vial 14 in such a way as to permit vapor gas between the interior of the vial and the outside environment during sterilization hydrogen peroxide vapor can enter the vial chamber by way of a 34 tortuous path 34 16 whereby the vapor must negotiate at least two 90 u00b0 bends in order to enter the vial the sterilant entering the chamber then acts upon the spore carrier 15 in order to expose the carrier to the same environment encountered by the other articles to be sterilized upon completion of the sterilization cycle the biological indicator 10 is removed from the sterilizer and thereby is exposed to a non sterile environment which may contain airborne microorganisms because there will be little or no pressure differential between the exterior and the interior of the container 10 there is minimal or extremely low probability that the non sterile environment e g air will flow into the chamber of the vial 14 carrying with it airborne microorganisms access to the chamber of the vial 14 by airborne microorganisms may be gained only if the microorganisms negotiate the tortuous path 16 such migration will be unlikely to occur because the microorganisms are incapable of independent movement and must be carried along by any negligible air currents that would traverse the tortuous path.