AFFF foam works by which mechanism?

• A. By removing air and cooling the fuel
• B. By insulating heat
• C. By thickening the fuel
• D. By dissolving the fuel

Answer: (A)

AFFF foam works by creating a foam blanket that covers the fuel surface, displacing air and providing cooling. The foam’s water content absorbs heat from the hydrocarbon burn, while the thin film formed by the foam reduces heat transfer and vapor emission. This combination both cools the fuel and keeps oxygen-rich air away from the surface, starving the flame of vapors that could reignite. That dual action—removing air from the immediate fuel surface and cooling the fuel—is the primary mechanism at work.

Other ideas aren’t the main way AFFF extinguishes fires: insulating heat is more about slowing heat transfer in some methods but isn’t the central action here; thickening the fuel would hinder flow and isn’t how foam extinguishes; dissolving the fuel isn’t something foam does.