High altitude illness

The higher you climb, the thinner the air.
That's because as altitude increases, barometric pressure drops and the volume of air expands. The oxygen molecules spread themselves out in the expanded volume of air, so you take in less oxygen each time you breathe. For example, at 3658 m (12,000 ft.), you get 40% fewer oxygen molecules in each breath.

Why is that dangerous?
Well, your body isn't getting as much oxygen as it's used to. So you breathe faster, but that doesn't compensate completely for the change in oxygen levels—creating a lot of stress on your system.

High-altitude climbers ascend mountains is stages, giving their bodies time to adjust. In addition, they often carry oxygen supplies with them to help their breathing. Without these precautions, climbers can get very sick: for reasons not entirely understood, the combination of high-altitude and low air pressure can cause fluid to build up in the lungs or brain, leading to a life-threatening condition known as edema.