An unidentified chemical substance settled during the night on cars and garden furniture in the vicinity of a chemical platform. The evidence consisted of a large number of brownish stains of a dry product that was very difficult to clean and embedded into the surfaces where it settled; some 20 individuals and one company located downwind of the platform were concerned. Samples were taken and the site’s various operators were requested to identify the eventual source of this discharge within their own equipment. It was advised not to consume vegetables from potentially affected gardens as a preventive step. Ultimately, one of the platform’s plants recognized the same type of deposits at the exhaust of a chimney located 26.5 m away. The stains were due to a dye (2-(2,4 diaminophenoxy)ethanol dihydrochloride) synthesised the previous day (one-week batch manufacturing carried out every 4 to 6 weeks). Displayed in colourless crystal form and only revealing its colour when in contact with water, this product was neither toxic nor carcinogenic, yet considered harmful if ingested in large quantity. The suction of a portion of product via the workshop’s ventilation system between the rotary dryer and packaging equipment caused the eventual atmospheric discharge: the finished product came in big chunks, and the sieving step prior to packaging lasted 3 hours instead of the 30 min typically allocated, which generated considerable dust, a large amount of which was suctioned and discharged onto the roof. To avoid repeat occurrence, suction at the level of the rotary dryer drainage station was discontinued and a new filtration system installed. The site’s other powder suction stations were also revised.