Abstract: At approximately 2:50 p.m. on August 19, 2004, Employees #1, #2, #3, and #4 were working in a scrubber processing unit at Sterigenics in Ontario, CA. Sterigenics utilizes ethylene oxide to sterilize medical equipment. Employees #2, #3, and #4 were performing normal scrubber operation duties, and Employee #1 was working in a supervisory capacity in the mezzanine control room. Chamber number 7 of the scrubber indicated that it had not received an ethylene oxide injection during a validation cycle. Maintenance employees were summoned to troubleshoot the error. After performing various checks on the chamber and system, the maintenance technician and maintenance supervisor calibrated its SEC IR spectrometer to verify the operation of the sterilization number 7 step. This procedure checks the ethylene oxide injection into the sterilization chambers and comprises of several verification phases. The gas inject phase involves the injection of approximately 125 lb of ethylene oxide into the chamber, which is followed by a gas dwell phase. In the vacuum phase, approximately 50 lb of the gas is evacuated from the room to a scrubber, and then the scrubber goes into a vacuum hold phase. The vacuum phase checks the ability of the chamber to maintain vacuum condition. It involves the injection of three gases, including nitrogen and or steam to remove the ethylene oxide from the sterilization chamber, and then the final release, in which a minimal amount of residual ethylene oxide is flushed using air. The residual ethylene oxide and air mixture is sent to a fired catalytic oxidizer that react the residual ethylene oxide to neutralize it. Employee #5 verified that the gas dwell phase and vacuum phase ran correctly and requested the permissive password for authorization to advance the procedure cycles to final release. Employee #5 received the permissive password from Employee #6 and advanced the cycles, bypassing the gas or "safety" washes. Employee #5 then proceeded to open the chamber door on chamber number 7. Opening the chamber door causes a blower to send air through the chamber and blow any residual gases to the fired catalytic oxidizer. A high concentration of approximately 62 lb of ethylene oxide was routed from the sterilization chamber number 7 to the fired catalytic oxidizer, resulting in an explosion and extensive structural and equipment damage. The explosion caused injuries to the employees located near by the scrubber processing unit. Some of the employees received injuries that included contusions and lacerations from flying glass and debris as a result of the explosion.