Why pure O₂ speeds up your deco

Posted 07 July 2026 · Field Note #25

Off-gassing is driven by a pressure difference. The inert gas dissolved in your tissues wants to move toward wherever its partial pressure is lowest — and the lowest place available is the gas in your lungs. The bigger that difference, the faster the gas leaves. Everything about smart decompression comes back to widening that gap.

Now switch to a high-oxygen deco mix. By filling your breathing gas with O₂, you crowd out the nitrogen and helium almost entirely — their partial pressure in your lungs drops toward zero. That opens the widest possible gradient between the inert gas in your tissues and the gas you're breathing. Bigger gradient, faster washout. This is the main reason a switch to EAN50 or pure O₂ shortens your stops so dramatically.

There's a smaller bonus on top. Your tissues constantly consume oxygen for metabolism, so the total gas tension in your venous blood sits a little below ambient pressure. That deficit is the oxygen window — a partial vacancy that gives inert gas a touch more room to leave solution. It's real, but modest, and it doesn't keep growing without limit as you pile on more O₂. The lion's share of the benefit is the steep inert-gas gradient, not the window itself.

So why not breathe pure O₂ everywhere? Because oxygen at pressure is toxic. Most guidance caps deco ppO₂ at a 1.6 ata ceiling, and pure O₂ reaches 1.6 at just ~6 m / 20 ft. Go deeper on it and you risk a CNS oxygen-toxicity convulsion — almost always fatal underwater.

The practical takeaway: carry your richest legal mix for the shallow stops, where its MOD allows it and the off-gassing payoff is greatest. EAN50 typically rides from ~21 m, pure O₂ from ~6 m. Switch as soon as each mix becomes safe, verify the cylinder before you breathe it, and let the steep gradient do the work.

All Field Notes · daily on @DecoLogFieldNotes

⚠ Educational only — not a substitute for training or a dive computer. Verify every number independently and dive within your certification.