Wildfires in parts of the US West could transform a benign form of chromium into its cancer-causing counterpart – potentially endangering first responders and surrounding communities, a new study shows.
The research, published Tuesday in the journal Nature Communications, identified high levels of the dangerous metal hexavalent chromium, or chromium-6, at specific types of combustion sites along California’s northern coast.
Chromium-6, also known as “Erin Brockovich’s chemical,” burst into the public eye in the 1990s after Brockovich — then a legal aid worker — discovered it was contaminating drinking water and making residents of Hinkley, California, sick.
This toxic substance, which increases the risk of cancer if inhaled or ingested, was not present at the sites relevant to the study before they burned.
Instead, the soils and plants in these locations were rich in naturally occurring trivalent chromium – chromium-3 – an essential nutrient that helps the human body break down glucose.
Although chromium-6 can also occur naturally in the environment, this toxic form of the metal more commonly contaminates communities through runoff and wastewater from industrial processes.
Smoke plumes from wildfires are known to transport dangerous pollutants such as aerosols, gases and particulate matter, but the researchers wondered if the same could be said for heavy metals, and what the risk might be to firefighters and those living downwind .
“In the complex mixture of gases and particles that wildfires emit as smoke and leave behind as dust, heavy metals such as chromium have been largely overlooked,” said senior author Scott Fendorf, a professor at Stanford University’s Doerr School of Sustainability, in a rack.
Laboratory experiments conducted by Australian researchers in 2019 had already shown that chromium-6 could quickly form from chromium-3 in torched surface soils.
This transformation occurs through a process known as oxidation – or in this scenario, a reaction between chromium and atmospheric oxygen that involves the loss of electrons.
Because that process happens slowly at low temperatures, it “effectively doesn’t happen,” Fendorf, who is also a senior fellow at the Stanford Woods Institute for the Environment, told The Hill.
“But when you start heating up the samples, in this case through wildfires, that causes that response to occur,” he said. “And so you have transformed from the benign to the truly toxic form.”
With that in mind, Fendorf and his colleagues decided to test the theory that forest fires could leave behind soils contaminated with chromium-6.
The scientists turned their attention to California’s northern coast, where they identified four recently burned ecological natural areas that contain chromium-rich rocks such as serpentinite.
While the southern Sierra Nevada typically contains more granite and less chromium, the northern Sierra has more serpentinite — California’s state rock, Fendorf noted.
“If you bring in the wildfire, it leads to much higher levels of chromium-6 being produced,” he said.
The four study sites — Pepperwood Preserve, White Rock Preserve, Modini Preserve and Sylvia McLaughlin Natural Reserve — were all partially or completely burned during the Kincade or Hennessey fires, which occurred in November 2019 and September 2020, respectively, according to the study.
Lead author Alandra Lopez, a postdoctoral researcher in Earth system sciences at Stanford’s Doerr School, collected soil from these sites and then separated out the smallest particles that are most sensitive to wind transport.
She then measured the levels of chromium-6 in this ultrafine dust from both burned and unburned areas, while also collecting data on local fire severity, prevailing soil conditions, underlying geology and ecosystem characteristics.
In chromium-rich sites where vegetation caused prolonged fires at high temperatures, the scientists found that toxic chromium concentrations were 6.5 times higher than those in unburned areas.
“Our study suggests that much more attention should be paid to wildfire-modified chromium, and we assume there are other metals as well,” Lopez said in a statement.
This, she added, is necessary to “more thoroughly characterize the overall threats that wildfires pose to human health.”
Regarding these potential threats, the authors say they believe that fire-related exposure to toxic chromium has the most acute effects on first responders and people living near the fires.
Compared to exposure to chromium-6 through contaminated drinking water, as discovered in Hinkley by Brockovich, Fendorf emphasized that “inhalation is much worse.”
“Toxicologists are very clear about that,” he said. “If you had the choice, you want to drink it before you inhale it. And not that they suggest either.”
The magnitude of the threat also varies depending on which plants fuel the fire, Fendorf said.
For example, grasslands don’t produce high enough temperatures to create much chromium-6, but shrubs and tree canopies do provide enough heat, he explained.
Even after the fires end, strong winds can expose nearby populations to fine particles of chromium-soaked soil, the researchers noted.
According to Fendorf, the risk associated with inhaling chromium-6 in the air likely decreases after the first major rainfall, which can wash the metal underground.
But in an arid environment like that of the US West, which is experiencing increasing and prolonged droughts due to climate change, exposure risks could persist for those rebuilding and replanting burned areas, he warned.
Although chromium is Fendorf’s main toxin because of “the radical transformation that takes place,” he says he’s also concerned about other metals, such as manganese, certain forms of iron, nickel and cadmium.
Fendorf emphasized the need for further research into heavy metal exposure from wildfires and noted that wearing an N95 at such a burn site could be useful for the time being.
Fendorf said he and his team are now creating predictive maps so firefighters can be more aware of where they might encounter fine particles and where they might also encounter chromium-6 exposure.
He and his colleagues have spent the past two years preparing for the deployment of active monitoring equipment capable of monitoring pollutants during a fire, but such fires have been minimal during this period.
Ed Burton, who led the 2019 research on the transformation of chromium-3 to chromium-6, cited the new study for showing “that wildfires cause a dramatic shift in the chemistry of chromium in fire-affected soil.”
“Significantly, the authors show that newly formed hexavalent chromium can persist in soils and surface ash for many months after bushfires,” Burton, professor of environmental geochemistry and mineralogy at Australia’s Southern Cross University, said in an e-mail. email The Hill.
In addition to describing the potential health risks for those exposed to soil particles or ash, he emphasized his “particular concern” about the implications of these findings for the Western US.
The region could be at particular risk because “large tracts of land are naturally rich in chromium in the soil and because wildfires appear to be increasing in severity and frequency due to climate change,” said Burton, who was not involved with the Stanford -research.
Dimitrios Alexakis, professor of geology and geochemistry at Greece’s University of Western Attica, also described “a broad global threat of dust-born metal and wildfire smoke to humans,” arising from the mix of these fires and chromium-rich soils.
“This study demonstrates a recognized human health threat related to the geology and fire severity,” he told The Hill in an email.
Alexakis, who was also not involved in this research, has studied both the spread of toxins from burned vegetation and the effects of fire-fueled land pollution on human health.
The Stanford study, he noted, could help “provide new perspectives on the reasons behind the increased health risks associated with exposure to wildfire smoke compared to pollution from other sources.”
Understanding how environmental factors contribute to the formation of chromium-6 from fire could also help scientists develop predictive tools to limit exposure risk and guide policymaking, Alexakis said.
As further research on this topic unfolds, the Stanford team emphasized that the results will apply not only to California’s northern coast, but also to fire-prone areas with metal-rich landscapes around the world.
“As wildfires are expected to increase in frequency and severity in many geographic regions due to a combination of climate change and past fire suppression, post-fire dust emissions are likely to increase,” the authors said.
Both the Pacific Rim and Mediterranean regions have geologies higher in chromium, as do other parts of Europe, Australia, South Africa and Brazil, the study said.
“Every continent has areas that are at pretty high risk, so it’s no small thing,” Fendorf said. “It really is a widespread problem.”
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