Air pressure in the cabin of a pressurized aircraft is not the same as at or near sea level. The pressurized cabin on a commercial jetliner is typically reduced to a pressure of between 6000 to 8000 feet. Although not the same as sea level pressure, this is enough for a person without lung disease to feel comfortable at rest. Since passengers are not exercising they do not notice the effective altitude as they might at the same elevation they might encounter in mountainous areas while hiking. Passengers on an airliner are exposed to a reduced level of oxygen tension because of the lower cabin pressure (cabin altitude for short). The air passengers are breathing is at the same concentration, approximately 21 percent but the oxygen tension is reduced. Oxygen tension is what supports metabolism(the production of energy). Passengers don’t notice the reduced oxygen tension. The development of stress such as might occur with a sudden illness on board the aircraft might make this reduction in oxygen tension important. Persons with marginal respiratory function should consider the effect of reduced oxygen effectiveness or oxygen tension when considering air travel.
The Boeing 787 Dreamliner and the Airbus A350 are pressurized to approximately the 6,000 foot cabin altitude. This can be accomplished because of the carbon fiber fuselage of these aircraft. Other aircraft like the Boeing 737 and Airbus A321 are pressurized to the 8000 foot cabin altitude. The airlines claim that this reduces the fatigue of long flights on these aircraft. This may be true but regardless, chemically and physically the passengers body is exposed to the lower cabin pressure in the carbon fiber fuselage.