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Icmesa chemical company, Seveso, Italy. 10th July 1976

Accident summary

At approximately 12:37 on Saturday 10th July 1976 a bursting disc on a chemical reactor ruptured. Maintenance staff heard a whistling sound and a cloud of vapour was seen to issue from a vent on the roof. A dense white cloud, of considerable altitude drifted offsite.

Among the substances in the white cloud was a small deposit of 2,3,7,8-Tetrachlorodibenzo-p-dioxin (‘TCDD’ or ‘dioxin’), a highly toxic material.

The release lasted for some twenty minutes. Over the next few days following the release there was much confusion due to the lack of communication between the company and the authorities in dealing with this type of situation.

The nearby town of Seveso, located 15 miles from Milan, had some 17,000 inhabitants. No human deaths were attributed to TCDD but many individuals fell ill. 26 pregnant women who had been exposed to the release had abortions. Thousands of animals in the contaminated area died and many thousands more were slaughtered to prevent TCDD entering the food chain.

Failings in technical measures

Operating Procedures: safe operating procedures

Relief Systems / Vent Systems: venting of excessive pressures, sizing of vents for exothermic reactions

Control Systems: Sensors Alarms / Trips / Interlocks: loss of cooling, agitator failure

Reaction / Product Testing: calorimetry methods, thermal stability

Design Codes - Plant: nature of hazardous releases

Secondary Containment: catchpots

Emergency Response / Spill Control: safety management system, site emergency plan


Lees, F.P., ‘Loss Prevention in the Process Industries – Hazard Identification, Assessment and Control’, Volume 3, Appendix 3, Butterworth Heinemann,
ISBN 0 7506 1547 8, 1996.