“Wildfires are associated with the release of toxic heavy metals into the environment, but until now these metals were assumed to come from natural sources like soil,” says Daniel McCurry, lead researcher of the study. “We now know that flame retardants can contribute to these metal releases.”

Forest extinguishing products, intended to inhibit fire activity before and after water evaporation, include fire retardants, water enhancers and foams. As wildfires have become more frequent and more severe, larger volumes of water as well as chemical and synthetic extinguishing agents – sprayed from the ground and dropped from airplanes – have been needed to put them out. Although manufacturers identify most of the active ingredients in suppressants, some components are proprietary. Additionally, previous researchers have observed increased concentrations of potentially toxic metals in soil and waterways after wildfires. So McCurry and his colleagues at the University of Southern California wondered whether extinguishing agents contained metals and could contaminate the environment.

The researchers tested samples of 14 fire suppression products sold by commercial retailers. They analyzed samples of 10 metals with known toxicity or regulated by the U.S. Environmental Protection Agency (EPA). Each product contained at least one metal whose concentration exceeded EPA maximum contaminant level regulations for drinking water. In particular, the two extinguishing agents classified as flame retardants contained eight metals (chromium, cadmium, arsenic, lead, vanadium, manganese, antimony, and thallium) that far exceeded EPA drinking water regulations. And one of the retarders exceeded California’s hazardous waste regulations for three of those metals. The researchers say these results indicate the potential for flame retardants to contaminate the aquatic environment and potentially drinking water, if these products enter water bodies.

Using the volume of fire retardants released during U.S. wildfires between 2009 and 2021, the researchers determined that the total amount of metals applied varied from year to year but generally increased over time. And for a Southern California wildfire, they estimate that the increased concentration of cadmium in a nearby stream could be explained by 31 percent of the fire retardant used to contain the fire. They say these results show that fire suppression activities could contribute to elevated levels of metals in the environment, but that additional work is needed to determine potential risks to human and environmental health.

The authors acknowledge funding from the U.S. National Science Foundation (NSF), an NSF Graduate Research Fellowship, and a University of Southern California Graduate Fellowship and Fellowship Women in Science and Engineering.