soil and rocks In the West, chromium-3, a harmless trace element, is found naturally in animals and plants, and in dietary supplements. “If you dig a hole outside right now, you’re going to find some amount of chromium-3,” said Matthew Polizotto, a professor of geosciences at the University of Oregon.
But researchers are learning that wildfires can change the chemical composition of elements, turning them from nutrients to poison. When heat from burning soil and plants combines with oxygen, it triggers a process that converts chromium-3 to chromium-6, a carcinogen linked to lung, nose, and sinus cancers. It can also cause asthma, dermatitis, and kidney and liver damage.
Chromium-6 (hexavalent chromium) can become airborne in smoke during a fire or become dust after a fire is extinguished, contaminating the air. It can also accumulate in burn scars for years and potentially leach into groundwater. Firefighters who inhale the toxic ash and local residents who drink contaminated well water are at risk, as are anyone else who encounters chromium-6.
Researchers have known since at least 2008 that wildfires can produce chromium-6, but now a new study has been published in an academic journal. environmental science and technology reported for the first time in November details, including how long they may remain in groundwater.
“If you dig a hole outdoors, wherever you are right now, you’ll find some amount of chromium-3.”
Research shows that chromium-6 can persist at dangerous levels in groundwater for up to two years after a wildfire. Lead author Chelsea Obeidi, currently an assistant professor of soil science at California State Polytechnic State University, Humboldt, worked on this study with Polizotto as a Ph.D. student. Obeidi burned soil samples in the lab and pumped water into the burnt soil to simulate rainfall. Although the experimental results should be investigated on a landscape scale, they “rang a bell for me,” Obeidi said.
Obeidi’s team also found that fires burning between 750 and 1,110 degrees Fahrenheit, which are considered more severe, produced the most chromium-6. Climate change-driven wildfires are intensifying across the West, killing trees, burning forest soil and exposing more areas to elemental-altering heat. Areas rich in serpentinite, such as the mixed chaparral forests of the Rogue River-Siskiyou National Forest in southern Oregon and northern California, are especially susceptible to high concentrations of chromium. The Rocky Mountains, California’s Coast Range, the Northern Sierras, and the Cascade Range of the Pacific Northwest all also feature serpentine soil deposits.
The amount of chromium-6 in soil fluctuates widely after fires, but Obeidi’s results show that soil samples at higher elevations, where the rocks are more weathered, had the highest concentrations. Low-intensity fires, of the kind that are carefully controlled during cultural and prescribed burns, do not appear to significantly activate chromium-6, according to Obeidi’s initial research.
Smoke from the Caldor Fire covers Lake Tahoe near Zephyr Cove, Nevada in 2021. At the time of the fire, air quality in the Tahoe Basin was among the worst in the nation. credit: max whitaker
anyone walking Burn scars can kick up ash and dust and release chromium-6 into the air as firefighters ensure the fire is fully extinguished and homeowners check their properties. “Such substances are especially dangerous if inhaled,” Polizotto said. Wind can also kick up dust containing chromium-6 and blow it into nearby communities, expanding the number of people at risk of breathing it.
The danger extends hundreds or even thousands of miles. Scott Fendorf, a professor of Earth system science at Stanford University, said Canadian wildfires in 2024 and 2025 burned areas with high chromium concentrations. He studies the effects of chromium-6 in the air and was not involved in Obeidi’s research. Some of that chromium would have oxidized to chromium-6, and the smoke would have settled on millions more people living in the northern and eastern United States.
“The material is especially dangerous if you breathe it in.”
There are no easy solutions. The only real solution is to address the wildfire crisis through climate action and increased prescribed cultural burning. In the meantime, Fendorf said, wearing an N95 mask can reduce exposure. And while techniques exist to remove some chromium-6 from soils contaminated by industries such as cement and steel manufacturing, Polizotto said these methods, which use chemicals to leach chromium-6 or reduce it to less toxic forms, do not work on a large scale.
But sometimes natural processes can help. Chromium-6 can revert to chromium-3 if there is enough organic matter in the soil and enough time, days, weeks, or years, depending on conditions. And if the burned area receives heavy rain (about an inch in 24 hours), that’s enough to dilute chromium-6 in the soil and runoff to non-toxic levels, Fendorf said.
New details about the appearance of chromium-6 after fires highlight the health risks wildfires pose to the environment and how little we still understand about them. We know that the dangerous legacy of wildfires extends far beyond the initial spark and smoke. What we don’t know yet, and what we may never know, is the full extent of the risks.
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This article is April 2026 print edition of the magazine The headline read, “Scorched Toxic Soil.”
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