Uncontrolled decompression
Uncontrolled decompression refers to an unexpected drop in the pressure of a sealed system. Where the speed of the decompression occurs faster than air can escape from the lungs, this is known as explosive decompression (ED), and is associated with explosive violence. Where decompression is still rapid, but not faster than the lungs can decompress, this is known as rapid decompression. Lastly, slow decompression or gradual decompression occurs so slowly that humans may not detect it before hypoxia sets in.
Generally uncontrolled decompression results from human error, material fatigue, engineering failure or impact, that causes a pressure vessel either not to pressurize, or to vent into lower-pressure surroundings.
Description
The term uncontrolled decompression here refers to the unplanned depressurisation of vessels that are occupied by people, for example an aircraft cabin at high altitude, a spacecraft or a hyperbaric chamber. For the catastrophic failure of other pressure vessels used to contain gas, liquids or reactants under pressure, the term explosion is more commonly used, or other specialised terms such as BLEVE may apply to particular situations.
Decompression can occur due to structural failure of the pressure vessel, or failure of the compression system itself.
The Federal Aviation Administration recognises three distinct types of decompression events in aircraft:
 Explosive decompression
Rapid decompression
Gradual decompression
Explosive decompression
Explosive decompression occurs at a rate faster than that at which air can escape from the lungs, typically in less than 0.1 to 0.5 seconds.
Paul Withey, an aviation expert, described an explosive decompression inside an aircraft cabin as similar to the explosion of a 500 pound (225 kilogram) bomb inside the cabin.["Comet Air Crash" ("Crash of the Comet"). Seconds From Disaster.]
Rapid decompression
Rapid decompression typically takes more than 0.1 to 0.5 seconds, allowing the lungs to decompress faster than the cabin.
Slow decompression
Slow, or gradual, decompression occurs slowly enough to go unnoticed and might only be detected by instruments.[ This type of decompression may also come about from a failure to pressurize as an aircraft climbs to altitude. This happened on a Ryanair, Boeing 737 flight in 2001 where the pressurization system was not activated by flight crew during pre-flight checks.]
Pressure vessel seals and testing
Seals in high-pressure vessels are also susceptible to explosive decompression; the O-rings or rubber gaskets used to seal pressurised pipelines tend to become saturated with high-pressure gases. If the pressure inside the vessel is suddenly released, then the gases within the rubber gasket may expand violently, causing blistering or explosion of the material. For this reason, it is common for military and industrial equipment to be subjected to an explosive decompression test before it is certified as safe for use.
Fallacies
Misunderstandings of the meaning of the words are quite likely to be a fuelling factor for a persistent myth that humans would explode if exposed to the non-pressure of outer space. Scenes in fictional movies such as Licence to Kill, where one of the character's head explodes after the hyperbaric chamber he was in was rapidly depressurized, have helped to fuel the myth. However accidents in space exploration research and high-altitude aviation have shown that while vacuum exposure causes swelling, human skin is tough enough to withstand a drop of one atm. This assumes that the person doesn't attempt to hold their breath (which is likely to cause acute lung trauma), the limiting factor on consciousness then being hypoxia after a few seconds.
Some believe that if a bullet is shot through the hull of an airplane, it will explosively decompress outwards, sucking chairs, baggage and people out of the hole. Using a high-pressure airplane and several scale tests, the television program, Mythbusters, demonstrated that fuselage design does not allow this to happen.[Rigorous study is necessary to fully validate their initial results.]
Decompression injuries
The following physical injuries may be associated with decompression incidents:
Hypoxia is the most serious risk associated with decompression, especially as it may go undetected or incapacitate the aircrew.
Barotrauma - an inability to equalize pressure in internal air spaces such as the middle ear or gastrointestinal tract, or more serious injury such as a burst lung.
Decompression sickness.
Physical trauma caused by the violence of explosive decompression, which can turn people and loose objects into projectiles.
Frostbite or hypothermia from exposure to freezing cold air at high altitude.
Notable decompression accidents and incidents
Decompression incidents are not uncommon on military and civilian aircraft (approximately 40-50 rapid decompression events worldwide annuallly[ Consequently where passengers and the aircraft do not suffer any ill-effects, the incidents tend not to be notable.][ Injuries resulting from decompression incidents are rare]
Implications for aircraft design
Modern aircraft are specifically designed with longitudinal and circumferential re-enforcing ribs in order to prevent localised damage from tearing the whole fuselage open during a decompression incident.
The FAA imposes a regulation[te web|url=http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/FED94F31539484AB852566720051AA5D?OpenDocument|title=Section 25.841: Airworthiness Standards: Transport Category Airplanes|publisher=]
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