Many households have invested in a CO2 extinguisher. A “clean agent” extinguisher, it does not leave behind a residue of extinguishing material. Not all fires respond to CO2, so understanding the fire classifications is paramount. Fires break into four major categories, including Class A for ordinary wood/paper/plastic; Class B for flammable or combustible liquids, Class C for electrical equipment or wiring, and Class D for combustible metals.