What Is Nitrogen Narcosis?

Also Known as the "Rapture of the Deep" Nitrogen Narcosis Can Be Dangerous

scuba diver swimming through a school of silver sides
Reviewing basic suba diving concepts will let you relax and enjoy your dives. © Getty Images

Guiding a group over a small shipwreck at ninety feet, I looked to my right and noticed that one of my divers was laying on his side in the sand. What in the world? I thought.

I swam to his side and flashed an “okay” sign at him. He looked at me, slightly cross-eyed, and grinned around his regulator. Then he giggled and pointed at the shipwreck. I had seen enough divers exhibit similar behavior to recognize that he was experiencing nitrogen narcosis.

In diver jargon, he was “narced”. I ended the dive and ascended. On the surface, he told me that during the dive he thought that he was upright, and that the shipwreck, the divers, and the ocean floor were all turned on their sides as some sort of silly joke.

What Is Nitrogen Narcosis?

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. (Learn how to manage nitrogen 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.

At What Depths Do Divers Experience Narcosis?

The average depth at which a diver experiences at least a mild narcosis is 100 feet of seawater. By 140 feet, 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 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.

How Does Narcosis Affect Scuba Divers?

Narcosis has an anesthetic effect on a diver. In most cases of narcosis, the anesthetic effects of are not extreme and the diver experiences a somewhat altered state without the complete loss of consciousness.

1. Emotional Effects of Narcosis on Divers

Depending upon the diver and the dive environment, narcosis may cause a diver to feel either positive, euphoric emotions or negative, stressful emotions (a "dark narc").

Both scenarios are dangerous.

A diver feeling overly relaxed and happy may fail to react appropriately to a dangerous situation because he feels that everything is fine. An example is a euphoric diver who notices that he has exceeded his tank reserve pressure, but decides to continue diving because he feels great and therefore isn't worried about running out of air.

A diver who experiences feelings of dread or stress may perceive problems which do not exist or may react inappropriately to those that do. An example is a stressed diver who notices that he has reached his tank reserve pressure. He panics, inflates his buoyancy compensator, and rockets to the surface because he is afraid that he will run out of air if he makes a normal controlled descent, even though he has more than sufficient air to do so.

2. Narcosis Slows and Impairs Mental Abilities

Narcosis affects a diver's ability to reason, evaluate situations, decide on appropriate courses of action, and recall information. Narcosis also slows a diver's thinking and reaction times. In effect, a diver experiencing narcosis thinks less clearly and more slowly than he normally does.

Foggy thinking and reasoning underwater is dangerous. Even normal situations can lead to potential disasters as a diver's mental abilities decline. As an example, a diver who is negatively buoyant may fail to inflate his buoyancy compensator because he doesn't recognize the problem (failing to evaluate the situation). Or, he may try to compensate for negative buoyancy by kicking himself up (failing to decide on an appropriate course of action).

3. Physical Impairment from Narcosis

Narcosis affects a diver's coordination. He may have trouble accomplishing tasks requiring precise movements on deep dives.

Another physical effect of narcosis is impaired thermoregulation (temperature control). The shivering reaction that helps to warm a diver's body is reduced with narcosis. Even though a diver experiencing narcosis may be dangerously chilled, he typically feels warmer than he is due to his changed perceptions and mental functioning. This leads to the possibility of hypothermia. Physical impairment due to narcosis tends to begin at greater depths than the mental and emotional effects of narcosis.

Do you know . . . What Is the Difference Between Nitrogen Narcosis and Decompression Sickness?

How to Recognize Narcosis When Diving

The threshold at which a diver becomes narced varies from diver to diver. Divers experiencing narcosis are frequently unaware that they are functioning at a sub-optimal level. A diver's altered perceptions may cause him to feel well enough during the dive that he does not realize that his motor skills and mental functioning are impaired, making narcosis difficult to self-diagnose. To make matters worse, the diver's buddy is likely to be experiencing the same narcotic effects as the diver himself, and may not be able to help him identify when he is narced.

To identify narcosis, note any unusual emotions (even good ones). Also be aware of difficulty perceiving information, such as reading your pressure gauge or dive computer. Many divers report having unusual thoughts during narcosis. For example, I once marveled at the huge, huge size of a butterfly fish and made sure to smile and wink at it so that it would know that I was friendly.

Divers have also reported bizarre effects such as salt water tasting sweet or seeing colors differently on their pressure gauge. The diver I mentioned in the introduction saw the world as sideways (which makes sense as he was laying on his side), but interpreted this information as everyone playing a silly joke on him and turning the whole world on its side. He didn't realize that he was the sideways one. While the effects of narcosis may feel enjoyable in certain circumstances, a diver should still take action to counteract narcosis the moment he notices it because he will not be able to efficiently and appropriately react to unexpected situations.

Narcosis and Diving Part II: How to Treat and Minimize Narcosis When Scuba Diving