During the unloading of a tanker lorry at an industrial laundry, 50 l of sulphuric acid were mistakenly poured into a tank containing 200 l of sodium bisulphite. Sulphur dioxide (SO2) immediately spread throughout the delivery area. The driver closed the valves, but the toxic cloud driven by the wind engulfed the rest of the plant. The establishment was evacuated and the gendarmes (French military police) set up a 200 m safety perimeter. The production section and the unloading area were ventilated. The vapours incapacitated around twenty employees. Twelve of them suffering from bronchial irritations were hospitalised for examination and discharged the same evening. Forty-two employees were placed laid off until the next day. Production losses were estimated at €5,000. The chemical trading company that had made the delivery removed and neutralised the contents of the tank of sodium bisulphite 4 days after the accident. Two chemical unloading procedures existed; the employee responsible for receiving the delivery was responsible for opening the padlocked unloading cabinet corresponding to the deliveryman’s request and remaining at his security post. Following this incident, the internal unloading procedure was modified to included imposed systematic joint checking throughout the entire duration of the operation. The enquiry also revealed that the unloading area had no retention system and that there was no hydrocarbon separator in the site’s stormwater network.