At around 11:00 p.m., phosgene (COCl2) and chlorobenzene began leaking inside the containment building of a chlorinated chemicals plant. The leak was detected at levels between 13 and 22 ppm by analyzers inside the containment. The analyzers’ control system automatically directed the air inside the containment towards the caustic scrubber. Technicians began looking for the cause of the leak. While one of them was suiting up to enter the containment, the analyzers detected an even larger leak (500 ppm) at 12:20 a.m. The technicians placed the circuit in a safe position by shutting off the pump (which stopped the leak) and isolating the containment, but not before 400 kg of phosgene and 600 kg of chlorobenzene spread inside the containment. Although the analyzers did not detect anything unusual, there was a noticeable odor outside, near the containment. At 2:00 a.m., the technicians began cleaning the containment by injecting air inside the scrubber. They could smell something in the air although none of the analyzers in the surrounding production facilities and labs detected any phosgene. At around 2:40 a.m., outside analyzers detected phosgene at levels between 0.12 and 0.2 ppm. The operator estimated that 4 – 5 kg of phosgene escaped to the atmosphere via the containment’s stack.