Nitrogen narcosis¹ is a condition experienced by divers breathing compressed air when they breathe nitrogen at a partial pressure greater than three times atmospheric pressure. Atmospheric pressure is measured in millibars, 1000 millibars being the standard pressure at sea level, or one bar. The actual pressure at sea level is 1013 millibars but we will stick with standard pressure as it makes the sums easier. What are a few millibars between friends?

Partial Pressure

Partial pressure is the total pressure exerted by a gas in a mixture of gases and is equal to the sum of the partial pressure that each member gas has and would alone have if the other gases were absent. The air we breathe is a mixture of 79% nitrogen, 20% oxygen and 1% other gases consisting mainly of argon but small but important traces of carbon dioxide and water vapour. Therefore, the partial pressure of nitrogen in the air at sea level is 790 millibars.

As a diver descends in water the ambient pressure increases at a rate of one bar for every ten metres. So at a depth of ten metres, the partial pressure of nitrogen is 1.6 bars, at 20 metres 2.4 bar and at 30 metres 3.2 bars. Therefore, a diver at a depth of 30 metres breathing compressed air is subject to nitrogen narcosis.

Effects

The effect varies on individuals as their tolerance varies depending on their metabolism. Some individuals with a fast metabolism can experience nitrogen narcosis at a depth of 25 metres, with others, the onset can be delayed to more than 30 metres. An individual's tolerance to nitrogen narcosis is similar to that of alcohol and the effects are very similar, but without the hangover.

As the diver descends, the effect becomes greater until they have reached a depth of around 60 metres when the effect becomes so pronounced that it is dangerous. It induces a feeling of euphoria and excitement, making the diver believe that he is capable of anything, even breathing like a fish. Divers with severe nitrogen narcosis have been known to discard their breathing apparatus with the obvious dangers that brings. Matched with this feeling of over-confidence is a corresponding decrease in the ability do even the simplest of tasks. Coordination gets progressively worse, leading eventually to unconsciousness.

Symptoms

The symptoms experienced by the diver are numbness in the lips, legs and feet. Other divers can recognise it by a deadpan look on the affected diver's face, as can be seen in many bars on a Saturday night. The effects quickly wear off when the diver ascends to a shallower depth where the only residual effect is amnesia surrounding the time when they were narcotic. Experienced divers can build up a tolerance to nitrogen narcosis with gradual exposure and learning to recognise the symptoms.

Nitrogen narcosis itself is not dangerous. Anymore than sitting at home in your favourite armchair with a bottle of your favourite tipple is dangerous. However, it is dangerous in the same way that driving a car after drinking your favourite tipple is. Consequently 60 metres is considered the absolute maximum depth for breathing compressed air.

Incidentally, at depths approaching 100 metres when the partial pressure of oxygen in compressed air is greater than two bars the oxygen becomes poisonous and that can be fatal. Commercial divers who regularly dive to depths of more than 100 metres in the North Sea get round the problems of nitrogen narcosis and oxygen poisoning by breathing a mixture of helium with a reduced percentage of oxygen. Helium brings its own problems, not least that it is a very good heat conductor and can cause the diver to lose body temperature at a very rapid rate.

How to Get it

One way to experience nitrogen narcosis in a controlled environment is in a decompression chamber. Decompression chambers are used to treat certain medical conditions and to treat decompression sickness or the bends. Decompression chambers are sometimes used to give divers experience of breathing high pressure air at an equivalent depth of around 50 metres. An added hazard, if you can call it that, is that in a decompression chamber talking to each other is possible. Because of the high pressure air, the voice turns Mickey Mouse-esque. To see a 14-stone man talking like Mickey Mouse and not being able to do a thing about it is guaranteed to have everyone rolling on the floor in uncontrollable laughter, helped by the effects of nitrogen narcosis.

On leaving the decompression chamber, you know you have had a good time but cannot remember much about why - just like a Sunday morning after a good night out.