It is typical of an air hostess to assure passengers just before the flight takes off that they need not panic in case the cabin pressure drops. It is because, in the event of a decompression, an oxygen mask will automatically drop down in front of the passengers which they need to pull towards them; place it firmly over their nose and mouth and breath in the oxygen, until the aeroplane descends and lands.
But first, how does the cabin pressure work?
Most passengers barely notice cabin pressure once they get into an aircraft. It is because once the flight takes off, cabin pressure gradually adjusts the air pressure inside the aircraft as it climbs in altitude, and then adjusts it again on the way down.
How does this happen? As the jet engines suck in air, some of the excess air is diverted into the aeroplane’s cabin. The air is both cooled and humidified (with moisture being added to it) after which it gets circulated throughout the cabin. Once the cabin achieves an ideal pressure level, it is regulated by a device called the air cabin pressure controller, according to the howstuffworks.com website.
The cabin pressurisation is important because of the nuances between low- and high-altitude air density. Air is less dense at high altitudes than at low altitudes. At ground level, the air pressure is a little over 14 pounds per square inch (PSI). When an aeroplane reaches its typical cruising altitude — usually about 30,000 feet to 40,000 feet — the air pressure may be just 4 to 5 PSI, according to the One Monroe Aerospace website.
The low air pressure associated with high-altitude flights can restrict passengers from receiving an adequate amount of oxygen unless the cabin is pressurised. Low air pressure means the air is less dense. Therefore, it contains less oxygen. If aeroplanes didn’t pressurise their cabins, it could lead to insufficient oxygen as well as related medical problems like hypoxia. Aeroplanes need pressurised cabins because it ensures passengers, as well as crew members, receive an adequate amount of oxygen in the air they breathe, the website pointed out.
(Source: Howstuffworks.com; manroeaerospace.com).