Version: 1
Last updated: January 2020

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Shallow water blackout is the loss of consciousness of a swimmer caused by a lack of oxygen to the brain (cerebral hypoxia) following hyperventilation and breath-holding. Without rapid rescue and first aid, a fatal drowning will occur.

A shallow water blackout occurs when the individual resists the urge to surface to breathe (intentionally or unintentionally) whilst oxygen levels in the body fall below the level required to maintain consciousness.

Unintentional hyperventilation may be caused by strenuous exercise, and the swimmer may experience no warning symptoms prior to the blackout. Therefore, the swimmer cannot be expected to recognise nearing the limit or calling for help or self-rescue.

Blackout due to cerebral hypoxia can also occur in any depth of water; therefore, this guidance applies to all breath-holding activities.

Guidance for RLSS UK Trainers and Candidates

To reduce the likelihood of shallow water blackouts, the following guidance should be applied to all RLSS UK activities:

  • Activities requiring breath holding (such as surface diving) should not be completed at a depth or duration greater than that required by the qualification, award, or event.
  • Hyperventilation should not be permitted prior to breath-holding or competitive events/activities.
  • Prolonged breath-holding activities (such as swimming underwater) should not immediately follow strenuous exercise, including sprint swims of more than
  • Breathing should return to a normal/relaxed state before prolonged breath-holding activities are repeated.
  • Lifeguards should be informed when breath-holding activities are taking place.
  • Lifeguards should intervene when activities occur without close supervision or when swimmers are swimming over 25 metres under the water.

Guidance for Public Awareness

A shallow water blackout is when someone loses consciousness underwater caused by a lack of oxygen. Breathing normally before diving under the water and keeping periods under the water short is the best way to avoid shallow water blackout.

  • Always allow your breathing to return to normal (not gasping) before you dive under the
  • Don’t compete with others to see who can hold their breath for the longest or to see who can swim the furthest underwater.
  • If you feel the urge to breathe, don’t resist it and return to the surface, and allow your breathing to return to normal.
  • Never swim alone (when no lifeguard is present).

Click here for other RLSS UK Guidance Statements