A Special Guest Article By Neal W. Pollock, Ph.D.

The risk of fatal loss of consciousness in fit and frequently highly competent swimmers was well described by Albert Craig in 1961.1,2 Blackout in swimming pools is not a new problem, but it is one that requires eternal vigilance. More importantly, terminology has recently become confusing and misleading. We can clear up some of the confusion; the need for vigilance will remain.

Metabolic gases (oxygen [O2] and carbon dioxide [CO2]) are fundamental components of our physiological processes. We consume O2 and produce CO2. While CO2 is commonly thought of as a waste product, it is critical in maintaining the acid-base balance in our tissues. For this reason, we maintain CO2 in the body at concentrations 140-160 times greater than the concentration in air.

The respiratory cycle regulates the levels of O2 and CO2 in our bodies by focusing on CO2. Expiration (breathing out) eliminates CO2 and inspiration (breathing in) restores O2. It is the rise of CO2 in the blood that stimulates breathing. When a breath-hold swimmer takes in a full breath of air and begins voluntary breath-hold, the urge to break the breath-hold is almost exclusively driven by rising CO2 levels. A normal healthy individual can hold his or her breath as long as possible with no significant risk. The point at which the urge to breathe is absolutely undeniable is reached far before the O2 level in the blood falls low enough to threaten consciousness. This is the exquisite nature of respiratory control.

What many swimmers who become interested in breath-hold quickly realize is that hyperventilation (ventilation of the lungs in excess of metabolic need) can dramatically increase breath-hold time. The effect works whether the hyperventilation is by faster or deeper breathing than that being demanded by normal body signals. Here, though, lies the start of the confusion. Contrary to the belief of some, hyperventilation is only trivially increasing to O2 stores in the body. What it does do is to dramatically reduce the CO2 content that is normally so much higher than in air. The result of lowering the body CO2 levels at the start of breath-hold is that it takes longer to reach the levels required to drive the urge to breathe. Excessive hyperventilation can delay the urge to breathe so long that the person can lose consciousness due to low O2 levels (hypoxia) with absolutely no warning. Stated another way, hyperventilation eliminates the safety buffer between the normal CO2-driven urge to breathe and minimum safe O2 levels. Excessive hyperventilation is effectively removing one of our most important protective mechanisms, a critical one when in water since the medium is unforgiving for an unconscious person.

The lack of postmortem physical evidence makes it difficult to confirm the use or magnitude of hyperventilation. While reasonable confidence can be established through witness reports or known patterns of victim practice, excessive hyperventilation must often remain only as a suspicion, even when blackout appears to have occurred in apparently healthy individuals with no other obvious insults or injuries. Still, even with conservative analyses, blackout is consistently one of the most common disabling agents found in fatal events captured in the Divers Alert Network breath-hold incident database.3,4

Terminology is another point of confusion. The term “shallow water blackout” has been picked up by the aquatics community to talk about almost any case of unexplained loss of consciousness. This is problematic because the term has already been used to describe two other conditions. The first was for problems observed in British closed-circuit oxygen rebreather divers. Device failures resulted in a buildup of CO2 that led to intoxication and loss of consciousness. More recently, for a problem experienced by breath-hold divers traveling vertically through a substantial depth range. In essence, descending through the water column compresses the gas in the lungs, driving more gas into the blood. Most importantly, this increases the amount of O2 available to be consumed. Problematically, though, as the breath-hold diver ascends through the water column the blood O2 level falls much faster than it would without the vertical excursion. Since the relative pressure change is greatest in the shallowest water, it is normal for blackout to occur in the final stage or just after surfacing, hence the adoption of the term. The blackouts occurring in most swimming pool cases, particularly those in shallow pools, are almost undoubtedly driven by excessive hyperventilation alone, with no meaningful contribution of pressure change to the event.

Turning to considerations for safe practice, it is important to note that, effectively, every respiratory cycle includes a brief period of functional breath-hold. Trying to ban breath-hold as some have proposed is a fairly irrational response to the problem. As discussed earlier, a normal person cannot voluntarily breath-hold long enough to lose consciousness without hyperventilation. The focus of safety programs should be to educate swimmers about the risk so they appreciate the importance of conservative practice. Not only is the understanding more likely to keep them safer than a ban that is impossible to enforce, it may help them explain the hazards to others who discover the effect of hyperventilation on their own.

Regarding safe limits, the available research suggests that hyperventilation restricted to no more than two or three full ventilatory exchanges (maximum breath in and out) is unlikely to reduce CO2 levels enough to produce a significant risk of loss of consciousness. Regarding terminology, “hyperventilation-induced blackout” is most appropriate if pre-breath-hold hyperventilation is known or suspected. Simply “blackout” or “hypoxic blackout” are also better alternatives to ‘shallow water blackout’ unless a significant vertical migration (perhaps greater than 15 feet in depth) was involved in the event and blackout occurred during the final stage of ascent.

Ultimately, the keys to breath-hold safety are proper education, thoughtful practice, and awareness of events. These keys hold for swimmers, instructors, lifeguards, and other leaders.



1. Craig AB Jr. Causes of loss of consciousness during underwater swimming. J Appl Physiol. 1961; 16(4): 583-6.

2. Craig AB Jr. Underwater swimming and loss of consciousness. JAMA. 1961; 176(4): 255-8.

3. Pollock NW, Dunford RG, Denoble PJ, Caruso JL. DAN Annual Diving Report – 2009 Edition. Durham, NC: Divers Alert Network, 2013; 153 pp.

4. Pollock NW, Dunford RG, Denoble PJ, Dovenbarger JA, Caruso JL. Annual Diving Report – 2008 Edition. Durham, NC: Divers Alert Network, 2008; 139 pp.


Dr. Pollock is a research physiologist at Duke University and Research Director at Divers Alert Network (DAN), in Durham, NC.


UPDATE:  Dr. Pollock answers questions regarding this article:  HERE