Below is more in depth information about SWB

Blackouts from O2 deprivation in shallow water can occur in 4 conditions:

  1. Low CO2 prior to the breath-hold: CO2 may be lowered with intentional hyperventilation and lowered with unintentional hyperventilation from rapid, deep breathing. Blackout from low O2 occurs prior to the trigger level of CO2 to breathe. This is most commonly thought to be associated with SWB.
  2. Low O2 prior to the breath-hold: O2 levels may be lowered with repetition (repetitive breath-holding) and exercise leading to exertion and exhaustion. Blackout from low oxygen can occur prior to the trigger level of CO2 to breathe or even with elevated CO2.
  3. Normal O2 and CO2 prior to the breath-hold: CO2 trigger level is reached prior to O2 levels to cause blackout, but the urge to breathe is intentionally ignored associated with competition and determination to win. The urge to breathe subsides giving one a feeling of empowerment similar to a “runner’s high.” Blackout occurs when critical hypoxia is reached.
  4. Low O2 and low CO2 prior to the breath-hold in a competitive situation: Combinations of the above (1, 2 and 3) are EXTREMELY DANGEROUS.

Please note: When a person free dives in deeper water (>16 ft) blackouts can occur secondary to the changes in the pressure gradient as one ascends from deep water. O2 can be directed away from the brain. This was originally noted in shallow water on ascent and is the origin of the term “shallow water blackout.” Now the term is applied to blackouts in shallow water (

**There are additional processes that can make breath-holding fatal. Spasm of the larynx can produce asphyxiation. Fatal cardiac arrhythmias can occur from long Q-T syndrome, pH changes in the blood, and “R-on-T phenomenon” (the superimposition of an ectopic beat on the T wave of a preceding beat) when hypoxia occurs. Ectopic beats or PVCs (premature ventricular contractions) can be fatal if they occur when the heart is re-polarizing. These PVCs can be thrown hours after breath-holding. Seizures may also be stimulated from hypoxia if one is predisposed. So there are many other causes of death besides drowning that can be precipitated by the hypoxia of breath-holding.

* Please note: The condition of the lungs after underwater blackout may be wet (from drowning) or dry (from spasm of the larynx.) Also, breath-holding may stimulate genetic triggers leading to various causes of death.


People who hold their breath while swimming or practicing breath-holding underwater in pools are at risk of “passing out” due to lack of oxygen. This phenomenon is commonly referred to as shallow water blackout (SWB) and it is the result of a SEVERE LACK OF OXYGEN TO THE BRAIN.

SWB may be the leading cause of swimmer death. The number of deaths that can be attributed to SWB is not fully known, as these deaths are often misdiagnosed as traditional drowning. When coroners rule a SWB death as “drowning” only, it masks the real problem of the prolonged breath-holding and the contributing factors of hyperventilation, competitive, and repetition. Adding to the confusion, as mentioned above other causes besides drowning, can lead to death from breath-holding or hypoxia. For this reason, SWB is not well known or understood by many of those who are most at risk.

When oxygen levels fall to critical levels, blackout is instantaneous and frequently occurs without warning. Most of the time, underwater swimmers have no clue they are about to be rendered unconscious and that they will be vulnerable to death within minutes. Swimmers who hyperventilate to excess before breath-holding are in particular danger. Hyperventilation is simply an increase in the amount of air moving in and out of the lungs. It may be due to rapid, shallow breathing, but deep slow breathing can also result in hyperventilation. Paradoxically, hyperventilation does not increase the amount of oxygen in the body, but it does decrease the amount of circulating carbon dioxide. Carbon dioxide levels in the blood are primarily responsible for the swimmer’s desire to breathe. When the level of carbon dioxide in the blood is driven to artificially low levels as a result of hyperventilation, the desire to breathe is diminished. This artificial method of fooling the body into thinking it does not need oxygen is deadly, as it lures the breath-holder into believing he can hold his breath longer than he safely can.


  • Underwater breath-holding and underwater swimming have been practiced for decades.
  • Coaches and military trainers teach hypoxic training and breath-holding, which can be deadly without professional, one-on-one monitoring.
  • Lifeguards typically do not have training to monitor breath-holding.
  • Lifeguards and parents routinely accept and encourage breath-holding drills conducted in swimming pools.


  • Swimming athletes who train and perform in swimming pools, particularly those who practice hyperventilation.
  • Swimmers who are physically exerted.
  • Swimmers who are not closely observed while engaged in breath-holding.
  • Swimmers who consecutively perform a repeated hyperventilation/breath-holding routine.
  • Swimmers playing breath-holding games.
  • Breath-holding swimmers who have unknown and underlying medical causes, i.e. long Q-T, RyR2, seizures, etc.
  • People who practice holding their breath in a pool while floating face down or sitting on the bottom. Since the individual is already in a state of relaxation, SWB becomes nearly impossible for an observer to detect.
  • Freedivers and spearfishermen (please see


  • DO NOT PRACTICE prolonged breath-holding. (The ARC, YMCA and USA Swimming warn of the dangers of hypoxic training and prolonged underwater swimming although far too many still practice it. Currently NYC pools and YMCAs ban prolonged breath-holding. The National Swimming Pool Foundation warns of breath-holding games).

  • Never swim alone.

  • Underwater breath-holding should never be encouraged, but if practiced the rule of thumb for safety is: One Breath-hold, One Time, One Lap, ONLY.

  • Never Hyperventilate.

  • Repetitive breath-holding increases risk of SWB. If Breath-holding under water, a buddy must be next to you tapping you on your shoulder so you can signal that you are OK. Their total focus needs to be you and your safety. They should never breath-hold with you. Do not rely on lifeguards. SWB is difficult to detect above water.

  • For freediving and spearfishing safety guidelines and safety courses visit and