Altitude training works because of the difference in atmospheric pressure between sea level and high altitude. At sea level, air is denser and there are more molecules of gas per litre of air. Regardless of altitude, air is composed of 21% oxygen and 78% nitrogen. As the altitude increases, the pressure exerted by these gases decreases. Therefore, there are fewer molecules per unit volume: this causes a decrease in partial pressures of gases in the body, which elicits a variety of physiological changes in the body that occur at high altitude.The physiological adaptation that is mainly responsible for the performance gains achieved from altitude training, is a subject of discussion among researchers. Some, including American researchers Ben Levine and Jim Stray-Gundersen, claim it is primarily the increased red blood cell volume.Others, including Australian researcher Chris Gore, and New Zealand researcher Will Hopkins, dispute this and instead claim the gains are primarily a result of other adaptions such as a switch to a more economic mode of oxygen utilization.