Hypercapnia is a potentially fatal condition for divers that can be caused by an excessive build-up of carbon dioxide in the respiratory and circulatory system. Elevated levels of carbon dioxide lead to headache, confusion, and eventually, loss of consciousness.
Hypercapnia is sometimes associated with full-face masks, rebreathers, and skip-breathing (open circuit). It is not very common in recreational open circuit diving, although it may still occur, especially when skip-breathing or hyperventilating at depth. Therefore, we recommend taking two or three slow, deep breaths before ascending and keeping a close watch on your buddy.