Backdraft

A backdraft is an explosive event at a fire resulting from rapid re-introduction of oxygen to combustion in an oxygen-starved environment, for example, the breaking of a window or opening of a door to an enclosed space. Backdrafts present a serious threat to firefighters,^[1] even those with a high level of experience. There is some controversy surrounding the question of whether backdrafts should be considered a type of flashover (see below).

How backdrafts occur

A backdraft is a situation which can occur when a fire's product-gases are starved 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 above 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 resulting in an explosive effect 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 incomplete combustion (more particulate matter in the gases darkens them). If the room contains a lot of soot (particulate matter), it indicates 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 cracks around in the room. The window might have cracked because of the heat. The windows of the structure may also have a slight vibration due to the varying pressure differentials. The surrounding environment (e.g. the hallway outside the suspected backdraft room) will 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, these puffs of smoke are sometimes sucked back into the enclosed space from which they emanate, which is where the term backdraft originates.

Backdrafts are very dangerous situations,^[1][2] often surprising firefighters, regardless of their level of experience. The most common tactic used by firefighters in defusing a potential backdraft is to ventilate from the highest point, allowing the heat and smoke to escape without igniting explosively.

Backdrafts and flashovers

Although ISO 13943^[3] 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 properly be considered flashovers.^[4] (A different viewpoint: ^[5]) The most common use of the term flashover is to describe the near-simultaneous ignition of material caused by heat reaching the autoignition temperature of the combustible material and gases in an enclosure; flashovers of this type are not backdrafts as they are triggered by thermal change. Backdrafts, on the other hand, are caused by the introduction of oxygen into an enclosure that may already be hot enough for ignition; thus, backdrafts are triggered by chemical change.

In popular culture

Backdrafts entered popular consciousness through the 1991 film Backdraft, where a serial arsonist was using them as a means of assassinating people.

An interesting variation on the phenomenon occurred after the 1906 San Francisco earthquake and fire. In 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, led to immediate and explosive combustion of the remaining intact contents.

See also

• Flashover
• Backdraft (film)
• Pyrolysis

References

External links

• A backdraft (still image)
• White Smoke Warning
• Daniel's Block Fire-BACKDRAFT