What is Phos-Chek, the pink powder used to fight California fire? Is it safe to use?

As the Los Angeles fires killed at least 24 people, displacing thousands more and destroying over 12,000 structures since they began last Tuesday, aircraft battling fires are now dropping more than water to contain the flames before they destroy more neighbourhoods.

Hundreds of thousands of gallons of Phos-Chek, a hot-pink fire suppressant, has been sprayed on Mandeville Canyon and other rugged areas that are difficult for firefighters on the ground to reach. Typically, the pink flame retardant is sprayed around a wildfire on fire-prone vegetation and land to prevent the blaze from spreading.

California Fire agencies have said that fire suppressants—most often used to fight forest fires—are invaluable tools.

What is Phos-Chek?

Phos-Chek is an aerial fire suppressant, generally a mixture of water, ammonium phosphate—essentially fertiliser—and iron oxide, which is added to make the pink flame retardant visible.

It has been used to fight fires in the US since 1963 and is the main long-term fire retardant used by the California Department of Forestry and Fire Protection.

According to a 2022 report, it is also the most-used fire retardant in the world.

How does Phos-Chek work?

The Forest Service, which has used 13 aircraft to dump suppressants on the Los Angeles fires, says it helps starve a fire of oxygen and slow the burn rate by cooling and coating vegetation and other surfaces.

According to Perimeter, the company that supplies pink fire retardant, the phosphate changes the way cellulose in plants decomposes and makes them non-flammable.

While Phos-Chek-like fire suppressants can be very effective, they do have limitations. The fire agency said, “Strong winds can make it too dangerous to fly at the low altitudes needed for drops and can dissipate the retardant before it hits the ground.”

Is Phos-Chek safe to use?

Pink-flame fire suppressants are generally considered safe for people, but many worry about their potential effects on wildlife.

The Forest Service has banned the use of aerial suppressants over waterways and endangered species habitats, “except when human life or public safety are threatened”, due to potential health effects on fish and other wildlife.

"The use of wildland fire retardant is the best way to save lives, protect communities and keep fires small," said Edward Goldberg, vice chairman of solutions at Perimeter.

Daniel McCurry, a civil and environmental engineering professor at the University of Southern California, said more study is needed on fire suppressants -- including in Los Angeles once the fires are out -- but he understands their value, “If there was a brush fire coming for my house, I'd still rather than paint a lot of (fire retardant) in front of it”.

What should be done after Phos-Chek is sprayed?

Perimeter has advised cleaning the powder off as soon as it is safe. "The longer the retardant dries, the more difficult it is to remove completely," they cautioned.

The company said that warm water and mild detergent are effective in removing it from small surfaces, while pressure washers can be used for larger surfaces.

(With Associated Press inputs)