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The World Health Organization defines sterilization as any process, physical or chemical, that leads to the destruction of all life forms, and in particular all microorganisms in vegetative form and spore state, both pathogens and saprophytes (bacteria and fungi).
The EN 556-1:2002 standard defines the concept of sterile: “An article or product is considered sterile when it is free of viable microorganisms”. The standard establishes the Sterility Assurance Level (SAL), according to which the likelihood of finding a surviving microorganisms inside a sterilization batch must be lower than 1 in 1 million (SAL = 10-6).
True sterilization is the end result of physical and/or chemical procedures that use standardized, repeatable, and documentable methodologies with the goal of destroying all living microorganisms, both pathogens and non, in vegetative phase and as spores. Theoretical and experimental models have shown that a microbial population subjected to a sterilization process is destroyed at a rate that is a constant percentage per unit of time: only infinitely long treatment can deliver certainty of zero microbial groups; what's more, this certainty cannot be acquired through sterility tests which are destructive in nature. Consequently, today we assign a relative value to the concept of sterilization, the SAL. Sterilization is therefore a process that ensures an acceptably low probability that one of the treated articles is not sterile. It is therefore correct to define sterility as a condition in which the survival of a microorganism is highly improbable. Because microbial load is strictly linked to the presence of organic material, complete removal of the latter is an essential condition for achieving sterilization.