Pure O
Tom and Veronika,
I can attest to what Veronika experienced with her trials of pure Oxygen. In 1998 I was invited by Duke University to the Hyperbaric Chamber Unit for a clinical study funded by DAN and none other than the US Navy for a cooperative study that was later published in some Journal of Science on the effects of pure oxygen on experienced breath hold divers. Basically, they were trying to figure out (DAN and the Navy) from a base premise of breath holding on pure oxygen how much variability you would have and what the side effects would be of pure Oxygen to later help them solve the mysteries of predicting and being able to accurately come up with somewhat accurate tables for mixed gas diving. In other words, trying to figure out if some guidelines depending on what data they gathered, could be produced for mixed gas diving.
My static breath-hold at the time was around 5 minutes and that was under the controlled environment at DUKE. In fact I never recommend anyone try to figure out their static breath hold in a pool alone or without someone watching closely. The tests were composed of a kill switch if I lost consciousness that would catapult me out of the water in the eventuality of a black-out, a catheter in my arm to draw blood and take samples, and under several sets of other conditions like bringing the chamber to one ATM (33ft) or pressure, exercising during breath hold, hyperventilation before breath hold and a few others...
For me personally with a minute of hyperventilation before hand with normal air and one final inhalation before breath hold of PURE 02 my best breath hold was nearly 12 minutes (11:49).
Its a very surreal calm feeling holding your breath for that long inside a trippy chamber with lights underwater. Compared to my best static breath hold, that was a remarkable difference compared again to a few of the recruited subjects who had longer static breath holds than I did whose breath hold time with a pure O2 inhalation only improved by a couple minutes.
What did these tests prove ultimately? I'm not sure since I never caught a chance to read the post study results and considerations made by the scientists and MD's, I think also because DAN and DUKE never came to any certain conclusions based on such discrepancies between divers breath hold times and differences.
What I can tell you is that you definitely run a great risk of killing yourself trying it spearfishing since pure O2 is toxic at depth and causes Oxygen toxisity
and our bodies have an amazing ability to keep us conscious and feed the brain with extremely low levels of oxygen before passing out. And lastly repeating what Veronika said: more than extra caution needs to be taken under an experiment of this sort (a dumb and dangerous one unless its for science) because instead of having pre black out signals/warnings/shortness of breath symptoms caused by C02 a pure O2 breath hold gives you a very short-or-no such warning or feeling of shortness of breath, and this can be lethal since you are actually on fatally low levels of Oxygen.
Clear waters,
Mark