Separation From Oxygen

• A blanket of foam separates the fuel from oxygen and smothers the fire. If all of the four elements are still present and the foam is removed, the fire will reignite. In basements and mines, a medium- or high-expansion foam is used to fill the space, effectively displacing the oxygen and smothering the fire. In the case of a gasoline or other hydrocarbon fire, a blanket of foam is lobbed on top of the liquid until it's fully covered, smothering the fire. Foam is an important tool for fighting this type of fire because water is heavier than the fuel, meaning it simply sink under the fuel if used. Eventually, the fuel would overflow its container, spreading the fire.
  
  

Cooling the Fuel

• Foam cools the fuel, removing the heat needed for combustion. In this case, the fire would not reignite unless the heat returned. The cooling effect is present in all applications of foam to some degree.
  
  

Insulation from an Ignition Source

• The fuel is insulated from the ignition source. Like separation, if the foam is removed, the fire could reignite. Insulation also prevents unburned surfaces from catching fire in the first place. Gel-like products are available to the public to insulate their homes in the event of a forest fire.
  
  

Suppression of Vapors

• The foam suppresses vapors from the fuel, preventing them from reaching a remote ignition source. For example, if an open tank of gasoline is located some distance away from an open flame, a foam layer would prevent its vapors from reaching that flame and causing a large fire.