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Rather than breathing compressed air at any depth greater than 40 metres, nowadays I always dive on a trimix diluent, which replaces a large element of the dangerous nitrogen in the breathing mix with helium, which has no discernable narcotic effect. Although nitrogen narcosis is no longer an issue for me, if you want to get an idea of what nitrogen narcosis can do, I recounted a hit I got in the chapter entitled ‘Bail out on the Cushendall’ in The Darkness Below. This was a 58-metre air dive in 3-metre visibility, in a current, on the wreck of the World War II casualty SS Cushendall which lies off north-east Scotland. Oh, the things we do when we are young …

Whereas oxygen is very therapeutic and beneficial in normal use, as the aqualung feeds a diver increased volumes of breathing gas on descending, this means that in addition to getting higher partial pressures (or concentrations) of nitrogen, the diver also gets increased volumes of oxygen.

Oxygen, the very stuff that keeps us alive on the surface (and of course underwater as well) becomes increasingly toxic in the larger volumes breathed by divers as they venture deeper. The risk of an oxygen toxicity hit becomes a very real danger. This starts off with twitching and spasms but rapidly develops to uncontrollable convulsions where a diver will amongst other things, rip off their mask and spit out the breathing regulator. In water, a hit nearly always results in drowning, unless the diver is wearing a full-face mask. A number of leading technical divers have sadly ‘ox-toxed’ over the years and died of the uncontrollable convulsions – so deadly underwater. Some had mistakenly breathed their shallow water oxygen-rich decompression gases at too great a depth, quickly bringing on a fatal oxygen toxicity hit.