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
  • The production cycle was interrupted, without any agitation or cooling, prolonging holding of the reaction mass. Also, the conduct of the final batch involved a series of failures to adhere to the operating procedures. The original method of distillation patent specified that the charge was acidified before distillation. However, in the plant procedures the order of these steps was reversed.
  • Relief Systems / Vent Systems: venting of excessive pressures, sizing of vents for exothermic reactions
  • The bursting disc was set at 3.5 bar to guard against excessive pressure in the compressed air used to transfer the materials to the reactor. Had a bursting disc with a lower set pressure been installed, venting would have occurred at a lower and less hazardous temperature.
  • Control Systems: Sensors Alarms / Trips / Interlocks: loss of cooling, agitator failure
  • The reactor control systems were inadequate, both in terms of the measuring equipment for a number of fundamental parameters and in the absence of any automatic control system.
  • Reaction / Product Testing: calorimetry methods, thermal stability
  • The company was aware of the hazardous characteristics of the principal exotherm. However, studies showed that weaker exotherms existed that could lead to a runaway reaction.
  • Design Codes - Plant: nature of hazardous releases
  • There was no device to collect or destroy the toxic materials as they vented.
  • Secondary Containment: catchpots
  • The bursting disc manufacturer recommended using a second receiver to recover toxic materials. No such vessel was fitted.
  • Emergency Response / Spill Control: safety management system, site emergency plan
  • Information on the chemicals released and their associated hazards was not available from the company. Communication was poor and failed both between the company and the local authorities and within the regulatory authorities.


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.

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