Fire science
How fire works
The chemistry, the physics and the behaviour — explained in plain language, with every claim sourced.
Information draws on guidance from NFPA, USFA / FEMA and UK MHCLG.
Cool it
Heat
The energy that raises fuel to its ignition temperature. Remove it — by cooling with water — and the fire stops.
In a real fire, get out first and call your local emergency number.
Visualised
Stages of a room fire
A conceptual curve showing how temperature, smoke and danger evolve. Data based on research summarised by NIST Fire Research Division and USFA.
Fire science · Explainer
How a room fire grows
A compartment fire typically moves through four stages. The window for escape and for using an extinguisher is in the first two — once flashover happens, the room is no longer survivable.
- Incipient — ignition; small, often smouldering.
- Growth — flames spread; hot smoke layer builds.
- Fully developed — after flashover; all fuel burning.
- Decay — fuel or oxygen runs out.
Explainers
Core fire science topics
The fire tetrahedron
Heat, fuel, oxygen and the chain reaction — remove one and fire stops.
NFPA reference ↗ Guide coming soonHeat transfer
How conduction, convection and radiation spread fire through a building.
UK Approved Document B ↗ Guide coming soonSmoke behaviour
Why smoke kills before flames — and how it moves through a structure.
CDC — CO poisoning ↗ Guide coming soonFlashover vs backdraft
Two dangerous phenomena that change a fire in seconds.
USFA fire stats ↗ Guide coming soonFire science FAQ
Common questions about how fire works
The fire tetrahedron has four sides: heat, fuel, oxygen and the uninhibited chain reaction. Removing any one element extinguishes or prevents fire. This model replaced the older fire triangle because it accounts for why dry-chemical agents work even when fuel and oxygen are still present.
Source: NFPA — The Fire Tetrahedron
Flashover is the near-simultaneous ignition of all exposed combustible surfaces in a room. It is triggered when the upper gas layer reaches approximately 500–600 °C. A typical room can go from a small fire to full flashover in under five minutes, often in two or three.
Once flashover occurs, survival inside the room is virtually impossible without full protective gear. This is why early detection and fast evacuation are the two most important fire-safety measures.
Conduction is heat travelling through solid material — for example a steel beam heating wall material on the other side. Convection is heat carried by rising hot gases and smoke — the main reason fire can spread rapidly to upper floors. Radiation is heat transferred as infrared waves across open space, which can ignite a building across a street in a wildfire.
Understanding all three helps explain why compartmentation (fire doors and walls) and early ventilation decisions matter. See: UK Approved Document B — Fire Safety
Backdraft happens when a sealed, oxygen-depleted fire suddenly receives fresh air — for example when a door or window is opened. The hot, unburned gases ignite explosively as oxygen rushes in. Flashover, by contrast, is driven by heat build-up, not by a sudden air supply.
Fire services train specifically to recognise backdraft warning signs: black smoke under pressure, staining around door edges, and pulsing smoke. Never open a door to a room showing these signs without calling the fire service first.
Most fire deaths are caused by smoke inhalation, not burns. Smoke is a mixture of hot gases, carbon monoxide, hydrogen cyanide and particulates. Carbon monoxide binds to haemoglobin 200× more readily than oxygen, causing rapid incapacitation. Hydrogen cyanide prevents cells from using oxygen even if CO levels are low.
Additionally, the thermal layer of hot smoke near the ceiling can reach 500–600 °C — hot enough to cause fatal burns to the respiratory tract on inhalation. Staying low and evacuating immediately are the most effective responses.
A room fire typically passes through four stages: incipient (initial ignition, small flame, low heat), growth (flames spread, hot gas layer forms at ceiling, temperature rising rapidly), fully developed (peak heat, flashover may occur at the transition from growth), and decay (fuel consumed, fire dying).
The growth phase is where smoke alarms and sprinklers have the greatest impact — intervening before the transition to fully developed fire dramatically improves survivability.