Backdraft

A backdraft is a dramatic event caused by a fire, resulting from rapid re-introduction of oxygen to combustion in an oxygen-depleted environment; for example, the breaking of a window or opening of a door to an enclosed space. Backdrafts present a serious threat to firefighters. There is some controversy concerning whether backdrafts should be considered a type of flashover (see below).

A backdraft can occur when a compartment fire has little or no ventilation, leading to slowing of gas-phase combustion (due to the lack of oxygen); combustible fuel gases (unburnt fuel vapor and gas-phase combustion intermediates such as hydrocarbons and carbon monoxide) and smoke (primarily particulate matter) remain at a temperature hotter than the auto-ignition temperature of the fuel mixture. If oxygen is then re-introduced to the compartment, e.g. by opening a door or window to a closed room, combustion will restart, often rapidly, as the gases are heated by the combustion and expand rapidly because of the rapidly increasing temperature.

Characteristic signs of a backdraft situation include yellow or brown smoke, smoke which exits small holes in puffs (a sort of breathing effect) and is often found around the edges of doors and windows, and windows which appear brown or black when viewed from the exterior. These darker colors are caused by the presence of large amounts of particulate matter suspended in the air inside the room due to incomplete combustion; it is an indication that the room lacks enough oxygen to permit oxidation of the soot particles. Firefighters often look to see if there is soot on the inside of windows and in any cracks in the window (caused e.g. by the heat). The windows may also have a slight vibration due to varying pressure within the compartment due to intermittent combustion.

If firefighters discover a room pulling air into itself, for example through a crack, they generally evacuate immediately, because this is a strong indication that a backdraft is imminent. Due to pressure differences, puffs of smoke are sometimes drawn back into the enclosed space from which they emanated, which is how the term backdraft originated.

Backdrafts are very dangerous, often surprising even experienced firefighters. The most common tactic used by firefighters to defuse a potential backdraft is to ventilate a room from its highest point, allowing the heat and smoke to escape without igniting.

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