Borate salts were used in the past to fight wildfires but were found to sterilize the soil, were toxic to animals, and are now prohibited.[16] Newer retardants use ammonium sulfate or ammonium polyphosphate with attapulgite clay thickener or diammonium phosphate with a guar gum derivative thickener. These are not only less toxic but act as fertilizers to help the regrowth of plants after the fire. Fire retardants often contain wetting agents, preservatives and rust inhibitors and are colored red with ferric oxide or fugitive color to mark where they have been dropped. Brand names of fire retardants for aerial application include Fire-Trol and Phos-Chek.

Some water-dropping aircraft carry tanks of a guar gum derivative to thicken the water and reduce runoff.