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 between and for ordinary combustibles and an incident heat flux at floor level of .
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.
In October 2013, the Fire department of Oak Ridge, Tennessee filmed a controlled demonstration of a flashover fire and uploaded it to the video-sharing platform YouTube for educational purposes. The description of the video emphasizes the importance of a "Home Escape Plan" in saving lives.
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, there are 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.
During the 1942 Cocoanut Grove fire in Boston, Massachusetts, which resulted in 492 fatalities, a sudden flashover occurred when superheated flammable gases, trapped in enclosed ceiling spaces, spontaneously ignited and raced through the nightclub.
Several tragic flashovers have occurred in history, including the Luoyang Christmas fire in China in 2000, which claimed 309 lives; the Beverly Hills Supper Club fire in Southgate, Kentucky in 1977, resulting in 165 fatalities; and the Dupont Plaza Hotel arson in San Juan, Puerto Rico, on New Year's Eve in December 1986, which killed 98 people. On ValentineâÂÂs night in 1981, 48 young people lost their lives in a fire at the Stardust Nightclub in Artane, Dublin. A flashover during a New Year's Eve fire at a bar in Crans-Montana, Switzerland, in 2026, resulted in 41 fatalities.
During the 2003 Station Nightclub fire in West Warwick, Rhode Island, which resulted in the loss of 100 lives, flashover conditions occurred approximately 90 seconds after ignition.
According to Brian Meacham, a fire safety engineering professor at Lund University in Sweden, "Almost every year, somewhere in the world, a major nightclub fire leads to significant loss of life due to a combination of factors: the presence of combustible sound insulation, tightly packed spaces, inadequate egress routes, and the lack of sprinkler systems."