Originally Posted by
CliffordK
One thing I read about hyperventilating while diving is that hyperventilating only marginally increases your O[SUB]2[/SUB], but significantly decreases the CO[SUB]2[/SUB]. The result is that it suppresses the breathing tendency to the point where a person could pass out.
It is ok to pass out in the open air, but very dangerous to do underwater.
So true, the term is known as "shallow water blackout" or SWB and sadly happens a lot to new free divers who just want to dive deeper and longer, but don't know the risk. To know and understand this limitation it's actually encouraged to practice at home on the couch so you can understand what it feels like before during and after you blackout. I've held my breath until I blacked out more than once. The seal who taught me how to dive said it's pretty common that they push so hard you blackout several times during the training.