Wildfires in some parts of the western United States may be converting a benign form of chromium into its cancer-causing counterpart, according to a new study; This could potentially endanger first responders and surrounding communities.
The research, published Tuesday in the journal Nature Communications, detected high levels of the hazardous metal hexavalent chromium, or chromium-6, in certain burn areas along California’s North Coast.
Chromium-6, also known as the “Erin Brockovich chemical,” came into public view in the 1990s after Brockovich, then a legal aid officer, discovered that it was contaminating drinking water and sickening residents of Hinkley, California.
This toxic compound, which increases the risk of cancer when inhaled or ingested, was not present in the areas of interest for the study before burning.
In contrast, soils and plants in these regions were rich in naturally occurring trivalent chromium (chromium-3), an essential nutrient that helps the human body break down glucose.
Although chromium-6 can occur naturally in the environment, this toxic form of the metal further contaminates communities through runoff and wastewater from industrial processes.
Wildfire smoke clouds are known to carry dangerous pollutants such as aerosols, gases and fine particulate matter, but researchers wondered whether the same could be said for heavy metals and what the risk might be to firefighters and those living downwind.
“Heavy metals like chromium have been largely overlooked in the complex mixture of gases and particles that wildfires emit as smoke and leave behind as dust,” said senior author Scott Fendorf, a professor at Stanford University’s Doerr School of Sustainability. expression.
Laboratory experiments conducted by Australian researchers in 2019 had already shown that chromium-6 could rapidly form from chromium-3 in flaming surface soils.
This transformation occurs through a process known as oxidation, or in this scenario, a reaction in which electrons are lost between chromium and atmospheric oxygen.
Fendorf, who is also a senior researcher at the Stanford Woods Institute for the Environment, told The Hill that this process “doesn’t really happen” because it is slow at low temperatures.
“But in this case, when you start heating the samples, wildfires cause this reaction to occur,” he said. “And so you went from the benign form to the truly toxic form.”
With this in mind, Fendorf and his colleagues decided to test the theory that wildfires could cause soils to become contaminated with chromium-6.
Scientists focused their attention on California’s North Coast; Here they identified four recently burned ecological reserves containing chromium-rich rocks such as serpentinite.
Fendorf noted that the southern Sierra Nevada tends to have more granite and less chromium content, while the northern Sierra has more serpentinite (California’s state rock).
“When you put out a forest fire, it leads to much higher levels of chromium-6 being produced,” he said.
According to the study, four study areas — Pepperwood Preserve, White Rock Preserve, Modini Preserve and Sylvia McLaughlin Preserve — were partially or completely burned during the Kincade or Hennessey fires that occurred in November 2019 and September 2020, respectively.
Lead author Alandra Lopez, a postdoctoral researcher in Earth system science at the Stanford Doerr School, collected soil from these areas and then separated out the smallest particles most susceptible to wind transport.
He then measured chromium-6 levels 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 properties.
Scientists found that in chromium-rich areas where vegetation fueled high-temperature, long-lasting fires, toxic chromium concentrations were 6.5 times higher than in unburned areas.
“Our study shows that much more attention should be paid to wildfire-modified chromium, and we hypothesize that additional metals are also present,” Lopez said in a statement.
Doing so was necessary “to more comprehensively characterize the overall threats posed by wildfires to human health,” he added.
When it comes to these potential threats, the authors said they believe fire-induced toxic chromium exposure has the most acute effects on first responders and people living near fires.
Compared to exposure to chromium-6 through contaminated drinking water discovered by Brockovich at Hinkley, Fendorf emphasized that “inhalation is much worse.”
“Toxicologists are really clear on this,” he said. “If you had a choice, you’d want to drink it before you breathe in. And they don’t recommend either.
The scope of the threat also varies depending on which plants are fueling the fire, according to Fendorf.
For example, grasslands don’t produce temperatures high enough to create much chromium-6, but shrubs and tree canopies provide enough heat, he explained.
Even after fires end, strong winds can expose nearby populations to fine soil particles containing chromium, the researchers noted.
According to Fendorf, much of the risk associated with inhaling airborne chromium-6 likely decreases after the first major rainfall that could draw the metal underground.
But he warned that in an arid environment like the U.S. West, which faces increasingly prolonged droughts due to climate change, exposure risks may persist for those rebuilding and revegetating burned areas.
While chromium is Fendorf’s toxin of greatest concern because of the “radical transformation that occurs,” he said he is also concerned about other metals such as manganese, certain forms of iron, nickel and cadmium.
Emphasizing the need for more research on exposure to heavy metals due to forest fires, Fendorf noted that it may currently be useful to wear N95 in such a burn area.
Fendorf said he and his team are now creating predictive maps so firefighters have greater awareness of where they may encounter fine particulate matter and where they may be at risk of exposure to chromium-6.
He and his colleagues have been preparing for the past two years to deploy active monitoring devices that can track contaminants in the middle of a fire, but such fires have been minimal during this period.
Ed Burton, who led the 2019 study on the conversion of chromium-3 to chromium-6, believed the new study showed that “wildfires cause a dramatic shift in chromium chemistry in fire-affected soil.”
“The authors significantly demonstrate that newly formed hexavalent chromium can persist in topsoil and ash for months after wildfires,” Burton, professor of environmental geochemistry and mineralogy at Southern Cross University in Australia, told The Hill in an email.
In addition to describing potential health risks to those exposed to soil particles or ash, he also emphasized his “particular concern” about the effects of these findings on the U.S. West.
Burton, who was not involved in the Stanford study, said the region may be particularly at risk “because large areas of land are naturally rich in soil chromium and the severity and frequency of wildfires are increasing due to climate change.”
Dimitrios Alexakis, a professor of geology and geochemistry at Greece’s University of Western Attica, similarly described “a broad global threat to humans from dust-induced metal and wildfire smoke” resulting from the mixture of these flames and chromium-rich soils.
“This study reveals an under-recognized health threat to humans related to the geology and fire severity,” he told The Hill in an email.
Alexakis, who was not involved in this research, examined both the distribution of toxins in burned vegetation and the effects of soil pollution from fire on human health.
He observed that the Stanford study could help provide “new insights into 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 chromium-6 formation through fire could also help scientists develop predictive tools to reduce exposure risk and guide policymaking, according to Alexakis.
As more research is conducted on the subject, the Stanford team emphasized that the results will apply not only to California’s North Coast, but also to fire-prone areas with metal-rich landscapes around the world.
“As the frequency and severity of wildfires are expected to increase in many geographic regions due to a combination of climate change and past fire management, post-fire dust emissions are also likely to increase,” the authors said.
The Pacific Rim and Mediterranean regions have geology higher in chromium, as do other parts of Europe, Australia, South Africa and Brazil, according to the study.
“There are areas on every continent that are at pretty high risk, so this is not a minor point,” Fendorf said. “This is a really widely distributed problem.”
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