PHOS-CHEK is manufactured by Perimeter Solutions, a Missouri-based company with deep roots in industrial chemicals that recently opened a manufacturing facility in Sacramento. The name refers to its key ingredient: ammonium phosphate, a compound also used in fertilizer. It works by coating vegetation in a film that helps prevent combustion. Its trademark red color, which comes from iron oxide—essentially rust—helps pilots see where it has already been dropped.
Despite its widespread use, PHOS-CHEK’s full list of ingredients remains secret. According to the material safety data sheet from the U.S. Forest Service, approximately 15% of the concentrated MVP-Fx formulation—the main version used by Cal Fire—consists of “performance additives” listed as trade secrets.
That lack of transparency has drawn concern from researchers like Daniel McCurry, an associate professor of environmental engineering at USC. McCurry helped lead a study that analyzed unused PHOS-CHEK samples, as well as samples from the Post and Highland fires. The results, published in 2024, found toxic heavy metals including cadmium, chromium and vanadium in concentrations 4–2,880 times greater than drinking water regulatory limits.
Those metals are known to accumulate in the environment and potentially harm both humans and wildlife. None of them are listed in public safety documentation for the product. When the findings were shared with Perimeter, the company said the trace metals were naturally occurring.
Perimeter Solutions maintains that its products are safe and meet rigorous federal guidelines. Jeff Emery, Perimeter Solutions President of Fire Safety, emphasized PHOS-CHEK’s role in mitigating wildfires, which he called one of the most impactful environmental threats year after year.
“(Wildfires) contribute to particulate pollution… and ash deposits that end up in waterways can deplete oxygen levels to the point that fish can’t survive,” Emery said. “The wildfire problem is very real in the environmental world, and retardant plays a critical role in minimizing the impact.”
He defended the product’s safety, calling it “the most environmentally friendly retardant that has ever been used,” and dismissed concerns about heavy metals, stating that the levels detected were in the parts per billion—far below EPA thresholds. He also disputed the USC study, claiming it tested an outdated formulation not used in California.
Environmental advocates argue federal safety standards lack independent verification and public transparency. Andy Stahl, executive director of Forest Service Employees for Environmental Ethics (FSEEE), has filed multiple lawsuits challenging the U.S. Forest Service’s use of fire retardant.
“The most popular fire retardants used by the forest service have very high levels of toxic heavy metals like cadmium,” Stahl said, citing USC’s findings. “If they were found at an industrial site, it would make it a Superfund site.”
FSEEE’s latest lawsuit, filed in May 2025, accuses the Forest Service of violating the Endangered Species Act by using PHOS-CHEK in sensitive habitats without required ecological assessments. The suit also names the U.S. Fish and Wildlife Service and National Marine Fisheries Service for failing to evaluate risks. Neither agency responded to requests for comment.
The U.S. Department of Agriculture said the agency does not comment on pending litigation. The U.S. Forest Service did not respond to a request for comment. Cal Fire did not provide a comment after multiple requests.
For wildfire survivor Teri Rubiolo, the debate is personal. She lost her home in the 2018 Camp Fire and now runs a donation center in Concow, near Paradise.
“I know they didn’t drop retardant on my creek, but they did up above on waterways when they could get in there,” she said, indicating that all the fish and turtles in the creek running through her property died during a fire in 2008. She said the same thing happened during the Camp Fire in 2018 even though retardant was not dropped nearby. She said the impact to her property was the same. “There’s not a lot of difference between the damage to the wildlife from the fire or from retardant. It just takes time for things to recover.”
Rubiolo said she supports fire retardant use in principle and wants even more investment in aerial firefighting capabilities. “If they could get it on faster, we’d lose less forest. That helps the animals and the people,” she said.
A 2014 NOAA study found two common PHOS-CHEK formulations were acutely toxic to Chinook salmon at concentrations lower than typical field use. There are other potential impacts to the environment when fire retardant is dropped. The research suggests PHOS-CHEK’s fertilizer-like properties can encourage invasive species and algal blooms when washed into waterways.
Despite concerns, federal policy is leaning toward expanded use. In June 2025, President Donald Trump signed an executive order encouraging retardant deployment and reducing regulatory barriers. Pending bills in Congress (H.R. 3300 and S.1672) would exempt retardant drops from Clean Water Act permits if listed on the Forest Service’s Qualified Products List.
PHOS-CHEK remains the only fully approved aerial fire retardant on the U.S. Forest Service’s Qualified Products List. A magnesium chloride-based competitor developed by Fortress North America was conditionally qualified in 2022 but removed from service in 2024 after inspections revealed aircraft corrosion.
Perimeter’s dominance is clear: in 2023, Cal Fire dropped 3.8 million gallons of retardant—compared to 8.2 million gallons of water. In 2021, a record fire year, nearly 18 million gallons were used statewide according to Cal Fire data.
Dr. Hugh Safford, a fire and vegetation ecologist at UC Davis, said while retardant is a valuable tool for battling wildfires, it is not always used appropriately.
“There's often very strong sociopolitical pressure on agencies to demonstrate somehow that they're doing something,” he said. “A large plane dropping huge amounts of red foamy material onto the ground, that's your standard newsreel that everyone gets with the way fires get fought.”
Safford said the retardant is an important tool for fighting wildfires, especially in low to moderate winds. “Retardant is a really important part of the toolbox for fighting fires,” Safford said. But in high wind conditions, “you can't get the planes in there to drop it, or if you do drop it, it's just gonna end up across the whole county.”
He also criticized agencies for withholding ingredient data. “One of the things that disturbs me is learning how reluctant agencies, particularly the Forest Service agencies are in releasing basic background information on what's in the material,” he said.
Still, Perimeter Solutions insists PHOS-CHEK is safe when handled properly. Emery advised residents who find retardant on homes or cars to avoid high-pressure sprays. “Use a garden hose to dilute it; it cleans off easily,” he said, adding that the product is safe for humans and pets alike.
For now, the red streaks across California’s hills remain a fire season staple. But as lawsuits mount and research continues, the fate of fighting wildfires in an environmentally responsible manner hangs in the air.