WHEN YOU TEACH DEEP DECOMPRESSION-DIVING ON AIR, you regularly see people in the grip of narcosis.
A diver leading a group will keep going beyond the turn point of the dive, finning away along the reef wall, oblivious to having left companions behind. Another will glance at her computer and, 10 seconds later, will look at it once more, then again 10 seconds after that.
A third will wait at the bottom of the line, staring fixedly at a branch of coral for a good two to three minutes instead of ascending, until roused from his reverie. Later, the diver will tell you that he thought it was a moray eel and that he was waiting for it to move.
Incidents such as these can be amusing, but there is a serious side to narcosis. Divers who head off into the blue without their buddies lose the support of the team.
If they are in an overhead environment, they risk getting lost.
Divers who can’t remember what their computer was showing just a few seconds after looking at it are suffering from short-term memory-loss, and this can reduce their ability to follow a dive plan.
And divers who delay their ascent on account of an imaginary eel are increasing their decompression burden for every extra second they remain at depth, and risking running out of gas in the final stages of their ascent.

Air is an intoxicating cocktail. Many people will tell you that they enjoy the “buzz” of going deep on air.
This feeling is partly a response to their subconscious awareness that they are no longer in complete control of their surroundings and that, if something were to go wrong, they might not be thinking clearly enough to handle it – something like the thrill of riding a roller-coaster.
However the “buzz” is also due to “anaesthetic potential”, a dangerous property of all gases. In the right quantity, any gas can knock you out. Some gases, like nitrous oxide (laughing gas) have long been known as excellent anaesthetics.
The major constituent gas in air, nitrogen, has anaesthetic potential and, as you dive deeper and as the partial pressure of the gas increases, the depressant effect on your central nervous system becomes greater. Divers call it narcosis. Jacques Cousteau called it “l’ivresse des profondeurs”, the drunkenness of the depths!
As Cousteau poetically suggested, the effect is similar to that of alcohol. And just as with alcohol, it is dose-related; the degree to which you are affected depends on the quantity you consume.
The effects of narcosis are progressive and increase with time and depth. At 30m divers breathing air will experience symptoms such as mild euphoria and slow reactions; by the time they get to 50m, their judgment will be significantly impaired.
The subjective symptoms differ from diver to diver and from dive to dive, but the objective effect is the same. Often, it manifests itself as overconfidence, fearlessness or over-relaxation, which is beneficial to a degree, but in an emergency this can produce confusion and slow response-time.
For instance, the diver who is deep on air when he finds that his regulator is free-flowing may, in his befuddled state, remain at depth while he tries to fix the problem and while his air is gushing quickly away into the ocean, instead of ascending immediately or swimming to a team-member for assistance.
Narcosis also affects the memory. When you ask people to tell you about their dives, the things they always remember most clearly are the things they saw during the comparatively shallow portions of the dive.

However, the good news is that to a certain extent you can train yourself to accommodate your impaired state.
The first step is to recognise that you are mentally impaired when you are at depth. The next step is to learn to concentrate on important issues such as time, depth and the tasks you need to carry out, and not let the euphoria cloud your mind.
Maintain a tight focus, slow down and exercise mental control over every movement. Success in performing a task at depth is far more likely if the action is performed in a rehearsed sequence of steps rather than in one continuous, flowing movement.
Professionals use memory cues. Technical divers always carry slates to write out run-times, decompression schedules and back-up plans, and instructors have checklists of the things they need to do during a training dive, and make notes for debriefing the students later. They know from experience that they cannot entirely depend on memory.
This is also one of the main reasons why it is essential that you should spend time rehearsing emergency drills and self-rescue skills until they become instinctive. Developing automatic correct responses to emergencies is the best way to combat the narcosis-assisted confusion that will prevent you finding the right solution if you rely on your intellect alone.

A number of factors can exacerbate a diver’s predisposition to narcosis. These include fatigue, alcohol, stress, cold and dark water.
Anticipating and mitigating the effect of these additional factors is the key to dealing with them - for instance, using a drysuit in cold water and minimising alcohol intake and getting a good night’s sleep before a deep-diving day.

There have been a number of studies suggesting that oxygen is also narcotic, although there are no conclusive data, and scientific opinion is divided.
Aside from academic interest, the debate would be significant for sport-divers only if oxygen were proven not to be narcotic, in which case nitrox might be a useful tool in reducing narcosis.
However, in the absence of such proof, the assumption is that nitrox and air are equally narcotic.

Helium is a very light, non-toxic, minimally narcotic gas. By adding it to your breathing gas, among other benefits you can reduce the percentage of narcotic gases in the mix, which means that you dive with a much clearer head.
Why don’t we all use helium when we dive? Mainly because there is a limited supply, and it is therefore expensive.
The extra cost is manageable for closed-circuit rebreather divers because they use much less gas than open-circuit divers.
For the rest of us, if we want to dive beyond the shallows narcosis is just something that we have to deal with as one of the inescapable facts of scuba-diving life.

Photo credits:
Jan Brown
William Bynum

Read more from Simon Pridmore in:
Scuba Confidential – An Insider’s Guide to Becoming a Better Diver
Scuba Professional – Insights into Sport Diver Training & Operations
Scuba Fundamentals – Start Diving the Right Way

All are available on Amazon in a variety of formats.