A flashover is the near-simultaneous ignition of most of the directly exposed combustible material in an enclosed area. When certain organic materials are heated, they undergo thermal decomposition and release flammable gases. Flashover occurs when the majority of the exposed surfaces in a space are heated to their autoignition temperature and emit flammable gases (see also flash point). Flashover normally occurs at 500 °C (932 °F) or 590 °C (1,100 °F) for ordinary combustibles and an incident heat flux at floor level of 20 kilowatts per square metre (2.5 hp/sq ft).
An example of flashover is the ignition of a piece of furniture in a domestic room. The fire involving the initial piece of furniture can produce a layer of hot smoke, which spreads across the ceiling in the room. The hot buoyant smoke layer grows in depth, as it is bounded by the walls of the room. The radiated heat from this layer heats the surfaces of the directly exposed combustible materials in the room, causing them to give off flammable gases, via pyrolysis. When the temperatures of the evolved gases become high enough, these gases will ignite throughout their extent.
The original Swedish terminology related to the term 'flashover' has been altered in its translation to conform with current European and North American accepted [scientific] definitions as follows:
Flashover is one of the most-feared phenomena among firefighters. Firefighters are taught to recognize the signs of imminent rollovers and flashovers and to avoid backdrafts. For example, they have certain routines for opening closed doors to buildings and compartments on fire, known as door entry procedures, ensuring fire crew safety where possible.
The following are some of the signs that firefighters are looking for when they attempt to determine whether a flashover is likely to occur.
Firefighters memorize a chant to help remember these during training: "Thick dark smoke, high heat, rollover, free burning."
The colour of the smoke is often considered as well, but there is no connection between the colour of the smoke and the risk of flashovers. Traditionally, black, dense smoke was considered particularly dangerous, but history shows this to be an unreliable indicator. For example, there was a fire in a rubber mattress factory in London in 1975 which produced white smoke. The white smoke was not considered dangerous, so firefighters decided to ventilate, which caused a smoke explosion and killed two firefighters. The white smoke from the pyrolysis of the rubber turned out to be extremely flammable.