Abstract: AN EMPTY 150 POUND ANMONIA CYLINDER, ONE OF TWO MANIFOLDED TO A WHITEPRINT MACHINE, WAS BEING EXCHANGED FOR A FULL ONE. THE DRIVER/DELIVERER, AFTER BEING TOLD WHICH CYLINDER WAS EMPTY, ASSUMED IT WAS READY TO BE REMOVED AND UNSCREWED THE T-HANDLE ON THE CYLINDER YOKE FITTING. AMMONIA MIST WAS UNEXPECTEDLY RELEASED, RESULTING IN RAPID EXCAVATION OF THE ROOM, EXCEPT FOR THE DRIVER. HE STAYED FOR 15 TO 20 SECONDS TO SCREW DOWN THE YOKE AND STOP THE LEAK, THEN LEFT. THE EMPLOYEE ASSISTING THE DRIVER ASSUMED THAT THE CYLINDER VALVE AND MANIFOLD ISOLATION VALVE WERE CLOSED, AS IS CUSTOMARY. IN FACT, THE DEPARTMENT SUPERVISOR DECLARED THAT HE SHUT BOTH VALVES THE DAY BEFORE, WHEN HE SHIFTED TANKS. WHY AND BY WHOM THE ISOLATION VALVE WAS OPENED IS NOT KNOWN. THREE COUNTY EMPLOYEES AND THE DRIVER WERE TAKEN TO A NEARBY HOSPITAL, EXAMINED AND RELEASED WITHOUT TREATMENT. THE PROCEDURE HAS BEEN CHANGED TO INCLUDE REMOVING THE YOKE AND CAPPING THE EMPTY CYLINDER AT THE TIME THE SHIFT IS MADE TO A FULL CYLINDER. CONSIDERATION IS ALSO BEING GIVEN TO INSTALLING A CHECK VALVE IN EACH DISCHARGE LINE. THIS WOULD PREVENT BACKFLOW IN CASE THE ISOLATION VALVE IS ACCIDENTALLY OPENED AFTER THE YOKE HAS BEEN REMOVED.