What is the primary mechanism by which carbon dioxide extinguishes a fire?

The primary mechanism by which carbon dioxide extinguishes a fire is by displacing oxygen. Fire requires three elements to sustain combustion: heat, fuel, and an oxidizing agent (usually oxygen). When carbon dioxide is introduced into the environment of a fire, it replaces the oxygen that the fire needs to continue burning. Since carbon dioxide makes up about 0.04% of the atmosphere and is non-flammable, its introduction effectively lowers the concentration of available oxygen, thereby suffocating the fire and preventing it from sustaining combustion.

This is especially effective in enclosed spaces where rapid displacement of oxygen can occur. As the oxygen level drops below what is necessary for flaming combustion, the fire cannot maintain itself, and this leads to extinguishment.

The method of displacing oxygen is particularly relevant in fire suppression systems that utilize carbon dioxide, such as those found in computer rooms or industrial applications, where protecting equipment from damage due to water or foam fire suppression methods is crucial. Other answers refer to different concepts but do not directly address the fundamental way carbon dioxide operates as a fire extinguishing agent.