A burn is a type of injury to flesh or skin caused by heat, electricity, chemicals, friction, or radiation. Burns that affect only the superficial skin are known as superficial or first-degree burns. When damage penetrates into some of the underlying layers, it is a partial-thickness or second-degree burn. In a full-thickness or third-degree burn, the injury extends to all layers of the skin. A fourth-degree burn additionally involves injury to deeper tissues, such as muscle or bone. The treatment required depends on the severity of the burn. Superficial burns may be managed with little more than simple pain relievers, while major burns may require prolonged treatment in specialized burn centers. Cooling with tap water may help relieve pain and decrease damage; however, prolonged exposure may result in low body temperature. Partial-thickness burns may require cleaning with soap and water, followed by dressings. It is not clear how to manage blisters, but it is probably reasonable to leave them intact. Full-thickness burns usually require surgical treatments, such as skin grafting. Extensive burns often require large amounts of intravenous fluid, because the subsequent inflammatory response causes significant capillary fluid leakage and edema. The most common complications of burns involve infection. While large burns can be fatal, modern treatments developed since 1960 have significantly improved the outcomes, especially in children and young adults. Globally, about 11 million people seek medical treatment, and 300,000 die from burns each year. In the United States, approximately 4% of those admitted to a burn center die from their injuries. The long-term outcome is primarily related to the size of burn and the age of the person affected.