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In fire-prone areas, water isn't the only thing used to quell blazes. Wildland firefighters also apply chemical or synthetic suppressants. Researchers in Environmental Science & Technology Letters explored whether these suppressants could be a source of elevated metal levels sometimes found in waterways after wildfires are extinguished.

Several products they investigated contained high levels of at least one metal, including chromium and cadmium, and could contribute to post-fire increases in the environment.

"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," says Daniel McCurry, principal investigator of the study. "We now know that fire retardants may contribute to these metal releases."

Wildfire suppressant products, which are intended to inhibit fire activity before and after water evaporates, include fire retardants, water enhancers and foams. As wildfires have become more frequent and severe, larger volumes of water along with chemical and synthetic suppressants鈥攕prayed from the ground and dropped from planes鈥攈ave been required to extinguish them.

Although manufacturers identify most of the active ingredients in suppressants, some components are proprietary. In addition, previous researchers have observed increased concentrations of potentially toxic metals in soil and streams after wildfires.

So, McCurry and colleagues at the University of Southern California wondered if the suppressants contain metals and could contaminate the environment.

The researchers tested samples from 14 fire suppression products sold by commercial retailers. They analyzed samples for 10 metals that have known toxicity or are regulated by the U.S. Environmental Protection Agency (EPA).

Every product contained at least one metal with a concentration that exceeded the EPA's Maximum Contaminant Level regulations for drinking water.

In particular, the two suppression products classified as fire retardants contained eight metals (chromium, cadmium, arsenic, lead, vanadium, manganese, antimony and thallium) that greatly exceeded the EPA's drinking water regulations. And one of the retardants exceeded California's hazardous waste regulations for three of those metals.

The researchers say these results indicate the potential for fire retardants to contaminate the aquatic environment and potentially drinking water, if these products enter bodies of water.

From the volume of fire retardants dropped on wildfires in the U.S. between 2009 and 2021, the researchers determined that the total amount of metals applied was variable year to year but generally increased over time. And for one Southern California wildfire, they estimate that the increased concentration of cadmium in a nearby stream could be explained by 31% of the reported fire retardant used to contain the fire.

They say these results show that fire suppression activities could contribute to elevated metal levels in the environment but that more work is needed to determine potential risks to human and environmental health.