science / medicine

A shallow water blackout is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive in water typically shallower than five metres (16 feet), when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers, and have not experienced problems before.

Many drowning and near drowning events occur among swimmers who black out underwater while free-diving or doing breath-hold pool laps. Blacking out, or browning out, near the end of a breath-hold dive is common. Although the mechanism is well understood, it is not common knowledge among breath-hold divers.

Shallow water blackout is related to, but differs from deep water blackout in its characteristics, mechanism and prevention; deep water blackout is precipitated by depressurisation on ascent from depth. Blackout may also be referred to as a syncope or fainting.