This European standard specifies the minimum performance requirements and test methods of two types of sterilizers employing ethylene oxide gas as the sterilant, either as a pure gas or in admixture with other gases (whether supplied ready mixed or mixed at the point of use) in a temporarily sealed chamber. These sterilizers are intended to be used for medical, dental, pharmaceutical, veterinary and industrial or related purposes. The two types of sterilizers have been designated Type A and Type B respectively using the following criteria: Type A sterilizers are capable of being programmed by the user; Type B sterilizers are of limited size and provided with one or more pre-set operating cycles which cannot be varied by the user. The clauses of this standard apply to both types of sterilizer unless it is specifically indicated within the clause that it applies only to one of the types in particular. This standard includes minimum performance and construction requirements for sterilizers working above or below atmospheric pressure: To ensure that the process is capable of being used to sterilize medical products; For the equipment and controls necessary to permit validation and monitoring of the sterilization process. This standard does not specify those tests which are necessary to determine the probability of a processed product being sterile, nor the routine quality control tests required prior to release of sterile product. These topics are addressed in EN 550. This standard does not specify the procedures and equipment which can be used to improve the efficacy and/or efficiency of the process before or after the sterilization cycle. Considerations of operator safety are addressed in EN 61010-1 + A2 and IEC 1010-2-042. This standard is applicable when: a) specified in a contract for supply of an ethylene oxide sterilizer; or, b) a sterilizer manufacturer declares compliance when intending to supply an ethylene oxide sterilizer. This standard is not intended as a checklist for suitability of an existing ethylene oxide sterilizer when assessing compliance with EN 550.