A deficiency in the body when blood fails to deliver the necessary level of oxygen to tissues and organs. It can be caused by a lack of oxygen in the atmosphere due to displacement or exhaustion, particularly by the release of gases in confined spaces, or due to carbon monoxide poisoning, which inhibits the ability of haemoglobin in the blood to release the oxygen bound to it. The symptoms include headaches, tiredness, and shortness of breath, to loss of consciousness and death. Cyanosis, a blue discolouration of the skin and in particular the lips, show this in severe cases.