The Importance of Hyperventilating Before You Go Underwater

Do not ever hyperventilate before you put your head under water. If you do that too much, the blood vessels to your brain constricts so much that perfusion to your brain drops too much and you pass out. Your oxygen levels may be fine, but your major stimulus to take the next breath is actually high carbon dioxide levels. You'll hit this point where your oxygen levels drop so low that you become so hypoxic that you passing out.