Sickness caused by being at a high altitude, usually above 2,400 meters (approximately 8,000 feet). The cause of altitude sickness is a matter of oxygen physiology. At sea level the concentration of oxygen is about 21 per- cent, and the barometric pressure averages 760 mm Hg. As altitude increases, the concentration remains the same, but the number of oxygen molecules per breath is reduced. At 5,400 meters (approximately 12,000 feet) above sea level, the barometric pres- sure is only 483 mm Hg, so there are roughly 40 percent fewer oxygen molecules per breath. In order to oxygenate the body effectively, the breath- ing rate must increase. This extra ventilation increases the oxygen content in the blood—but not to sea level concentrations. Because the amount of oxygen required for activity is the same at high alti- tude as at sea level, the body must adjust to having less oxygen. In addition, high altitude and lower air pressure cause fluid to leak from the capillaries, which can cause fluid buildup in the lungs and the brain. Prevention measures for altitude sickness include avoiding or retreating from high-altitude areas, gradual acclimatization, and medication. The acclimatization process is inhibited by dehydration, overexertion, and intake of alcohol and depressant drugs. Preventive medications include acetazo- lamide (brand name- Diamox) and dexamethasone (a steroid).