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Dunn's Dispatch: Explosions on the Fireground

Rookie firefighters sometimes confuse the terms backdraft and flashover. These two dangerous violent events are different and knowing these differences helps us understand each one better.

"Do you know the definition of an explosion?" Fire protection engineers define the term explosion as an "effect" produced by a sudden violent expansion of gases. Some "effects" of an explosion are shock waves which shatter windows, blow down firefighters and collapse walls, heat, sound blast and flame.

Rookie firefighters sometimes confuse the terms backdraft and flashover. These two dangerous violent events are different and knowing these differences helps us understand each one better. There are four main differences.

First of all, backdrafts do not happen often at fires. You may experience only one or two during your entire career. Flashovers - sudden full room involvement in flame - happen often. You will probably see one at your next fire. A second difference is that a backdraft is an explosion; a flashover is not. There will be shock waves during a backdraft that will break the confining structure around the explosion. Windows may break blasts of smoke and flame may blow out a doorway or a part of the structure may collapse.

Flashover is rapid-fire development, but it stops short of an explosion's speed of chemical reaction third difference is the triggering or cause of a backdraft. Air sets off the backdraft explosion. As firefighters enter a confined smoke filled area and bring fresh air with them, sometimes a backdraft or smoke explosion happens. The trigger or cause of a flashover is heat, not air. The theory of flashover is that heat, which is reradiated back into a burning room from the ceiling and upper walls, raises the gases and furnishings in the room to the auto-ignition temperature and triggers a flashover.

The final difference between a backdraft and flashover is the stage of fire growth in which they occur. There are three stages to a fire: the growth stage, the fully developed stage and the decay stage. Backdraft explosions occur when there is smoke in a confined space, that is, during the first and third stage of a fire. During the growth and decay stages, smoldering, incomplete combustion can take place and generate explosive CO gas. Flashovers, on the other hand, occur in the growth stage of a fire and signal the end of the growth stage. The term backdraft is used too frequently by the fire service. It is usually used to describe any explosion that occurs at a fire; when actually a backdraft is only one type of explosion and it does not happen that often.

In fact, most explosions are not backdrafts. They are explosions caused by leaking gas piping, meters or cylinders, or they are BLEVES of cylinders or containers or they are explosions caused by flammable vapors leftover from an arsonist's accelerant. Before a fire chief or investigator declares an explosion at a fire to be a backdraft-smoke explosion, a post-fire analysis must rule out all other possibilities. If the gas piping is intact, if there are no ruptured containers found and there are no traces of an accelerant residue, then the explosion is a backdraft.

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Vincent Dunn, a Firehouse Magazine contributing editor, is a 42-year veteran of the FDNY and a deputy chief (ret.), serving as division commander for midtown Manhattan. A nationally renowned lecturer, he is the author of the best-selling text and video series Collapse of Burning Buildings and the textbooks Safety and Survival on the Fireground and Command and Control of Fires and Emergencies. A new book, Strategy of Firefighting - How to Extinguish Fires will be published in April. Dunn has a master's degree in urban studies, a bachelor's degree in sociology and an associate's degree in fire administration from Queens College, City University of New York. He can be reached at 1-800-231-3388 or via through his website at