There are many factors that can contribute to ear pressure. The pressure regulation system within the ear is quite vulnerable to the environment and viruses, making altitude, diving underwater, and ear infections prime causes of pressure in the ear. When the pressure regulator becomes blocked for any reason, pressure builds in the ear.
The Eustachian tubes are what connect the middle ear to the throat. These tubes are responsible for removing bacteria from the middle ear and also for maintaining equal pressure between the middle ear and outside of the ear by allowing airflow. The pressure balance is what can be felt when a person swallows or yawns and there is a crackling or popping sensation in the ear.
A middle ear infection, also called otitis media, is one of the most common causes of pressure in the ear. Ear infections are usually caused by Eustachian tube dysfunction. When the tubes are blocked, they can't properly remove bacteria from the middle ear. Bacteria multiplies very quickly within the middle ear, which leads to infection.
The Eustachian tubes sometimes become blocked during a cold, allergies, or flu. During upper respiratory illnesses, mucus production is increased, and often the sinuses and by proximity, the Eustachian tubes, become inflamed. The blocked tubes create an imbalance in ear pressure, which can be severely painful.
The airflow within the Eustachian tubes keeps the middle ear from fluid buildup. Fluid left in the ear due to Eustachian tube dysfunction causes an imbalance in pressure. Fluid in the ear can take several weeks to completely dissipate, which is why the pressure in the ear can take so long to regulate.
Altitude is another common cause of feeling pressure in the ears. Just about everyone has experienced uncomfortable ear pressure on a flight or driving on a mountain road. Normally, pressure can be regulated by chewing, yawning, or swallowing, but sometimes the pressure isn't released properly. This results in very painful pressure in the ears, called barotrauma.
Pressure in the ears and barotrauma can also be caused by diving underwater. Again here, swallowing or yawning usually can equalize the pressure in the ear with the pressure of the surrounding water. Unfortunately, the pressure equalization system isn't perfect, and it can fail, causing discomfort. Divers may avoid some instances by not diving while they are congested, but there's no sure way to maintain equal pressure.
Occasionally, small tumors at the base of the skull can block the Eustachian tubes. This is a fairly uncommon place to develop tumors. It does happen, but it is a relatively rare cause of ear pressure.