Foam stability is measured via two ways: one of them is the drainage resistance (keeping the foam wet with the consequent cooling effect)and the other one is the consistency of its tridimensional structure. This way, the extinguishing is done by displacing the oxygen “asphyxiating” the fire. It is crucial that on medium expansion nozzles (~120) and high expansion monitors (~800) the foam is homogeneous because this would confer better resistance against the heat. Medium expansion can be used indoors and outdoors, while high expansion foam should only be used indoors, as its density is so low that the wind could carry the foam out of the fire. In high expansion foam it is very important that the foam can be used with foul air , as well as being used with fresh and sea water.