Soil and rocks throughout the West naturally contain chromium-3, a harmless trace element also found in animals and plants — and even included in dietary supplements. “You could dig a hole outside, wherever you are right now, and you’re going to find some amount of chromium-3,” said Matthew Polizzotto, professor of earth sciences at the University of Oregon.
But researchers are learning that wildfires change the element’s chemical composition, turning it from a nutrient into a poison. Heat from burning soils and plants, combined with oxygen, triggers a process that converts chromium-3 into chromium-6, a carcinogen linked to lung, nasal and sinus cancers. It can also cause asthma, skin irritation and kidney and liver damage.
Chromium-6, or hexavalent chromium, can become airborne in smoke during a blaze or be stirred up in dust after the fire is out, contaminating the air. It can also loiter in a burn scar for years, leaching into groundwater. Firefighters breathing in toxic ash and rural residents who drink contaminated well water are at risk, along with anyone else who encounters chromium-6.
Researchers have known since at least 2008 that wildfires can create chromium-6, but a new study, published in the journal Environmental Science and Technology in November, is the first to report details such as how long it might persist in groundwater.
“You could dig a hole outside, wherever you are right now, and you’re going to find some amount of chromium-3.”
Chromium-6 can linger in groundwater at unsafe levels for up to two years post-wildfire, according to the research. Lead author Chelsea Obeidy, now an assistant professor of soil science at California State Polytechnic University, Humboldt, worked with Polizzotto on the study as a Ph.D. student. Obeidy burned soil samples in a lab, then simulated rainfall by pumping water through the scorched soil. Though the lab results should be investigated on a landscape scale, Obeidy said, they “rang a bell for me.”
Obeidy’s team also found that fires that burn between 750 and 1,110 degrees Fahrenheit, considered high severity, created the most chromium-6. Increasingly hot climate change-driven wildfires across the West are killing trees and incinerating forest soil, subjecting more areas to this element-altering heat. Areas rich in serpentine rock, like the mixed chaparral forests of the Rogue River-Siskiyou National Forest in southern Oregon and Northern California, are especially prone to high concentrations of chromium. The Rocky Mountains, the Coast Ranges of California, the Northern Sierras and the Cascade Range in the Pacific Northwest also all feature serpentine soil deposits.
The amount of chromium-6 in soil varies wildly after a fire, though Obeidy’s results show that soil samples from higher elevations, where rocks are more weathered, have the highest levels. Lower-intensity fire, the kind carefully controlled during cultural and prescribed burns, does not appear to significantly activate chromium-6, according to Obeidy’s initial research.
ANYONE WALKING through a burn scar — a firefighting crew making sure a blaze is fully extinguished, homeowners checking on properties — can stir up ash and dust, sending chromium-6 into the air. “Breathing in any of that material would be especially dangerous,” Polizzotto said. Winds can also whip up chromium-6-laced dust and sweep it into neighboring communities, expanding the number of people at risk of inhaling it.
The danger extends hundreds or even thousands of miles. Canadian wildfires in 2024 and 2025 burned across areas high in chromium, said Scott Fendorf, a professor of earth system science at Stanford University who has studied the airborne effects of chromium-6 and wasn’t involved in Obeidy’s study. Some of that chromium would have oxidized into chromium-6 and then traveled via smoke and settled over millions of people living in the Northern and Eastern U.S.
“Breathing in any of that material would be especially dangerous.”
There’s no easy fix: The only real solution is to get a handle on the wildfire crisis through climate action and ramping up prescribed and cultural burning. In the meantime, wearing an N95 mask can help reduce exposure, Fendorf said. And while the technology exists to remove some chromium-6 from soil contaminated by industries like cement and steel manufacturing, these methods — using chemicals to leach it out or reduce it to its less toxic form — don’t work on a large scale, Polizzotto said.
Natural processes can help, however: Given enough organic material in the soil and enough time — days, weeks or years, depending on conditions — chromium-6 can convert back to chromium-3. And if a burned area gets heavy rains — around an inch in 24 hours, Fendorf said — that’s enough to dilute chromium-6 to non-toxic levels in soil or runoff.
The new details on the emergence of chromium-6 post-fire underscore the environmental health hazards wildfires leave behind, and how little we still understand them. We know that the dangerous legacy of wildfire extends far beyond the initial sparks and smoke. What we don’t yet know, and may never know, is the full extent of the risks.
We welcome reader letters. Email High Country News at editor@hcn.org or submit a letter to the editor. See our letters to the editor policy.
This article appeared in the April 2026 print edition of the magazine with the headline “Scorched and poisonous soil.”
Republish our articles for free, online or in print, under a Creative Commons license.
