After some recent intense wildfires in Northern California, scientists tested samples of scorched soil and were disturbed by their findings: It was loaded with a carcinogenic metal called hexavalent chromium.
Scientists believe the heat from severe wildfires could transform a benign version of the metal, common in California soil, into a notorious carcinogen, according to new research published Tuesday in the journal Nature Communications.
As climate change intensifies wildfires, scientists are trying to figure out how dangerous their smoke could be to human health. Researchers have found dangerous metals in previous fires – from burned-out cars, homes and farms. The new finding adds a surprising twist to the growing body of research, suggesting that wildfires in natural areas can also pump smoke laced with a toxic metal into the atmosphere.
“I think it changes our risk assessment when you think about exposure to wildfire smoke,” said Scott Fendorf, professor of earth system sciences at Stanford University and author of the study.
Climate change could increase the risk: Wildfires that burn hotter and longer are more likely to turn harmless soil into carcinogenic dust and ash.
“Forest fires are becoming more common due to climate change and the severity of fires is increasing,” Fendorf said. “You get more exposure and you get exposure to materials that are going to be more toxic.”
According to the International Agency for Research on Cancer, hexavalent chromium is a Group 1 carcinogen, meaning it is known to cause cancer in humans. Exposure to high amounts of hexavalent chromium is linked to lung cancer, according to a toxicology assessment of the substance by the Environmental Protection Agency, which evaluated decades of workplace exposure in people who worked in chromium plating and chromate pigment factories.
In a study of mice exposed to hexavalent chromium in drinking water for two years, some developed tumors in their mouth, small intestine and liver.
Hexavalent chromium is a known contaminant because it was the central chemical in the class action lawsuit depicted in the film “Erin Brockovich” about chromium contamination in water in Hinkley, California, where the metal had been used to prevent corrosion in the cooling tower water at a natural gas compressor station.
About 600 Hinkley residents settled an initial case with Pacific Gas & Electric Company (PG&E) for $333 million. According to the Associated Press, PG&E paid an additional $315 million to settle other lawsuits.
The California Air Resources Board adopted a rule earlier this year to phase out hexavalent chromium from industrial facilities, saying in a news release that there was “no known safe exposure level.”
In its trivalent form, chromium is relatively harmless and abundant. But heat over 390 degrees Fahrenheit can catalyze chemical reactions that transform it into its more dangerous form, hexavalent chromium, the new study says.
Researchers visited the sites of wildfires in California’s North Coast Range, including the 2019 Kincade Fire and the 2020 Hennessey Fire, to look for hexavalent chromium. They took soil samples in four ecological reserves shortly after firefighters finished putting out the fires, then returned about a year later for follow-up data.
Some areas sampled had ‘metal-rich geologies’ – hotspots for trivalent chromium; others don’t. The researchers collected a total of about 38 soil cores from both locations that had burned and locations that had not.
They found “dangerous” levels of hexavalent chromium in locations where wildfires burned intensely in chaparral shrubs growing in areas with “serpentine” soils that were relatively rich in the metal, the study said.
Areas without as much metal or where the fires burned at a lower intensity – such as grasslands where the fire passed quickly – tested much lower for hexavalent chromium.
Alandra Marie Lopez, a postdoctoral researcher at the Stanford Doerr School of Sustainability, said she spent several hours in fields sampling arid, ashen landscapes, only to find high concentrations of chromium in the laboratory.
“That really fueled my concerns,” she said. “Firefighters spend hours in the landscape clearing burned areas.”
The researchers believe that hexavalent chromium could travel in wildfire smoke, kick up as dust after a fire is extinguished, and persist for months afterward.
More research is needed to understand the risk. The researchers are pursuing air sampling for hexavalent chromium during wildfires and trying to predict risks based on geology and vegetation, Fendorf said.
Serpentine rock is common in fire-prone areas along the coastal mountains that line the Pacific coast.
Researchers have been concerned for years that wildfires cause toxic metal pollution.
After the 2018 Camp Fire, which burned nearly 19,000 buildings, researchers found elevated levels of lead, zinc, calcium, iron and manganese in the smoke.
Some metals traveled more than 150 miles. During the fire, levels of lead — a potent neurotoxin — in Chico were about 50 times above average, California Air Resources Board researchers found.
“Wildfire toxins are a huge concern,” said Barbara Weller, a pulmonary pathologist and toxicologist with the California Air Resources Board’s Research Division. “If a vehicle burns and if a house burns, very different components may be released than in a wildfire where trees and grassland are on fire.”
She said the administration and academic researchers are trying to get a handle on how dangerous wildfire smoke is. This finding adds a new wrinkle.
“Toxins will always remain a health concern, whether they are produced by a natural source or man-made source. This brings to the fore ongoing concerns about wildfires and the impacts of climate change,” Weller said. Serpentine minerals are “found all over California, so that certainly adds to the concern.”
This article was originally published on NBCNews.com