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…