This guideline specifies a method for determining the numerical concentration of inorganic fibrous particles in indoor air or ambient air and the assignment of these particles to particular classes of fibres (chrysotile, amphibole asbestos, calcium sulphate, other inorganic fibres). The fibre counting and assignment are carried out by the SEM/EDXA method (scanning electron microscope/energy -dispersive X-ray analyis). The limit of detection of the method under the standard conditions specified in this guideline is 300 fibres/m3. Information on the differentiation of product fibres and the determination of organic fibres is provided.