Nitrogen narcosis is an altered state of mind caused by breathing nitrogen at a high partial pressure. 

The deeper a diver descends, the higher the partial pressure of nitrogen and other gasses in his air will be. For this reason, nitrogen narcosis is usually thought of as a function of depth. The deeper a diver goes, the greater the narcosis. Although nitrogen is the principle component of air (79%), other gases in a diver’s tank are also narcotic at great depths, such as oxygen and carbon dioxide. For this reason, many training agencies are now referring to the narcosis caused by breathing compressed air at depth as “inert gas narcosis” rather than “nitrogen narcosis”. Of course, oxygen and carbon dioxide are not inert gases, so perhaps the best term to use is simply “narcosis”. Whatever you call it, the point is that more than one gas may influence a diver’s level of narcosis underwater.

Narcosis has been called the “rapture of the deep” and many divers compare narcosis to a feeling of pleasant drunkenness. In fact, divers sometimes use the “Martini Rule” to roughly estimate the effects of narcosis during a dive. Depending upon the source, the Martini Rule states that for every 30 or 60 feet of depth, a diver experiences the narcotic effect of drinking one martini.

Narcosis and Depth

The average depth at which a diver experiences at least a mild narcosis is 100 feet (30m) of seawater. By 140 feet (42m), most divers will experience significant narcosis. Diving beyond 140 feet (the recreational diving depth limit) while breathing air is strongly discouraged by most training organizations. Some divers will make dives up to 160 – 190 feet (48m – 58m) on air, but such dives require deep air training, and are generally frowned upon. If a diver exceeds a depth of 200 feet wile breathing air, he is likely to experience debilitating narcosis – even unconsciousness.