Cyanosis is a physical sign causing bluish discoloration of the skin and mucous membranes. It is caused by a lack of oxygen in the blood, and is associated with cold temperatures, heart failure, lung diseases, and smothering. Cyanosis is seen in infants at birth as a result of heart defects, respiratory distress syndrome, or lung and breathing problems.

Blood contains a red pigment (hemoglobin) in its red blood cells. Hemoglobin picks up oxygen from the lungs, then circulates it through arteries and releases it to cells through tiny capillaries. After giving up its oxygen, blood circulates back to the lungs through capillaries and veins. Hemoglobin, as well as blood, is bright red when it contains oxygen, but appears dark or "bluish" after it gives up oxygen.

The blue discoloration of cyanosis is seen most readily in the beds of the fingernails and toenails, and on the lips and tongue. It often appears transiently as a result of slowed blood flow through the skin due to the cold. As such, it is not a serious symptom. However, in other cases cyanosis is a serious symptom of underlying disease.

The blue color of the skin and mucous membranes is caused by lack of oxygen in the blood. Low blood oxygen may be caused by poor blood circulation, or heart or breathing problems. It can also be caused by being in a low-oxygen environment or by carbon monoxide poisoning. More rarely, cyanosis can be present at birth as a sign of congenital heart disease, in which some of the blood is not pumped to the lungs where oxygen would make the blood a bright red color. Instead, the blood goes to the rest of the body and remains unoxygenated. Cyanosis also may be caused by poisoning from chemicals, drugs, or contaminated food and water.

Other signs of low blood oxygen may accompany cyanosis, including feeling lightheaded or fainting.

If the underlying condition (such as heart or lung disease) can be properly treated, the skin will return to its normal shade.