The striking pink and red substance that blanketed vegetation near Beacon Hill during the Upriver fire this week is a phosphate-based fertilizer mixture that has been used in aerial firefighting for more than six decades — and experts say it poses minimal risk to structures it coats.
More than 50,000 gallons of the retardant were dropped in the area as crews worked to slow the fire’s spread. The product, most commonly sold under the brand name Phos-Chek, consists primarily of phosphate salts and clay suspended in water. Planes release it from roughly 250 feet in the air, where it forms a foamy coating on vegetation, forest canopy, or wooden structures ahead of an advancing fire line.
How the Chemistry Works
The retardant is not designed to extinguish an active fire directly. Instead, crews drop it outside the fire’s perimeter to create a chemical barrier on unburned fuel. When heat reaches the treated material, the ammonium phosphate in the mixture begins to decompose, generating an acid and ammonia gas that work together to cool the combustion process. The reaction also leaves behind a black carbon coating that helps suppress further burning.
Ryan Prosser, a chemical toxicology researcher at the University of Guelph, explained one practical advantage of the vivid coloring: “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.”
The red aerial retardant was first developed by Monsanto in the early 1960s and is now manufactured by Perimeter Solutions. The Upriver fire was the only one among several regional blazes this week that required aerial retardant drops, underlining how the tool is reserved for situations where ground crews need additional support holding a fire line.
For homeowners or property owners whose structures were coated during the operation, Ryan Rodruck, a spokesperson for the Washington Department of Natural Resources, offered reassurance: “It should just wash off with a hose.”
Environmental Restrictions on Use
Despite its long track record in firefighting, Phos-Chek has not been without controversy. In 2024, the U.S. Forest Service formally banned aerial application of the retardant near bodies of water, with an exception carved out for situations involving an immediate threat to human life or public safety. The restriction was the result of a regulatory process that traces back to a 2008 lawsuit filed by the Forest Service Employees for Environmental Ethics. A federal judge ruled at that time that the agency was obligated to avoid retardant use in ecologically sensitive areas, particularly around waterways where the phosphate-heavy mixture can disrupt aquatic ecosystems.
The Upriver fire operations appear to have centered on the Beacon Hill area, away from the sensitive riparian corridors that prompted those federal rules.
Aerial retardant has been in heavy use across the inland Northwest this week as hot, dry conditions pushed multiple fires across the region. A separate wildfire along Highway 95 north of Lewiston burned approximately 108 acres before crews managed to halt its spread, while a vehicle fire on Lewiston Hill ignited a 90-acre blaze that threatened homes nearby. Conditions across Nez Perce County and surrounding areas remain primed for rapid fire growth, with dry grass and elevated temperatures continuing to stress the landscape.
What Comes Next
Fire managers will continue monitoring the Upriver fire and other regional blazes as summer heat builds. The phosphate coating left behind by this week’s retardant drops will remain on treated vegetation until rainfall or manual washing removes it. Property owners who find the substance on structures, fencing, or equipment are encouraged to rinse surfaces with a garden hose as conditions allow. Anyone near active fire areas should monitor official updates from the Idaho Department of Lands and local emergency management offices for any changes in evacuation or shelter-in-place guidance.