The Altitude Penalty

Climb a mountain and air pressure drops — there's simply less atmosphere above you pushing down. At the summit of Mount Everest (8,849 m), atmospheric pressure is only about one-third of sea-level pressure.

At 2,500 meters, most people feel slightly breathless during exercise. At 5,000 meters, altitude sickness is common — headaches, nausea, confusion. Above 8,000 meters is the death zone: your body consumes oxygen faster than breathing can supply it.