Firefighters are trying to stop the damage from several wildfires by dropping thousands of gallons of red flame retardant into the hills around Los Angeles.
When I see pictures of bright red clouds covering trees and sagebrush, I want to know what it is and what’s inside it.
NPR talks to Daniel McCurry, an associate professor of civil and environmental engineering at the University of Southern California. “It’s mostly a product called Phos-Chek LC95, which is sold by a company called Perimeter.”
“It’s been used for decades by the Forest Service,” he adds. “It’s one of maybe only two products currently approved for aerial use by the Forest Service.”
Since the 1950s, firefighters have been able to get to hard-to-reach places with the help of air tankers and chemical fire retardants. According to the U.S. Forest Service, retardants “slow the rate of spread by cooling and coating fuels, depriving the fire of oxygen, and slowing the rate of fuel combustion as the retardant’s inorganic salts change how fuels burn.”
So, what’s in it?
“It’s basically a mixture of water, fertilizer, and then the red color that you see is just rust,” he says. In the wild, the color fades over time because it is in the sun.
“You might see a gum or thickening agent just to change the viscosity, how sticky this stuff is,” he says. He also says, “But the business end of it really is ammonium phosphate fertilizer.” Thickeners keep the material from moving off-target.
Anyone who has heard of fertilizer being used in bombs knows what ammonium nitrate is. There are two main types of salt in Phos-Chek: diammonium phosphate ([NH4]2HPO4) and ammonium polyphosphate ((NH4PO3)n).
Ammonium phosphate is often found in crystal-making kits for kids. It is also used as a fertilizer to give plants enough nitrogen and phosphorous.
So, how does it stop fires from spreading?
In normal situations, heating plant matter breaks down the cellulose in it, making compounds that can catch fire. The company that makes Phos-Chek says that the reaction between the retardant and cellulose uses up the heat energy from the fire and makes carbon material that is not flammable.
We want to stop or slow the fire’s spread, especially if there are homes or other buildings in the area.
“They don’t tend to drop [retardant] directly in the center of an ongoing fire,” he says. As a way to stop the disease from spreading, they tend to drop right at the edge of it. And the reason these products might be useful for that is because the active ingredient is not water. Yes? The fertilizer is it. That red line will stay there for days or weeks after the water evaporates.
Big air tankers can drop as much as 9,400 gallons of Phos-Chek on trees and other things that could be used as fuel. Because there are so many of them, McCurry and other researchers are thinking about how they might hurt people. McCurry says that people in a fire zone probably have more important things to worry about.
“In terms of immediate concern, the main thing I’d be worried about, regardless of whether there’s fire retardant used or not, is just air pollution — like particulate matter,” he adds. “It’s likely still very smokey if you’re walking around in an area that has been burned. That’s why you should wear a respirator.
Is it safe?
The Forest Service doesn’t allow aerial retardant drops in certain areas because they are bad for the environment. These areas include waterways and habitats for endangered species (map). It is against the law “except when human life or public safety are threatened.” People who use retardants in restricted areas need to be reported so that the problem can be fixed.
According to the company that made Phos-Chek, it doesn’t have any of the substances that are on California’s Proposition 65 list of materials that are known to cause cancer and other harms.
The Forest Service’s rules for wildfire retardants don’t allow PFAS, which are also called “forever chemicals,” or any other compounds that aren’t allowed.
McCurry and his colleagues published a study last year that said some fire retardants, like Phos-Chek, may contain heavy metals like cadmium and chromium that are harmful to humans. Big metals aren’t added to fire retardants, according to the Forest Service. “But may be present as naturally occurring impurities in the retarding salts (which come from the same source as crop fertilizers),” Boise State Public Radio reported in November.
When the air tankers fly over burning hillsides, it looks like the pilots are the ones who are most at risk right now.
“It’s a really heroic job,” McCurry says of the pilots who fly big planes over difficult terrain at low altitudes. Many of them are former military. “You know, some of those guys die every year.”
A 2015 report from the Centers for Disease Control and Prevention says that 78 people died in plane crashes while trying to put out wildfires between 2000 and 2013.