Most people carry a canister of oxygen when they climb because there's not much oxygen on Mount Everest. As you climb higher and higher, it gets harder to breathe. You cannot stay long on the top of the mountain because you cannot breathe, and plus its really freezing cold on Mount. Everest.
At 10,000 to 14,000 feet, the available oxygen is only 60 to 70 percent that of sea level. At this elevation, climbers can experience illnesses that may be caused by the higher than normal altitude. One of our teachers at school experienced this problem. She was hiking in Nepal and had all of the symptoms from the high altitude.
On Everest's summit, about
30 percent of the climber's oxygen that he/she
takes in, goes to the physical activity of breathing. In other
words, this means that a climber is using of 30 percent of their
energy just to breathe. A normal person, at sea level, that is
exhausted because of exercise, only gives 7 percent of his/her
energy to the activity of breathing.