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Effects of breathing high pressure gas
Decompression sickness
The diver must avoid the formation of gas bubbles in the body, called decompression sickness or 'the bends', by releasing the water pressure on the body slowly at the end of the dive and allowing gases trapped in the bloodstream to gradually break solution and leave the body, called "off-gassing." This is done by making safety stops or decompression stops and ascending slowly using dive computers or decompression tables for guidance. Decompression sickness must be treated promptly, typically in a recompression chamber. Administering enriched-oxygen breathing gas or pure oxygen to a decompression sickness stricken diver on the surface is a good form of first aid for decompression sickness, although fatality or permanent disability may still occur.
Nitrogen narcosis
Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in divers who breathe high pressure gas at depth. The mechanism is similar to that of nitrous oxide, or "laughing gas," administered as anesthesia. Being "narced" can impair judgement and make diving very dangerous. It occurs at any depth, but in most cases doesn't become noticeable until deeper depths; typically when breathing air at around 30m/100 ft. Jacques Cousteau famously described it as the "rapture of the deep". Nitrogen narcosis occurs quickly and the symptoms typically disappear during the ascent, so that divers often fail to realize they were ever affected. It affects individual divers at varying depths and conditions, and can even vary from dive to dive under identical conditions.
cpi: The relation of depth to narcosis is informally known as "Martini's law": It's like one martini per 10 meters below 20 meters.
Oxygen Toxicity
Oxygen toxicity occurs when oxygen in the body exceeds a safe "partial pressure" (PPO²). In extreme cases it affects the central nervous system such as to cause a seizure, which can result in the diver spitting out his regulator and drowning. The condition is preventable provided one never exceeds the established maximum depth of a given breathing gas. For deep dives, (generally past 130 feet/39 meters) "hypoxic blends" containing a lower percentage of oxygen than atmospheric air are utilized.
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