Effects of Altitude
Although aircraft cabins are pressurized, that pressure (called barometric pressure) is less than that on the ground. For most flights the cabin pressure is the same as that at 5,000-8,000 feet above sea level. In other words, when you are flying, the atmosphere within the aircraft is like that on the 5,000-8,000 feet peak of a small mountain. This has two effects: there is less oxygen available because the pressure of oxygen becomes lower; and, gas within our body cavities expands. Both of these phenomena are usually well tolerated by healthy passengers.
A) Oxygen: There is less oxygen absorbed into the blood and circulated throughout the body during flight as compared to ground level due to a decrease in oxygen with an increased cabin altitude. As long as you are in reasonably good health, your body has physiological mechanisms that compensate for this decreased quantity of oxygen. On the other hand, passengers with significant heart, lung, and blood diseases may not well tolerate lower amounts of oxygen. Therefore, they should consult their physician before air travel to evaluate their capability to travel and to determine if there is a need for medical oxygen or other special assistance. Medical oxygen can be arranged with most airlines and it is important to check with your carrier several days in advance of the flight. Furthermore, the combination of low oxygen, alcohol, inactivity and sleep can generate unpleasant side effects like dizziness and/or fainting if one stands up too fast after awakening. Arm and leg exercises before standing up will usually prevent this.
B) Gas Expansion: The body contains air in the middle ear (inside of the ear drum) and sinuses. As the aircraft ascends, the air in these cavities will expand to the outside via tubes connecting them to the nose. On descent the reverse occurs with air flowing from outside to these cavities via the same tubes. This is well tolerated as long as the air can flow into and out of these cavities freely. To facilitate the free flow of air, particularly on descent, it is helpful to periodically swallow, chew, or yawn. (This is why it is important that passengers stay awake during descent.) Give something to drink to young children or a pacifier to infants. Individuals with ear, nose and sinus infections should avoid flying because the congestion prevents the air from flowing freely in and out of these cavities which could result in pain, bleeding, and possibly a ruptured ear drum. Also, avoid gas forming foods or liquids before flight.