Dioxin, dy OK suhn, is any of 75 related chemicals, all of which consist of carbon, chlorine, hydrogen, and oxygen. However, the word dioxin is most commonly used to refer to only one of these chemicals, the compound 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD). Some scientists consider TCDD to be the most toxic synthetically produced chemical.