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Many divers do in fact use EAN80 (80 per cent oxygen and 20 per cent nitrogen) for decompression. This higher oxygen level is very beneficial for decompression, but can only be breathed from about 12 metres up to the surface. To breathe EAN80 deeper than 12 metres or to breathe EAN50 deeper than 20 metres, or EAN100 (100 per cent pure oxygen) deeper than 9 metres, risks a potentially fatal oxygen toxicity hit. Thus, every deco mix, be it EAN50, EAN80, EAN100 or whatever, all have their own depth limits where the amount of oxygen in the mix becomes toxic – and potentially fatal.

In a contrast to open-circuit diving, Paul and I, in common with the majority of technical divers, have for a long time been using closed-circuit rebreathers (CCRs).

Whereas in open-circuit diving the exhaled breathing gas is vented to the surface, a closed- circuit rebreather continuously recirculates the same breathing gas – there is no venting to the surface. One of the benefits of using a rebreather is that a diver can program their onboard computer to never let the PO2 in the breathing gas loop drop below a certain level.