Altitude sickness occurs when someone goes to a high altitude where there is less oxygen than he's used to. Although the concentration of oxygen in the air remains the same (21 percent) at differing altitudes, the number of oxygen molecules per breath goes down as the altitude increases. People have to breathe faster in order to accommodate, and even so won't get as much oxygen as they would at a lower altitude.
Symptoms of altitude sickness include headache, dizziness, shortness of breath, nausea, vomiting, weakness and malaise. An additional side effect is leakage of fluid from the capillaries, which may build up in the lungs or brain and become serious or even life threatening.
Your body's effort to cope with less oxygen than it's used to can cause changes to your electrolytes (salts) and your body's balance of fluid and salts. In fact, studies have found a direct inverse correlation between hydration and symptoms of altitude sickness. This means that it is extremely important to drink enough water and other fluids (but not alcohol) when traveling to high altitudes, especially because of the potential loss of fluids from your capillaries. Make sure to drink at least to 17 to 25 cups (four to six liters) a day to keep properly hydrated. If you begin to experience altitude sickness, make sure not to exercise because you will lose even more fluid this way.
Anyone can acclimatize (adapt) to pretty much any altitude if they give it enough time. It takes one to three days to acclimatize to any given altitude. If you give yourself enough time to adapt as you climb you should not get altitude sickness, especially if you are well hydrated. Altitude sickness arises when going too high to fast or stay up very high for too long. If you do develop altitude sickness and are not able to acclimatize, descend to a lower altitude.