Dear Straight Dope:
Aren't the passenger cabins of airliner jets pressurized? I have my doubts. Why else would it feel like my ears were about to explode every time I commute?
Huh? is sort of a difficult nickname to incorporate into an answer, so I’ll just call you Patrick, how’s that?
Jet cabins are certainly pressurized. If they weren’t, we’d have a lot of dead commuters landing after each flight longer than a few minutes, and the movie “Airport” would have undoubtedly been a box office bust. The question isn’t *whether* the cabins are pressurized, it’s to what *degree* they’re pressurized.
First, planes are pressurized in much the same way that you put air into a balloon. Engine compressors pump enough air into the cabin, and a valve allows a controlled leak, until there is enough air in the cabin to produce the desired pressure.
Air pressure at ground level is 1013 millibars, or 29.92 inches of mercury, or 14.7 pounds per square inch, depending on whose scale you like most. There are local variations, as these figures are based on an ambient temperature of 59 degrees Fahrenheit (which is also 15 degrees Celsius, just in case you’re into that alternate scale thing here, too).
Jet cabins are typically pressurized at sea level only up to a certain altitude. By normal cruising altitude for the big boys, say 32,000 feet, the air inside the cabin is pressurized to the air pressure found at 7,000 to 8,000 feet, which is roughly 750 to 800 millibars. (Nowadays this is done automatically by computer-controlled mechanisms.)
As the plane climbs, the cabin pressure slowly decreases to that found at 8,000 feet above sea level. The air pressure inside your ear is at the higher level, present on the ground at the departure airport. The discomfort is caused by the air inside your inner ear pushing outwardly against the eardrum. The discomfort is relieved by reducing the air pressure inside your head until it equals the air pressure outside your head. The body is a wonderful thing.
The same thing happens in reverse during descent. However, the Eustachian tube (the tube that connects the inner ear to the back of your mouth) seems to have a harder time equalizing pressure when the pressure is lower inside the ear than outside. This is, of course, the case on descent, as the cabin is pressurized to equal the atmospheric pressure at your destination. The higher pressure of the outside air now presses against the eardrum, causing discomfort. If the disequilibrium continues, the pain you feel can increase. The danger is a ruptured eardrum, which relieves the pressure differential immediately, but you’ll need a doctor.
But why aren’t plane cabins pressurized to the atmospheric pressure at sea level, you may ask? Basically, it’s a compromise: if the cabin is pressurized to sea level pressure, the forces exerted upon the “skin” of the plane are much greater, increasing the risk of mechanical failure; if the cabin is pressurized to less than the pressure at 7,000 or 8,000 feet, passengers who may be susceptible to altitude sickness, which usually kicks in at 9,000 or 10,000 feet, may start to experience symptoms. Since vomiting is one symptom, this is undesirable whether you personally are susceptible to altitude sickness or not.
Some common ways of equalizing the pressure within the various cavities in your head while travelling by air include yawning, sucking on candy, chewing gum, swallowing with your mouth open, and beating on the passenger next to you. While this last one isn’t very effective, it can be fun, especially if he’s being rude to the flight attendant.
A better last ditch effort is the Valsalva maneouver. You must be very careful and gentle with this; pinch your nose with thumb and forefinger like you’re going underwater. Take a breath through your mouth, and, VERY SLOWLY AND GENTLY, build up pressure like you’re GENTLY blowing your nose. This buildup of pressure in your mouth and nose often pops open a recalcintrant Eustachian tube. But be careful: if you blow too hard or too abruptly, you could damage your ear.
One final related note: you often hear that you should not fly if you have a cold or sinus trouble. This is good advice, and you should take it whenever possible. If the Eustachian tube is blocked (common with a cold), you can wind up with a bubble of high pressure air trapped in your ear. Ouch. A co-worker was recently on a flight where the eardrum of the guy sitting next to him burst in flight, projecting blood and pus through the canal, suddenly cranking up the discomfort and damage caused by his ear infection. The flight attendant indicated that this happened more often than you might like to believe.
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