Dear EarthTalk: How safe are the fire retardants that are used to quell forest fires across the American West?
—Barbara, Minneapolis, MN
So-called long-term fire retardants—those usually dropped from airplanes over forest fires—are comprised of water mixed with a slurry of chemicals, thickeners and corrosion inhibitors designed to prevent plants on the ground from igniting, keep the ingredients from separating and dispersing during targeted drops, and ensure that the harsh chemicals on board the plane don’t endanger the flight’s safety.
Firefighters sometimes add iron oxide to make the fire retardant turn red when applied so they can see where they have already covered. Ammonium phosphate and ammonium sulfate, known for their use as agricultural fertilizers, are also often added to provide nutrients to help the forest regenerate after a burn.
In recent years, where global warming and droughts have exacerbated forest fires across the American West, federal and state firefighting agencies have upped their cumulative annual use of long-term fire retardants to some 20+ million gallons a year spread across tens of thousands of individual fly-overs.
While such chemicals have been valuable in minimizing the damage of forest fires, their use comes with a price. The nitrogen in ammonium phosphate and ammonium sulfate can wreak terrible havoc on aquatic ecosystems, creating algae blooms that kill fish by choking out their oxygen. A 1998 study by the U.S. Geological Survey’s Northern Prairie Wildlife Research Center, found long-term fire retardants to be "very toxic to aquatic organisms including algae, aquatic invertebrates and fish." The study also said that fire-fighting chemicals "could cause substantial fish kills depending on the stream size and flow rate."