In a rubber manufacturing plant, a leak of acrylonitrile and butadiene occurred, and 6 m³ of non-stripped latex containing 30 kg of acrylonitrile were lost as well as 7.5 m³ (18 kg) of gaseous butadiene. The day before, a licensed external contractor removed a pipe on the “continuous” unit in the “metering” building. The mechanic from this company touched the key of the adjacent manual valve 1, and a few drops of non-stripped latex stripped came out. He immediately closed the valve. However, one of the unit explosimeters reached 40% of the LEL and activated a pre-alarm in the control room. The foreman decided to place blind flanges on the two valves located on either side of the piece to be modified (valves 1 and 2). The next day, a work permit was signed by the morning shift for the replacement of the modified line. The external company started its intervention early in the afternoon. At 1:59 pm, the incident alarm coming from the unit explosimeters was activated. A supervisor and two technicians entered the building and saw the non-stripped latex on the ground and the mechanic who had been sprayed in the face. He was placed in the safety shower and then taken to hospital. The accident was caused by opening manual valve 1. Given that the line being reassembled was relatively heavy, it was raised onto an overhead hoist. During manoeuvring, the hoist chain caught the key of the valve, causing it to open and gradually empty a tank containing non-stripped latex containing butadiene. The foreman and two technicians left the building with the door open to ventilate the room. One of the technicians closed the upstream and downstream valves (7 and 6). The foreman told the technicians to equip themselves with anti-gas outfits to close valve 1. The IOP was activated and the unit’s sprinkler system turned on to dilute the gas. The latex flowed into the gutters towards the water treatment plant. It was diverted to the extinction water recovery basin to prevent contamination of the RHINE with acrylonitrile. Soil contamination outside the retention basin was avoided. The water, loaded with latex, was pumped and treated internally. The intervention ended at around 6:30 pm. The latex leak was due to poor blinding of the installation: the plates should have been placed at the valves 6 an 7 (valves further away from the modified pipe than valves 1 and 2) instead of placing blind flanges on valves 1 and 2. In this case, opening valve 1 would not have had any effect. Procedures for working on piping containing toxic or hazardous products were reviewed: a diagram of the lines was to be established to identify which valves to close, plates to be installed and segments to be drained and flushed. This schematic diagram was attached to the work permit.