Technically, the dive agencies all teach that breathing O2 below 20 feet is "toxic". Practically speaking, that is only possible under certain conditions, and then, not always. I breathed 100% O2 at 24 feet on my last dive Saturday because the place we were going to do our deco was below 20 feet. So I input 92% as my breathing gas into my computer before the dive and breathed 100% O2 at 24 feet for 6 minutes at that depth and then on to the surface for a total of about 11 minutes. Technically, the ppO2 was greater than 1.6, but that does not necessarily mean you will get a reaction or suffer a seizure. In a chamber, 100% O2 is breathed down to simulated depths of 60 feet in that controlled environment. It is thought that this is safe in the chamber because a seizure can be addressed immediately by a medical professional, and since a person is at rest in the chamber, it is easier to do.
I am not advocating to anyone else doing what I did without a thorough understanding of the consequences and without someone available to help in the event of a problem. I do extreme diving, and in this case, it would have been difficult to perform a deco stop at 20 feet in this particular cave. The truth is, everyone has a different metabolism and may react differently to breathing 100% O2 deeper than 20 feet. The standards (i.e., don't breathe 100% O2 below 20 feet) were developed based on data from different divers under different conditions and circumstances, and they are thought by the standards agencies to be conservative and safe for 99% of all divers. Some divers may be able to breathe O2 at 30 feet all day, and some may suffer O2 toxicity after 30 seconds at 21 feet.
Yeah, I actually know all that... I just didn't want to say it like that so newbies (like Frank0 wouldn't breath off a tank with 100% O2 in it at 40fsw or something.
