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A new study from McGill University finds higher incidence of lung cancer and brain tumors in people exposed to wildfires. The study, which tracks over 2 million Canadians over a period of 20 years, is the first to examine how proximity to forest fires may influence cancer risk.

“Wildfires tend to happen in the same locations each year, but we know very little about the long-term health effects of these events. Our study shows that living in close proximity to wildfires may increase the risk of certain cancers,” says Scott Weichenthal, an Associate Professor in the Department of Epidemiology, Biostatistics, and Occupational Health at McGill University.

Published in The Lancet Planetary Health, the study shows that people living within 50 kilometers of wildfires over the past 10 years had a 10% higher incidence of brain tumors and 4.9% higher incidence of lung cancer than people living further away.

Impacts of climate change on human health

With the changing climate, wildfires are predicted to become more prevalent, severe, and longer in duration in the future—and they are increasingly recognized as a global health problem. “Many of the pollutants emitted by wildfires are known human carcinogens, suggesting that exposure could increase cancer risk in humans,” says Jill Korsiak, a Ph.D. student in Professor Weichenthal’s lab who led the analysis.

Wildfires typically occur in similar regions each year, and as a result, people living in nearby communities might be exposed to carcinogenic wildfire pollutants on a chronic basis, warn the researchers.

Lingering harmful pollutants

In addition to impacts on air quality, wildfires also pollute aquatic, soil, and indoor environments. While some pollutants return to normal concentrations shortly after the fire has stopped burning, other chemicals might persist in the environment for long periods of time, including heavy metals and hydrocarbons. “Exposure to harmful environmental pollutants might continue beyond the period of active burning through several routes of exposure,” adds Professor Weichenthal.

Still, more research is necessary to understand the complex mixture of environmental pollutants released during wildfires, note the researchers. They also note that further work is needed to develop more long-term estimates of the chronic health effects of wildfires.

“Long-term exposure to wildfires and cancer incidence in Canada: a population-based observational cohort study” by Jill Korsiak, Lauren Pinault, Tanya Christidis, Richard Burnett, Michal Abrahamowicz, and Scott Weichenthal was published in The Lancet Planetary Health.


Wildfires are increasing in size and frequency worldwide, due in part to the hotter and drier conditions caused by global climate change [1,2]. While heat from fires can cause bodily injury, the smoke is also concerning due to the large amounts of carbon dioxide, carbon monoxide, and fine particulate matter released into the air as vegetation burns [1].

Previous research and literature reviews have focused on acute health effects immediately following a wildfire, such as reduced lung function, and exacerbation of chronic lung disease [3,4]; as well as neurological and cardiovascular effects and increased mortality [4]. In the U.S., the nine most devastating fire seasons have occurred since 2005, when record keeping began [5]. To date, the record-setting 2020 and 2021 fire seasons in the western U.S. have resulted in 7.7 million acres burned and poor air quality across the nation [6], [7], [8].

Historically dry conditions brought on by drought and compounded by record-breaking heat wave events, all due to a changing climate, are responsible for fires spreading more quickly and burning more land area [9,10]. However, as the risk and extent of wildfires, particularly in the western U.S., continues to increase, it is imperative that research shift beyond the acute effects of wildfire exposure to the longitudinal health outcomes of populations exposed to wildfires and wildfire smoke.

Wildfire smoke contains a number of components known to be harmful to human health, including fine particulate matter, carbon monoxide, ozone, and many hazardous air pollutants (HAPs) [11]. The known health effects of exposure to wildfire include increased respiratory morbidity, including respiratory infections, asthma, chronic obstructive pulmonary disease, and all-cause mortality [12].

Fine particulate matter, such as PM2.5, refers to particles which are 2.5 micrometers in diameter or smaller. The particles can be made up of hundreds of different chemical compounds, and once inhaled can cause cardiovascular disease, lung disease, or premature mortality [13]. PM2.5 is the primary air pollutant of concern in wildfire smoke [11]. Within the contiguous U.S., it is expected that PM2.5 levels due to wildfires will increase 55–190% by 2100, accounting for over 50% of all ambient PM2.5 [14].

In an effort to understand the future impact of these increasingly frequent natural disasters, several studies have sought to assess current and projected human exposure to wildfire smoke. A recent study predicts that the current rate of population exposure to wildfire smoke (less than 10 per 1,000 persons exposed from 1981 to 2010) will increase to between 12 and 20 per 1,000 persons across Europe in the years 2071–2100 [15].

Mendoza et al. [16] found that at baseline, PM2.5 indoors is equal to 25–33% of the PM2.5 outdoors; however, indoor PM2.5 pollution increases to 77–78% of that found outdoors in the presence of wildfire activity. Another study found that while wildfires burned outdoors, ambient PM2.5 levels increased to the 90th percentile and correlated with increased levels of ambient carbon monoxide (CO), ozone (O3), and nitric oxide (NO) [17].

Previous longitudinal studies have focused on the long-term health effects of occupational exposure to wildfires in firefighters, a vulnerable group characterized by chronic exposure to wildfire smoke. A systematic review found that occupational exposure to smoke may increase the risk of hypertension [18]. The review did not, however, find sufficient evidence to comment on the long-term respiratory impacts and increased cancer risk of firefighters, possibly due to the challenges of following firefighters from one season to the next, leading to poor follow-up.

There are additional challenges to understanding the health effects of wildfires. Firstly, the combination of pollutants in smoke, such as fine particulate matter and HAPs, may be significantly different depending on the type of vegetation burned in a wildfire, and whether houses and other structures are also burned.

Additionally, researchers must distinguish between ambient air pollution caused by natural sources such as wildfires and aeolian dust (particulate matter transported by wind through suspension in the air), and anthropogenic emissions, such as those associated with fossil fuel use, in order to avoid confounding [19].

This scoping review assessed the state of the current literature on the long-term health effects of wildfires. Research questions included: What evidence exists on the longitudinal health effects of wildfire exposure for impacted populations, and what are the unmet health needs for populations residing in geographic areas characterized by high risk of exposure to wildfires? Results will map gaps in the science, inform future population health research priorities, and evaluate the needs for a public health response to the increasing threat of wildfires.

reference link :https://www.sciencedirect.com/science/article/pii/S2667278221001073


More information: Long-term exposure to wildfires and cancer incidence in Canada: A population-based observational cohort study, The Lancet Planetary Health (2022). DOI: 10.1016/S2542-5196(22)00067-5

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