Breath hold blackout (BHB), also referred to as hypoxic blackout or shallow water blackout, is a potentially lethal consequence of breath hold activities. BHB can result from oxygen deprivation to the brain after extended breath holding, causing loss of consciousness and consequently death by drowning.¹³ Elevated arterial concentration of carbon dioxide occurs during oxygen deprivation and is an important physiological trigger to breathe. Pre-submersion hyperventilation (an increased rate or depth of breathing compared to regular breathing at rest), decreases arterial carbon dioxide concentration, referred to as hypocapnia. Hypocapnia resulting from hyperventilation, removes the natural trigger to breathe,^{14, 15} with potentially lethal consequences if loss of consciousness subsequently occurs underwater. This risk is further increased for underwater swimming drills compared to surface swimming when there is an urge to breath, swimmers are more likely to take a breath. Another risk factor for BHB is the rest period between breath holds. A decreased arterial oxygen concentration following a completed breath hold, potentially coupled with hypocapnia from an increased breathing rate after an earlier breath hold can increase the risk of BHB in subsequent breath holds, even where breath hold duration is not maximal. It is imperative that water-based athletes, especially swimmers, do not ignore the urge to breathe. Coaches supervising BHT must be vigilant for the risks and must take precautions to mitigate the inherent dangers of this training strategy. BHT should never be undertaken in unsupervised environments due to the risk of BHB and subsequent drowning.