Prolonged breath holding is a dangerous practice that can lead to drowning if you hyperventilate before you go under water.
Hyperventilation is the repeated inhalation of fast, full breaths of air and rapid exhalation. The effect of hyperventilation is to wash C02 out of your blood, resulting in an extremely low C02 level.
While the underwater swimmer burns up oxygen through exertion, you never get the signal from the brain, the "bursting lung" sensation, because of the low level of C02 in the lungs and blood. Without the C02 stimulus, the brain doesn’t recognize the need to breathe and you black out from hypoxia, a lack of oxygen to the brain.
In order to understand why breath holding poses a threat, it is necessary to know something about human respiration. A high level of carbon dioxide in the blood is what actually triggers air hunger, not the lack of oxygen.
When you are actually starving for oxygen, your body doesn't notice it because the carbon dioxide levels are normal. Oxygen-starved, you may black out and drown before anyone realizes what is happening. This phenomenon is sometimes known as shallow water blackout.
Shallow Water Blackout is most commonly associated with skin diving, but the more swimming coaches we speak with, the more we realize that underwater fainting is not at all uncommon in the world of competitive aquatic sports.
Carbon dioxide serves as a stimulus to breathing by informing the brain that a person's body requires another breath of air. This process occurs in any prolonged holding of breath, including underwater swimming.
Normally there is no danger because the carbon dioxide build up in the blood signals the brain that you need to take a breath by giving you the sensation that your lungs are going to burst if you don't get a breath.
The problem comes when a swimmer or diver manipulates the brain’s automatic breathing control device through hyperventilation. The blackout victim is in an extremely dangerous position at the point of unconsciousness.
Seeing him underwater will fool observers as the unconscious swimmer often makes seemingly coordinated movements even after the fainting. He does not appear to be in difficulty.
Irreversible physiological brain damage from a lack of oxygen is only minutes away, even if he's saved from death.