Some don't realize oxygen depletion is a problem as one scales Everest. Son did documentary of a group going up the mountain and some had to go back down when oxygen became more scarce. The body couldn't deal with it. Son had to go in areas around the world to make doc. films so he ran and lifted weights to keep his body in good shape to handle various problems, like the north pole, Siberia, and deserts, jungles, mountains, etc.