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Trials have shown that a PO2 of 1.4 is relatively safe, but above a PO2 of 1.6, you are entering an area where the oxygen concentration in your body is starting to become toxic – and if the levels increase or if that same level is breathed for more than a certain time, you risk an oxygen toxicity hit, convulsions and death. That’s why we put a maximum depth limit on breathing EAN50 of 20 metres, where the PO2 is 1.5 bar.

But EAN50 has a fixed percentage of oxygen in it at all times – 50 per cent. Thus, at 10 metres, the PO2 is twice 0.5 = 1.0 bar. There’s less therapeutic oxygen in the breathing mix compared to breathing, say, EAN80 with 80 per cent oxygen, where the PO2 is 1.6 bar. So, although EAN50 is good because you can start breathing it deeper, at 20 metres, and start reducing the level of nitrogen in your body early, in the shallows it is not giving you as much oxygen as you could safely breathe. You can breathe pure 100 per cent oxygen from 9 metres to the surface, which is extremely good at accelerating decompression. Thus, in the shallows EAN50 is not such an effective decompression gas.