At the altitude at which many jet aircraft fly, the air pressure is extremely low —less than the pressure inside the human body. This makes it impossible for the body to take in air. There is also very little oxygen, so the air inside the plane has to be pressurized in order to simulate the level of air pressure on the surface.
Although aircraft cabins are pressurized, cabin air pressure at cruising altitude is lower than air pressure at sea level. At typical cruising altitudes in the range 11 000–12 200 m (36 000–40 000 feet), air pressure in the cabin is equivalent to the outside air pressure at 1800–2400 m (6000–8000 feet) above sea level. As a consequence, less oxygen is taken up by the blood (hypoxia) and gases within the body expand. The effects of reduced cabin air pressure are usually well tolerated by healthy passengers.
Oxygen and hypoxia
Cabin air contains ample oxygen for healthy passengers and crew. However, because cabin air pressure is relatively low, the amount of oxygen carried in the blood is reduced compared with that at sea level. Passengers with certain medical conditions, particularly heart and lung diseases and blood disorders such as anaemia (in particular sickle-cell anaemia), may not tolerate this reduced oxygen level (hypoxia) very well. Some of these passengers are able to travel safely if arrangements are made with the airline for the provision of an additional oxygen supply during flight. However, because regulations and practices differ from country to country and between airlines, it is strongly recommended that these travellers, especially those wishing to carry their own oxygen, contact the airline early in their travel plans. An additional charge is often levied on passengers who require supplemental oxygen to be provided by the airline.
Gas expansion
As the aircraft climbs in altitude after take-off, the decreasing cabin air pressure causes gases to expand. Similarly, as the aircraft descends in altitude before landing, the increasing pressure in the cabin causes gases to contract. These changes may have effects where air is trapped in the body.
Passengers often experience a “popping” sensation in the ears caused by air escaping from the middle ear and the sinuses during the aircraft’s climb. This is not usually considered a problem. As the aircraft descends in altitude prior to landing, air must flow back into the middle ear and sinuses in order to equalize pressure. If this does not happen, the ears or sinuses may feel as if they are blocked and pain can result. Swallowing, chewing or yawning (“clearing the ears”) will usually relieve any discomfort. As soon as it is recognized that the problem will not resolve itself using these methods, a short forceful expiration against a pinched nose and closed mouth (Valsalva manoeuvre) should be tried and will usually help. For infants, feeding or giving a pacifier (dummy) to stimulate swallowing may reduce the symptoms.
Individuals with ear, nose and sinus infections should avoid flying because pain and injury may result from the inability to equalize pressure differences. If travel cannot be avoided, the use of decongestant nasal drops shortly before the flight and again before descent may be helpful.
As the aircraft climbs, expansion of gas in the abdomen can cause discomfort, although this is usually mild.
Some forms of surgery (e.g. abdominal surgery) and other medical treatments or tests (e.g. treatment for a detached retina) may introduce air or other gases into a body cavity. Travellers who have recently undergone such procedures should ask a travel medicine physician or their treating physician how long they should wait before undertaking air travel.
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