In the operating routine the expressions pure steam as well as clean steam are used repeatedly. But it is seldom clear whether the same is meant with these two expressions.
Indeed, it is not really simple to answer this question. The GMP guidelines very often use the term 'clean steam' without giving any further explanation. The steam's quality is defined by its use and its quality in turn is regulated in standards and pharmacopoeias. This means that pure steam and clean steam are the same from a pharmaceutical point of view. It was clearly defined in the USP for the first time in the year 2006 and referred to as pure steam. The quality of this steam is defined by its condensate which must have WFI quality. This steam quality is applied for the sterilisation of primary packaging material for sterile dosage forms or for the sterilisation of equipment parts in contact with the product during sterile production, for example. The term 'clean steam' is quite often used with the same intention but without having its own pharmaceutical definition.
Furthermore, DIN 58950-7 defines a steam quality which is used for the sterilisation of goods that are not in direct contact with the product or for the sterilisation of parenterals in closed containers. This steam is called sterilisation steam. In contrast to the steam described in the pharmacopoeia the limit for the conductivity of the condensate has a maximum of 10 µm/cm and is above the limit of WFI. But a limit is indicated for the non-condensable gases. The USP also mentions the non-condensable gases but without any specifications. These gases can cause the occurrence of gas bubbles in areas which are difficult to access where they prevent the heating up to the temperature required for sterilisation.