Hypercapnea

The symptoms are disorientation, panic, hyperventilation, unconsciousness and death.

In a SCUBA diving context, carbon dioxide gas is a product of the body's metabolism, which is normally expelled through the lungs. If it cannot be removed from the breathing cycle, it may be re-inhaled causing an increase in concentration in the breathing gas and poisoning.

There are a variety of reasons the carbon dioxide is not being expelled completely when the diver exhales:

• the lungs are fully emptying due to tight fitting clothing around the chest
• the diver is exhaling into a vessel such as a long snorkel, full face diving mask or diving helmet and then re-inhaling from that vessel
• the scrubber in the rebreather the diver is using has failed and is not removing carbon dioxide fromt the loop
• the diver is working too hard and therefore producing more carbon dioxide, breathing quicker and possibly not exhaling fully
• the diver is skip breathing to conserve breathing gas