What substance is used in firefighting foam to smother fires?

AFFF, or Aqueous Film Forming Foam, is specifically formulated to combat flammable liquid fires by creating a blanket of foam that smothers the flames. This foam forms a film on the surface of the burning liquid, which suppresses the release of flammable vapors and prevents the fire from spreading. It also enhances fire suppression by cooling the surface and sealing it to prevent oxygen from reaching the fuel.

In contrast, although water is a common extinguishing agent, it is less effective for certain types of fires, particularly those involving flammable liquids, as it may not adequately smother the flames and can sometimes spread the fire. ABC powder is a dry chemical extinguishing agent that's effective for various types of fires, but it functions differently than foam and does not smother in the same way. Carbon dioxide is also an extinguishing agent that works by displacing oxygen, but it does not create a barrier like AFFF does. Therefore, AFFF is the most suitable choice for effectively smothering flammable liquid fires.