Who knew there were so many different ways to die? Shallow water blackout is a condition in which your body actually doesn’t realize that it needs to breathe leading to people losing consciousness while still underwater where they drown unless someone rescues them.
According to Wikipedia the main culprit is when swimmers planning to stay underwater for an extended time–such as swimming laps underwater in a pool–and decide to hyperventilate themselves in the mistaken belief that this will increase the oxygen in their lungs and/or bloodstream.
Hyperventilation, or over-breathing, involves breathing faster and/or deeper than the body naturally demands and is often used by divers in the mistaken belief that this will increase oxygen (O2) saturation. Although this appears true intuitively, under normal circumstances the breathing rate dictated by the body alone already leads to 98-99% oxygen saturation of the arterial blood and the effect of over-breathing on the oxygen intake is minor. What is really happening differs from divers’ understanding; these divers are extending their dive by closing down the body’s natural breathing mechanism, not by increasing oxygen load.
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The primary urge to breathe is triggered by rising carbon dioxide (CO2) levels in the bloodstream. CO2 builds up in the bloodstream when O2 is metabolized and it needs to be expelled as a waste product. The body detects CO2 levels very accurately and relies on this to control breathing. Hyperventilation artificially depletes this (CO2) causing a low blood carbon dioxide condition called hypocapnia. Hypocapnia reduces the reflexive respiratory drive, allows the delay of breathing and leaves the diver susceptible to loss of consciousness from hypoxia. For most healthy people the first sign of low O2 is a greyout or unconsciousness: there is no bodily sensation that warns a diver of an impending blackout.