The fire triangle is a basic conceptual model used in fire science and safety education to visually represent the three components necessary for a fire to ignite and sustain itself. Fire is a chemical process called combustion, which is a rapid, exothermic oxidation reaction. This process requires the continuous presence of three specific elements; if any one is removed, the combustion reaction will cease. Understanding this relationship provides the foundation for preventing and effectively suppressing fires.
Fuel Sources
Fuel is any combustible material that can undergo combustion. These materials exist in three physical states: solid, liquid, or gas. For combustion to occur, the fuel must be in a gaseous or vaporized state, as the reaction takes place between the fuel vapor and oxygen.
Solid fuels, such as wood or paper, must first undergo pyrolysis, where heat causes them to decompose and release flammable vapors. Liquid fuels, like gasoline or oil, produce flammable vapors from their surface that mix with the air, rather than burning as liquids. Gaseous fuels, such as propane or natural gas, are already in the necessary state and mix readily with air, making them instantly combustible once heat is applied.
Ignition Temperature
Heat provides the energy required to raise the fuel to its ignition temperature. This temperature is the lowest point at which a substance will spontaneously ignite and sustain combustion without an external spark or flame. This energy input starts the chemical reaction and causes the fuel to release enough flammable vapors to burn. Once established, the heat generated by the flame itself raises adjacent fuel to its ignition temperature, allowing the fire to become self-sustaining.
The ignition temperature is distinct from the flash point, which is the lower temperature at which a liquid produces enough vapor to briefly ignite when an external ignition source is introduced. The ignition temperature, often called the autoignition temperature, represents the minimum energy required to initiate sustained combustion without reliance on a separate spark. The initial heat source can come from various forms of energy, including friction, electrical current, or an open flame.
The Oxidizing Agent
The oxidizing agent, typically atmospheric oxygen, is the third component enabling the chemical reaction. Oxygen is highly reactive and combines rapidly with fuel vapors during combustion, releasing heat and light. While normal air contains 21% oxygen, a fire requires a specific concentration to sustain itself.
For most combustible materials, the oxygen concentration must remain at or above 16% for flaming combustion to continue. If the oxygen percentage drops below this limiting concentration, the fire will extinguish or transition to a slow, smoldering state. This dependency on sufficient oxygen highlights the importance of restricting airflow as a method of fire suppression.
Preventing and Extinguishing Fire
Understanding the three components of the fire triangle provides the blueprint for fire prevention and suppression strategies. By controlling or removing one of the three sides—fuel, heat, or oxygen—the combustion reaction is interrupted and the fire is extinguished. Prevention focuses on keeping these three elements separated, such as storing flammable materials away from heat sources.
Firefighting tactics are categorized into three methods, each targeting a specific component. The first method is cooling, which involves removing heat, most commonly by applying water. Water absorbs heat energy from the burning material, lowering the temperature below the ignition point and stopping the production of flammable vapors. The second method, smothering, works by removing the oxygen supply.
Smothering agents, such as fire blankets, carbon dioxide, or foam, displace or block the surrounding air, reducing the oxygen concentration below the 16% threshold required for combustion. The final method is starvation, which focuses on removing the fuel source. This is achieved by physically separating the unburned fuel from the flame, such as clearing a fire break in a wildfire, or by shutting off the flow of gas in a natural gas fire.