The ear can be divided into three sections: the outer ear, the middle ear, and the inner ear. The middle ear is an air-filled chamber that is connected to the nose and throat via a channel called the eustachian tube.

Normally, air moves through this tube to equalize the pressure in the middle ear with that of the air around us. This is especially important when there are large changes in the external air pressure, such as in flying or scuba diving. For example, as an airplane rises in altitude, the air pressure in the cabin gradually drops. This causes the pressure in the middle ear to seem relatively high, and the eardrum bulges slightly outward.

When the pressure difference between the cabin and the middle ear reaches about 15 millimeters of mercury (mm Hg), the eustachian tubes open with a popping sensation and air is released from the middle ears. Under normal circumstances, this happens about once every five hundred to one-thousand feet during ascent.

Conversely, as the plane descends and the pressure in the cabin increases, the middle ear pressure seems relatively low and the eardrum is pulled slightly inward. Now when the eustachian tubes open, air rushes into the middle ears to equalize the pressure.

Sometimes during descent a manual technique must be used to equalize middle ear pressure. For example, swallowing, chewing gum, or blowing the nose can help open the eustachian tubes if the pressure difference becomes uncomfortable.

Illnesses such as colds and ear infections can cause the membranes surrounding the eustachian tubes to swell. This may make pressure equalization more difficult, producing pain in the middle ear.

Caution should be taken to avoid flying or scuba diving when one of these conditions is present, which may affect the middle ear. This is especially important in children, who are more susceptible to ear infections and are less skilled in equalizing their ear pressure.