Typically, an airplane depressurizes due to damage that compromises the plane's airtight seal. If this happens at cruising altitude, you'll only have 1-2 minutes of air before you succumb to hypoxia, or a lack of oxygen.
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If airplanes didn't pressurize their cabins, it could lead to insufficient oxygen as well as related medical problems like hypoxia. Airplanes need pressurized cabins because it ensures passengers, as well as crew members, receive an adequate amount of oxygen in the air they breathe.
Most aircraft cabins are pressurized to an altitude of 8,000 feet, called cabin altitude. Aircraft pilots have access to the mode controls of a cabin pressure control system and – if needed – can command the cabin to depressurize.
If the plane is somehow kept pressurized, the cabin pressure is usually equivalent of 6000–8000 ft, which is lower pressure than the outside at sea level. Which means as soon as you unlock the door, it will swing inwards with a lot of force, as air from outside rushes in to re-pressurize the cabin.
Pilot and Vietnam War veteran Pete Jordan knows exactly what happens when a pressurized cabin decompresses 30,000 feet in the air at 300 to 600 mph: There's no oxygen, and it gets damn cold in a hurry. An open door would release the cabin's ball of pressure, causing an immediate suction explosion.
The Cabin Altitude of a pressurised aircraft is normally maintained at and altitude of 8,000 ft or less as a compromise between the physiological needs of the crew and passengers and the structural limitations of the aircraft. At 8,000 ft the use of supplemental oxygen is not required.
Just like a bag of peanuts mid-flight, air pressure changes will cause you to inflate. A build-up of gas can lead to bloating, constipation and stomach pains. Lack of movement leads to fluid build-up around the body, increasing the risk of deep vein thrombosis (DVT).