E2science

Flashover is the near-simultaneous autoignition of all combustible materials in an enclosed space. This is what a textbook, and Wikipedia, will tell you.

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 survivable event. This is what a fire instructor will tell you.

The phenomenon of flashover owes itself to the same processes that give human beings body heat. The compounds released by the flames in a flashing room are much the same as those released by breathing. Humans and fire share several lines in the poetry of science. This is what grace will tell you.

Solids and liquids do not burn. Only gases burn.

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, you will see what appears to be smoke escaping from the spout at the top. If you hold a match to the escaping gases, they will ignite.

When sufficiently heated, hydrocarbons will begin to vaporize. With most hydrocarbons (read: paper, wood, plastic) this temperature requirement is relatively high. With hydrocarbons considered 'flammable' (read: gasoline, kerosene, propane), the temperature requirement is relatively low. Vaporized hydrocarbons are replete with free radicals, tragic little souls with empty outer valences.

Hit up Google and pull an image of the Periodic Table. Assuming you have zero knowledge of chemistry, you will notice that the elements run left to right. Among many things, an element's position on the table denotes how many electrons occupy its outermost shell, described alternatively by the term "valence." (I know: quantum this, probability that. I know.) Electrons like to travel in even numbers. Electrons like symmetry.

Take sodium, on the left. It's on the left because its outer valence denotes only one electron. This valence is desperate 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 the moisture of the open air.

This is the nature of the free radical. Now imagine a room filled with them. Talk to a firefighter who's been on the job a decade or two, and you will likely find that he has kept a melted helmet as a trophy.

Oxidation 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 back through the lungs as carbon dioxide, heat, and water.

In fire, Flick's law draws oxygen into the reaction zone. The heat catylizes 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, resulting from an imperfect oxygen/fuel ratio. Smoke consists of incompletely-burned substances expelled from the fire by the convection column: tars, hydrogen sulfides, carbon monoxides, hydrogen cyanides.

Smoke burns. I toured a facility in Beaumont, California 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 can be defined as the ignition of matter without direct flame involvement. Spontaneous combustion would be a good analogy, but with an actual, verifiable cause.

The flashpoint of any given material is the temperature at which said material will autoignite.

Many of the things we interact with are hydrocarbons. More obviously, gasoline. Less obviously, carpet, wood, plastic, polypropylene. Many of these everyday substances have similar flashpoints.

All things absorb heat before re-radiating heat, vaporizing and igniting. In hydrocarbons, this is largely dependent on the porousness and surface area of the material relative to its mass.

Try to light a solid oak log with a match. It will not happen. The log will absorb the heat without a second (first) thought and you will burn your fingers.

Now, imagine that same log as a pile of wood chips. The same quantity of matter with many times more surface area. Blammo.

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. This is flashover. All hydrocarbons reach flashpoint simultaneously and ignite — this is flashover. The smoke itself, thrown off from what was once a weak curtain fire, ignites — this is flashover. Free radicals find each other in the dark and produce enough heat to soften steel. Imagine that you are there, that chemistry dictates your breath is the same substance billowing out the doorways, the windows. 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 also aware that the term "rich flashover" is interchangable with "backdraft." Firefighters tend to differentiate flashover and backdraft rather stubbornly, as backdraft is caused by the sudden introduction of oxygen rather than simultaneous autoignition of everything in an enclosed space.

As a biologist, I feel compelled to point out that two of the organs offered as vestigial are not actually recognized as such today.

Sacrum

The sacrum serves at least two vital functions. First, the sacral nerves need to exit the spine below the juncture of spine and pelvis to innervate the buttocks, legs, and other *ahem* important *ahem* organs. Without the sacrum descending below the rest of the spine as protection, these nerves would be much more prone to damage and paraplegia would be correspondingly more common.

Further, the structure of the perineum necessitates an attachment point for muscles which might be regarded as "important" (defecation would be either impossible or "automatic" depending on which muscles were detached) and *ahem* important *ahem*. The coccyx, which articulates with the distal end of the sacrum, provides these anchorage points.

Appendix

The appendix serves at least two known important purposes. First, it is one of the densest patches of immune tissue in the human gut. Gut-associated lymphoid tissue (GALT) is one of the first lines of defense in the body, as the gut is the system most exposed to the outside world (What? You thought it was the skin? Nope. Skin cells are protected from direct contact by a thick armour plating of dead, keratinized cells.) and thus most often exposed to pathogens. GALT both produces and stores the immune cells which respond to potential pathogens.

The pocket-like "dead end" structure of the appendix allows it to capture small amounts of matter as digestion proceeds and "examine" it for longer than most other GALTs. As generation of "matching" immune cells is a probabilistic process (immune cells are generated at random and ones that find a "matching" pathogen proliferate), this "lag time" assists in the detection of and defense against pathogens.

As well, the fact that the contents of the appendix are only slowly "turned over" and replaced makes it a haven for essential gut flora ("good germs") in case of catastrophic (from the gut's perspective) events such as diarrhea which flush most of the flora from the gut.

For these reasons, people with intact appendices both suffer a lower incidence of gastrointestinal infection and recover from such infections much more quickly.

While no studies that I've been able to find — granted, in only about fifteen minutes of searching the literature — seem to deal with the consequences of a missing palmaris longus muscle, it may contribute (like other weak muscles which work alongside stronger muscles) to stability rather than to strength. I'll leave that for those who could actually research it, though.

If you still think the sacrum is vestigial, though, I'll remove yours for you.

The vocal cords or vocal folds are necessary for speech, and we generally think of this as their prime function. This might even be their prime function, as they have many more fancy design features to maximize their the speech production than for any other function. But other functions they do have, some of them quite important.

The vocal cords can clamp shut very tightly, and do so automatically if anything coming down the windpipe touches them. This, together with the choking and coughing reflexes, have saved you from some very nasty lung infections, and has probably saved your life. Every time you 'swallowed something the wrong way', whether it was a sip of water or a splinter of bone, a death was narrowly averted.

Somewhat less dramatically, the vocal cords clamping down will also trap air in the lungs. This allows us to hold your breath, and it also provides a structural function. Filling your lungs with air and tightly shutting your vocal cords will 'brace' your thorax, providing a stiff framework for your muscles to work off of. This is called thoracic fixation, and we use it unconsciously, but constantly.

Perhaps the most obvious example of thoracic fixation is seen when you are lifting a heavy weight, and you grunt. This grunt pops out as your body braces itself by clamping down the vocal chords. You may also find yourself gasping or giving a deep sigh when dropping a heavy weight, and your body relaxes after a hard fixation.

Another good example of thoracic fixation is giving birth. Bracing the thorax gives the abdominals something to push from, and will make the baby come out faster. This is why you might hear the expectant mother being told to "breathe!" No, she isn't going to suffocate without the doctor's instructions. Concentrating on taking deep breaths will help the mother keep from pushing with the contractions until it's time for the baby to come out.

And now you know why the rest of the animal kingdom has vocal cords.

I thought it might be interesting (well, to some) to point out that you might also use thoracic fixation during a particularly difficult bowel movement, but I couldn't quite work it in to the paragraph on giving birth. Anyway, now you know.

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