Climbers use supplemental oxygen to give them an edge while pushing to the summit of a mountain like Everest at 8850 meters. At that altitude, the available oxygen is 33% of that at sea level. It is like running up a staircase while holding your breath 2 out 3 steps. To summit Everest without using any supplemental oxygen anytime on the climb is rare, it is estimated less than 100 out of the over 6500 summits have been accomplished in this pure manner.