Fires are divided into 4 classes, each requiring special treatment. The essential point in extinguishing all types is exclusion of air from the fire by an effective means. Class A includes fires in combustible materials, such as wood, paper, and cloth, where the quenching and cooling effect of quantities of water or of solutions containing a high percentage of water is of primary importance. Fire extinguishers utilizing the pressure of carbon dioxide to throw a stream of water onto the fire are the most widely used for this class. In the soda-acid extinguisher, the carbon dioxide is generated within the cylinder at the time of use. In another type, carbon dioxide gas is stored in the cylinder under pressure and is released by opening a valve. Class B includes fires in flammable liquids where a blanketing or smothering effect is essential. Carbon dioxide gas, dry chemical, or foam are suitable. Water should not be used. Class C includes fires in electrical equipment. The use of carbon dioxide gas or dry chemical extinguishers is recommended. Water should not be used. Class D fires are burning metals. A powder formulation such as “Met-L-X” powdered graphite or trimethoxyboroxine will extinguish a metal fire. Water should not be used. In general, for small fires, salt (sodium chloride) and sodium bicarbonate, either dry or in concentrated solution are effective. Carbon tetrachloride and methyl bromide should be avoided as extinguishing agents because of the toxicity of their decomposition products, for example, phosgene.See Foam, fire-extinguishing.
