What causes shallow water black out to divers?

Answer 1

I believe shallow water black out in divers is not as common as in Snorkelers or free divers.

Shallow water black out happens when the impulse to breath is delayed to the point that you genuinely do run out of oxygen before you need to succomb to the impulse to breath. In an extream case the diver may be totally unaware that they are dangerously depleating their blood oxygen before the carbon dioxide levels increase sufficiently to stimulate them to surface and gasp for air. Low blood Oxygen may lead to the diver passing out - with fatal consequences if thiere swimming or diving partner cant get them to the surface and revive them in time. Its for this reason that snorklers and divers are told not to flush their lungs excessivly by taking many many deep breaths before trying to do some prolonged underwater swimming.

It can happen in the case of an excited diver breathing rapidly and then staying relatively stationary and simply forgeting to breath from their aqualung though generally this is less likely.

The classic cause is by some one breathing rapidly before a dive to flush carbon dioxide out of their lungs - this is often presented as "storing oxygen" so that they can swim under water for longer - in fact taking a few deep breaths will not significantly increase the amount of Oxygen you store in your blood - it will however flush out relatively more of the Carbon dioxide. Then when swimming you appear to have more air as the impulse to breath is delayed. Infact this isnt because you have more air its because the carbon dioxide in your blood is taking longer to build up. Its Carbon Dioxide that creates the impulse to breath and reminds the diver to surface. and this reflex to the buildup of Carbon Dioxide is independant of the oxygen levels in your blood. It just so happens that if you have too much carbon dioxide - you probably need to breath more oxygen as well.