This reduces the amount of oxygen that reaches the organs of the body.

VOA: special.2009.07.14

At about nineteen-thousand meters, the air is so thin and the amount of oxygen so small that a person needs a lot of special equipment to survive.

VOA: special.2011.04.06

This makes it more difficult for the body to effectively use the same amount of oxygen as it would on the ground.

VOA: special.2009.07.14