Fire is a rapid oxidation process involving the combination of oxygen with a fuel source to produce heat, light, and combustion products. This reaction is sustained by the fire tetrahedron, which requires heat, fuel, an oxidizing agent (typically oxygen), and a chemical chain reaction. Fire development is systematically categorized into four distinct stages for understanding its behavior, predicting its spread, and effectively managing incident response. These phases describe the fire’s progression from its initial spark to its final extinguishment.
Stage One Ignition
The ignition stage is the point where heat is applied to the fuel and sustained combustion begins. Solid fuels do not burn directly. Instead, applied heat causes pyrolysis, breaking the material down into flammable gases and vapors. Ignition occurs when the concentration of these gases reaches its lower flammable limit and the temperature is sufficient, resulting in a visible flame.
This initial phase is characterized by a minimal heat release rate, keeping the fire small and localized to the source material. The fire’s behavior is entirely fuel-controlled, meaning its size and intensity are limited by the physical properties and arrangement of the burning material. Visibility is usually clear in the surrounding area, and the heat emitted is low compared to later stages. If detected quickly, the fire is easiest to suppress using portable equipment.
Stage Two Growth
The fire moves into the growth stage as it consumes more available fuel and oxygen, leading to a rapid increase in size and intensity. Heat is transferred through convection and radiation, causing nearby combustible materials to reach their ignition temperature and ignite. This creates a plume of hot gases and flames that rises toward the ceiling of an enclosed space.
Thermal layering develops, where hot gases collect and bank down from the ceiling while a cooler layer of air remains near the floor. The heat release rate accelerates significantly, and the fire begins to spread horizontally across the ceiling. As the temperature increases and the hot gas layer descends, the fire transitions from purely fuel-controlled to potentially ventilation-controlled if the oxygen supply within the compartment becomes restricted. This stage precedes a sudden and dramatic shift in fire behavior.
Stage Three Fully Developed
The transition to the fully developed stage is often marked by flashover. Flashover is a rapid, near-simultaneous ignition of all exposed combustible surfaces within a compartment. This occurs because radiant heat feedback from the hot gas layer has heated all the contents in the room to their auto-ignition temperature.
Once flashover occurs, the fire reaches its maximum size and intensity, consuming all available fuel and producing the highest heat release rate. Temperatures within the compartment soar, often reaching between 700°C and 1200°C (1,292°F to 2,192°F). The fire is now ventilation-controlled, meaning its intensity is limited only by the amount of oxygen drawn into the space. Flames are widespread, and the environment is hostile, with dense, dark smoke and extreme structural damage occurring.
Stage Four Decay
The decay stage begins when the fire’s intensity and heat release rate start to decrease. This reduction is caused by the exhaustion of available fuel or, in enclosed structure fires, by a depletion of the oxygen supply. The fire transitions back to a lower energy state, with visible flames diminishing and temperatures dropping.
Despite the decrease in visible flames, the fire remains hazardous, continuing as smoldering combustion that produces high heat and toxic gases. A defining risk of the decay stage is backdraft, a smoke explosion. Backdraft occurs when oxygen is suddenly introduced into a hot, fuel-rich, oxygen-depleted environment, causing the accumulated superheated, uncombusted gases to ignite violently and explosively. This potential for a rapid, dangerous change requires careful consideration by responders when opening doors or windows in a decaying fire environment.