Abstract: At 10:00 a.m. on August 7, 1986, Employee #1 decided to exchange the gas pressure regulator in use on a cylinder of chlorine liquid/gas that was located outside the plant and inside a storage cabinet near an open truck loading doorway. He was going to replace the regulator with a regulator that had been removed from another cylinder. Thinking that he had shut off the supply valve on top of the cylinder, he removed the regulator. The valve was, in fact, open and when the seal was broken chlorine gas escaped in front of Employee #1. He moved back and away from the leaking cylinder and radioed the maintenance dispatcher to report the chlorine leak. The dispatcher arrived wearing an emergency respirator and closed the valve. Approximately 10 lb of chlorine escaped during the 12 1/2 minutes the value was open. Employees #1 through #3 inhaled the escaped gas and were taken to area hospitals.