Shallow water blackout: what it is and why it gives no warning

Shallow water blackout is loss of consciousness caused by low blood oxygen during a breath hold in water. It usually arrives with no warning, most often near the surface or on the way up. Hyperventilating first makes it more likely, because it removes the carbon-dioxide signal that would otherwise force you to breathe. Strong swimmers drown this way.

Why it happens

Oxygen falls for as long as the hold lasts. Rising carbon dioxide contributes to the urge to breathe that sends you back to the surface, alongside oxygen, lung volume and respiratory-muscle activity, so the urge and the oxygen level can come apart. Blow the carbon dioxide off with fast deep breaths beforehand and the urge arrives late, while the oxygen keeps dropping. DAN's review of hypoxia in breath-hold diving describes the same mechanism, and adds that on the way back to the surface the falling pressure drops blood oxygen faster still. That is why divers black out in the last few metres.

Intentional hyperventilation is not required. Underwater Hypoxic Blackout Prevention states that blackout can happen without it, and that hypoxia can feel pleasant rather than alarming.

Each repeat leaves less margin than the one before. In a 2023 study in the European Journal of Applied Physiology, 18 participants performed five repeated static apneas with cold facial immersion. When each hold was preceded by 15 seconds of hyperventilation, oxygen saturation fell further with every repeat, averaging 90.6% against 93.6% without it, and four participants dropped below 75% on the fifth hold.

Why there is usually no warning

Contractions and comfort are not a gauge. Involuntary breathing movements vary between people and between attempts, and they do not measure blood oxygen. Reduced discomfort does not prove oxygen is safe; it often means the warning has gone. Royal Life Saving Society UK guidance puts it directly: the swimmer may experience no warning symptoms before the blackout.

So there is no checklist of signs the swimmer can act on. What a bystander can see is undramatic: a swimmer lying still underwater, face down, who has not come up when they should have. StatPearls notes that victims typically lose consciousness underwater without preceding distress, which is why nobody nearby reacts in time.

Who it kills

Pool breath-holding games, coached hypoxic sets, freedivers and spearos coming back up, children, and fit competitive swimmers. No body publishes a reliable annual national figure, so every number below carries its publisher and its date.

The CDC's MMWR of 22 May 2015 reviewed drowning and unconsciousness from breath-holding in New York State from 1988 to 2011. It found 16 cases across 15 incidents, 4 fatal and 12 nonfatal. Ages ran from 7 to 47 with an average of 17, and 13 of the 16 were male. All four who died were advanced swimmers aged 17 to 22. At regulated facilities, all but one incident happened with a lifeguard on duty. That is a 16-case state series, not a national count.

The International Life Saving Federation's Medical Position Statement MPS-16, approved 5 September 2011, reports that 12 of 60 Australian snorkelling deaths between 1987 and 1998, 20%, were attributed to hypoxia from breath-holding and hyperventilation, citing Edmonds and Walker in the Medical Journal of Australia, 1999. The same statement says records of incidence are vague because there is no standard definition. For scale only, CDC drowning facts reviewed on 27 January 2026 give about 4,000 fatal and 8,000 nonfatal drownings a year in the US. No source attributes a share of those to blackout, and neither does this page.

Scuba divers are not exposed to this by their gear, because it is a breath-hold mechanism.

Shallow water blackout, hypoxic blackout, and the name argument

The American Red Cross, USA Swimming and the YMCA of the USA reject the term "shallow water blackout" in their joint statement of 31 October 2022 and use "hypoxic blackout" instead. DAN and StatPearls still use the older term. The charity that campaigns on it renamed itself Underwater Hypoxic Blackout Prevention. The two names describe the same event.

The older name misleads. It does not only happen in shallow water, and shallow water is not the safe end of it. Deep water blackout is the same hypoxia in a diver returning from greater pressure. Drowning is what happens next if nobody gets the airway out of the water; StatPearls notes the term is often misused as a synonym for drowning, and it is not one.

What to stop doing

Those pool rules come from national bodies. RLSS UK guidance says hyperventilation before breath-holding should not be permitted, that lifeguards should intervene past 25 m underwater, and that extended breath-holding belongs only in a structured session run by a competent coach in a closed pool. Pool lifeguards are not trained to supervise hypoxic drills. The Red Cross, USA Swimming and YMCA statement calls hyperventilation before underwater swimming dangerous and potentially deadly, and limits instructors to a single inhalation before any submersion drill.

If it happens

Call emergency services. Get the person's airway out of the water and support the head. Start CPR if they are not breathing. Written instructions are not rescue training, and reading this paragraph has qualified nobody to pull an unconscious swimmer out of a pool. Take a certified course.

What this page does not do

• It does not teach breath-holding in water, at any depth, for any duration.
• It does not give a safe hold time, a safe depth, or a warning-sign checklist you can train against, because blackout commonly arrives with none.
• It is not medical advice and not a rescue qualification. Get in-water rescue skills from a certified course.
• It does not replace a lifeguard, a coach's supervision, or your national governing body's pool rules.
• BreathHold is a dry-land training app. It cannot make in-water breath-holding safe.

Some people arrive here searching for shallow water blackout training, technique, depth, or a blackout vest. Those searches are asking how to stay under for longer, and this page will not answer them. The dry-land starting guide and the CO2 table generator are where that work belongs.

Why BreathHold is dry-land only

BreathHold runs CO2 and O2 tables, timers and baseline tests on land, away from water. That is the whole scope. For supervised in-water instruction, use the freediving schools map; for the rules that apply to any session, the breath-hold safety guide.

Sources and review

• Hypoxia in Breath-Hold Diving, Marty McCafferty, DAN Alert Diver, 1 May 2016.
• Drowning and unconsciousness from breath-holding, New York State 1988 to 2011, CDC MMWR 64(19):518 to 521, 22 May 2015.
• Drowning facts, CDC, reviewed 27 January 2026.
• Underwater Hypoxic Blackout Prevention, formerly Shallow Water Blackout Prevention.
• Repeated apneas with hyperventilation and facial immersion, European Journal of Applied Physiology, 15 April 2023.
• Hypoxic blackout joint statement, American Red Cross, USA Swimming and YMCA of the USA, 31 October 2022.
• GS005 and GS012, Royal Life Saving Society UK guidance, undated.
• Shallow Water Blackout, StatPearls, NCBI Bookshelf, 2026 edition.
• Medical Position Statement MPS-16, International Life Saving Federation, approved 5 September 2011.

Written and reviewed by MJ Kühn, Freediving Instructor Trainer and Functional Breath Coach. Last reviewed 10 September 2026. Next review due by 10 March 2027, owner MJ Kühn.

This page gives general risk-reduction information. It is not medical advice, it cannot assess your risk, and it does not replace certified freediving instruction or emergency-response training. Ask a qualified clinician about your own circumstances.

If you want the dry-land training that stays out of the water, BreathHold runs the tables and timers on land only.