(a) Cabin Air Pressure
Although aircraft cabins are pressurized, the cabin air pressure is not the same as on the ground. It is equivalent to pressure at the altitude of 1500-2500 meters above sea level. Consequently, the available oxygen is reduced and gases within the body expand.
The effects of reduced cabin air pressure are usually well tolerated by healthy passengers.
(b) Oxygen and Hypoxia
Cabin air contains ample oxygen for healthy passengers. The oxygen saturation of the blood is slightly reduced, leading to mild hypoxia (i.e. reduced supply of oxygen to the tissues) as a result of reduction in cabin air pressure.
The effect of alcohol on the brain is increased by hypoxia. Passengers with cardiovascular, respiratory disease or certain disorders of the blood, such as anemia or sickle cell disease may not tolerate hypoxia well and should consult their physician before travelling or consuming alcoholic beverages during flight.
(c) Gas Expansion
Air expands in all air-filled body cavities as a result of reduction in cabin air pressure. Abdominal gas expansion may cause moderate discomfort, whereas expansion of the air in the middle ear and sinuses may cause pain particularly during descent.
As the aircraft ascends, air escapes from the middle ear and the sinuses, usually without causing problems.
As the aircraft descends, air must be allowed to flow back into the middle ear and sinuses in order to equalize pressure differences (“clearing the ears”). This is sometimes difficult, but discomfort can be alleviated by swallowing, chewing or yawning. If the problem persists, forceful expiration against a closed nose and mouth will usually help.
People with ear, nose and sinus infections should avoid flying because injury may result from inability to equalize pressure differences. If problems arise during flight nasal drops may sometimes be helpful.
Relative humidity is low in aircraft cabins, usually less than 20%. Low humidity may cause discomfort in the eyes, mouth and nose but presents little risk to traveler health and does not result in central dehydration as long as normal fluid intake is maintained. Discomfort can be alleviated by maintaining good fluid intake before and during the flight, the use of a skin moisturizing lotion, use of a saline nasal spray to moisturize nasal passages and the use of eye glasses rather than contact lenses.
Measures should be taken to prevent dehydration on long flights. Adequate intake of non-alcoholic beverages (water and fruit juices) should be maintained before and throughout the flight. Consumption of caffeinated beverages (including tea, coffee and cola) and alcohol cause dehydration and should be restricted and preferably avoided before and during the flight.