White phosphorus bombs are weapons that use white phosphorus, a chemical element that is used in smoke, illumination, and incendiary munitions. White phosphorus is a waxy solid substance that ignites spontaneously in air and is extremely toxic to humans. It can be used as a smokescreen, to mark targets, or as an incendiary weapon that can spread fire over an area up to several hundred square yards. When used as a weapon, white phosphorus can cause severe burns to people and objects in the vicinity, and its effects can be long-lasting. It is not considered a chemical weapon because it operates primarily by heat and flame rather than toxicity. White phosphorus bombs can cause injuries that are more serious and harder to treat than injuries from conventional bombs, and medical professionals need special training to deal with these types of injuries and to protect themselves from phosphorus burns during treatment. The use of white phosphorus in densely populated areas violates the requirement under international humanitarian law that parties to the conflict take all feasible precautions to avoid civilian injury and loss of life.