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Freezers designed to store blood bank products and samples at temperatures below the point at which most or all biological activity ceases (sometimes defined as the glass transition temperature of water) about -130 degrees Celsius (-202 degrees Fahrenheit). These freezers typically use liquid nitrogen as the cooling agent; some specially designed electromechanical systems are also capable of cooling at these temperatures. Products may be stored either directly in liquid nitrogen, which provides a temperature of -180 degrees Celsius (-292 degrees Fahrenheit) or colder with minimal temperature fluctuations; or they may be stored at the vapor phase above liquid nitrogen at a temperature of about -140 degrees Celsius (-220 degrees Fahrenheit). Storage in the vapor phase decreases the risk of cross-contamination between stored products. Red blood cells (RBCs) preserved with a low concentration of a cryoprotective agent such as glycerol at 20 percent or dimethyl sulfoxide may be stored for long periods, typically up to 10 years. Cryogenic blood bank freezers are used mainly to store rare types of RBCs.
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