Abstract: At approximately 7:00 p.m. on June 12, 2000, Employees #1 through #5 were working on the third shift in the casting department when the chlorine rupture disc in the chlorine mixing station building activated. Approximately 240 lb of vaporized chlorine was released through an open piping system to the outside of the building and was carried by the wind approximately 75 ft to the casting department. The chlorine went unnoticed until Employees #1 through #5 smelled it, after which the gas activated the monitors at the drop area for aluminum ingots, near the furnace. These monitors at the furnaces were used for detecting chlorine leaks in the chlorine piping system for fluxing the pots and drops. The hazmat team manually turned off the valve on the one-ton chlorine cylinder, preventing its entire contents from being released. Employees #1 through #5 were exposed to the chlorine vapor and were transported to the hospital. Employees #1 through #3 were admitted for observation; Employees #4 and #5 were treated and released. This incident was likely caused by a malfunctioning argon heater and a bad chlorine regulator. The malfunctioning equipment probably caused the temperature and pressure of the chlorine in the line to rise, rupturing the rupture disc.