A heavy metal that can cause cancer may be spread far and wide via the smoke of wildfires, new research has found.
The carcinogenic form of the metal chromium and other cancer-associated chemicals were found to be present in large quantities in ash after wildfires across Northern California, a new paper in the journal Nature Communications reveals.
In dry environments, this ash may easily be blown towards population centers in the wind, and could therefore pose a long-term public health hazard.
The researchers analyzed the soil after wildfires across Northern California in 2019 and 2020, including in Sonoma, Napa and Lake Counties, finding that there were dangerous levels of chromium in the ash. The chromium was discovered to be most prevalent in areas with an underlying metal-rich geology, and in areas where the fires were more severe.
A stock image of a wildfire near South Lake Tahoe in California. Wildfires have been found to leave behind ash rich in the carcinogenic form of the metal chromium. ISTOCK / GETTY IMAGES PLUS
“We demonstrate that high temperatures during California wildfires catalyzed widespread transformation of chromium to its carcinogenic form in soil and ash, as hexavalent chromium, particularly in areas with metal-rich geologies (e.g., serpentinite),” the authors, hailing from Stanford University, wrote in the paper.
“In wildfire ash, we observed dangerous levels (327-13,100 µg kg−1 [micrograms per kilograms]) of reactive hexavalent chromium in wind-dispersible particulates,” they said.
Chromium is an essential element in the human body, but only in the form of trivalent chromium (Cr III). Hexavalent chromium (Cr VI), however, is carcinogenic, and is the form of chromium that has been found in the wildfire ash. According to the United States Environmental Protection Agency, Cr VI exposure is strongly linked to lung cancers in humans and animals, as well as acute effects including coughing and wheezing, and even gastrointestinal and neurological effects in high dosages.
The researchers also found that drier weather in the aftermath of the fires caused the chromium to stay in the soil and dust for nearly a year.
“Relatively dry post-fire weather contributed to the persistence of elevated hexavalent chromium in surficial soil layers for up to ten months post-fire,” the authors wrote.
Wildfires release large quantities of other dangerous substances, including PM2.5 pollution. PM2.5 particles are 2.5 micrometers or less in diameter, and include soot, black carbon, ammonia, sodium chloride and mineral dust, which can all cause respiratory irritation.
A stock image of a man wearing a face mask to protect himself against wildfire smoke in Australia in 2020. Wildfires release large quantities of dangerous substances that can cause respiratory irritation. ISTOCK / GETTY IMAGES PLUS
The researchers suggest that the chromium left behind in the ash may also pose a large health hazard to people living nearby and first responders, as it will get blown around in the wind. While the study focused on California wildfires, the authors estimate that other regions of the world where soils are rich in heavy metals and fires are frequent face similar problems, including large swaths of West, East and Southeast Africa, Northern Australia and Russia.
As the chromium appears to persist in the soil for longer in drier environments, the effects of climate change may therefore play into both the frequency and severity of wildfires, as well as exacerbating the damaging effects of wildfire pollution in the future.
“Our findings provide new insights into why wildfire smoke exposure appears to be more hazardous to humans than pollution from other sources,” the authors wrote.
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