As a diver descends, the water pressure on the diver increases. With the increase in pressure, nitrogen from the air the diver is breathing dissolves in the diver’s blood and other body tissues. The quantity of dissolved nitrogen increases with depth (pressure) and time.
When ascending at the end of a dive, the nitrogen dissolved in the body tissues is released. If the diver ascends too quickly, the released nitrogen forms bubbles in the divers body before it can be expelled through the lungs. It is these nitrogen bubbles which cause decompression sickness or the bends. The symptoms of decompression sickness include joint pains and paralysis.
To compensate for the amount of nitrogen accumulated over a long and deep dive, divers make slow ascents and pause for calculated periods at various depths during the ascent to prevent nitrogen being released too quickly. These breaks in the ascent are known as decompression stops. Many divers carry instruments called dive computers to automatically calculate their decompression stops.
Normal air contains 21% oxygen and 79% nitrogen. To speed up decompression, divers can breathe nitrox during decompression stops, a mixture of oxygen and nitrogen with more than 21% oxygen. Commonly used mixtures for decompression are 50%, 80% and 100% oxygen. Because there is less nitrogen in the gas the diver is breathing, nitrogen released during the ascent from a dive is expelled from the diver’s body more readily and the risk of decompression sickness is reduced.
Nitrogen is also narcotic under pressure. At depth, the narcotic affect of nitrogen leads to nitrogen narcosis, with symptoms that are often similar to drunkenness. For exceptionally deep dives, some divers use a mixture of oxygen, nitrogen and helium called trimix. Including helium in the breathing gas reduces the affect of nitrogen narcosis, however, like nitrogen, helium also dissolves into the divers body tissues, so a diver using trimix still has to make decompression stops.