Near the summit, solar radiation is brutal. Climbers who breathe with their mouth open can burn the roof of their mouth, and dehydration and headaches are common.

At above 17,000 feet, the body literally consumes itself for energy:

• Faces swell overnight, particularly around the eyes
• Coughing can be so violent that people crack ribs
• Cracks appear in fingers and tongues
• Sleeping becomes a problem
• Weight loss occurs

At high altitudes, the human body reacts to the lack of oxygen by producing more red blood cells. As a result, climbers absorb more oxygen per breath, but as their blood thickens, they’re also twice as likely to have a heart attack or stroke, or a cerebral edema -- a swelling of the brain that causes disorientation, temporary insanity and even death.

Climbers on the verge of the Everest summit will take as many as 15 breaths to move just one step.

On summit day, climbers can burn 12,000 to 15,000 calories - ten times what the body uses on an average day.