<P> The effects of high altitude on humans are considerable . The percentage oxygen saturation of hemoglobin determines the content of oxygen in blood . After the human body reaches around 2,100 m (7,000 feet) above sea level, the saturation of oxyhemoglobin begins to decrease rapidly . However, the human body has both short - term and long - term adaptations to altitude that allow it to partially compensate for the lack of oxygen . Athletes use these adaptations to help their performance . There is a limit to the level of adaptation; mountaineers refer to the altitudes above 8,000 metres (26,000 ft) as the "death zone", where it is generally believed that no human body can acclimatize . </P> <P> The human body can perform best at sea level, where the atmospheric pressure is 101,325 Pa or 1013.25 millibars (or 1 atm, by definition). The concentration of oxygen (O) in sea - level air is 20.9%, so the partial pressure of O (pO) is 21.136 kPa . In healthy individuals, this saturates hemoglobin, the oxygen - binding red pigment in red blood cells . </P>

What happens to the air when exercising at an altitude that is above sea level