Carbon dioxide (CO₂) retention, or hypercapnia, is a known risk of diving that 
can cause mental and physical impairments leading to life-threatening accidents. 
Often, such accidents occur due to elevated inspired carbon dioxide. For 
instance, in cases of CO₂ elimination system failures during rebreather dives, 
elevated inspired partial pressure of carbon dioxide (PCO₂) can rapidly lead to 
dangerous levels of hypercapnia. Elevations in PaCO₂ (arterial pressure of PCO₂) 
can also occur in divers without a change in inspired PCO₂. In such cases, 
hypercapnia occurs due to alveolar hypoventilation. Several factors of the dive 
environment contribute to this effect through changes in minute ventilation and 
dead space. Predominantly, minute ventilation is reduced in diving due to 
changes in respiratory load and associated changes in respiratory control. 
Minute ventilation is further reduced by hyperoxic attenuation of 
chemosensitivity. Physiologic dead space is also increased due to elevated 
breathing gas density and to hyperoxia. The Haldane effect, a reduction in CO₂ 
solubility in blood due to hyperoxia, may contribute indirectly to hypercapnia 
through an increase in mixed venous PCO₂. In some individuals, low ventilatory 
response to hypercapnia may also contribute to carbon dioxide retention. This 
review outlines what is currently known about hypercapnia in diving, including 
its measurement, cause, mental and physical effects, and areas for future study.