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Basically, the more oxygen in your deco mix, the more you can shorten – or accelerate – your decompression stops. But there are certain depth limitations for different levels of oxygen in your deco mix, as these increased oxygen levels when you are diving can be dangerous at different depths.

The air you are breathing just now, reading this book on the surface, is comprised of 79 per cent nitrogen and about 21 per cent oxygen. Although largely inert on the surface, at high pressure levels nitrogen has a narcotic effect – the nasty diving problem called nitrogen narcosis. So both of the elements that make up ordinary air, nitrogen and oxygen, can become problematic when you are diving deep.

Nitrogen narcosis is a creeping (and at first largely unnoticeable) debilitating effect, which starts for me (when I’m diving on air) at a depth of about 30–35 metres. You need to know a little about the mechanics of diving to understand how it becomes a problem.

As a diver descends, the increasing weight of water surrounding them tries to compress internal air spaces such as their lungs, which are, simplistically, just bags of air. Imagine taking an air-filled crisp bag down underwater – it would very quickly be compressed to a fraction of its size by the surrounding water pressure. To avoid this eventually fatal effect happening to a diver’s lungs, an aqualung (or breathing regulator) delivers increasing amounts of compressed air with each breath as they descend. The aqualung delicately and rather cleverly keeps the air pressure in the diver’s lungs exactly equal to the increasing water pressure around the diver. The lungs stay the same size as topside, and no catastrophic collapse happens.