Which extinguishing agent is least effective on Class A fires?

Carbon dioxide (CO2) is least effective on Class A fires, which involve ordinary combustible materials like wood, paper, and fabric. Class A fires rely heavily on cooling and soaking the fuel to extinguish the flames.

While CO2 is effective for fires involving flammable liquids (Class B fires) and electrical fires (Class C fires), its use on Class A fires is limited because it primarily works by displacing oxygen, which can be less effective against solid materials that require heat removal to be extinguished. This is particularly important as Class A fires often produce embers and localized pockets of heat, where the presence of CO2 alone is insufficient to cool the materials down. In contrast, foam agents, water, and dry chemicals work well on Class A fires by either cooling the fire (water), preventing re-ignition (foam), or interrupting the chemical reaction of the fire (dry chemicals).