A generalised power failure at 4.20 pm in the entire geographical sector caused the units of a plant manufacturing surfactants, polyesters, polyols and polyurethanes to stop. When the units were re-started, 180 kg of 6% sulphur trioxide (SO3) was released into the atmosphere for 10 min when a sulphonation unit pump did not re-start. The irritant cloud drifted away and effected a 7-storey building attracting 15 complaints from residents. The fire-fighters confirmed the irritating nature of the cloud, took measurements that revealed no anomaly and ended operations at 7.30 pm. During the pollution incident, the local network Air Normand monitoring atmospheric pollution (measurement frequencies: 10 s) did not detect any abnormally high concentrations of sulphur dioxide and sulphur trioxide.