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by Wing Commander, D.M.S. Karunaratne

High altitude air travel and space travel is associated with risk of human exposure to reduced atmospheric pressure. Space shuttle Colombia disaster is the most recent tragedy. In addition to the effects of speeds and heat, decompression of unprotected crew will produce unconsciousness rapidly.

How would the unprotected human body react to the vacuum of outer space? Would it inflate to bursting?

Unprotected healthy human is safe up to initial 10,000 feet from the ground.

Unpressurized aircraft are flying within this envelope. High altitude flying aircraft have pressurized cabins to provide protection against decompression. In case of cabin decompression aircraft can reach safe altitudes with minimal effects to the passengers.

When a person is suddenly exposed to rapid decompression, event is generally known as "explosive decompression," and apart from the simple effect of vacuum on the body, the explosive decompression event itself will be hazardous. Expanding gas inside the body will produce damaging effects. One of the potential dangers during a rapid decompression is the expansion of gases within body cavities.

Because of the relatively large volume of air normally contained in the lungs and the delicate nature of the lung tissue, it is recognized that the lungs are potentially the most vulnerable part of the body during a rapid decompression.

If the escape of expanding air from the lungs is blocked or seriously impeded during a sudden drop in the cabin pressure, it is possible for a dangerously high pressure to build up and to over distend the lungs and thorax.

No serious injuries have resulted from rapid decompressions with open airways, even while wearing an oxygen mask, but disastrous, of fatal, consequences can result if the lung passages are blocked, such as forceful breath-holding with the lungs full of air.

The trapped air is forced through the lungs into the thoracic cage, and air can be injected directly into the ruptured blood vessels, with massive air bubbles moving throughout the body and lodging in vital organs such as the heart and brain.

The movement of these air bubbles is similar to the air embolism that can occur in SCUBA diving and submarine escape when an individual ascends from underwater to the surface with breath-holding (Pulmonary Over Inflation Syndrome).

Other problem of rapid decompression is Decompression Sickness. (also known as "Bends"). Because of the rapid ascent to relatively high altitudes, the risk of decompression sickness is increased. But person may not survive till full blown decompression sickness is established.

Situation within the aircraft

When explosive decompression occurs there will be an explosive noise within the cabin. The rapid rush of air out of the aircraft cabin on decompression has such force and items that are not secured will be extracted out of the ruptured hole in the pressurized compartment. Items such as maps, charts, flight logs, and magazines will be blown out.

Dirt and dust will affect vision for several seconds. Visibility will be more deteriorated by fogging. Air at any temperature and pressure has the capability of holding just so much water vapour. Sudden changes in temperature or pressure, or both, change the amount of water vapour the air can hold.

In a rapid decompression, temperature and pressure are reduced with a subsequent reduction in water vapour holding capacity. The water vapour that cannot be held by the air appears in the compartment as fog.

This fog may dissipate rapidly, as in most fighters, or not so rapidly, as in larger aircraft. Cabin temperature during flight is generally maintained at a comfortable level; however, the ambient temperature gets colder as the aircraft flies higher. If a decompression occurs, temperature will be reduced rapidly. Chilling and frostbite may occur if proper protective clothing is not worn or available.

If the immediate mechanical effects of rapid decompression on occupants of a pressurised cabin are minimal, the effects of subsequent low level of oxygen (hypoxia) become more serious with increasing altitudes. Hypoxia will be the immediate problem following rapid decompression.

Some degree of consciousness will probably be retained for few seconds. Aviation medicine defines the "time of useful consciousness", that is, how long after a decompression incident pilots will be awake and be sufficiently aware to take active measures to save their lives. The time of useful consciousness after loss of cabin pressure will be reduced due to off gassing of oxygen from venous blood to the lungs.

Above 50,000 feet

Above 50,000 feet and in to the space atmospheric conditions are functionally equal to that of a vacuum. Effects of hypoxia will be prominent up to 60,000 feet and additional effect of vacuum incapacitate the human above 60,000 feet. Above 40,000 feet (12 km), the time of useful consciousness is 12 to 25 seconds. For complete vacuum, this will be slightly less; 9 to 11 seconds. Linda Pendleton says: "An explosive or rapid decompression will cut this time in half due to the startle factor and the accelerated rate at which an adrenaline-soaked body burns oxygen."

