Ear pain is the most common complaint from scuba divers. Some divers call it "ear squeeze." As a diver goes deeper under water and the outer environment pressure increases, the pressure in the middle ear (the part behind the ear drum) is "squeezed" by the increasing pressure of the water from outside.
The middle ear is an air-filled space formed by bone and the tympanic membrane or eardrum. It is connected to the back of the nose by a tunnel called the eustachian tube. Outside air passing through the eustachian tube keeps the pressure in the middle ear equal to that of the outside world. If the eustachian tube malfunctions and a pressure difference occurs across the eardrum, pain or ear squeeze occurs.
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Ear pain occurs during the descent portion of a dive -- as the diver drops deeper underwater. The squeezing ear pain most often occurs near the surface where the relative pressure changes are greatest. Each foot below the surface places continuing pressure on the diver. For every 33 feet under water, atmospheric pressure increases in the amount of 1 atmosphere (this can be compared to the pressure of 1 atmosphere for anyone at sea level).
Normally, the eustachian tube will open and allow the pressure behind the eardrum to equalize with the outside pressure of the seawater in the ear canal. But, if the eustachian tube can't do its job, then as the seawater pressure in the ear canal increases, the eardrum is pushed inward, stretching and inflaming the eardrum and causing pain. If the pain is ignored and the diver drops deeper, the pressure will continue to increase and the eardrum may burst, allowing cold seawater to rush into the middle ear. Nausea, vomiting, dizziness, and confusion may follow.
Rapid ascents or descents in a car or commercial air flight may also cause pressure equalization problems in the ear, but not to the same degree as in a dive. You may get an ear pop but not an ear squeeze.
There are many reasons for the eustachian tubes not to equalize the pressure: