Swedish Customs (Tullverket) has assigned FOI CBRN Defense and Security to describe how to use the equipment PortaCount Pro+ 3038, TSI for fit testing of protective masks. PortaCount is useful for determining suitable size of e.g. Skyddsmask 90 (Protective mask 90) for current user and to examine and optimize the fit of the mask for the user. User recommendations and methods for fit testing at the occasions; at retrieval, over time and just before a special event is also included in the project. Before preforming a fit test it is important to prepare the test person as well as the protective mask and the PortaCount equipment. The test person should be informed about the test procedure and should not smoke or eat a certain period before the test. In addition, beard and extensive make-up should also be avoided. An adapter is required for connecting the protective mask to PortaCount during a fit test. The adapter is used for sampling the particle concentration inside the mask. This concentration is then compared with the ambient particle concentration, in order to determine the fit factor of the mask. FOI CBRN-Defense and Security recommends that Swedish Customs acquire either one or both of the two adapters that the Swedish Armed Forces use for Skyddsmask 90. PortaCount is prepared by mounting an alcohol cartridge. As the instrument is initialized, the user can either manage PortaCount by its touch screen (Stand Alone) or by a computer (External Control) installed with the software FitPro. The ambient particle concentration must be high enough, in order to determine the fit factor with high certainty. For that reason, particles must be generated prior to a fit test. The location for particle generation must be enclosed and not too large, since generating high particle concentrations then becomes very time consuming. FOI CBRN Defense and Security recommends that a salt aerosol generator (Particle Generator Model 8026, TSI) is used inside the fit test tent utilized by the Swedish Armed Forces. At least 50 000 particles/cm3 must be generated in order to determine a fit factor of 10 000. The salt generator, in combination with the fit test tent, should always be used during fit testing at retrieval and over time. At these occasions, PortaCount is managed either by its touch screen menus or by a computer, depending on the presence of experienced PortaCount users. Prior to a special event, it is possible to light a tea light as an alternative particle generator. PortaCount should then be managed by the touch screen menus and fit testing can be performed in an improvised room e.g. a vehicle.