Above the raging flames, these planes can release huge canisters full of bright pink fire retardant in just 20 seconds. They have long been considered vital in the fight against forest fires.

But new research showed that the millions of gallons of retardant sprayed on the landscape to tame wildfires each year come with a toxic payload because they contain heavy metals and other chemicals that are harmful to human health and the environment.

Toxicity is a big dilemma. These tankers and their cargo are a powerful tool for taming deadly fires. However, as forest fires intensify and become more frequent in the era of climate change, firefighters are using them more often and releasing more harmful chemicals into the environment.

Some environmental groups have questioned the effectiveness of retardants and the possibility of harm. The effectiveness of the fire retardant was difficult to measure, as it was one of the barrage firefighting tactics used in the large fire. Once the flame goes out, it’s hard to take credit.

The frequency and severity of wildfires has increased in recent years, particularly in the western United States. Scientists have also discovered that fires across the region they became faster in recent decades.

There are also long-term health effects of exposure to fire smoke, which can penetrate the lungs and heart, causing disease. AND a recent global survey of the health effects of air pollution from wildfires found that in the United States, exposure to wildfire smoke has increased by 77 percent since 2002. Globally, wildfire smoke is estimated to be responsible for up to 675,000 premature deaths per year.

Fire retardants add to the burden on health and the environment because they represent 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 a recent study on their heavy-metal content.

The United States Forest Service said Thursday that nine large retardant-spraying planes, as well as 20 water-dropping helicopters, were deployed to fight the Southern California wildfires, which have displaced tens of thousands of people. Also in use are several amphibious aircraft of the “water cooper” type, capable of flying over the surface of the sea or other bodies of water to fill their tanks.

Two large DC-10 aircraft, dubbed “Very Large Air Tankers” and capable of delivering up to 9,400 gallons of retardant, will also join the fleet soon, said Stanton Florea, a spokesman for the National Interagency Fire Center in Boise, Idaho, which coordinates the national effort in fighting wildland fires across the West.

Sprayed in front of the fire, the retardants cover vegetation and prevent oxygen from burning, Mr. Florea. (Red paint is added so firefighters can see the retardant against the landscape.) And the retardant, usually made from a salt like ammonium polyphosphate, “lasts longer. It doesn’t evaporate, like when water drips,” he said.

New research by 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 exceed California’s hazardous waste requirements.

Federal data shows that more than 440 million gallons of retardant were applied to federal, state and private lands 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 ground. environment from fire suppression, a third of that in Southern California.

Both the federal government and the retardant’s maker, Perimeter Solutions, disputed that analysis, saying the researchers evaluated a different version of the retardant. Dan Green, a spokesman for Perimeter, said the retardants used for aerial firefighting have undergone “extensive testing to confirm they meet stringent aquatic and mammal safety standards.”

Still, the findings help explain why heavy metal concentrations tend to rise in rivers and streams after wildfires, sometimes hundreds of times. As scrutiny of fire suppression agents grew, the Forest Service set up buffer zones around lakes and rivers, though own data shows the retardant is still unintentionally floating in 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 that covers accidental spraying into waterways.

The judge ruled that the agency should have received a permit. But it allowed the use of retardant to continue to protect life and property.