The vivid red and pink clouds falling from aircraft near Beacon Hill this week were not paint or foam — they were more than 50,000 gallons of a fertilizer-based compound that firefighters have relied on for more than six decades to slow the spread of wildfires. The Upriver fire prompted the aerial retardant drops, making it the only fire among several burning in the region this week to require that type of air support.
What’s in the Mixture
The product used is called Phos-Chek, the primary retardant formulation in use by fire agencies across the country. Its active ingredients are ammonium phosphate salts, sulfate, and iron-rich clay. That clay is responsible for the product’s distinctive red color — a feature that serves a practical purpose. Ryan Prosser, a chemical toxicology researcher at the University of Guelph, explained that “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.”
Water is mixed into the fertilizer compound to create a foam suitable for aerial application. Aircraft can release the retardant from as high as 250 feet above ground. The strategy centers on getting the product onto vegetation and structures before the fire arrives. “You want to coat the material ahead of the fire to prevent it from igniting,” Prosser said. Crews typically drop retardant outside the fire’s perimeter to create a barrier rather than directly on active flames.
The chemistry behind its effectiveness is straightforward. When heated, ammonium phosphate breaks down into phosphoric acid and ammonia gas, both of which act to cool the fire. The decomposition also leaves behind a black, nonflammable carbon coating on wood, further slowing combustion.
Phos-Chek traces its origins to early 1960s research by Monsanto and is now manufactured by Perimeter Solutions.
Safety and Environmental Concerns
For residents who may find the substance on their homes, vehicles, or skin, officials say there is little cause for alarm. Phos-Chek is biodegradable, will not permanently stain surfaces, and can be washed off with a garden hose. Rain will naturally clear it from outdoor surfaces over time. Health impacts to humans are considered minimal — eye irritation is possible from direct spray exposure, and consuming large quantities could cause digestive symptoms including nausea, abdominal pain, and diarrhea, though such ingestion scenarios are unlikely under normal conditions.
Environmental concerns are more significant when the retardant reaches water. As a fertilizer compound, Phos-Chek can trigger elevated bacterial and algae growth in streams and lakes, disrupting aquatic ecosystems. Those concerns are particularly relevant in North Idaho, where rivers and lakes support fish populations already under pressure from drought and rising summer temperatures. State wildlife officials have warned this summer that Idaho fish populations face elevated risk from heat and low water conditions.
In response to documented ecological impacts, the U.S. Forest Service in 2024 banned aerial retardant application near bodies of water, with an exception when human life or public safety is directly threatened. That policy follows a 2008 federal court ruling that directed the agency to avoid retardant drops in ecologically sensitive areas.
The Upriver fire’s use of Phos-Chek near Beacon Hill comes as North Idaho contends with an active early summer fire season. A separate blaze, the Gold Run Fire, reached 75 percent containment this weekend after strong winds posed a renewed threat over the weekend, illustrating the volatile conditions facing firefighting crews across the region.
What Comes Next
Firefighting crews will continue to assess conditions around the Upriver fire and determine whether additional aerial retardant operations are needed. Residents in affected areas are advised that any Phos-Chek deposits on structures or vehicles pose no lasting damage and can be removed with water. Ongoing fire conditions across Bonner County and the broader North Idaho Panhandle will depend heavily on summer precipitation and wind patterns in the weeks ahead.