The purpose of this document is to provide guidelines for the user, designer and tester of gas tight fans. This document will help users to choose the most suitable class of tightness for their process with the appropriate ways to achieve this and to measure the leakage. The smaller the permissible leakage, the higher is the cost. It must be remembered that perfect tightness is difficult to obtain and especially to maintain for a long time. This document is available in English.