As wildfires rage across Los Angeles, an unusual sight accompanies the flames bright pink slurry dropped from air tankers over burning homes, cars, and forests. This strikingly vivid substance is fire retardant, primarily Phos-Chek, a mixture of ammonium phosphate, iron oxide (rust), and various additives. Its purpose is to help contain fires by slowing their spread, making it easier for firefighters to gain control.
Phos-Chek has been used by the US Forest Service since the 1960s, and its distinctive color makes it easy for pilots to track drops, ensuring they cover all areas without missing spots. The retardant’s effectiveness lies in its ability to continue working even after the water it’s mixed with evaporates, unlike water drops that lose their impact once they dry up. “It’s amazing stuff,” says Jason Colquhoun, a pilot with HeliQwest, a company that specializes in aerial firefighting.
However, the scale at which it has been deployed in recent days has raised concerns. With fires in Los Angeles reaching unprecedented levels, Phos-Chek has been used more extensively, including in residential neighborhoods, leading to questions about its safety. The substance contains fertilizers and rust, both of which are generally considered non-toxic to mammals and aquatic life when used in accordance with the Environmental Protection Agency’s guidelines. But that doesn’t mean it’s without risk.
The older formulation of Phos-Chek, LC95, contained high levels of heavy metals, which could contaminate water sources. This formulation was phased out in December 2024, and the new version, MVP-Fx, is touted as less toxic. Still, experts caution that its environmental impact, particularly when dropped in large quantities over urban areas, is not entirely understood.
While the Forest Service maintains that the retardant is “practically non-toxic,” studies suggest that its widespread use could pose risks, especially to ecosystems and water quality. The full effects of such massive deployments, especially in urban areas, remain uncertain, and with fires growing more intense, the balance between fire suppression and environmental protection continues to be a critical question.
