The Seveso disaster was an industrial accident that occurred in Seveso , Italy on July 10, 1976 in a small chemical manufacturing plant of ICMESA .

During the production of 2,4,5-trichlorophenol , a herbicide, fungicide, and chemical intermediate, an uncontrolled reaction occurred and the chemical reactor ruptured. Probably several kilograms of the dioxin TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) were released into the atmosphere and the chemical was spread throughout a large section of the Lombard Plain between Milan and Lake Como. 3,000 pets and farm animals died and 70,000 animals were slaughtered later to prevent dioxin from entering the food chain. 193 people in the affected areas suffered from chloracne and other symptoms.

Due to the fact that dioxin is highly toxic, many were concerned about the long-term outlook for the residents of Seveso. Fortunately, in the long run human symptoms were not grave. Cancer rates were increased only slightly, if at all, and other problems relating to reproduction and mental illness were far less than many had worried at the time of the accident. Scientists still have not been able to prove that dioxin was the cause of these illnesses.

Treatment of the soil in the effected areas was so complete that it now has a dioxin level below what would normally be found. The disaster lead to the Seveso Directive , which was issued by the European Community and imposed much harsher industrial regulations.