ABSTRACT
The logic behind the operation of an isolator at a pressure above or below the surrounding atmosphere is that, should there be any leakage, it will always be in the correct direction. The product will be protected in the sterile, positive isolator, and the operator will be protected in the toxic, negative isolator. However, this is not necessarily the case, due to a phenomenon known as induction leakage. This may occur in two forms: (1) at the site of undetected holes and (2) at the site of known apertures, such as product output mouseholes (see Chapter 3, “Dynamic Mousehole”). The mousehole should be engineered to take into account induction leakage, but, in the case of an inadvertent hole, there could be a problem. If the movement of air close to the wall of an isolator has sufficiently high velocity, then its dynamic pressure might reduce and, in extreme cases, nullify the differential pressure across the isolator wall. If this occurs at the site of a hole, then there is potential for contamination to move against the static pressure differential.
