Hypercapnia or hypercapnea (from the Greek hyper = "above" and kapnos = "smoke"), also known as hypercarbia, is a condition where there is too much carbon dioxide (CO2) in the blood. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs.
Hypercapnia normally triggers a reflex which increases breathing and access to oxygen, such as arousal and turning the head during sleep. A failure of this reflex can be fatal, as in sudden infant death syndrome.
Hypercapnia is the opposite of hypocapnia.