Cabin Pressurization - Need For Cabin Pressurization

Need For Cabin Pressurization

Pressurization becomes necessary at altitudes beyond 10,000 feet (3,000 m) 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; it also serves to generally increase passenger comfort. The principal physiological problems are as follows:

Hypoxia. The low partial pressure of oxygen at an altitude reduces the alveolar oxygen tension in the lungs and subsequently in the brain, leading to sluggish thinking, dimmed vision, loss of consciousness, and ultimately death. In some individuals, particularly those with heart or lung disease, symptoms may begin as low as 5,000 feet (1,500 m), although most passengers can tolerate altitudes of 8,000 feet (2,400 m) without ill effect. At this altitude, there is about 25% less oxygen than there is at sea level. Hypoxia may be addressed by the administration of supplemental oxygen, either through an oxygen mask or through a nasal cannula.

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 (CO₂) to out-gas, raising the blood pH and inducing alkalosis. Passengers may experience fatigue, nausea, headaches, sleeplessness, and (on extended flights) even pulmonary oedema. 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 envelopes the body in a pressurized environment; this is clearly impractical for commercial passengers.

Decompression sickness. The low local partial pressure of gases, principally nitrogen (N₂) 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 for 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 of the bends. 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 (aerotitus) or paranasal sinuses by a blocked Eustachian tube or sinuses. Pain may also be experienced in the 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 and can exacerbate or precipitate pre-existing medical conditions, such as pneumothorax.

Pressurisation 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.