Researchers at Stanford University have shown that wildfires can transform metals in soil into cancer-causing substances. These compounds can become inhalable and be carried into the air, posing risks to firefighters and communities living downwind. The findings come from a study summarized in Nature Communications, underscoring how wildfire smoke carries more than just heat and ash.
Chromium exists in two common forms in the environment. The trivalent form, Cr(III), is widespread and generally less harmful to human health. When temperatures rise above roughly 200 degrees Celsius, Cr(III) can be oxidized by atmospheric oxygen to Cr(VI), a form known for greater toxicity. Investigations during fires indicate that as much as a third of the chromium present in soils can be converted to Cr(VI) under such conditions, raising concerns about airborne exposure during wildfire events (Nature Communications, annual fire context and metal transformation data referenced).
In California, scientists collected soil and ash samples from wildfires at multiple intervals over nearly a year. Even ten months after the fires, traces of toxic metals persisted in the form of fine, windblown particles. Measured concentrations ranged from a few hundred to well over ten thousand micrograms per kilogram, highlighting that toxic residues can linger in the environment long after flames die down and continue to pose inhalation risks for nearby residents and responders (California wildfire study reference).
The researchers emphasize the need for further work to determine the precise levels of Cr(VI) in wildfire smoke and to examine the potential toxicity of other metals found in soils, such as manganese and nickel. In the meantime, protective measures are advised for people living in fire-prone areas and for those who respond to fires, including the use of appropriate masks and the consideration of air purification in indoor spaces (author team notes and ongoing research calls).
A cross-border collaboration involving scientists from Cambridge University in the United Kingdom and the Australian National University reported that between 2001 and 2021, forests worldwide experienced up to 25 million hectares of burning area. The quantity of annual forest fires has tripled over the last decade. The analysis identifies Brazil, Australia, Canada, the United States, and Siberia as among the most affected regions, illustrating a global trend toward more frequent and extensive wildfire seasons (Cambridge and ANU study overview).
One potential driver of the increasing fire danger is timber harvesting, which can leave forests more vulnerable to high-intensity fires. Observations in parts of southeastern Australia show that intact forests tend to burn with lower intensity compared to recently felled areas, suggesting that forest management practices influence fire behavior and outcomes in some landscapes (regional fire-impact notes).
Elsewhere in the scientific literature, researchers have documented that wildfires burn areas that, in total, can exceed the size of India each year. This scale underscores the global reach of wildfire impacts and the importance of understanding how heat, smoke, and fire-derived materials affect air quality, public health, and ecosystem resilience across multiple continents (global fire area assessments).