Some wildfire suppressants may be unleashing heavy metals into the environment: Study

The widespread use of certain chemical or synthetic fire suppressants may be leading to heavy metal contamination in wildfire-prone areas, a new study has found.

Wildland firefighters often supplement the use of water with suppressant products — such as fire retardants, water enhancers and foams — which are meant to prevent fire activity before and after water evaporates, the study authors explained.

In an investigation of a range of these products, published Wednesday in Environmental Science & Technology Letters, the researchers found that several contained high levels of at least one metal, including chromium and cadmium.

“Wildfires are associated with the release of toxic heavy metals to the environment, but until now, it was assumed that these metals came from natural sources like soil,” senior author Daniel McCurry, an assistant professor of civil and environmental engineering at the University of Southern California, said in a statement.

“We now know that fire retardants may contribute to these metal releases," McCurry added.

As wildfires have become more frequent and intense, so too have the amounts of suppressants required to extinguish them, the researchers noted.

Yet as firefighters spray these materials from the ground and drop them from planes into woodlands, their precise chemical makeup is not always clear. While manufacturers must list most active ingredients, other components remain proprietary, the researchers warned.

Although all product formulation components must be disclosed to the U.S. Forest Service, publicly available safety data information describes up to 20 percent of these formulations as "proprietary" or a "trade secret," according to the study.

In an attempt to unravel this mystery, the researchers tested samples from 14 fire suppression products sold by commercial retailers — looking for 10 metals that have known toxicity or are regulated by the Environmental Protection Agency (EPA).

Every product contained at least one metal with a concentration that surpassed the EPA's toxicity thresholds for drinking water, the scientists found.

They flagged two particular products, both classified as retardants, that showed exceedances of eight such metals: chromium, cadmium, arsenic, lead, vanadium, manganese, antimony and thallium.

Looking at the volume of fire retardants dropped on blazes across the U.S. from 2009 to 2021, the researchers determined that the amounts of metals varied from year to year but generally rose over time.

"As rates of aerial fire retardant application have grown, likely so too have loadings of toxic metals released into the environment from their use," the authors concluded.

Stressing that this trend "may intensify if wildfire frequency and intensity continues to increase," they advocated for further research on "the environmental fate of metals released by aerial fire suppression” — with a goal of understanding just how much they contribute to ecological and human health risks.