History of decompression research and development gets too high when compared to the ambient pressure. These bubbles, and products of injury caused by the bubbles, can cause damage to tissues known as "decompression sickness" or "the bends". The immediate goal of controlled decompression is to avoid development of symptoms of bubble formation in the tissues of the diver, and the long-term goal is to also avoid complications due to sub-clinical decompression injury. The symptoms of decompression sickness are known to be caused by damage resulting from the formation and growth of bubbles of inert gas within the tissues and by blockage of arterial blood supply to
Scuba set or use lower proportions of oxygen than air. In these situations divers often carry additional scuba sets, called stages, with gas mixtures with higher levels of oxygen that are primarily used to reduce decompression time in staged decompression diving. These gas mixes allow longer dives, better management of the risks of decompression sickness, oxygen toxicity or lack of oxygen (hypoxia), and the severity of nitrogen narcosis. Closed circuit scuba sets (rebreathers) provide a gas mix that is controlled to optimise the mix for the actual depth at the time. Gas cylinders used for scuba diving come in various sizes and