Abstract: At approximately 10:30 a.m. on June 20, 1999, Employee #1 and two coworkers, production technicians/operators for the Great Lakes Chemical Corporation, were changing the catalyst on the fourth floor of the trifluoropropane-I unit. A Monel rupture disk burst, and chlorine escaped to the atmosphere through a Teflon gasket flange between the rupture disk and the chlorine expansion tank. The R & D supervisor donned PPE and respiratory equipment, and was able to tighten the leaking flange and stop the release; however, the technicians had inhaled chlorine vapors while exiting the fourth floor. They immediately received emergency oxygen and were transported by ambulance to the local medical center. Employee #1 was kept overnight for observation and was released the following morning. He had undergone approximately 4 months of onsite training for his present job. This included respirator training along with fit testing, pulmonary function testing, and training on SCBAs. The company provided sketches and drawings, and others were drawn during the course of the investigation and were included in the case file.