Narcosis while diving (also known as nitrogen narcosis, inert gas narcosis, raptures of the deep, Martini effect) is a reversible alteration in consciousness that occurs while scuba diving at depth. It produces a state similar to alcohol intoxication. It can occur during shallow dives, but usually does not become noticeable until greater depths, beyond 30 meters (100 ft). Apart from Helium, all gases that can be breathed have a narcotic effect. As depth increases, the effects may become hazardous as the diver is increasingly impaired. Although divers can learn to cope with the effects, it is NOT possible to develop a tolerance. While narcosis affects all divers, predicting the depth at which narcosis will affect a diver is difficult, as susceptibility varies widely from dive to dive and amongst individuals. The condition is completely reversed by ascending to a shallower depth with no long-term effects. For this reason, narcosis while diving in an open water recreational situation rarely develops into a serious problem as long as the divers are aware of its symptoms and ascend to manage it.
Enriched Air Nitrox Diving in contrast to many assumptions made by novice divers mandates a diver to dive shallower not deeper than when diving on air but for longer. Indeed some argue that the narcotic effects of Oxygen is actually greater than Nitrogen making Enriched Air Nitrox more narcotic than air at a given depth.
Hence the only way to eliminate or reduce the narcotic effect of diving deeper is to reduce the Nitrogen in the breathing gas, fix the Oxygen at a level that is safe to dive at the given depth and fill the remainder of the tank with Helium which has little narcotic effect. This leads to a breathing gas consisting of Oxygen, Helium and Nitrogen hence the name Trimix. Due to the cost of Helium usually some Nitrogen is left in the breathing gas, however when a pure Helium/Oxygen breathing gas is used it is called Heliox.
