Hamilton, ON, Jan. 30, 2025 – Researchers have shown that plumes of wildfire smoke can carry contaminants hundreds of kilometres, leaving a toxic and lingering footprint which has the potential to be re-released into the environment.
The frequency and severity of wildfires is expected to continue increasing due to climate change. In recent weeks, catastrophic wildfires have devasted Los Angeles, scorching tens of thousands of acres.
Canada's 2023 wildfire season was the most destructive ever recorded, with an estimated 18.5 million hectares burned. The 2024 season was the second worst on record, with more than 5 million hectares burned according to the Canadian Interagency Forest Fire Centre, which tracks national statistics.
Wildfire events may become an increasingly dominant and troubling source of pollution in urban areas, say researchers.
Wildfire smoke features a complex mixture of pollutants, including polycyclic aromatic hydrocarbons (PAHs), a class of carcinogenic compounds that can also cause mutations in nature. PAHs are produced whenever incomplete combustion occurs, including when wood burns.
In the new study, published today in the journal Environmental Science & Technology , researchers at McMaster University report the potential impact of PAHs extends far downwind of wildfires, even hundreds of kilometres.
"This study was motivated by the large increase in wildfire frequency and severity in Western Canada," explains Iris Chan, a graduate student in the Department of Chemistry & Chemical Biology and lead author of the study. "There is a great deal of public awareness and research on air quality related to North American wildfires, but the long-term impact of smoke drifting into cities is virtually unknown."
Urban landscapes are dominated by impermeable structures and surfaces such as buildings and roads, she explains. Over time, these surfaces accumulate what is known as "urban grime," a buildup of deposited particles and other chemical compounds that can hold and re-release pollutants such as PAHs.
For this study, researchers enlisted volunteers in Kamloops and Calgary to collect samples in their backyards from August to November 2021.
They set out specially designed kits containing glass beads, which mimic impervious urban surfaces like windows. The samples were regularly collected and analyzed at McMaster.
The team looked for correlations between surface-grime PAHs and evidence of fire activity in measurements of local air quality such as carbon monoxide and fine particulate matter.
In the Calgary samples, researchers found toxin levels nearly doubled when smoke from fires in neighbouring Saskatchewan arrived from about 500 kilometres away. There were no other large-scale pollution events in Calgary at that time, suggesting the increase was linked.
In Kamloops, they pinpointed a sharp increase in toxins even when there were no significant wildfire events in the region. Based on the specific composition of samples, researchers concluded the uptick was due to a hyper-local burn, likely a neighborhood campfire.
"We should be mindful that the minor things people do every day, like using their barbeque or having a campfire in the backyard, can have a significant and long-lasting impact on their local environment," says Sarah Styler, who supervised the study and holds the Canada Research Chair in Atmospheric Chemistry at McMaster.
The accumulation problem grows worse when there isn't sufficient rainfall to wash away the grimy buildup. A reservoir of toxins can, in principle, grow for long periods.
"We would then expect precipitation to release pollutants into stormwater runoff, with the potential for adverse consequences for downstream water bodies, sediments and aquatic life," says Styler.
The team is currently following up by analyzing samples from multiple cities in Canada and the United States collected during the 2022 wildfire season. Additionally, they have recently begun a pilot project with Environment Hamilton to collect and analyze dust and grime samples in city neighbourhoods to determine how much is falling in different areas and what it might contain.
