Answer:
Air Pressure
Step-by-step explanation:
The lower the air pressure, the more difficult it is to breathe
The higher the height above sea level, the less air pressure there is in the atmosphere
According to National Geographic, 'Mount Everest's Summit (29,000 ft ASL) is approximately 1/3 the air pressure that exists at sea level'
Scientists have estimated that around 19,000 ft and above, the human body isn't capable of remaining indefinitely.
Frost-bite is also definite if you don't climb prepared. Due to the lower air pressure, the heart has to work harder to pump blood and deliver the needed heat throughout your body.