Above the roaring fires devastating parts of Los Angeles last month was a surprising sight. Air tankers dropped gallons of bright red and Barbie-pink slurry over forests, homes, cars and anything else that might lie in the blazes’ path.
The substance, vivid against the grey smoke and charred landscape, is a fire retardant, a substance that is used to slow down or stop the spread of fire. Much of it is a product called Phos-Chek that has been used by the US Forest Service since the 1960s.
“You can see it so easily … it’s amazing stuff,” said Jason Colquhoun, a pilot with HeliQwest, a charter helicopter company specialising in putting out fires.
But in the last month, it’s been dropped on residential areas at an “unprecedented” scale, said Daniel McCurry, an associate professor of civil and environmental engineering at the University of Southern California.
Los Angeles wildfires force thousands to evacuate as toxic smoke spreads
How safe is it?
Colquhoun said that Phos-Chek comes as a powder and is mixed in giant paddling pools before being loaded onto aeroplanes and helicopters for coordinated drops.
Perimeter Solutions, the company that sells the substance, said its colour can look different depending on the time of day, lighting or smoke. This helps pilots know where to make the next drop.
McCurry said he had seen photos “where a brush fire burned right up to a Phos-Chek line and then stopped”, but expressed some caution. He cited a former firefighter telling him that in a high-intensity fire, it’s “not much use” and said that the high winds that have fanned the fires in Los Angeles may have limited its effectiveness.
Meeting environmental norms
The Forest Service said it only uses retardants that “meet the Environmental Protection Agency’s criteria for being ‘practically non-toxic’ to mammals, including humans, and aquatic species”.
It prohibits drops in waterways and areas home to vulnerable species – unless the fire is threatening human life or public safety, and the retardant could be “reasonably expected” to stave off that threat, spokesman Wade Muehlhof said. But accidents do happen, he added, “whether through wind drift or an inadvertent drop”.
The service said it phased out Phos-Chek’s older formulation, LC95 – which McCurry’s study showed had high levels of heavy metals that can contaminate drinking water – nationwide as of December 31.
It now uses a new, less toxic formulation called MVP-Fx. Perimeter added that Phos-Chek contains no forever chemicals or substances “known to cause cancer or other harms” under California law.
McCurry said the Forest Service has been successfully sued in the past on environmental grounds and that Phos-Chek is probably harmful to the environment.
“On the other hand, the human health impact is still a little unclear,” he said. He added that it would take “a lot” of retardant to poison a reservoir, for example.
“However, we’ve seen it dropped on neighbourhoods at an unprecedented scale [because of the Los Angeles fires],” he continued, adding that it’s more often used further from populated areas or in lower amounts.
What gives Phos-Chek its distinct colour?
Phos-Chek is a bright red colour because it contains iron oxide. Iron oxide makes the red colour permanent, lasting for many years. Sometimes, a temporary red colour is used. This comes from a special dye that changes colour when exposed to light.
Phos-Chek consists mainly of ammonium phosphate (see graphic). Other ingredients include thickeners made from gum. This makes the retardant thicker so it falls more accurately and sticks to the plants better. It ensures the Phos-Chek solution is delivered effectively and remains in place to suppress wildfires.
Calcium phosphate, which is commonly found in common table salt, is also used as a flow conditioner. The flow conditioner acts as an anti-caking agent. It prevents the fine particles of the dry powder from clumping together. This makes mixing the powder with water and making the retardant solution simple.