Bedsores are caused from a lack of blood flow and from mechanical stress to the skin and tissues over a bony area that has been under pressure for a prolonged period. If blood supply is cut off to an area of skin for more than 2 or 3 hours the skin is deprived of oxygen and begins to die. In addition, when slowly sliding down a bed or chair, friction to the outer skin layer such as from wrinkled bedding and clothing contribute to skin injury and ulcers. Excessive exposure to moisture such as sweat, blood, urine or faeces also increases the likelihood of bedsores.