Abstract: Employee #1, an engineer, had just changed a cylinder of ethylene oxide (EtO) according to proper procedures when he accidentally tripped a lever, releasing EtO into the sterilizer. Four alarms sounded. He turned off the lever and then reset the alarms. Employee #1 was sent to the doctor for an examination and medical surveillance; his pulmonary function and blood tests were satisfactory and he was released. He had not been instructed to wear a full-face, gas canister respirator when changing cylinders. After this incident, all employees were instructed to wear gas canister masks when changing cylinders.