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The trick is to use a nitrox mix which has the right amount of oxygen to safely accelerate decompression – and to use it at the right depth where the nitrox does not become toxic. The consequences of getting it wrong can be fatal.

A commonly used enriched air nitrox (EAN) mix that is suitable to breathe and shorten decompression (compared to breathing standard compressed air all the way to the surface) from a depth of 20 metres upwards is EAN50, which comprises 50 per cent oxygen and 50 per cent nitrogen. The increased amount of oxygen and reduced amount of dangerous nitrogen shortens (or accelerates) the time needed for decompression before surfacing.

At a depth of 20 metres, the water pressure on your body is three times what the atmospheric pressure on your body is as you read this right now. So the aqualung feeds the diver three times as much compressed breathing gas to keep the pressure in the lungs exactly the same as the surrounding water pressure – and avoid a lung collapse. This means that in every breath the diver is breathing in three times as much oxygen as on the surface. If each breath is 50 per cent oxygen, or half of the total mix, we could say that at the surface that the partial pressure of oxygen (abbreviated to PO2) is 0.5. At 20 metres, breathing three times as much oxygen the partial pressure is 3 x 0.50 = PO2 of 1.5 bar.