In section “B” of the ICMESA factory on Saturday, 10 July 1976 at five in the morning, the tetrachlorophenol production cycle was stopped. This was unlike the usual procedure when this was done towards the end of the last work shift before the week-end break. At 12.37 an uncontrollable rise of the temperature inside reactor A101 had provoked excessive pressure resulting in the breakage of a safety valve, with the consequent emission into the air of a toxic cloud composed of about 400 kilos of reactive products in the form of a spray. The cloud formed was composed mainly of tetrachlorophenol and caustic soda, with also a percentage of Dioxin (2,3,7,8 tetraclorodibenzodioxin).
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The cloud was dragged south-eastward by the wind and deposited on the soil of an area of about 1,810 hectares, covering the municipalities of Meda, Seveso, Cesano Maderno and Desio.
On Thursday, 15 July, five days after the incident and following the death of many animals and cases of skin lesions on children in the polluted zone, the mayors of the municipalities of Seveso – Francesco Rocca – and Meda – Fabrizio Malgrati – issued the first ordinances to safeguard the health of the population.
After days of agonising uncertainty on the gravity of the event, also due to the silence of ICMESA’s officials, on Saturday, 24 July 1976 the mayors ordered the evacuation of the zone that had been most polluted, by the next Monday. The area was fenced out and access was prohibited. This part of the territory was called “Zone A” and today constitutes the area on which the Oak Wood stands. In the following days also “Zone B,” a zone which was less polluted was fenced out – and so was “Zone R” or the zone of regard – which was not polluted or polluted with values lower than 5 microgrammes per square metre. “Zone A” was later isolated with a fine mesh metallic net, 4 metres high and lined with yellow fiberglass to impede the dispersive action of the wind and access to animals. The entire perimeter was fenced out with barbed wire.
