Dioxin is one of a number of poisonous petroleum-derived chemicals which are produced when herbicides (substances used for killing plants) are made or when plastics are burned. Dioxins are chemically dibenzo-p-dioxins. One of the best known is TCDD (2, 3, 7, 8-TCDD). Dioxins cause skin disease (chloracne). In animal tests, dioxins are teratogens (agents that cause birth defects and miscarriages), mutagens (agents that cause mutations) and carcinogens (agents that cause cancer). In 2001 the US government added the TCDD to the list of substances that are known carcinogens based on “sufficient evidence of carcinogenicity from studies in humans.” The National Institutes of Health (NIH) concluded that: “It is now clear that there is a causal relationship between exposure to TCDD and human cancer.” A factory explosion in northern Italy in 1976 polluted the town of Seveso with dioxin. Some defoliants such as Agent Orange, which was used in the Vietnam war, contain dioxin.