Before 1953, not a single person had climbed to the summit of Mount Everest, the highest point on earth. Because the air at the summit is so thin -- it contains 1/3 less oxygen than air at sea level -- many people doubted that it was even possible. Physiologists speculated that a climber, even if he or she were breathing bottled oxygen, would be unable to reach the top because the amount of supplemental oxygen required on the ascent would simply be too much for one person to carry.

The doubters were proven wrong in 1953, when Edmund Hillary and Tenzing Norgay climbed the peak breathing bottled oxygen. They were proven wrong again in 1978, when two other climbers became the first to reach the peak without supplemental oxygen. However, the doubts expressed by the nay-sayers were not without merit. Most climbers and physiologists agree that if Everest were just a bit higher -- as little as 1,000 feet higher -- it would be unclimbable without bottled oxygen. Indeed, even well below the 29,028-foot summit, humans can't survive for more than a few days. Climbers call the area above 26,000 feet the "Death Zone" because of the debilitating and ultimately fatal effects this environment has on the body.

The low air pressure at extreme altitudes not only reduces the amount of oxygen in the air (by reducing the density of air), but also makes the transfer of the oxygen from the lung tissue into the lung capillaries a less efficient process. There is simply less pressure to push the air through the capillary walls. To compound this problem, as a climber works harder at extremely high altitudes, his or her heart beats faster. This causes blood to pass more rapidly through the capillaries, giving the oxygen less time to move from the lung tissue into the bloodstream.

High-altitude climbers suffer in a variety of ways from the low-oxygen conditions they experience on peaks like Everest. Despite the body's efforts to compensate - by increasing breathing rate and, over time, increasing the production of red blood cells - tissues become starved of oxygen and slowly begin to die. Even below the Death Zone, experienced climbers suffer from the lack of oxygen. Most obviously, they tire more quickly than they do at sea level because the oxygen their cells require to burn food molecules are in short supply.

Tests, like the memory test presented to climbers on Everest, also show that the brains of climbers function more slowly at high altitudes. With too little oxygen fueling their brain cells, the ability of climbers to process information slows. They find it difficult to remember instructions, recognize familiar objects and images, and solve simple logic problems. In general, say physiologists, the higher a person climbs, the slower he or she thinks. This slow thinking, some believe, can have dire consequences in an environment in which technical proficiency and rational thought are critical to one's survival.