Shallow Water Blackout

Shallow water blackout refers to a loss of consciousness due to dangerously low levels of oxygen in the bloodstream. This rarely occurs when scuba diving, but is a risk to skin divers who stay underwater too long without surfacing for a breath.

Shallow water blackout usually occurs as the skin diver nears the surface during ascent. As the ambient pressure decreases, the pressure of oxygen in the bloodstream also decreases. If the oxygen's pressure drops below the safe limit, the diver loses consciousness without any warning or signal to breathe. This results in drowning if the diver is not immediately removed from the water.

Hyperventilation and Shallow Water Blackout

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.

breathe hold diving with no hyperventilation

Preventing Shallow Water Blackout

The best prevention against shallow water blackout is to avoid excessive hyperventilation. Limit yourself to two or three breaths, then take and hold a final breath before the skin dive.

Like scuba diving, skin diving should always be performed with a buddy. But unlike scuba diving, skin divers take turns diving below the surface while the other remains at the surface. This ensures that help will be available in the event a blackout occurs.

Finally, avoid deep skin dives, and always keep track of the time you've spent underwater. Many skin divers have become preoccupied with the life they see while underwater, and completely forget to surface for a breath of air before it's too late.

Carbon Dioxide