It is possible this should be moved to physics, depending on the nature of the answer (i.e. if the reason is physical). But I suspect the explanation is biological, so I posted it here.
Sometimes technical divers dive with 100% oxygen. I don't understand how can this not burn, oxidize or at least damage their lungs. To begin with, even if someone would breath 100% oxygen in atmospheric pressure, I'd suppose he'd damage his lungs, because the 79% of the volume that would normally be filled with inert nitrogen is now filled with oxidizing oxygen.
If, however, someone dives to, say, 30 meters, with pure oxygen, he'd have to fill his lungs with oxygen at 4 atmospheres to equalize the pressures. That means the pure oxygen filling his lungs is even more concentrated. How can this be? A friend of mine who dived with pure oxygen even says the breathing feels just normal. How is this possible? How are the lungs not damaged?