I am talking about breath hold induced hypoxic state. Basically at what point during such an exercise would you expect the risk of fainting to be significant or can it happen at any saturation level? Can I assume, that at 50% the danger of fainting is very high, like 90% and at 70% saturation it is extremely low? Is it possible to train to be able to withstand lower saturation levels and not faint? For example would an experienced breath holder be able to not faint at 40% saturation? And finally, if you regularly do breath holding exercises, would you expect your red blood cell count to rise, similar to altitude exposure?

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    Are you asking about fainting purely from hypoxia, as opposed to vaso-vagal syncope induced by breath holding etc? – DoctorWhom Jul 25 '19 at 4:44
  • Yes, you can train this. Saying from the head, I think that some divers can go without oxygen for about 10 minutes or more, which suggests that you can faint at very different tissue oxygen saturation levels. – Jan Jul 25 '19 at 14:38
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    Although top freedivers can go an amazingly long time under water, the sport is quite dangerous and people have died doing it. In my EMS experience, O2 sats under about 70% begins to produce mental confusion, and under about 55% unconsciousness is likely. I doubt that anyone can remain conscious with O2 sats of 40%. – Carey Gregory Jul 25 '19 at 23:57
  • Isn't CO2 buildup in the body more of a problem than O2 saturation? – Count Iblis Jul 26 '19 at 2:36
  • @CountIblis They're both a "problem" but lack of O2 will disable and kill brain cells long before excess CO2 does. – Carey Gregory Jul 26 '19 at 14:32

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