While unloading a lorry in a special industrial waste (SIW) treatment centre, a chemical reaction occurred when 2-bromoisobutyroyl bromide was being added to a tank prior to its destruction in an incinerator. The pressure in the tank rose and a cloud of irritating vapours was released into the atmosphere. Thermal runaway occurred when transferring the bromide substance from a 200 l drum into an 11,000 l tank. In agreement with fire-fighters, the tank’s vents were opened to prevent any explosion. Strong winds pushed the whitish cloud of alkyl bromide (a corrosive and irritating product) and hydrogen bromide (HBr) which had formed. This cloud, which could be seen from several km away, headed towards a sawmill and 4 warehouses located in an uninhabited area. The site’s safety departments assisted by several dozen fire-fighters sprayed the tank in order to cool it and water down the irritating vapours; the diluted vapours were recovered in the site’s retention basins. The prefecture activated a crisis unit and a medical unit went to the site. The internal emergency plan was activated at 12:25 pm and a safety perimeter was defined. The containment measures deployed concerned almost 180 people: the site’s employees, those from companies located within a 1 km radius and around sixty pupils from a nursery school 4 km outside the safety perimeter. The chemical reaction was controlled at around 1:30 pm. The strong wind eventually dispersed the cloud. The safety perimeter and the internal emergency plan were lifted at around 4 pm. Eighteen employees from neighbouring companies complained of headaches and breathing difficulties; 7 were treated, 4 were given oxygen, but no one was hospitalised. Atmospheric measurements showed that toxic thresholds were not reached. The health services warned health professionals in the sector concerned about the toxic risk incurred by the population in order to draw their attention to any clinical symptoms that could appear. The pressure build-up was due to the presence of a contaminant in the tank, which caused the initial product to decompose. The tank involved was immobilised for sampling and analyses. The operator contacted its subcontractor to obtain information about the condition of the tank: quality of the cleaning carried out, history of substances contained, etc. The 8 drums present on the site and which contained substances of the same type, will need to be inspected and monitored.