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?

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    $\begingroup$ I think you might be misinformed about the gases that divers use. chemistry.stackexchange.com/questions/61406/… No one is really breathing 100% oxygen at depth, they are at most using a 100% oxygen supply. And yeah, the answers here aren't really based on biology. Maybe physics/chemistry, but you are probably going to get a better answer at a more specialized source for that particular niche of diving. $\endgroup$ – Bryan Krause Apr 5 '18 at 16:55
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    $\begingroup$ From what I recall, nitrox is at max 40% oxygen. Do people really dive with 100% oxygen? Can you please give a link that support this claim? Do they breath from it directly or do they use these tanks in a rebreather system? $\endgroup$ – Remi.b Apr 5 '18 at 17:00
  • $\begingroup$ @Remi.b It's a rebreather. They are recycling the nitrogen/other gases, scrubbing the CO2, and replacing the metabolic O2 from a 100% O2 tank to reduce mass. $\endgroup$ – Bryan Krause Apr 5 '18 at 19:25
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    $\begingroup$ I am VTC because I think the question is based upon OP's misunderstanding of diving equipment. Outdoor.SE might be of help to you to improve your understanding of diving equipment. $\endgroup$ – Remi.b Apr 5 '18 at 19:28