New study sounds alarm about harmful toxin found in wildfire smoke, ash and dust left behind by intense infernos
Scientists at Stanford University have revealed yet another reason to be alarmed by exposure to wildfire smoke and the ash and dust left behind by high intensity fires.
It turns out that catastrophic wildfires like those experienced across the North Bay can transform a naturally occurring metal in rock, soil and plants into the toxic, carcinogenic stuff that put environmental activist Erin Brockovich on the map: chromium 6, or hexavalent chromium.
The new study, published Tuesday in the journal “Nature Communication,” was based on research samples taken locally, in the footprints of the November 2019 Kincade Fire and the Hennessy Fire, part of the LNU Lightning Complex that broke out around the region in fall 2020.
It found that high heat drastically accelerates the conversion of naturally occurring heavy metal called chromium 3 into chromium 6, particularly in metal-rich geologic areas, like serpentine, which is common to the North Coast Range.
Production of the toxic form also is amplified by abundant fuels and certain plant types, like chaparral and forest, both because they raise the temperature of the fire and provide greater volumes of chromium 3 to transform.
Particles of chromium 6 can then be transported in the smoke or lie on the ground in surface soil for nearly a year, or until a sufficient rainfall dilutes it back into the soil, said Scott Fendorf, professor of Earth System Science at Stanford and senior associate dean of Integrative Initiatives at the Doerr School of Sustainability
"In addition to local (and possibly distal) communities that may be exposed by transport of Cr (VI)-bearing PM (particulate matter) during and after wildfires, wildland fire crews and first responders are particularly at risk,“ the paper states. ”Given the persistence of Cr (VI) after wildfires, continued dust exposure serves as a continued human health threat to land workers, recreationalists, and local residents.“
That’s particularly true if whatever they’re doing on a site with high chromium 6 levels stirs up or disrupts the soil, Fendorf said. Stanford postdoctoral researcher Alandra Marie Lopez and Research Associate Juan Lezama Pacheco were co-authors.
The risk is marked for wildland firefighters, who often hike around in fire zones with out the respirators common to urban firefighters and take in more toxic smoke through their lungs and even through their skin.
“They wear a bandana at best, right?” said Fendorf. “And I recognize for them, the risk calculus probably isn’t going to change it.”
But for others, “your risk calculation could make quite a big difference.”
The study should encourage people to wear masks, especially N-95s, any time there’s smoke in the air, if they go outside and to try to stay indoors, with an air purifier running, Fendorf said.
“All the things we actually hear from the health community? You should just take that way more seriously,” he said.
The kind of rock in the burning or burned landscape matters, though, based on sampling across different geologic zones. Granite, for instance, has far lower metal content than what are called mafic or ultramafic rock areas.
Prescribed fires, usually conducted in cool, moist conditions, are by design low intensity burns and would not generate the necessary heat to transform chromium 3, Fendorf said.
In what is ultimately a call for more investigation — and a warning to those who work in, around and after wildfires — the new paper notes significant awareness around the cancer-causing toxins contained in smoke from structure fires, as a broad range of chemicals and substances incinerate.
The wildland equivalent is less understood and “under appreciated,” the study says.
The new research is one of many studies spurred by a spate of catastrophic wildfires that erupted around California over the past decade and did particular damage in the North Bay, burning hundreds of thousands of acres around the Sonoma, Lake, Napa and Mendocino counties since 2015.
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