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A decompression stop is a pause in a diver's ascent made to allow the body to express dissolved gases in the blood. Without decompression stops, these gases would expand, turning into bubbles and causing decompression sickness. Decompression stops are a critical part of safe deep water diving, with the length and depth of such stops varying depending on the depth and length of the dive. Typically, more than one decompression stop is needed.
When people dive, their bodies are subjected to immense pressure as they go underwater. For every 30 feet (10 meters), an additional “atmosphere of pressure” is added, meaning that someone 60 feet (20 meters) below is experiencing the equivalent of three times the pressure at sea level. At a certain point, the pressure becomes so intense that the human body cannot survive, although no one has determined the precise point at which someone would die from the pressure. Along the way to the bottom, the gases in the human body dissolve into the blood, thanks to the immense pressure.
As a diver ascends, these compressed gases start to expand. If a diver ascends abruptly, the gases expand so quickly that the body cannot eliminate the gas safely, and the diver develops decompression sickness. Therefore, divers make a series of decompression stops to allow their bodies to acclimate to the decreased pressure. At each decompression stop, the diver breathes normally, allowing the dissolved gases to be expressed.
Many divers use computer software to calculate their decompression stops, although it is also possible to do the math by hand. Since most people dive with a buddy or group for safety reasons, people usually do their calculations independently and then compare to confirm that they have established a safe schedule of decompression stops. At regularly used dive spots, there may even be markers in the water to indicate sites for decompression stops, and a decompression stop may even have a decompression trapeze for a diver to rest on for the duration of the stop.
A decompression stop can sometimes be shortened by breathing a special oxygen-rich gas formulation known as “decompression gas” or “deco gas.” Breathing gas with a high oxygen level is dangerous in deep water, so deco gas is usually very clearly labeled so that a diver does not use it by accident. Decompression can also be accomplished in a hyperbaric chamber, a chamber which can be pressurized and controlled, allowing the diver to slowly become accustomed to the pressure at sea level.
Intriguingly, some divers have developed decompression sickness when they fly immediately after a dive. This is because even with decompression stops, the body may still be acclimating to the pressure at sea level, and most planes are underpressurized, so flying is the equivalent of ascending very quickly from a deep dive. For this reason, it's a good idea to wait at least twelve hours and sometimes longer to fly after a deep dive or series of dives.