Tools · Dalton's law

Every breath, taken apart

Dalton's law says the pressure of a gas mixture is just the sum of the pressures each gas would exert on its own — its partial pressure. Air is roughly 21% oxygen and 79% nitrogen, so at the surface oxygen pushes with 0.21 atmospheres and nitrogen with 0.79. Each gas's share is simply its fraction of the whole:

Underwater the total pressure climbs by one atmosphere every 10 metres, and here's the catch the percentages hide: the mix never changes, but every gas's partial pressure rises right along with the depth. The same 21% oxygen pushes five times as hard at 40 metres as it did at the surface. Pick a mix and a depth and watch each gas grow.

Oxygen is the one with a ceiling. Breathed at a partial pressure much above about 1.4 ATA, it turns from life-support into a central-nervous-system hazard — which is why every mix has a maximum operating depth, the depth where its oxygen hits that line. For plain air that's around 57 metres, comfortably past where recreational divers go, so air never bumps the oxygen limit — though the gas turns too thick to breathe a good deal shallower. Enriched air — Nitrox, with extra oxygen — hits 1.4 much shallower, so its depth ceiling rises toward you. The equation is just the law solved for depth:

So Nitrox isn't "deeper air" — it's the opposite. The reason to breathe it is the other gas: more oxygen means less nitrogen, and nitrogen is what dissolves in and loads your tissues and fogs your head with narcosis. Less of it means more no-decompression time — you load nitrogen more slowly, so you reach the limit later — and a slightly clearer mind at the same depth. That's the whole trade in one sentence: dial the oxygen up and you buy bottom time and clarity, but you pay for it with a shallower ceiling. Two gases, one law, pulling in opposite directions.