Abstract: To establish predetermined humidity levels within a sterile treatment chamber, and maintain sterility, water, preferably deionized, is conducted to an evaporator outside of a container defining the treatment chamber, to be there heated to superheated steam, for example in the order of 300.degree. C., and conducted to a bypass duct (1) in atmospheric communication with the treatment chamber (10), for mixing with the atmosphere within the treatment chamber. Preferably, the treatment chamber is surrounded by heat exchanger elements, such as electrical heating wires and/or cooling coils, the heaters being controlled to rapidly heat the treatment chamber to a temperature in the order of 180.degree. C. during pauses of treatment for sterilizing the interior of the container, and/or maintaining temperature levels at a desired treatment level in operation.