Narcosis! Are you up for the challenge?

To help explain this, I have produced a graph which compares levels of nitrogen narcosis for a diver breathing air at depth against the equivalent levels of alcoholic intoxication. For convenience, the end point of the graph is assumed to be unconsciousness or stupor at around 120m/400 ft depth as against having a blood alcohol level of 310mg/100ml.


The actual resultant blood alcohol levels indicated will vary with the person’s age, weight, sex and duration over which the hypothetical drinks were consumed. The blood alcohol levels used in the graph were calculated using the BAC calculator which is based on the widmark method of calculating blood alcohol content.

The comparison is inevitably approximate. However, the opinion expressed is based on my long-term interest in diving neurophysiology.

I know of no direct, objective research to confirm the opinion expressed, so the graph is presently no more than a subjective opinion. So how close is it to reality? Hence my challenge! What we need is verifiable research data to confirm or refine the graph. Can anyone do it?**

This is important because useful analogies and conclusions can be drawn from such a graph.


Not least is the observation that whilst breathing air at 50 metres, which is the maximum depth a professional diver can work on air (in the UK), the diver is suffering from nitrogen narcosis equivalent to having drunk more than four pints of beer, or nearly a whole bottle of red wine, resulting in an equivalent blood alcohol level of about 130mg/100ml. A car driver would be arrested, jailed and branded a criminal with this level of alcohol in his bloodstream?

The present permitted limited for blood alcohol whilst driving a vehicle in the UK is 80 mg/100ml. (It is the same in New Zealand if you’re aged 20 years or over). If the same limit or the equivalent level of nitrogen narcosis was used to limit the depth of a professional air diver, he would be limited to a depth of about 30m/100 ft.

Food for thought!

** Needless to say, it would be potentially dangerous and inadvisable to attempt to experiment with levels towards the extreme end of the graph.

By John Bevan, editor Underwater Contractor International

(reprinted Underwater Contractor International Jul/Aug 2013,


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