Altitude training (traditionally training at an altitude camp, or now commonly using altitude simulation tents) is the practice by some endurance athletes of training at high altitude, usually over 2,500 m (8,000 ft) above sea level, for several weeks. At this altitude, there is only 74% as much oxygen available as there is at sea level.http://www.higherpeak.com/altitudechart.html Table of effective oxygen percentage as a function of altitudehttp://www.altitudephysiology.org/oxygenphysiology/mounteverest/airpressurehighaltitude.htm Online high altitude oxygen and pressure calculator The body adapts to the relative lack of oxygen by increasing the concentration of red blood cells and hemoglobin. Proponents claim that when such athletes return to sea level (where they are competing) they will still have a higher concentration of red blood cells for 10-14 days.