Editing Backdraft (section)

==Cause==
[[File:USMC-071117-M-1283D-079.jpg|thumb|left|Incompletely-combusted smoke can ignite explosively.<ref name="smokereading"/>]]
A backdraft can occur when a compartment fire has little or no ventilation. Due to this, little or no oxygen can flow into the compartment. Then, because fires reduce oxygen, the oxygen concentration decreases. When the oxygen concentration becomes too low to support combustion, some or all of the combustion switches to pyrolysis. However, the hydrocarbons and smoke (primarily [[particulates|particulate matter]]) remain at a temperature hot enough to [[Autoignition temperature|auto-ignite]]. If oxygen is then re-introduced to the compartment, e.g. by opening a door or window to a closed room, while the gasses are still hot enough to auto-ignite, combustion will restart, often abruptly or even explosively, as the gasses are heated by the combustion and expand rapidly because of the rapidly increasing temperature, combined with the energy released from combustion.

The colour and movement of smoke is used by firefighters to infer fire conditions, including the risk of backdraft.<ref name="smokereading">{{cite web |last1=Brouwer |first1=Ed |title=Trainer's Corner: The science of reading smoke |url=https://www.firefightingincanada.com/trainers-corner-the-science-of-reading-smoke-2139/ |website=www.firefightingincanada.com |publisher=Firefighting in Canada |date=6 June 2008 |access-date=30 July 2019 |archive-date=30 July 2019 |archive-url=https://web.archive.org/web/20190730043543/https://www.firefightingincanada.com/trainers-corner-the-science-of-reading-smoke-2139/ |url-status=live }}</ref> Characteristic warning signs of a backdraft 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 due to soot from incomplete combustion. This 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 sucking 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,<ref name="essentials_of_firefighting">{{cite book|author1=Hall, Richard  |author2=Adams, Barbara |name-list-style=amp |year=1998|title=Essentials of firefighting|edition=4th|publisher=International Fire Service Training Association|isbn=0-87939-149-9}}</ref> often surprising even experienced firefighters. The most common tactic used by firefighters to defuse a potential backdraft is to [[Ventilation (firefighting)|ventilate]] a room from its highest point, allowing the heat and smoke to escape without igniting.

Common signs of imminent backdraft include a sudden inrush of air upon creating an opening into a closed compartment, no visible signs of flame in a hot compartment (fire above its upper [[flammability limit]]), "pulsing" smoke plumes from openings, and auto-ignition of hot gases at openings as they mix with oxygen in the surrounding air.