Abstract: On March 21, 1995, Employees #1 and #2, of EBMUD, were exposed to high levels of chlorine gas during a routine maintenance operation on a valve in the chlorination piping system. The scheduled work was to have been performed on CPRV #1 on evaporator #1 in the evaporator room. The line to be opened had been evacuated of chlorine and all incoming chlorine ability had been locked out and tagged out. Both employees had been trained on the procedure and had reviewed the procedure with their supervisor. Both employees were equipped with supplied-air full face mask respirators. Instead of opening CPRV #1, they opened CPRV #2, which was under 90 lb of chlorine gas pressure. Employee #1 was hit directly in the face mask with chlorine gas, and some entered the mask. Employee #2 was unaffected. Alarms sounded and the release was stopped from outside the room. Employee #1 was seen at a hospital and released with no permanent injury.