Because symptoms of hypoxia become more severe as the air thins, an aircraft cabin must be pressurized at altitudes above 4,600 m (15,000 ft) unless a supplemental oxygen supply is available to the flight crew and passengers. In actual practice, to avoid hypoxia (or asphyxiation at higher altitudes), commercial aircraft cabins are pressurized at takeoff and remain pressurized throughout the flight. A cabin typically is pressurized to about 75% of the average sea-level air pressure.
Although the cabin is pressurized, people commonly feel the effects of changing air pressure in a rapidly ascending or descending aircraft by a popping sensation in their ears. Rapid ascent or descent in an express elevator or on mountain roads often produces the same sensation. Ear popping is symptomatic of a natural response that helps protect the eardrum from damage.
The eardrum separates the outer ear from the middle ear chamber. As an air- craft takes off and cabin pressure drops, however, the air pressure on the outer ear decreases. As air pressure on the outer ear changes, the eardrum becomes distorted unless a compensating pressure change takes place in the middle ear. If the pressure does not equalize between the outer and middle ear, the eardrum bulges outward (see Figure 4.12a). On the other hand, when an aircraft descends and cabin pressure increases, air pressure in the outer ear increases. Without a compensating pressure change in the middle ear chamber, the eardrum bulges inward (see Figure 4.12b). In both circumstances, deformation of the eardrum not only causes physical discomfort, but the bulging eardrum does not vibrate efficiently and sounds are muffled. If the air pressure difference between the middle ear and outer ear continues to increase, the eardrum could rupture, perhaps causing permanent hearing loss. Fortunately, the body has a natural mechanism that alters the air pressure in the middle ear chamber. The Eustachian tube connects the middle ear with the pharynx, which, in turn, leads to the outside via the oral and nasal cavities (see Figure 4.12c). Normally, the Eustachian tube is closed where it enters the pharynx, but it opens if a sufficient air pressure difference develops between the middle ear and the pharynx.
Opening of the Eustachian tube allows the air pressure in the middle ear to quickly equilibrate with the external air pressure and the eardrum pops back to its normal shape. Vibrations of the eardrum that are associated with the rapid change in its shape are what a person hears as "ear-popping." Ear popping is thus the body's way of preventing a permanent hearing loss when experiencing a rapid change in air pressure. Yawning or swallowing hastens the opening of the Eustachian tube thereby reducing the period of discomfort. For this reason, air travelers are advised to chew gum during ascent and descent of the aircraft. On some flights, cabin attendants hand out hard candy. Chewing gum, sucking on hard candy, and even the smell and taste of these substances signals the brain, which, in turn, accelerates the release of saliva from the salivary glands (located in the jaw region). Build up of saliva in the oral cavity triggers a swallow reflex which opens the Eustachian tube and equalizes air pressure on both sides of the eardrum.
