A thick cloud was observed for 45 min near a plant producing titanium dioxide. The gases stemming from the attack on ores (slag) by sulphuric acid were normally treated in a processing and regeneration unit. This attack and the gas suctioning had stopped subsequent to an electrical outage; 600 kg of sulphur dioxide/trioxide and H2SO4 were released into the atmosphere by a valve in order to avoid a pressure surge in the reactor, with the exothermic reaction continuing at a slow rate until the attack on the ore load came to an end. A similar incident had occurred 2 months prior following controller malfunction. Children at a nearby school complained of breathing difficulties. The unit’s safety report was completed (electricity supply cut-off, transmission and water supply circuits). An efficient detection apparatus, even in the event of electrical outage, and emergency response resources (electric generating sets, washers, etc.) were to be proposed.