Pool blackout is a cerebral hypoxia that causes unconsciousness. Usually, a self-inflicted hyperventilation prior to diving is the cause.
If carbon dioxide levels raise in the blood, usually a growing breathing reflex is set off. It will grow so strong that a breath takes place before losing consciousness. In self-inflicted hyperventilation carbon dioxide levels of the blood are strongly reduced while the oxygen levels of the blood only increase relatively weakly.
Because of the missing breathing reflex oxygen levels can sink so low that an unconsciousness occurs before the respiratory stimulus grows so strong that breathing can occur automatically. If the person is under water, water is aspirated due to the unconscious respiratory reflex.
