The firefighters were contacted by the manager of a bakery plant who reported that employees on the bun-making line in building A began smelling a pungent and irritating odour at around 9:00 a.m. The odour was probably caused by a gas leak. The firefighters arrived and, at around 10:00 a.m., measured concentrations of 45–50 ppm for ammonia, 70 ppm for carbon monoxide, and 0 ppm for chlorine in the air at the production floor. The employees were evacuated and the production floor was shut down. An inspector of classified facilities arrived at the scene.

The following day, the firefighters measured concentrations in the air of the production floor and found concentrations of 12 ppm for ammonia (42 to 40 ppm at breast height) and 140 to 170 ppm for carbon monoxide (height of 2.5 m).

The operator’s analyses revealed that the burner of the unit’s furnace, which is fired with natural gas, displayed an intermittent operating fault. The air damper’s drive shaft was warped. As a result, when the burner operated at full power, the control linkage would push the air damper beyond its fully open position, decreasing the supply of air and causing incomplete combustion, which produces carbon monoxide in the combustion gases. Measurements showed that the carbon monoxide content in the combustion gases was 4000 ppm after the faulty burner operated for 3 minutes (versus between 10 and 15 minutes with a normal burner). Because the damper in the furnace’s flue gas pipe was continuously open, carbon monoxide was able to seep into the air of the production floor. This damper, which in theory is automatically controlled by pressure drops in the pipe, controls the stack draught.

Because the air-extraction system for the production floor was shared by the nearby production line named “Mécatherme” and the nearby drying chamber, it drew in the carbon monoxide. The air mixed inside the drying chamber and continued to be distributed into the air of the production floors even after the furnace burner was turned off. The drying chamber contains a trap and a wastewater pipe. As a result, small amounts of carbon monoxide were found inside the production floor’s wastewater system. Both of these unforeseen factors interfered with the investigations. The operator checked and confirmed this scenario by visualising the flows with a thermographic camera during a re-enactment. The ammonia levels measured were caused by the use of detergents to unclog the pipes in building A.

The furnace is periodically checked. Nothing unusual had been found during the last check, which had been conducted shortly before the incident. The defective components were repaired. The operator purchased carbon-monoxide detectors.