Fire is a rapid chemical reaction called combustion, characterized by the release of energy as heat and light. This exothermic process requires a continuous supply of specific elements to exist. Historically, these requirements were represented by the Fire Triangle: fuel, oxygen, and heat. The more complete scientific model, the Fire Tetrahedron, adds a fourth element—the chemical chain reaction—to explain how fire becomes self-sustaining.
Fuel: The Material that Burns
Fuel is the material consumed by the fire, providing the combustible vapors necessary for the reaction. Fuel can be a solid (like wood), a liquid (like gasoline), or a gas (like methane). Only the gaseous form of the fuel actively participates in the burning process. Solid and liquid fuels must first be heated to release these flammable vapors—a process called pyrolysis for solids or vaporization for liquids. For instance, liquid gasoline does not burn; the vapor rising from its surface ignites when mixed with air. Fuel characteristics, such as moisture content, size, and shape, influence how easily it converts into this gaseous state.
Oxygen: The Oxidizing Agent
Oxygen acts as the oxidizing agent, chemically combining with fuel vapors to release energy. Air, the most common source, contains approximately 21% oxygen. However, fire does not require this full concentration to burn. To ignite and sustain an open flame, the atmosphere must contain a minimum of about 16% oxygen. If the concentration drops below this threshold, the fire will cease, even if fuel and heat remain. Higher oxygen concentrations result in a much more vigorous and intense combustion reaction.
Heat: Reaching and Maintaining Ignition Temperature
Heat is the energy required to initiate the combustion reaction and ensure its continuation. The primary function of heat is to raise the fuel to its specific ignition temperature. This is the minimum temperature at which the fuel spontaneously ignites without an external spark or flame. For example, wood typically ignites between 450 to 500 degrees Fahrenheit (232 to 260 degrees Celsius). The second function of heat is to maintain the reaction by continuously vaporizing new fuel. The heat generated by the fire transfers back to the unburned material, driving the release of fresh combustible vapors and keeping the fire burning.
The Sustaining Element: The Chemical Chain Reaction
The chemical chain reaction is the fourth component necessary for a fire to be self-sustaining. While fuel, oxygen, and heat are needed for ignition, this element explains how combustion continues dynamically. The intense heat breaks down fuel and oxygen molecules into highly reactive, short-lived components called free radicals. These free radicals rapidly react with other molecules, generating more heat and forming new free radicals in a continuous, self-propagating cycle. This exothermic chain reaction feeds the fire, allowing it to generate enough heat to vaporize more fuel. Without this uninhibited cycle, the fire would quickly die out even if the other three components were available.
Extinguishing Fire by Removing a Need
Understanding the four elements of the Fire Tetrahedron provides the foundation for all fire suppression techniques. A fire is extinguished by removing any single component, a strategy known as “breaking the tetrahedron.” Removing the fuel source (starvation) is achieved by separating the fire from unburned material, such as creating a firebreak. Smothering removes oxygen by covering the fire with foam or a blanket to reduce the concentration below the 16% minimum. Cooling removes heat, typically by applying water to lower the temperature below the fuel’s ignition point. Finally, chemical agents, like dry chemicals or halons, interfere with the chain reaction by neutralizing the free radicals.