Aubervilliers residents noticed the presence of strong odours. The departmental sewer service office informed the Classified Facilities Inspectorate of pollution in the wastewater network that seemingly originated at a pharmaceuticals plant 3 km away. The operator of the cited plant remarked that 2 storage tanks A1 and A2, each of 100cu.m capacity and containing yeast, were abruptly drained on the same day following a malfunction of one of the drainage pumps two weeks prior. These tanks had typically been drained on a continuous basis with the simultaneous injection of bleach by a feed pump at the sewer outfall for treating odours. Anaerobic matter content was thus drained over the course of one day instead of flowing out regularly for the 2 weeks during which the malfunction had gone undetected. On the day of this incident, the down hypochlorite injection feed pump had to be urgently repaired. The operator halted effluent discharge from tanks in the drainage network at the request of sewer services and then injected large quantities of clean cooling water to create a flushing effect and thereby mitigate odours. Around 5:50 pm, the results of analyses performed amounted to 26 ppm H2S in the sewer network collector pipe in Aubervilliers. Fire-fighters ventilated the pipe to lower the concentration to 10 ppm; this value was reached at about 8 pm. It should be noted that the Threshold Limit Value (TLV) and Average Exposure Value established by the Labour Ministry for H2S at 10 ppm (14 mg/m³) and 5 ppm (7 mg/m³), respectively, had been surpassed, thus suggesting that the site’s network outlet concentration was very high. On 4th October 2005, an analytical laboratory confirmed the presence of methyl isobutyl ketone (MIBK) in samples extracted on the day of the incident. The sewer services and administrative authorities logged this incident.