Cabin air circulation. Space station systems Space station systems include technology that enables humans to live in outer space for a prolonged period of time. Such technology includes filtration systems for human waste disposal and air production. Skylab Skylab used 72% oxygen and 28% nitrogen at a total pressure of 5 psi. Salyut and Mir The Salyut and Mir space stations contained an air-like Oxygen and Nitrogen mixture at approximately sea-level pressures of 93.1 kPa (13.5psi) to 129 kPa (18.8 psi) with an Oxygen content of 21% to 40%. Bigelow commercial space station The life-support system for the Bigelow Commercial Space Station is being designed by Bigelow Aerospace in Las Vegas, Nevada. The space station will be constructed of habitable Sundancer and BA 330 expandable spacecraft modules. "human-in-the-loop testing of the environmental control and life-support system (ECLSS)" for Sundancer has begun. Natural systems Natural LSS like the Biosphere 2 in Arizona have been tested for future space travel or colonization. These systems are also known as closed ecological systems. They have the advantage of using solar energy as primary energy only and being independent from logistical support with fuel. Natural systems have the highest degree of efficiency due to integration of multiple functions. They also provide the proper ambience for humans which is necessary for a longer stay in outer space. Underwater and saturation diving habitats Underwater habitats and surface saturation accommodation facilities provide life-support for their occupants over periods of days to weeks. The occupants are constrained from immediate return to surface atmospheric pressure by decompression obligations of up to several weeks. The life support system of a surface saturation accommodation facility provides breathing gas and other services to support life for the personnel under pressure. It includes the following components: Underwater habitats differ in that the ambient external pressure is the same as internal pressure, so some engineering problems are simplified. Gas compression, mixing and storage facilities Chamber climate control system - control of temperature and humidity, and filtration of gas Instrumentation, control, monitoring and communications equipment Fire suppression systems Sanitation systems Underwater habitats balance internal pressure with the ambient external pressure, allowing the occupants free access to the ambient environment within a specific depth range, while saturation divers accommodated in surface systems are transferred under pressure to the working depth in a closed diving bell The life support system for the bell provides and monitors the main