Ammonia was released through a vent within a carbon dioxide liquefaction workshop at a chemical plant. 300 employees had to be evacuated. A total of 24 individuals suffered discomfort, four of whom required hospitalisation as a precaution. The workshop was devoted to liquefying CO2 with support provided by a refrigeration circuit delivering 5 tonnes of ammonia. With the shop in a down period, a pressure transmitter had been disassembled the previous day in order to proceed with maintenance on the high-pressure NH3 compression circuit. This transmitter, which was performing a dual function, made it possible to regulate refrigeration circuit pressure at a recommended value of 13 bar and thereby ensure installation safety with a trigger value set at 14 bar. The workshop resumed operations the following morning while the transmitter was still being repaired. With no regulation or safety protection in place, the system diverged and the NH3 circuit rose in both temperature and pressure. The circuit safety valve was activated, and 200 kg of NH3 were discharged through a vent 17 m high.