White phosphorus bombs are weapons that use white phosphorus, a chemical element that is used in smoke, illumination, and incendiary munitions. White phosphorus is pyrophoric, meaning it ignites on contact with air, and burns fiercely, igniting cloth, fuel, ammunition, and other combustibles. It is commonly used to create a dense smoke screen or mark targets, and is also used as an incendiary substance that can severely burn people and set structures, fields, and other civilian objects in the vicinity on fire. White phosphorus bombs are incendiary weapons that can spread fire over an area up to several hundred square yards, and the phosphorus continues to burn until it is all gone. White phosphorus is harmful by all routes of exposure, and can cause deep and severe burns, penetrating even through bone, and has been known to reignite after initial treatment. The usage of white phosphorus is restricted under international humanitarian law, and it must never be fired at, or in close proximity to, a populated civilian area or civilian infrastructure, due to the high likelihood that the fires and smoke it causes spread.