Re: why are airplanes pressurized?

Hello John: Cabins are pressurized in transport airplanes to maintain a 
suitable environment for passengers and cargo. Modern aircraft fly at very 
non-friendly altitudes where the air molecules aren’t as densely packed as 
compared to normal sea level conditions. Although the air composition by 
percentage stays relatively the same, there are fewer oxygen molecules in 
any given breath to provide for the body requirements. Also the air 
temperature is very cold, another reason for windows to have a large air 
gap between outer and inner surfaces for insulation. 
Sophisticated “environmental conditioning systems” (ECS) ensure adequate 
warmth and oxygen to passengers and crew during flight.
To reduce the amount of expansion and contraction that the fuselage must 
endure (and reduce metal fatigue) the cabins are usually regulated for an 
equivalent of 8,000 ft of altitude. As added benefit, the 
equivalent “pressurized” environment relaxes the body and easier to “get 
sleepy”. People suffering from allergies, colds, etc mostly notice the 
conditioned environment. The differential pressure created on the eardrums 
due to plugged Eustachian tubes is sometimes uncomfortable. Lookup this 
reference in the Internet:
www.funkandwagnalls.com/encyclopedia/getpage.asp?
abspage=/articles/007001b/007001557.asp

All airplanes have a certain amount of pressurization leakage. This 
leakage is carefully monitored and tested as part of the aircraft 
maintenance. During rare instances the aircraft may lose cabin 
pressurization at high altitude. Emergency oxygen masks automatically 
deploy to keep passengers in good health while the aircraft undergoes a 
very steep descent to 10,000 ft, an altitude that can be tolerated without 
supplemental oxygen.

Each time the aircraft completes a flight, the skin and retaining 
structure experience expansion and contraction because of necessary 
fuselage flexibility. This factor accounts for “aging” factor, or the 
lifetime of the airplane. Fortunately airframes can be rejuvenated by 
replacing skin panels and other parts of the structure.

Violent loss of pressurization occurs because of structural failure. At 
high altitude the result is explosive and if the failed structure is large 
enough and nearby, it is possible that someone (with unfastened seatbelt) 
can be ejected for a likely death. Rapid decompression also causes the 
cabin air to expand so fast that the ambient cools and becomes very foggy. 
Let me stress that this failure is very remote; air travel has proven to 
be the safest of all other means for transportation.

A little background about the atmosphere for reference:
Air is a combination of gases, approximately nitrogen 78 % and oxygen 21%. 
The atmospheric gases in the remaining 1% are: argon 0.9 %, carbon dioxide 
0.03 % and varying amounts of water vapor, and trace amounts of hydrogen, 
ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.
Look-up Britannica on the Internet:
www.britannica.com/bcom/eb/article/5/0,5716,5245+1+5193,00.html
www.britannica.com/bcom/eb/article/1/0,5716,118221+1,00.html
The atmosphere may be divided into several layers. In the lowest one, the 
troposphere, the temperature as a rule decreases upward at the rate of 
5.5° C per 1000 m (3° F per 1000 ft). This is the layer in which most 
clouds occur. It extends up to 10 miles at the equator, where the 
temperature is -110° Fahrenheit.

Now, a short tutorial on the human body versus altitude: http://members.aol.com/Rosendalhe/pressure.htm
The normal barometric pressure at Mean Sea Level (MSL) is 14.7 pounds per 
square inch, or 28.98 inches of mercury column. As the altitude increases 
the total pressure exerted by the atmosphere decreases, although the 
mixture content of the air stays the same. Humans do well from MSL to 
about 8,000 ft of altitude. Beyond that altitude, the partial pressure of 
oxygen starts exerting negative effects to an unconditioned body. This is 
evidenced by the “lack of breath” when exerting energy, for example, 
running. For practical reasons the “top ceiling” or highest altitude for 
flight without supplemental oxygen is 10,000 ft above MSL. Lack of oxygen 
is insidious when the level of activity is low. Typically the oxygen-
starved person gets lethargic and by the time that normal vision begins 
to “tunnel” it is too late to react. The other BIG factor as we saw above 
is the cold.
Interestingly enough, the body manifests the lack of oxygen in various 
ways before oxygen starvation precludes reaction. The most common form 
is “feeling very flushed”. Military and commercial pilots undergo altitude 
chamber training periodically to reinforce identification of their bodies’ 
reaction to lack of oxygen. Typically the air chamber pressure is dropped 
to 25,000 ft while the subject is wearing the oxygen mask. At that time 
the mask is removed (under the watchful eyes of the instructors) the 
subject is given a piece of paper and pencil with simple problems to 
solve, for instance, adding two and two, signature, etc. The subject feels 
like he/she is in slow motion but not uncomfortable at all. As the vision 
starts to tunnel, most subjects cannot replace their own oxygen masks 
before becoming unconscious; the instructors do so. Only takes a breath or 
two once the mask is back on to regain full consciousness. Looking at the 
problem sheet, the first line maybe looks OK. The second is garbage. The 
third, worse. However the body gives a very good cue as soon as the mask 
was removed. That is the signal that pressurization or oxygen flow failed, 
allowing enough time to react before is too late.
Your MAD.SCI Micro.