A couple of minor corrections to the comments on diving with oxygen. Pure oxygen is neurotoxic above 2 atmospheres pressure, approx 10m depth with pure oxygen. The only folk who use pure oxygen are:
a) military divers, shallower than 10m and using rebreathing systems (no bubbles on the surface; a balance of risks) and
b) the rare really deep divers who use a complex gas mixture- at extreme depth this oxygen pressure equates to low single figure percentages of oxygen by volume, so the gas mixtures are carefully crafted using single gas cylinders.