Breath-holding and water: why over-breathing can drown you

The rules

• Never breathe fast or deeply on purpose before swimming under water. Not even a few breaths.
• Never practise breath-holds in water alone. Freedivers and spearfishers train with a club and a buddy watching them from the surface.
• Never do breathing exercises with breath-holds in the bath, a pool, a hot tub, a cold plunge or open water.
• Never practise fast breathing or long holds while driving, cycling or standing where a faint could injure you.
• No breath-holding competitions in the pool, for adults or children.

Why it happens

Your urge to breathe is driven mainly by rising carbon dioxide, not by falling oxygen. When you over-breathe, you blow off carbon dioxide, so when you then hold your breath it takes much longer to build up to the point where you feel you must breathe.

Meanwhile, your body keeps using oxygen, especially if you are swimming. The level of oxygen in your blood can fall low enough for you to lose consciousness before the urge to breathe arrives. There is no warning: people describe feeling fine, even euphoric, right up to the blackout. Under water, a person who blacks out will drown within minutes unless someone gets them out immediately. Because it often happens in shallow water, near the end of a length, it is called shallow water blackout. See carbon dioxide and over-breathing.

What the research shows

After two healthy young men drowned at a New York City pool in 2011, having over-breathed before going under, public health officials reviewed drownings in New York State from 1988 to 2011. They found 16 cases linked to what they called dangerous underwater breath-holding behaviours, including intentional over-breathing before going under, and found that doing more than one of these behaviours increased the risk of death. New York City has since brought in rules and education to discourage them.

Breathing methods and water

Some popular methods combine over-breathing and long breath-holds, most notably the Wim Hof method, which also involves cold exposure. The breathing must only ever be done on dry land, sitting or lying down, and never in the same session as getting into cold water. Getting into cold water also causes a gasp and fast breathing of its own, called cold water shock, which adds to the danger.

Slow breathing exercises without long holds, such as resonance breathing, don’t carry this risk. But there is no reason to practise any breathing exercise in water.

If someone blacks out in water

Get them out of the water at once, call 999, check their breathing and start CPR if they are not breathing normally. Even if they come round quickly, they should be checked in hospital, as water in the lungs can cause problems hours later.