Why is it dangerous to a swimmer to hyperventilate in order to hold breath for a longer period of time?

When you hyperventilate, you lower the CO2 levels in your body which normally gives you that urge to breathe after holding your breath for awhile. You can have a shallow water blackout when this occurs because you have little or no warning that you need air because that urge is not there. My breath hold record is 3 minutes and 27 seconds underwater but not by hyperventilating.

It's because of the risk of greying or blacking out.

Because you can get light headed ans pass out - under water!

i think Michael Phelps could give you the best answer