Dropping fire retardant solutions from planes is a crucial strategy in wildfire fighting, but some experts are concerned about their impact on the environment, including aquatic ecosystems. Once dropped, these retardants leave a rust-coloured or pinkish liquid coating on roads, roofs, and vegetation. This technique, commonly used since the mid 20th century, helps fire crews contain blazes by depriving flames of oxygen, explains Jen Baron, a postdoctoral researcher at the University of British Columbia’s Centre for Wildfire Coexistence. Since 2009, the B.C. Wildfire Service has relied on Phos-Chek LC-95A, a widely used retardant from Perimeter Solutions… Its main ingredient, ammonium polyphosphate, is a common fertilizer. Additives, like iron oxide, give the retardant its signature red colour, helping crews to see where it has been dropped. …Uldis Silins, a professor of forest hydrology at the University of Alberta says studying retardants like Phos-Chek remains crucial, especially because their specific chemical composition is proprietary and therefore unknown.