Burns are a common injury in daily life, production work and warfare. Most people think that heat is the only cause of burns; however, certain chemicals and electrical currents can also cause burns. The skin is often only part of the body that is burned; the subcutaneous tissue can also be burned, and even when there is no skin burn, there can be internal organ burns. For example, drinking very hot liquids or corrosive substances (such as acids) can burn the esophagus and stomach. In a building fire, inhalation of smoke or hot air can cause burns to the lungs. Burnt tissues may become necrotic. In tissue burns, fluid leakage from the blood vessels causes tissue edema. In large burns, abnormal vascular permeability and loss of large amounts of fluid may cause shock. In shock, blood pressure is very low and blood flow to the brain and other vital organs is reduced. Electrical burns are caused by the flow of electric current through the body that generates high temperatures above 5000°C. The skin is often completely destroyed and burned at the site where the current enters the body (see section 278). Because of the high resistance of the skin in contact with the charged body, a large amount of electrical energy is converted into heat there to make the surface burn. Most electrical burns also severely damage the subcutaneous tissue, and the extent and depth of the burn varies. The area affected may be much larger than the area of burned skin. Severe electroshock can cause apnea and cardiac arrhythmias, causing dangerous heart rate disturbances. Chemical burns can be caused by a variety of irritating and toxic chemicals, including strong acids, strong bases, phenol, toluene (organic solvents), mustard gas, phosphorus, etc. Chemical burns can cause tissue necrosis and slowly expand a few hours after the burn.