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A backdraft is an explosive 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 fire's product-gases are depleted of oxygen; consequently combustion slows (due to the lack of oxygen) but the combustible fuel gases (primarily carbon monoxide) and smoke (primarily hydrocarbon free radicals and particulate matter) remain at a temperature hotter than the ignition-point of the fuel gases. If oxygen is re-introduced to the fire, e.g. by opening a door (or window) to a closed room, combustion will restart, often explosively, 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 combustion 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 the varying pressure differentials. The surrounding environment (e.g. the hallway outside the suspected backdraft room) is likely to be extremely hot.
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 explosively.
Although ISO 13943 defines flashover as "transition to a state of total surface involvement in a fire of combustible materials within an enclosure," a broad definition that embraces several different scenarios, including backdrafts, there is nevertheless considerable disagreement regarding whether or not backdrafts should be properly considered flashovers. (A different opinion:) The most common use of the term flashover is to describe the near-simultaneous ignition of material caused by heat attaining the autoignition temperature of the combustible material and gases in an enclosure; flashovers of this type are not backdrafts as they are caused by thermal change. Backdrafts, however, are caused by the introduction of oxygen into an enclosure that may already be hot enough for ignition; thus, backdrafts are caused by chemical change.
An interesting variation of the phenomenon occurred after the 1906 San Francisco earthquake and fire. During the days after the blaze several business people opened warm safes to see if their papers were safe. The sudden inrush of oxygen into the still-hot gas of the interiors that now contained pyrolyzed fuel gases due to the heated papers (hydrocarbons) therein caused immediate and explosive combustion of the remaining intact contents.