From above the raging flames, these planes can unleash immense tankfuls of bright pink fire retardant in just 20 seconds. They have long been considered vital in the battle against wildfires.
But emerging research has shown that the millions of gallons of retardant sprayed on the landscape to tame wildfires each year come with a toxic burden, because they contain heavy metals and other chemicals that are harmful to human health and the environment.
The toxicity presents a stark dilemma. These tankers and their cargo are a powerful tool for taming deadly blazes. Yet as wildfires intensify and become more frequent in an era of climate change, firefighters are using them more often, and in the process releasing more harmful chemicals into the environment.
Some environmental groups have questioned the retardants’ effectiveness and potential for harm. The efficiency of fire retardant has been hard to measure, because it’s one of a barrage of firefighting tactics deployed in a major fire. After the flames are doused, it’s difficult to assign credit.
The frequency and severity of wildfires has grown in recent years, particularly in the western United States. Scientists have also found that fires across the region have become faster moving in recent decades.
There are also the longer-term health effects of exposure to wildfire smoke, which can penetrate the lungs and heart, causing disease. A recent global survey of the health effects of air pollution caused by wildfires found that in the United States, exposure to wildfire smoke had increased by 77 percent since 2002. Globally, wildfire smoke has been estimated to be responsible for up to 675,000 premature deaths per year.
Fire retardants add to those health and environmental burdens because they present “a really, really thorny trade-off,” said Daniel McCurry, an assistant professor of civil and environmental engineering at the University of Southern California, who led the recent research on their heavy-metal content.
The United States Forest Service said on Thursday that nine large retardant-spraying planes, as well as 20 water-dropping helicopters, were being deployed to fight the Southern California fires, which have displaced tens of thousands of people. Several “water scooper” amphibious planes, capable of skimming the surface of the sea or other body of water to fill their tanks, are also being used.
Two large DC-10 aircraft, dubbed “Very Large Airtankers” and capable of delivering up to 9,400 gallons of retardant, were also set to join the fleet imminently, said Stanton Florea, a spokesman for the National Interagency Fire Center in Boise, Idaho, which coordinates national wildland firefighting efforts across the West.
Sprayed ahead of the fire, the retardants coat vegetation and prevent oxygen from allowing it to burn, Mr. Florea said. (Red dye is added so firefighters can see the retardant against the landscape.) And the retardant, typically made of salts like ammonium polyphosphate, “lasts longer. It doesn’t evaporate, like dropping water,” he said.
The new research from Dr. McCurry and his colleagues found, however, that at least four different types of heavy metals in a common type of retardant used by firefighters exceeded California’s requirements for hazardous waste.
Federal data shows that more than 440 million gallons of retardant were applied to federal, state, and private land between 2009 and 2021. Using that figure, the researchers estimated that between 2009 and 2021, more than 400 tons of heavy metals were released into the environment from fire suppression, a third of that in Southern California.
Both the federal government and the retardant’s manufacturer, Perimeter Solutions, have disputed that analysis, saying the researchers had evaluated a different version of the retardant. Dan Green, a spokesman for Perimeter, said retardants used for aerial firefighting had passed “extensive testing to confirm they meet strict standards for aquatic and mammalian safety.”
Still, the findings help explain why concentrations of heavy metals tend to surge in rivers and streams after wildfires, sometimes by hundreds of times. And as scrutiny of fire suppressants has grown, the Forestry Service has set buffer zones surrounding lakes and rivers, though its own data shows retardant still inadvertently drifts into those waters.
In 2022, the environmental nonprofit Forest Service Employees for Environmental Ethics sued the government in federal court in Montana, demanding that the Forest Service obtain a permit under the Clean Water Act to cover accidental spraying into waterways.
The judge ruled that the agency did indeed need to obtain a permit. But it allowed retardant use to continue to protect lives and property.
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