changes in hematologic parameters. Hematocrit and hemoglobin concentrations remain in general unchanged. There are a number of companies who provide altitude training system, most notably Hypoxico, Inc. who pioneered the artificial altitude training systems in the mid-1990s. A South African scientist named Neil Stacey has proposed the opposite approach, using oxygen enrichment to provide a training environment with an oxygen partial pressure even higher than at sea level. This method is intended to increase training intensity. Principles and mechanisms 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. Increased red blood cell volume At high altitudes, there is a decrease in oxygen hemoglobin saturation. This hypoxic condition causes hypoxia-inducible factor 1 (HIF1) to become stable and stimulates the production of erythropoietin (EPO), a hormone secreted by the kidneys, EPO stimulates red blood cell production from bone marrow in order to increase hemoglobin saturation and oxygen delivery. Some athletes demonstrate a strong red blood cell response to altitude while others see little or no gain in red cell mass with chronic exposure. It is uncertain how long this adaptation takes because various studies have found different conclusions based on the amount of time spent at high altitudes. While EPO occurs naturally in the body, it is also made synthetically to help treat patients with kidney failure and to treat patients during chemotherapy. Over the past thirty years, EPO has become frequently abused by competitive athletes through blood doping and injections