Phossy jaw, formally known as phosphorus necrosis of the jaw, was an occupational disease affecting those who worked with white phosphorus (also known as yellow phosphorus) without proper safeguards. It was most commonly seen in workers in the matchstick industry in the 19th and early 20th centuries. It was caused by white phosphorus vapour, which destroys the bones of the jaw. Modern occupational hygiene practices have since eliminated the working conditions that caused this disease.