What is the most common agent used to extinguish structure fires?

Water is the most common agent used to extinguish structure fires primarily due to its availability, effectiveness, and versatility. When applied to fires, water cools the burning material and reduces the temperature below the material's ignition point, effectively stopping the combustion process. Additionally, water can absorb a significant amount of heat, making it an efficient tool for firefighting in various types of structural materials.

Using water is also advantageous because it can be delivered easily through hoses and firefighting equipment, and it is cost-effective compared to other extinguishing agents. While other agents, like foam and carbon dioxide, may be used in specific scenarios—such as flammable liquid fires or confined spaces—the widespread and unrestricted access to water makes it the preferred and most reliable choice in structure firefighting.