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

But new research has shown that the millions of gallons of fire retardants sprayed on the landscape each year to control wildfires are linked to toxic pollution because they contain heavy metals and other chemicals that are harmful to human health and the environment .

Toxicity presents a major dilemma. These tankers and their cargo are an effective tool for containing deadly fires. But as wildfires increase and become more frequent in the era of climate change, firefighters are deploying them more frequently, releasing more and more harmful chemicals into the environment.

Some environmental groups have questioned the effectiveness and potential for harm of the retarders. The effectiveness of fire retardants is difficult to measure because it is one of the many firefighting measures used in a major fire. After the flames are extinguished, it is difficult to assign blame.

The frequency and severity of wildfires has increased in recent years, particularly in the western United States. Scientists have also found that fires across the region have been spreading at an increasing rate in recent decades.

There are also longer-term health effects of exposure to wildfire smoke, which can penetrate the lungs and heart and cause illness. A recent global survey on the health effects of air pollution caused by wildfires found that in the United States, exposure to wildfire smoke has increased by 77 percent since 2002. It is estimated that wildfire smoke is responsible for up to 675,000 premature deaths worldwide each year.

Flame retardants contribute to these health and environmental burdens because they represent “a really, really delicate trade-off,” said Daniel McCurry, an assistant professor of civil and environmental engineering at the University of Southern California, who led recent research on their heavy-metal content .

The U.S. Forest Service said Thursday that nine large ordnance-spraying aircraft and 20 water-dropping helicopters were being deployed to fight fires in Southern California that have displaced tens of thousands of people. Several amphibious aircraft are also used, skimming over the surface of the sea or other bodies of water to fill their tanks.

Two large DC-10 aircraft called Very Large Airtankers, capable of delivering up to 9,400 gallons of firefighting agent, also should be added to the fleet soon, said Stanton Florea, a spokesman for the National Interagency Fire Center in Boise, Idaho. which coordinates national firefighting efforts in wild areas in the West.

The repellent sprayed before the fire covers the vegetation and prevents the oxygen from burning, Mr. Florea said. (Red dye is added so firefighters can see the inhibitor in the image.) And the inhibitor, typically made of salts like ammonium polyphosphate, “lasts longer. It doesn’t evaporate like dripping water,” he said.

The new research by Dr. However, McCurry and his colleagues found that at least four different types of heavy metals, including chromium and cadmium, contained in a type of retardant commonly used by firefighters exceeded California’s hazardous waste requirements.

Federal data show that between 2009 and 2021, more than 440 million gallons of retardants were applied to federal, state and private lands. Using that number, researchers estimated that between 2009 and 2021, more than 400 tons of heavy metals were released into the country’s environment through firefighting, a third of them in Southern California.

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

Still, the results help explain why concentrations of heavy metals in rivers and streams tend to increase after wildfires, sometimes by a factor of 100. And as control over fire suppressants increases, the Forest Service has established buffer zones around lakes and rivers, even though its own data shows that fire suppressants are still unintentionally leaking into these 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 in waterways.

The judge ruled that the agency actually had to obtain approval. But it allowed the use of retardants to continue to protect life and property.