A class A fire is a fire involving ordinary combustible materials. These materials include cloth, wood, paper, rubber and many plastics. Fire extinguishers labeled with the letter A are designed for use on such fires.
Fires also come in classes B, C, D and K. Class B fires involve flammable and combustible liquids, including alcohol, diesel oil, gasoline, oil-based paints and lacquers. Class C fires involve energized electrical equipment. Class D fires involve combustible metals, including titanium, magnesium and sodium. Class K fires involve animal oils, vegetable oils or fats in cooking appliances. The class K classification is for commercial kitchens in restaurants, catering facilities and cafeterias.
Different agents work best to extinguish different fire classes. This is why all fire extinguishers do not contain the same ingredients. Water puts out class A fires most effectively. Agents that deplete oxygen and create a smothering effect work best to extinguish class B fires. Class C fires require a nonconductive agent, such as carbon dioxide, to extinguish them.
Dry-powder extinguishing agents work by smothering and heat absorption, and they are the best choice to put out a Class D fire. Class K fires require an ingredient called Purple K to put them out. Purple K typically appears in kitchen or galley extinguishers.