In a plastics plant, a rupture disc burst on a pre-polymerisation reactor, and monomer vinyl chloride (MVC) was released into the atmosphere. MVC polymerisation was performed in 2 stages: running the liquid MVC through a preliminary polymeriser with a catalyst (organic peroxides), before being split into several autoclaves. This preliminary polymeriser was equipped with one main bursting disc and 2 backup discs. The main disc was replaced twice a year, while the backup discs were disassembled and tested at 13 bar once a year and then replaced every 5 years. Polymerisation required a 4-phase process: filling of raw material, heating of the pre-polymeriser, the actual polymerisation step, and lastly drainage to an autoclave. During the drainage phase, a malfunction occurred at 9:40 pm, and the product proved difficult to transfer. Rupture of the disc was observed around 10:15 pm by the teams on-call, and they proceeded by switching over to the backup disc. In the meantime, 3 tonnes of MVC escaped during a 35-min period via the vent line downstream of the rupture disc, at a height of 25 m. Released vertically without carrying any PVC powder, the gaseous discharge diluted in the atmosphere (10 km/hr winds). The pre-polymeriser was degassed and drained for inspection; it exhibited no deficiencies when tapping the disc. The rupture occurred at a pressure of 4.6 bar, which was considerably less than the nominal bursting pressure (15.2 bar), the operating pressure (11.5 bar) or the test pressure (13 bar). An examination of records revealed that since installation of the disc 4 months prior, the maximum operating pressure in the pre-polymeriser had never exceeded 11.5 bar. The hypothesis of a mechanical disc failure was thus retained. A new disc from another batch than the burst disc was installed. The unloaded backup discs were not replaced. The pre-polymeriser unit was restarted the next day around 6 pm. The operator issued a press release. The 6 discs from the same batch were all replaced. An information exchange with the disc manufacturer’s distributor indicated that bursting pressures had remained within the specified tolerances ranging from 14.4 to 15.9 bar during the control procedure (rupture test on 2 discs from a batch of 6); visual inspection of the burst disc displayed no assembly problems (given the state of the support and the operating protocol); this assembly operation was performed by an employee specifically assigned to monitor this category of equipment. The disc manufacturer conducted an expert appraisal using destructive controls, which revealed corrosion on the interior disc surface. The following guidelines were adopted: replacement of the main disc every 3 months; evaluation of the disc removal mode in case of transfer difficulties; and replacement of all backup discs in the event of loading. The operator examined the deployment of resources to limit discharges during vent ruptures and assessed the health impacts from this dischar