Would your blood boil?

No. Your blood is at a higher pressure than the outside environment. A typical blood pressure might be 75/120. The '75' part of this means that between heartbeats, the blood is at pressure of 75 Torr (equal to about 100 mbar) above the external pressure. If the external pressure drops to zero, at a blood pressure of 75 Torr the boiling point of water is 46 degrees Celsius (115 F).

This is well above body temperature of 37 C (98.6 F). Blood won't boil, because the elastic pressure of the blood vessels keeps it at a pressure high enough that the body temperature is below the boiling point - at least, until the heart stops beating (at which point you have other things to worry about!). However, the effect of small pockets of localised vapour is to increase the pressure.

In places where the blood pressure is lowest, the vapour pressure will rise until equilibrium is reached. The net result is the same. It has been demonstrated that a properly fitted pressure suit elastic garments can entirely prevent "Blood Boiling" (ebullism) at pressures as low as 15 mm Hg absolute [Webb, 1969, 1970].)

Has anybody ever survived vacuum exposure in real life?

There are several cases of humans surviving exposure to vacuum worth noting. In 1966 a technician at NASA Houston was decompressed to vacuum in a space-suit test accident. NASA stated "At Manned Spacecraft Center (now renamed Johnson Space Center) we had a test subject accidentally exposed to a near vacuum (less than 1 psi) in an incident involving a leaking space suit in a vacuum chamber back in '65. He remained conscious for about 14 seconds, which is about the time it takes for Oxygen deprived blood to go from the lungs to the brain.

The suit probably did not reach a hard vacuum, and we began repressuring the chamber within 15 seconds. The subject regained later reported that he could feel and hear the air leaking out, and his last conscious memory was of the water on his tongue beginning to boil."

Another space incident reported as follows: "Incidentally, we have had one experience with a suit puncture on the Shuttle flights. On STS-37 during one of my flight experiments, the palm restraint in one of the astronaut's gloves came loose and migrated until it punched a hole in the pressure bladder between his thumb and forefinger.

It was not explosive decompression. Just a little 1/8 inch hole, but it was exciting down here in the swamp because it was the first injury we've ever had from a suit incident.

Amazingly, the astronaut in question didn't even know the puncture had occurred; he was so hopped on adrenalin it wasn't until after he got back in that he even noticed there was a painful red mark on his hand.

He figured his glove was chafing and didn't worry about it....What happened when the metal bar punctured the glove, the skin of the astronaut's hand partially sealed the opening. He bled into space, and at the same time his coagulating blood sealed the opening enough that the bar was retained inside the hole."

In 1960, during a high-altitude baloon parachute-jump, a partial-body vacuum exposure incident occurred when Joe Kittinger, Jr. lost pressurisation in his right glove during an ascent to 103,000 ft. (19.5 miles) in an unpressurised baloon gondola. Despite the depressurisation, he continued the mission and although the hand became painful and useless after he returned to the ground, his hand returned to normal. This is a very minimal degree decompression related incident.

How fast will a spaceship decompress if it gets punctured?

The decompression time will depend on size of the hole, pressure difference between inside and outside and cabin size.

Smaller the cabin, bigger the hole and higher the pressure difference, rate of decompression will be higher (rate of decompression will be reduced according the pressure gradient).

For a quick (and only roughly accurate) rule of thumb, if you put a one square centimetre hole in one cubic meter volume, the pressure will drop by a factor of ten every hundred seconds, and this time scales up proportionately to the volume, and scales down proportionately to the size of the hole.

This the reason for making window size of Concord smaller. Time available for pilot should be adequate to reach safe altitude, considering the rate of decompression and physical strength of the aircraft frame for G-Force tolerance. (Concord was flown in altitude of 50,000-55,000 feet). In aircraft there are other methods available for prevention of hypoxia for flying crew such as "quick donning emergency oxygen masks".

In Colombia space shuttle incident, following failure of the wing, chances of structural failure leading to breaking of integrity of the sealed cabin is very high. This was supported by the thermal effects of hot plasm and travelling speed. If the astronauts are with space suits and life support system, they may have survived the rapid decompression.

But due to the effects of heat, death will occur. If Apollo type "re-entry capsule" is made available within the space shuttle for re-entry, as a secondary escape system, there was a high chance of survival.

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