Abstract: On February 18, 1987, Employee #1 and a water/sewer operator trainee changed a chlorine gas cylinder. The employee leak tested the system with ammonia and, detecting no leaks, put a chlorine gas cylinder "on line" at 9:00 a.m. A water/sewer operator checked the chlorine monitor at 12:00 midnight. At approximately 6:00 a.m., the paging system alerted the water sewer operator of an activated alarm, but he did not respond. After 30 minutes, the paging service alerted Employee #1 and the trainee. Employee #1 arrived first and entered the panel room. Smelling chlorine, he went into the maintenance room to telephone for help. In approximately 10 minutes, two coworkers arrived and telephoned the fire department. Employee #1 was treated for chlorine gas inhalation at the hospital and released. At the time of the inspection, the employee was off from work.