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An altitude tent is something that stimulates the body to an existing altitude. Red blood cells carry oxygen to the body; athletes benefit from increased delivery of oxygen to the muscles, and mountain climbers can avoid altitude sickness by better utilizing the diminished amount of oxygen found at higher altitudes.
The basic concept of living or training at altitude is to cause the body to adapt to the lower oxygen content by producing more oxygen-carrying red blood cells and hemoglobin. This improves the athlete’s ability to perform work, because more oxygen is available to the working muscles.
Sleeping in a simulated altitude environment allows the body to achieve some of the positive adaptations to altitude while still permitting the athlete to perform workouts at an oxygen-rich lower altitude where muscles can perform at their normal work level. An altitude tent is one way to enable athletes living at any elevation sleep in a high altitude-like environment. A more expensive option gaining popularity amongst professional athletes is to convert their entire bedroom to altitude.
Rather than simulating altitude with actual low air pressure (which would require substantial engineering and the use of an airlock to prevent implosion), the altitude tent remains at normal air pressure, substituting low concentration of oxygen for low pressure. While normal air contains 20.9% oxygen independent of altitude, the air in an altitude tent contains as little as 12% oxygen (the remainder being nitrogen). The partial pressure of oxygen inside the tent is the same as it is at the natural elevation that the tent is simulating.
Most altitude tents create the low-oxygen environment with a “hypoxic air generator” outside the tent pumping the hypoxic (low oxygen) air into the tent. This displaces the more oxygen-rich air inside the tent and with it the excess carbon dioxide exhaled by the occupant(s). Most athletes use altitudes between 8,000 and 15,000 feet. To cause a physiological response, the altitude must be sufficient to reduce blood oxygen saturation (sometimes measured by a pulse oximeter) to approximately 90%.
The tents themselves come in several styles. Unlike camping tents, altitude tents cannot have much ventilation, and often substitute clear plastic windows for the typical nylon and mesh vents. Displaced air escapes the tent through small outlets, seams, or zippers. Air delivery can be through hose long enough to allow the generator to be placed in a different room, reducing noise. Tents are typically placed on the bed, with the floor of the tent between mattress and box springs.
One challenge with altitude tents is the build up of heat and humidity. Because of the use of plastic panels to reduce exchange with the room, heat and humidity can build up in an altitude tent. Some tents allow the use of air conditioners to maintain comfortable conditions. Carbon dioxide from exhaled air can also build up to uncomfortable levels if air exchange is too low. In recent years, advances in altitude tent design, and in the performance of the hypoxic air-supply units, have all resulted in greater air-exchange and significantly lower noise levels.
An alternative to the sealed altitude tent is the altitude canopy, which drapes over the user's bed, and features a weighted edge instead of a tent floor. The canopy maintains high altitude even though there may be small gaps around the edge by maintaining a high flow rate of hypoxic air through the canopy, effectively preventing room air from entering the gaps. By virtue of smaller internal volumes, canopies come to altitude faster than conventional altitude tents.
The ethics of the use of these devices by athletes has been discussed by the World Anti-Doping Agency (WADA), which claimed that it could be equivalent to blood doping and therefore they should be banned; however, on September 16, 2006, Dick Pound of the WADA announced that "...the overwhelming consensus of our health, medicine and research committees – was that, at this time, it is not appropriate to do so,"  No explanation was given as to how WADA would have enforced a ban.
The USADA report on doping in the Lance Armstrong case also indicates that sleeping in an altitude tent can be used to hide doping using EPO, as natural Erythropoietin production is increased, confusing the tests.
See also 
- * The Australian September 17th 2006.