Wildfires can release toxic metal particles from soils, study finds

Wildfires can turn a harmless metal found in soil and plants into toxic particles that spread easily through the air, according to a new study from Stanford University.

Published on December 12 in Natural communications, research documents elevated levels of a dangerous form of metallic chromium at wildfire sites with chromium-rich soils and certain types of vegetation compared to adjacent, unburned sites. Known as hexavalent chromium or chromium 6, it is the same toxin made famous by the 2000 film Erin Brockovich.

“Our study suggests that much more attention should be paid to wildfire-modified chromium, and we hypothesize other metals as well, to better characterize the overall threats that wildfires pose to human health,” a said the study’s lead author, Alandra Lopez, a postdoctoral researcher at Earth. systems science at the Stanford Doerr School of Sustainability.

An overlooked danger

Smoke plumes from wildfires are known to carry dangerous air pollutants, including gases, organic aerosols and fine particulate matter, which can trigger asthma attacks, heart attacks and premature death.

Scientists and regulators have paid less attention to the potential damage caused by metals like chromium, which is common in soils in the western United States, Australia, Brazil, Europe, from Indonesia and South Africa. As wildfires are expected to become more frequent and more severe due to climate change, the health risks posed by airborne chromium to firefighters, downwind residents and others will need to be better understood, the researchers said.

“In the complex mix of gases and particles that wildfires emit as smoke and leave behind as dust, heavy metals such as chromium have been largely overlooked,” said the lead author of the paper. study, Scott Fendorf, Terry Huffington Professor at the Stanford Doerr School of Sustainability. .

A scientific opportunity presents itself

In nature, chromium occurs primarily in a form known as trivalent chromium or chromium 3, an essential nutrient that our bodies use to break down glucose. Chromium 6, which increases the risk of cancer when inhaled or ingested through contaminated drinking water, most often results from industrial processes. High levels of chromium 6 have historically entered the environment from industrial runoff and wastewater.

Although natural chemical processes can trigger this transformation, laboratory experiments by researchers at Southern Cross University in Australia proved in 2019 that chromium 6 could also form rapidly from chromium 3 in heated surface soils. by forest fires.

Intrigued by these findings, Fendorf and Lopez set out to test the theory that wildfires can leave soils contaminated with chromium 6. Focusing on California’s North Coast Range, they identified sites in four ecological reserves that recently burned on soils naturally rich in chromium. rocks, such as serpentinite.

Lopez collected the soil from the reserves and separated the smallest particles, the most sensitive to wind transport. She measured concentrations of hexavalent chromium in this ultrafine dust from burned and unburned areas and gathered data on local fire severity and the prevailing soil, underlying geology and ecosystem types, ranging from grasslands open to dense forests.

Researchers found that all of these factors influenced chromium-6 levels in the soil. Even more dramatically, in chromium-rich areas where vegetation allowed fires to burn at high temperatures for long durations, concentrations of toxic chromium were about seven times higher than in unburned areas, suggesting that significant quantities of chromium 6 could be airborne.

Mitigate Risk

In terms of exposure risks, fire-induced toxic chromium would initially be encountered by first responders and people living near fires. Even after the fires subside, local communities located downwind could be exposed, as strong winds can carry fine soil particles containing chromium.

Much of the risk of inhaling airborne hexavalent chromium would likely diminish after the first big rains wash away the metal and the subsoil, said Fendorf, who is also a senior scientist at the Stanford Institute Woods for the environment.

However, before the rains arrive – which could take several months, especially as climate change leads to more frequent and more severe droughts in the American West – risks of exposure would loom for people working in revegetation or reconstruction of burned areas, as well as for recreationists checking burn scars. Additional research is needed to understand potential threats to ecosystems and human health if fire-induced chromium 6 enters streams or groundwater.

Fendorf said future research on wildfire-related toxic chromium exposure could help inform public health guidance, such as recommendations to wear an N95 mask when visiting a burn site.

To learn more, Lopez is now contributing to an assessment of firefighters’ health and exposure to metal-containing dust.

With support from the Stanford Woods Institute for the Environment’s Environmental Ventures program, Fendorf is working with colleagues to develop geospatial tools to predict threats of toxic chromium generation and downstream exposure. These tools could eventually help researchers develop better ways to limit exposure to chromium and other neglected metal pollutants.

“Although chromium is one of the metals of greatest concern, we are sure it is not the only one,” Fendorf said. “We expect future studies to confirm additional risks of inhalation exposure to metals posed by wildfires.”