Developments in the aerosol technology allow to have access to filter fractional efficiency. The Eurovent Working Group 4b has developed a new method of testing air filters used in general ventilation for determination of fractional efficiency, adapted for a better use of filters in industrial process and in the field of indoor air quality. This new method Eurovent 4/9 has replaced the old Eurovent 4/5 method. This is the English version of the Recommendation