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Aqueous film-forming foam extinguishers, or AFFF can be used on class A and class B fires. These contain a mixture of AFFF concentrate and water, and have a special nozzle that aerates the foam solution as it is expelled. The tank is pressurized with compressed air or nitrogen. When using this type of extinguisher, the foam should be rained down onto the burning material to create a blanket of foam to smother the fire and suppress the vapors.