An increase in altitude — by foot, car, plane, or hot-air balloon — changes the atmospheric environment around you. The higher you go, the less oxygen there is. At about 18,000 feet above sea level, air pressure is reduced by 50 percent. This is accentuated at higher latitudes because the earth's atmosphere is thicker at the equator and thinner at the poles. Thus, the effects of altitude on the summit of Denali in Alaska are about 15 percent greater than at the same altitude in the Himalayas.

At a constant altitude, the amount of oxygen in the air is also constant. It does not fluctuate significantly with the temperature, time of day, season, or any other routine environmental changes. Your"Big Three" body systems become accustomed to this. Your rate of respiration, the number of red blood cells in your circulation, and other physiological parameters are in balance with your environment, whether you're a lobsterman or a mountain guide.

The air at Denali (pictured) is thinner than the
air in the Himalayas at the same altitude.

There are compensatory mechanisms which allow you to change altitude, within limits, without getting out of balance. You can move from sea level to about 8000 feet with minimal effect. In the short term, there will be an increase in respiratory rate with associated chemical changes (elevated pH) in the blood. If you stay several days, your kidneys will re-balance the pH of the blood, resetting your system to your new environment.

If you were to continue even higher, your body would compensate and re-balance again. Over the course of weeks, you would undergo further physiologic changes, such as an increase in red-blood-cell production. Ultimately, you would reach the limit of your body's ability to compensate.

This ability of your body to adapt to altitude and the speed with which it happens vary widely from person to person. They appear to have no relationship to physical fitness or gender. Some people adapt to altitude better than others. However, everyone's ability to adapt to higher altitudes is reduced by dehydration, alcohol and other depressants, and overexertion.

The best way to adapt to higher altitude is to take your time, maintain hydration, stay away from depressants, and take it easy. Allow your normal compensatory mechanisms the time necessary to work by ascending in stages. Climb no faster than your body can adapt. Do not overexert on the first day at the new altitude, and plan to remain for two to three days before proceeding higher. If you pay attention to what your body is trying to tell you, you should be able to avoid the more severe forms of altitude sickness.