As skin divers hold their breath, carbon dioxide levels increase while oxygen levels decrease. Normally, carbon dioxide levels rise to a level that creates a stimulus to breathe long before the oxygen level drops to a dangerous level. This is illustrated by the chart to the left.

To extend dive time, skin divers often take several deep, rapid breaths prior to the dive. This is called hyperventilation, and lowers the carbon dioxide level in the body. As a result, it takes longer for the carbon dioxide to stimulate the body to breathe, thereby allowing the skin diver to remain underwater for an extended period of time. Click here to view this example in the chart to the left.

When divers excessively hyperventilate, their oxygen levels drop below the safe limit before they feel the need to take a breath. This results in the loss of consciousness known as shallow water blackout, and can lead to drowning. Click here to view this example in the chart to the left.