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Cabin pressurization is the process of compressing and regulating air in the inside of a vessel that ascends or descends very quickly. In most instances, cabin pressurization is discussed in the context of commercial air travel. Airplane cabins are all pressurized, which allows passengers to breathe as easily on the ground as they do at the maximum cruising altitude. Space shuttles and submarines must also be pressurized.
The human body requires consistent levels of oxygen in order to survive and to optimize organ and brain functions. Oxygen levels on earth are highest right around sea level and decrease slowly with altitude. People usually only start to notice changes in oxygen levels on the ground when ascending steep hills or peaks. Without cabin pressurization, humans would not be able to breathe in airplanes past a certain point.
Most airplanes fly at about 35,000 feet (about 10,668 meters) above sea level. The oxygen levels at that altitude are too thin to sustain life. In small airplanes, particularly fighter jets used for military purposes, pilots wear oxygen masks and pressurization helmets to counter the altitude. This is not usually a practical solution for commercial airliners.
Cabin pressurization is a means of regulating the air pressure and quality within the main cabin of an airplane. A pressurized airplane’s fuselage is built specifically to withstand and resist changes in outside air pressure. The thinner the oxygen in the air, the thinner and less compressed the air. Most aircraft are built with flexible steel frames, reinforced and specially sealed shells, and thick windows.
Pressurized air is not just a threat for airplane integrity. High altitudes often cause peoples’ blood vessels to constrict and can trigger a variety of altitude-related illnesses. Hypoxia, in which all of the body’s tissues and cells begin to constrict from lack of oxygen, is the most common side effect of altitude. Barotrauma is a similar altitude sickness through which the body's organs constrict in relation to outside pressure. It is barotrauma that causes the ears to pop, and in extreme circumstances, is what makes the ear drums rupture.
Decompression sickness can also be a consequence of unpressurized flight. As pressure returns to normal, dissolved gasses flow into the bloodstream, which often causes extreme nausea. A pressurized cabin significantly reduces the likelihood of passengers experiencing these or any other altitude ailments.
On most planes, cabin pressurization begins as soon as the wheels leave the ground. The engines begin sucking in air from the outside and funneling that air through a series of chambers. This both heats the air and pressurizes it. Before the air can be forced into the cabin, it must be cooled, which happens in what is known as an air cycle cooler. Air from this cooler flows constantly into the cabin through an overflow valve.
The overflow valve is a essentially a small hole in the plane’s fuselage through which compressed air is constantly both forced in and released out. It would not work to completely seal the cabin’s air, since humans exhale carbon dioxide. With as many people as most airliners hold, a sealed cabin would quickly run out of air.
Cabin pressurization depends on a lot of different factors to be successful. Although pressurization problems are rare, they are serious. Most governments require national airliners to provide oxygen masks for passengers in case of cabin pressure loss.
The process of pressurization is different for other vessels, such as submarines and space craft. These pressure vessels must be designed for the specific concerns of both deep-sea and zero-oxygen scenarios. Space suits and diving helmets are often used in conjunction with cabin pressurization to ensure the health and safety of all passengers on these craft.
Submarines do not have to be pressurized. The hatches are shut, and the ambient pressure is maintained because the submarine is obviously airtight, or water would get in. Hull compression will cause an insignificant increase in overall air pressure as the boat dives.