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Cabin pressurization
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==Need for cabin pressurization== [[File:Cabin pressure and Bleed air control panels on a Boeing 737-800.jpg|upright|thumbnail|right|The pressurization controls on a [[Boeing 737-800]]]]{{More citations needed section|date=July 2024}} Pressurization becomes increasingly necessary at altitudes above {{cvt|10000|ft|0}} above [[sea level]] to protect crew and passengers from the risk of a number of physiological problems caused by the low outside air pressure above that altitude. For private aircraft operating in the US, crew members are required to use oxygen masks if the cabin altitude (a representation of the air pressure, [[#Cabin altitude|see below]]) stays above {{cvt|12500|ft|0}} for more than 30 minutes, or if the cabin altitude reaches {{cvt|14000|ft|0}} at any time. At altitudes above {{cvt|15000|ft|0}}, passengers are required to be provided oxygen masks as well. On commercial aircraft, the cabin altitude must be maintained at {{cvt|8000|ft|0}} or less. Pressurization of the cargo hold is also required to prevent damage to pressure-sensitive goods that might leak, expand, burst or be crushed on re-pressurization.{{citation needed|date=March 2019}} The principal physiological problems are listed below.{{Citation needed|date=January 2024}} ; [[Hypoxia (medical)|Hypoxia]] : The lower [[partial pressure]] of oxygen at high altitude reduces the [[Pulmonary alveolus|alveolar]] oxygen tension in the lungs and subsequently in the brain, leading to sluggish thinking, dimmed vision, loss of consciousness, and ultimately death:{{Citation needed|date=January 2024}} In some individuals, particularly those with heart or lung disease, symptoms may begin as low as {{cvt|5000|ft|0}}, although most passengers can tolerate altitudes of {{cvt|8000|ft|0}} without ill effect. At this altitude, there is about 25% less oxygen than there is at sea level.<ref name="02_calc">{{cite web | author=K. Baillie and A. Simpson | title=Altitude oxygen calculator | url=http://www.altitude.org/air_pressure.php | access-date=2006-08-13}} – Online interactive altitude oxygen calculator</ref> : Hypoxia may be addressed by the administration of supplemental oxygen, either through an [[oxygen mask]] or through a [[nasal cannula]]. Without pressurization, sufficient oxygen can be delivered up to an altitude of about {{cvt|40000|ft|0}}. This is because a person who is used to living at sea level needs about {{cvt|0.20|bar|kPa psi}} [[Partial pressure of oxygen|partial oxygen pressure]] to function normally and that pressure can be maintained up to about {{cvt|40000|ft|0}} by increasing the [[mole fraction]] of oxygen in the air that is being breathed. At {{cvt|40000|ft|0}}, the ambient air pressure falls to about 0.2 bar, at which maintaining a minimum partial pressure of oxygen of 0.2 bar requires breathing 100% oxygen using an [[oxygen mask]]. : Emergency oxygen supply masks in the passenger compartment of airliners do not need to be pressure-demand masks because most flights stay below {{cvt|40000|ft|0}}. Above that altitude the partial pressure of oxygen will fall below 0.2 bar even at 100% oxygen and some degree of cabin pressurization or rapid descent will be essential to avoid the risk of hypoxia. ; [[Altitude sickness]] : [[Hyperventilation]], the body's most common response to hypoxia, does help to partially restore the partial pressure of oxygen in the blood, but it also causes [[carbon dioxide]] (CO<sub>2</sub>) to out-gas, raising the blood pH and inducing [[alkalosis]]. Passengers may experience fatigue, [[nausea]], headaches, sleeplessness, and (on extended flights) even [[pulmonary edema]]. These are the same symptoms that mountain climbers experience, but the limited duration of powered flight makes the development of pulmonary oedema unlikely. Altitude sickness may be controlled by a full [[pressure suit]] with helmet and faceplate, which completely envelops the body in a pressurized environment; however, this is impractical for commercial passengers. ; [[Decompression sickness]] : The low partial pressure of gases, principally nitrogen (N<sub>2</sub>) but including all other gases, may cause dissolved gases in the bloodstream to precipitate out, resulting in ''gas embolism'', or bubbles in the bloodstream. The mechanism is the same as that of compressed-air divers on ascent from depth. Symptoms may include the early symptoms of "the bends"—tiredness, forgetfulness, headache, stroke, thrombosis, and subcutaneous itching—but rarely the full symptoms thereof. Decompression sickness may also be controlled by a full-pressure suit as for altitude sickness. ; [[Barotrauma]] : As the aircraft climbs or descends, passengers may experience discomfort or acute pain as gases trapped within their bodies expand or contract. The most common problems occur with air trapped in the [[middle ear]] (aerotitis) or [[paranasal sinuses]] by a blocked [[Eustachian tube]] or sinuses. Pain may also be experienced in the [[Human gastrointestinal tract|gastrointestinal tract]] or even the teeth ([[barodontalgia]]). Usually these are not severe enough to cause actual trauma but can result in soreness in the ear that persists after the flight<ref name="Harvard">{{Cite web|url=https://www.health.harvard.edu/a_to_z/barotrauma-a-to-z|title=Barotrauma What is it?|date=December 2018|website=[[Harvard Health Publishing]]|publisher=[[Harvard Medical School]]|quote="On an airplane, barotrauma to the ear – also called aero-otitis or barotitis – can happen as the plane descends for landing."|access-date=2019-04-14}}</ref> and can exacerbate or precipitate pre-existing medical conditions, such as [[pneumothorax]].
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