Wildfires can exacerbate a toxic-metal threat to human health

Although wildfire is a natural process that preserves the long-term well-being of an ecosystem, it can devastate human health. Inhalation of smoke and particles can activate inflammation in the body and damage DNA. Fine (less than 2.5-µm-diameter) particles are problematic because they can penetrate deep into the lungs. In a new study, researchers have identified a particular phase of the metal chromium as an especially hazardous product of severe wildfire.

Alandra Lopez, her former thesis adviser Scott Fendorf, and their colleague Juan Lezama Pacheco (all at Stanford University) analyzed the soils and ash produced by 2019 and 2020 wildfires in California’s North Coast Range. Chromium is naturally abundant in its trivalent Cr(III) form in the metal-rich parts of the area. Whereas that ion is nontoxic, its oxidation at temperatures exceeding 200 °C is known to catalyze the formation of the carcinogenic Cr(VI) phase. Exposure to hexavalent chromium in industrial settings is linked to an increased risk of lung, sinus, and nasal cavity cancers, according to the National Cancer Institute.

Using microscale x-ray techniques, the researchers found large concentrations of hexavalent chromium in wind-dispersed ash throughout the burned areas of California’s Cr-rich chaparral landscapes. Those regions typically have evergreen shrubs, bushes, and small trees. After being excavated from the soil, the Cr(VI) turned out to be most concentrated within 2 cm of the surface, where wildfire temperatures peaked in the soil. In severely burned sites, the metal phase’s concentration reached 200 µg/kg, which was 6.5 times as high as that of unburned soils. The US Environmental Protection Agency’s recommended maximum level for residential soils is 300 µg/kg. Blanketing the ground in burned sites was a surface layer of ash whose concentration of Cr(VI) was much higher—up to 3335 µg/kg.

California’s geology is hardly unique. Between 2001 and 2020, hundreds of wildfires in metal-rich regions have occurred on every continent save Antarctica. What’s more, to judge by soil sampling done by the researchers, relatively dry weather can encourage elevated levels of Cr(VI) in surface-soil layers until the next major rainfall; in the case of the California wildfires, the researchers found Cr(VI) persisting for up to 10 months postfire. The susceptibility to wind erosion of particles in those layers makes it all the more important to understand the risks for people who live near the affected area or frequently hike downwind. (A. M. Lopez, J. Lezama Pacheco, S. Fendorf, Nat. Commun. 14, 8007, 2023 .)