What is the red fire retardant dropped from the air during the Upriver fire? Fertilizer
More than 50,000 gallons of pink and red fire retardant rained from the sky near Beacon Hill on Tuesday to slow the spread of the Upriver fire.
The mixture of phosphate salts and clay coated trees, homes, cars and even people trying to evacuate from neighborhoods.
Air tankers can deliver thousands of gallons of fire retardant at a time. The spray protects trees and homes from burning in a wildfire but leaves behind its iridescent red mark. That leaves the question for those who return to their lives - what is the substance that saved their home?
The red foam covering much of Beacon Hill is "primarily a fertilizer," said Ryan Prosser, chemical toxicology researcher at the University of Guelph in Ontario.
Most formulations of the retardant consist of a series of ammonium phosphate salts not dissimilar to what one might find in a garden fertilizer. Other materials found in the retardant include sulfate and an iron-rich clay that gives the foam its characteristic red hue.
The retardant is primarily sold as a product called Phos-Chek, where water is added to the fertilizer mixture to create a foam that can be shot from 250 feet in the air. The bright red aerial retardant was first developed by Monsanto in the early 1960s and is currently made by firefighting chemical manufacturer Perimeter Solutions.
The retardant is not applied directly on an ongoing fire. It is typically dropped just outside of the fire's perimeter as a kind of barrier.
"The red color makes it very clear where it has been applied, which makes it easier to know where the coating still needs to be applied," Prosser said.
The retardant works best when applied to plant material - either over a forest canopy or over wooden homes at risk of catching fire. When the ammonium phosphate of the retardant is heated, it decomposes into an acid and an ammonia gas. This combination cools the fire and leaves behind a black, nonflammable carbon coating.
The char created by Phos-Chek may burn the outside surface of wood but prevents a fire from going further into the wood or using the plant material as fuel. When tankers take to the air, they fight fires through a combination of water drops directly on flames and fire-retardant drops in areas where the fire may go in the future.
"You want to coat the material ahead of the fire to prevent it from igniting," Prosser said.
Phos-Chek will not leave stain on homes, cars or people, according to Washington Department of Natural Resources spokesperson Ryan Rodruck.
"It should just wash off with a hose," he said.
The red foam will only leave a mark if left on a surface too long in the hot sun, he added. The mixture is biodegradable and should break down over time and be washed away on its own by rain.
Though several fires started this week across the region, only the Upriver fire required aerial fire retardant. Later in the season, use of retardant may be delayed where limited supplies are needed in multiple fires.
"Here it was easy to get a ton of retardant because other places did not require it. That may not always be the case. The priority for its use will be if a lot of structures are at risk or the interstate," Rodruck said.
Environmental, health risks
Material used in Phos-Chek does not present significant health impact to those who come into contact with it, according to the Department of Natural Resources. Rodruck called the red foam "completely nontoxic" to humans and said to "just not let your dog drink it."
The spray may irritate if it comes into contact with the eyes. Symptoms related to ingestion would be the same for any phosphorous-based fertilizer, including diarrhea, abdominal pain and vomiting, if ingested in large quantities.
"Obviously you don't want to ingest it, but the risk to people is relatively low," Prosser said. "And there's no long-term increased risks of cancer or birth defects or anything like that."
Phos-Chek does present environmental concerns if applied to or near water systems. The fertilizer of the retardant causes an increase in bacterial and algae growth. The excess of organisms disrupts water systems' natural ecosystems and depletes oxygen from the water.
"We don't want to be providing excess nutrients to lakes and rivers because you can get algae blooms, which cause the dissolved oxygen in the water to go down. The end result is the fertilizer will kill many of the fish in the body of water," Prosser said.
In 2024, the U.S. Forest Service banned aerial retardant use near bodies of water "except when human life or public safety are threatened." Any retardant that is used in these restricted areas must be reported and remediated.
Many of these regulations around fire retardant use were prompted by the Forest Service Employees for Environmental Ethics. A federal judge ruled in the organization's 2008 lawsuit that the Forest Service avoid using retardant in ecologically sensitive areas and conduct an environmental impact study of the red spray.
The organization remains skeptical of the fire retardant's ecological safety and transparency over its use, said Kevin Hood, director of Forest Service Employees for Environmental Ethics.
"Even if there is some toxicity in fire retardant, maybe it's worth the trade-off if it is effective. But we would like to see more studies on how effective it actually is. If it's safe, effective and legal, then it should be a tool in the toolbox. We just want the Forest Service to be open and transparent," Hood said.
The environmental activist is specifically concerned fire retardant may inadvertently contain heavy metals. A 2024 study published by the American Chemical Society found that there were 850 kilograms of toxic metals in every 1 million gallons of fire retardant . Vanadium and chromium accounted for 52% and 32% of the heavy metals found in the retardant, respectively. The study did not address if exposure to fire retardant also can cause significant exposure to heavy metals.
According to the Forest Service, heavy metals are not intentionally added to fire retardants but "may be present as naturally occurring impurities in the retarding salts," a Forest Service official told Boise State Public Radio in 2024.
"Some of these metals are carcinogens. Some are neurotoxins. And when you drop fire retardant, it can disperse and so we just want an honest disclosure of what exactly is in this fire retardant," Hood said.
The Forest Service bans PFAS from use in aerial fire retardant. PFAS, which stands for perfluoroalkyl and polyfluoroalkyl substances, is a carcinogenic chemical that does not easily break down in the environment. It was commonly used in airport firefighting foam specifically designed for airplane-related fires, Prosser said.
"In recent years, there has been a real crackdown on the use of material that persists in the environment for a long time, like products with PFAS," Prosser said. "While further research into the retardants may be needed, the risk to those who come into contact with the red foam spray remains low."
"It can suck that your house is covered in this stuff, but your house burning down is definitely going to be a bigger problem," Prosser said. "Having this red material everywhere is going to be a pain in the butt to clean, but I'll take a little bit of cleanup and potential staining if my house is still there."
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