After recent intense wildfires in Northern California, scientists tested samples of burned soil and were disturbed by their findings: It was loaded with a cancer-causing metal called hexavalent chromium.
Scientists think the heat of raging wildfires may turn a benign version of the metal commonly found 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 understand how dangerous the smoke could be to human health. Researchers have found hazardous metals from burned cars, homes and farms in previous fires. The new finding adds a surprising twist to a growing body of research and suggests that wildfires burning in natural areas may also be pumping smoke containing toxic metals into the atmosphere.
“I think it changes our risk analysis when you consider exposure to wildfire smoke,” said Scott Fendorf, professor of earth system science at Stanford University and author of the study.
Climate change can increase the risk: Hotter and longer-lasting wildfires are more likely to turn harmless soil into carcinogenic dust and ash.
“Due to climate change, forest fires are more frequent and the severity of the fires is greater,” Fendorf said. “You get more exposure and you get exposed to materials that are going to be more toxic.”
According to the International Agency for Research on Cancer, hexavalent chromium is a group I carcinogen, meaning it is known to cause cancer in humans. Exposure to large amounts of hexavalent chromium is associated with lung cancer, according to an Environmental Protection Agency toxicology review that evaluated decades of workplace exposure of people working in chrome plating and chromate pigment plants.
In a study of mice exposed to hexavalent chromium in drinking water for two years, some mice developed tumors in their mouths, small intestines, and livers.
Hexavalent chromium is a well-known contaminant because it was the main chemical in a class-action lawsuit featured in the movie “Erin Brockovich” over chromium contamination of water in Hinkley, California, where the metal was used to prevent corrosion in a cooling tower. Water at natural gas compressor station.
Nearly 600 Hinkley residents settled an initial lawsuit with Pacific Gas & Electric Company (PG&E) for $333 million. PG&E paid another $315 million to resolve other lawsuits, according to the Associated Press.
The California Air Resources Board adopted a rule earlier this year to phase out hexavalent chromium in industrial facilities, stating in a press release that “there is no known safe exposure level.”
In its trivalent form, chromium is relatively harmless and abundant. But according to the new study, heat above 390 degrees Fahrenheit can catalyze chemical reactions that convert it into the more dangerous hexavalent chromium.
Researchers visited wildfire sites in California’s North Coast, including the 2019 Kincade Fire and the 2020 Hennessey Fire, to look for hexavalent chromium. They took soil samples at four ecological preserves immediately after firefighters finished fighting the fire and returned about a year later for follow-up data.
Some areas sampled had “metal-rich geologies”; these were hotspots for trivalent chromium; others did not. Researchers collected a total of 38 soil cores from both burned and unburned areas.
The study stated that they found “dangerous” levels of hexavalent chromium in bushes growing in regions with “serpentine” soils that are relatively rich in metals, in areas where forest fires burn intensely.
Areas where there wasn’t as much metal or where fires burned at lower intensity (for example, grasslands where 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 the fields sampling barren, ashy land, but found high levels of chromium in the laboratory.
“That really triggered my concerns,” he said. “Firefighters spend hours in the field clearing burned areas.”
Researchers believe hexavalent chromium can travel in wildfire smoke, blow like dust after the fire is out, and persist for months afterward.
More research is needed to understand the risk. Fendorf said researchers are trying to sample air for hexavalent chromium during wildfire events and try to estimate risk based on geology and vegetation.
Serpent rock is common in fire-prone areas along the coastal mountains up and down the Pacific coast.
Researchers have been concerned for years that wildfires create toxic metal pollution.
Following the Camp Fire in 2018, which burned nearly 19,000 buildings, researchers found high levels of lead, zinc, calcium, iron and manganese in the smoke.
Some metals traveled more than 150 miles. California Air Resources Board researchers found that levels of lead, a potent neurotoxin, in Chico at the time of the fire were nearly 50 times above average.
“Toxics in wildfire are a major concern,” said Barbara Weller, a pulmonary pathologist and toxicologist in the research division of the California Air Resources Board. “When a vehicle burns and a house burns, there are potentially very different components released from that compared to a forest fire where trees and grassland are burned.”
He said the board and academic researchers are trying to understand how dangerous wildfire smoke is. This finding adds a new wrinkle.
“Whether toxic substances are produced from a natural source or a man-made source, they will always pose a health concern, raising the ongoing concern about the impacts of wildfire and climate change,” Weller said. Serpentine minerals “are found all over California, which certainly adds to the concern.”
This article first appeared on NBCNews.com.