Abstract: PLANT MAINTENANCE WORKERS OPENED A 3 INCH VALVE IN AN AMMONIA REFIGERATION SYSTEM PIPE TO DRAIN THE REMAINING AMMONIA. EQUIPMENT WAS TO BE ADDED TO THE SYSTEM. THE SYSTEM INCLUDES MORE THAN 500 FEET OF PIPE AT BETWEEN 25 AND 30 PSI OF PRESSURE. AFTER PUMPING THE AMMONIA OFF, A TEMPERATURE INVERSION OCCURRED, CAUSING THE FORMATION OF AN AMMONIA CLOUD. THE CLOUD DRIFTED DOWN OVER THE PLANT GUARD BUILDING AND INTO THE PARKING LOT. TWO GUARDS, A TRUCK DRIVER, AND A PRODUCTION FACILITY EMPLOYEE WHO HAPPENED TO BE IN THE AREA WERE ENVELOPED BY THE CLOUD AND TAKEN TO THE HOSPITAL. THEY WERE RELEASED LATER THE SAME DAY. AN INVESTIGATION REVEALED THAT STANDARD PROCEDURE HAD BEEN USED AND THAT THE TASK WAS PERFORMED TWO OR THREE TIMES A YEAR, AS NEEDED.