Air pressure in the cabin of a pressurized aircraft is not the same at or near sea level. The pressurized cabin on a commercial jetliner is typically reduced to a pressure to 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 or even walking along a flat surface. Passengers on an airliner are exposed to a reduced level of oxygen supporting metabolism but don’t notice. The development of stress such as might occur with a sudden illness might make this reduction is oxygen tension important. Persons with marginal respiratory function should consider the effect of reduced oxygen effectiveness or oxygen tension when considering air travel.