Flashover is the near-simultaneous
autoignition of all combustible materials in an enclosed space.
A firefighter in full PPE has between two and five seconds to exit a space undergoing
flashover before death. This is why, when exploring a burning structure, one
never enters an enclosed space more than can be recovered in two to five
seconds. Temperatures in a flashing room approach 1000°F; flashover is not a
time. Fill a test tube
with matches; top
the tube with a rubber stopper containing a spout. Hold a torch
to the bottom
of the tube. Eventually vapor will escape
the spout, and if you hold a match to the escaping gases, they
Only gases burn.
When sufficiently heated, hydrocarbons vaporize. With most hydrocarbons (paper, wood, plastic)
this temperature requirement is relatively high, while with others (gasoline, kerosene, propane) it is relatively low. Vaporized
hydrocarbons are replete with free radicals,
tragic things with empty outer valences.
If you please, Google Image
the Periodic Table. Note that the elements run
left to right. Among many things, an element's position on the table indicates
how many electrons occupy its outermost shell, described alternatively by the
term "valence." (I know, quantums and probabils too).
Electrons like to travel in even numbers.
on the left. It's on the left because its outer valence holds one electron. This valence is primed to bond with other compounds, thereby evening itself
out. When this reaction takes place electrons shift and energy is released as
heat. Sodium is so reactive that it hisses when exposed to moisture in air.
A fire chief I took a class under said most firefighters who've been at it a decade or two keep at least one melted helmet on display, as a trophy.
is, intuitively, the process by which substances bond with oxygen.
Mitochondria oxidize energy-rich substances. Oxygen
introduced into the bloodstream by the lungs combines with glucose
retrieved from food by the digestive tract. The chemical reaction produces
energy, and the byproducts are expelled through the lungs as carbon dioxide,
heat, and water.
In fire, Flick's law
draws oxygen into the reaction zone. The heat catalyzes the exchange of
electrons between hydrocarbons and oxygen. Fire is a sustained chain-reaction
of rapid oxidation. In a perfect burn — like that attempted by your car's
engine — the by-products are carbon dioxide, heat, and water.
All things burn imperfectly: this is why there is smoke. An imperfect oxygen/fuel ratio generates incompletely-burned substances, expelled from the fire by the
convection column: tars, hydrogen sulfide, carbon monoxide,
Smoke burns. I toured a facility in Beaumont which disposes of waste by first burning it, then enclosing
the smoke in a chamber which recreates the conditions of flashover. The
resulting explosion of heat is converted to electrical energy which is then
released onto the grid.
Autoignition is the ignition of matter without direct
flame involvement. Spontaneous combustion would be a good analogy, but with an
of any given material is, therefore, the temperature at which said material will
Many of the things we interact with
are hydrocarbons. More obviously, gasoline. Less obviously, carpet, blinds,
toasters, mattresses. Many of these everyday substances have similar
All things absorb heat before re-radiating heat, vaporizing, and igniting. In
hydrocarbons, the temperature at which this happens is largely dependent on the porousness and surface area of
the material relative to its mass. So it is that you cannot light a solid log with a match.
But, the log as wood chips--the same quantity of matter with many times more surface area--is, as they say, kindling.
When a fire burns in an enclosed
space--say, a bedroom--a certain amount of the heat is absorbed into the
walls. Eventually, the walls radiate the heat back into the enclosed space. The smoke itself, thrown off from what was
curtain fire, ignites--this is flashover. Free radicals find each other en masse and produce enough heat to soften steel.
Firefighters call the licks
of flame which signal the beginning of flashover "angel fingers."
Excellent video footage of flashover
can be found here.
As well as the visual stimuli, you will be provided with a good timeline/scale
for the stages of an enclosed residential blaze.
Be aware also that the term
"rich flashover" is interchangable with "backdraft."
Firefighters tend to differentiate flashover and backdraft stubbornly,
as backdraft is caused by the sudden introduction of oxygen rather than
simultaneous autoignition of everything in an enclosed space.