In an un-pressurised aircraft climbing to higher and higher altitudes, your body is exposed to less and less pressure upon its outer surfaces. Because the pressure inside your body is still the same as it was on the ground, strange things begin to happen. Gases trapped in the body cavities start expanding in an effort to equalize the pressure with that of the environmental gas (that is, air). This phenomenon can cause you some discomfort. Trapped in such places as the sinuses, behind the ear drum, and in the stomach the expanding gas may lead to a headache, ear pain, or a feeling of abdominal fullness.

At 8,000 feet, the gases in your body expand to a volume of about 20% greater than that at ground level. If your rate of climb is gradual and your physical condition is good, you can usually adjust to this change easily and comfortably. At 18,000 feet, the wet gas bubbles more than double their normal size and the expansion continues as the un-pressurised aircraft gains altitude. A very rapid change of altitude is naturally more hazardous and uncomfortable than a slow change.

You can usually reduce the discomforts resulting from the expansion of trapped gases by slowing your rate of ascent. If they persist, descend to a lower altitude where the atmosphere is denser. Most of the gas in the intestines is swallowed air, but some is formed by the digestive process. The amount of gas varies with the individual and with the type of food eaten. If you expect to fly at high altitude, the following "Diet Don'ts" may help to minimize abdominal gas;

      1. Don't eat too quickly before a flight.
      2. Don't eat too much. (Swallowed air increases with each bite)
      3. Avoid large quantities of fluid, especially cokes, pop, and beer.
      4. Don't eat gas-forming foods. (Beans, cabbage, onions, raw apples, cucumbers, melons, or any greasy foods)
      5. Avoid chewing gum on the way up - it may result in your swallowing a great deal of air.

In addition to gases trapped in the body cavities, a considerable volume of gas (primarily nitrogen) exists within the body, not in its normal state, but in solution. That is, it is dissolved in the blood and other body tissues, especially fat. When the outside pressure falls, these gases tend to come out of solution, forming gas bubbles - just as carbonated beverages release bubbles when you remove the cap and let the pressure escape. These bubble can produce severe pain. Pain caused by bubble formation around the joints or muscles is called "bends."  The same bubble formation in the lung tissue is called the "chokes" and is recognized by a burning sensation or stabbing pain the chest area, a cough, and difficulty in breathing. Needless to say, the effects upon your ability to operate the aircraft can be disastrous.