Abstract: On May 13, 2003, an employee operating reaction tanks was working at a chemical company in an area when a chlorine release occurred during the manufacture of 5000 gallons of 15.5% sodium hypochlorite. One of the employees operating the tank failed to close a manual valve on a second partially empty reaction tank, causing the remaining product to release chlorine vapor due to rapid product decomposition. Approximately 0.75 to 4.0 pounds of chlorine was released into the plant. Three employees working in the area were hospitalized for treatment of severe respiratory injuries resulting from acute exposure to chlorine.