Nitrogen narcosis can appear from 30 metres and impair your judgement without you noticing. Learn about the Meyer-Overton theory, why helium does not cause narcosis and how to respond.
Jacques Cousteau called it the rapture of the deep, and the description is not an exaggeration. Nitrogen narcosis is an altered state of consciousness produced by the effect of nitrogen at high pressure on the central nervous system. Understanding what it is, why it occurs, and how to manage it is an essential part of any diver's education.
The first symptoms can appear from 30 meters, although the depth varies considerably between individuals. This individual variability is precisely one of the factors that makes narcosis dangerous: there is no universal safe depth.
The most widely accepted scientific explanation is the Meyer-Overton theory: there is a direct correlation between the solubility of a gas in lipids and its narcotic potency. Nitrogen has sufficient lipid solubility to alter neuronal function at the pressures reached during deep recreational dives.
Helium, used in trimix or heliox, has much lower lipid solubility and produces virtually no narcosis. This is why technical divers who descend to 60 meters or more use helium-containing mixes.
Factors that intensify narcosis: cold, physical and mental fatigue, alcohol in the hours preceding the dive, anxiety, and stress. A well-rested diver who is well hydrated and alcohol-free will consistently have fewer problems.
Symptoms include deceptive euphoria, disorientation, difficulty reading instruments, confusion about the direction of ascent, and cognitive impairment. In severe cases there may be hallucinations or completely irrational decisions.
Narcosis is completely reversible: ascending a few meters causes the symptoms to disappear within seconds or minutes. No antidote or special treatment is needed. But the mistakes made while narcotized can have permanent consequences.
The best strategy is prevention and communication: agree on mutual check signals with your buddy before deep dives. If symptoms are detected, signal to each other and ascend together. Never ignore symptoms or try to push through them.