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.