Explanation of Shallow Water Blackout

While there is considerable controversy about the term Shallow Water Blackout due to the fact that it can happen at any depth of water our Foundation will continue to refer to it for the time being.

Shallow Water Blackout (SWB) is a loss of consciousness caused by cerebral hypoxia toward the end of a prolonged breath-hold submersion under water. It can be caused by taking several very deep breaths, or hyperventilating (see below) just before submerging under water. Because hyperventilation reduces the normal amount of the body’s carbon dioxide it does not surpass the normal CO2 threshold (the body’s signal to breathe) the brain does not experience an urgent need to breathe; therefore, the brain passes out due to a lack of sufficient oxygen.  The brain’s natural reflex is to then take a breath, which causes the swimmer to drown.


  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.


Unexplained blackouts underwater have been associated with the practice of hyperventilation. Survivors of shallow water blackouts often report using hyperventilation as a technique to increase the time they can spend underwater. Hyperventilation, or over-breathing, involves breathing faster and/or deeper than the body naturally demands and is often used by divers in the mistaken belief that this will increase oxygen (O2) saturation. Although this appears true intuitively, under normal circumstances the breathing rate dictated by the body alone already leads to 98-99% oxygen saturation of the arterial blood and the effect of over-breathing on the oxygen intake is minor. What is really happening differs from divers' understanding; these divers are extending their dive by closing down the body's natural breathing mechanism, not by increasing oxygen load. The mechanism is as follows:

The primary urge to breathe is triggered by rising carbon dioxide(CO2) levels in the bloodstream.CO2 builds up in the bloodstream when O2 is metabolized and it needs to be expelled as a waste product. The body detects CO2 levels very accurately and relies on this to control breathing. Hyperventilation artificially depletes this (CO2) causing a low blood carbon dioxide condition called hypocapnia. Hypocapnia reduces the reflexive respiratory drive, allows the delay of breathing and leaves the diver susceptible to loss of consciousness from hypoxia. For most healthy people the first sign of low O2 is a greyoutor unconsciousness: there is no bodily sensation that warns a diver of an impending blackout.