"Evan Gatehouse" wrote in message
...

Shallow water blackout occurs when you hyperventilate too much
before a freedive, and then when down, you run out of O2. Your
body uses increasing levels of CO2 to tell you when to breathe,
and hyperventilating reduces blood CO2. So you don't get the
urge to breathe, and pass out underwater.

Thanks for that one Evan. I wasn't aware. I've had a dizzy spell
or two, and hadn't thought through the cause too carefully.

JimB