Abstract: At approximately 8:45 a.m. on March 4, 1986, Employee #1 was attempting to remove chlorine gas cylinders from a distribution point, located beneath a stair well. Eight of the cylinders were mostly full and simply in storage. The system had not been used for five years. One 150-lb cylinder was still connected to an advance gas chlorinator regulator made by Capital Controls Co. Calmar, PA, Model Number 201 and the manifold. While Employee #2 was removing a loose cylinder to another area, Employee #1 attempted to remove the connected cylinder. When Employee #1 could not turn the cylinder valve with the wrench, he assumed it had been closed years ago, when the systems were shut down. He did not observe the indicator on the chlorinator, which should have shown that there was still pressure on the unit. When Employee #1 loosened the single bolt on the chlorinator, there was an immediate full flow of chlorine gas, which spread quickly into a production area on the first and second floor. In addition to Employees #1 and #2, Employees #3-#10 were exposed either while attempting to stem the flow of gas or in passing through the gas cloud as they exited. No respiratory protection was used by any of the employees involved. All ten employees were hospitalized.