Backdraft
Definition
A deflagration event triggered when air is suddenly admitted to a compartment where a fire has been oxygen-depleted but has continued to produce flammable pyrolysis gases. The resulting rapid combustion produces an outward pressure wave and rolling flame ball.
- Trigger
- Oxygen suddenly admitted to oxygen-starved compartment with accumulated flammable gases
- Physical signature
- Outward pressure wave and rolling flame ball
- Key gases involved
- Unburned pyrolysis gases and carbon monoxide
Common questions
What happens in a backdraft?+
A backdraft occurs when unburned fuel gases and carbon monoxide accumulate in a room where fire has been consuming oxygen. When oxygen suddenly enters the space (like when a door or window opens), these gases ignite rapidly. This creates an explosive deflagration that produces a pressure wave and rolling flames outward.
Why is backdraft dangerous to first responders?+
When firefighters or emergency personnel open a sealed room with an active fire, they can trigger a backdraft without warning. The sudden inrush of oxygen into an oxygen-depleted space with accumulated pyrolysis gases causes violent, explosive combustion that puts rescuers at serious risk.
How is backdraft different from an explosion caused by a bomb?+
Backdraft is a rapid combustion event (deflagration) caused by the sudden introduction of oxygen to accumulated gases. It can look explosive and create a pressure wave, but it originates from fire dynamics and accumulated unburned fuel, not from a detonating device. Investigators must distinguish these causes during arson investigation.
Related terms
- Flashover
- The near-simultaneous ignition of all exposed combustible surfaces in a compartment, occurring when the upper gas layer reaches approximately 600°C and radiative...
- Fire Tetrahedron
- The model of sustained combustion requiring four concurrent elements: fuel, oxidiser, heat, and an uninhibited radical chain reaction. Replacing the older fire...
- NFPA 921
- US National Fire Protection Association Guide for Fire and Explosion Investigations. The internationally accepted methodology Indian fire investigators and SFSL fire divisions...
- ASTM E1188
- Standard Practice for Collection and Preservation of Information and Physical Items by a Technical Investigator, the companion evidence-preservation standard to NFPA 921...
- Deflagration
- A subsonic combustion front propagating through an explosive by thermal conduction. Characteristic of low explosives such as black powder and smokeless powder;...
- Equivalence Ratio
- The ratio of actual fuel-to-air ratio to the stoichiometric fuel-to-air ratio. An equivalence ratio above 1 (fuel-rich) produces CO, soot, and unburned...
- Equivalence Ratio (Phi)
- The ratio of actual fuel supply rate to the stoichiometric fuel supply rate for the available air. Values below 1 indicate fuel-controlled...
- Fire Point
- Slightly above flash point, the temperature at which sustained combustion begins after ignition.
- Fire Triangle
- The three components needed for combustion: fuel, heat and oxygen. Remove any one and the fire stops.
- Flash Point
- Lowest temperature at which a liquid gives off enough vapour to form an ignitable mixture in air, but the vapour does not...
- Fuel-Controlled Fire
- A compartment fire in which the rate of energy release is limited by the fuel supply (surface area, arrangement, reactivity) rather than...
- Hot Gas Layer
- The stratified layer of buoyant combustion gases that accumulates at ceiling level in a compartment fire. Its temperature rise drives flashover when...
Explained in these topics
- Fire and Arson Investigation: Cause, Origin and Burn PatternsSudden, sometimes explosive ignition of accumulated unburnt pyrolysis gases when oxygen is admitted into a previously oxygen-starved compartment. Risk to first...
- Fire Chemistry, Pyrolysis and the Arson Investigation FrameworkThe explosive ignition of accumulated unburned fuel gases and carbon monoxide in an oxygen-depleted compartment when ventilation is suddenly provided. Can be m...
- Fire Dynamics: Flashover, Backdraft and Ventilation-Controlled BurningA deflagration event triggered when air is suddenly admitted to a compartment where a fire has been oxygen-depleted but has continued to produce flammable pyro...