The Seveso disaster was an industrial accident that occurred around 12:37 PM on 10 July 1976, in a small chemical manufacturing plant approximately 20 kilometres (12 mi) north of Milan in the Lombardy region of Italy. It resulted in the highest known exposure to 2,3,7,8-tetrachlorodibenzodioxin (TCDD) in residential populations, which gave rise to numerous scientific studies and standardized industrial safety regulations, including the European Union's Seveso III Directive. This accident was ranked eighth in a list of the worst man-made environmental disasters by Time magazine in 2010.
Location of disaster The Seveso disaster was named after Seveso, the community most affected, which had a population of 17,000 in 1976. Other affected neighbouring communities were Meda (19,000), Desio (33,000), Cesano Maderno (34,000) and to a lesser extent Barlassina (6,000) and Bovisio-Masciago (11,000). The industrial plant, located in Meda, was owned by the company Industrie Chimiche Meda Società Azionaria (Meda Chemical Industries S.A., or ICMESA), a subsidiary of Givaudan, which in turn was a subsidiary of Hoffmann-La Roche (Roche Group). The factory building had been built around 30 years earlier and the local population did not perceive it as a potential source of danger; they were unaware of what was being manufactured there. Moreover, although several exposures of populations to dioxins had occurred before, mostly in industrial accidents, they were of a more limited scale.
Chemical events The accident occurred in the chemical plant's Building B. The chemical 2,4,5-trichlorophenol (2) was produced there from 1,2,4,5-tetrachlorobenzene (1) by the nucleophilic aromatic substitution reaction with sodium hydroxide. The 2,4,5-trichlorophenol was intended as an intermediate for hexachlorophene.
The reaction temperature was achieved by passing the steam exhaust from the onsite electricity generation turbine through an external heating coil installed on the chemical reactor vessel. The exhaust steam pressure was normally 12 bar with a temperature of 190 °C, which resulted in a reaction mixture temperature of 158 °C, very close to its boiling point of 160 °C. Safety testing showed the onset of an exothermic (heat-releasing) side reaction if the reaction mixture temperature reached 230 °C. Crucially, no steam temperature reading was made available to plant operators responsible for the reactor. The chemical-release accident occurred when a batch process was stopped before the completion of the final step of removal of ethylene glycol from the reaction mixture by distillation. The process was stopped due to conformance with an Italian law requiring the shutdown of plant operations over the weekend. Other parts of the site had already started to close down as the processing of other batches finished, which reduced power consumption across the plant, causing a dramatic drop in the load on the turbine and a consequent increase in the temperature of the exhaust steam to around 300 °C. This hotter-than-normal steam then heated the portion of the metal wall of the accident reactor above the level of the liquid within it to the same temperature. Not having a steam temperature reading among their instruments, the operators of the reactor were unaware of the presence of this additional heating, and they stopped the batch as they normally would by isolating the steam and turning off the stirrer in the reactor vessel. The abnormally hot upper region of the reactor jacket then heated the adjacent reaction mixture. With the stirrer not running, the heating was highly localised to just the portion of the upper layers of the reaction mixture adjacent to the reactor wall. The local temperature increased to above the critical temperature for the exothermic side reaction seen in testing. Additionally, the critical temperature proved to be only 180 °C, 50 °C lower than believed. At that lower critical temperature, an exothermic decomposition began, releasing more heat and leading to the onset of a rapid runaway reaction when the temperature reached 230 °C seven hours later. The reactor relief valve eventually opened, causing the aerial release of 6 tonnes of chemicals, which settled over 18 km2 (6.9 sq mi) of the surrounding area. The cloud of substances released contained sodium hydroxide, ethylene glycol, sodium trichlorophenate, and approximately 15 to 30 kg of TCDD (3). At the nominal reaction temperature, TCDD is normally seen only in trace amounts of less than 1 ppm (parts per million). However, in the higher-temperature conditions associated with the runaway reaction, TCDD production reached 166 ppm or more.
Immediate effects The affected area was split into zones A, B, and R in decreasing order of surface soil concentrations of TCDD. Zone A was further split into 7 sub-zones. The local population was advised not to touch or eat locally grown fruits or vegetables.
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