Guideline VDI 3861 Part 2 specifies a method for the determination of low numerical concentrations of inorganic fibrous particles in dry particle-gas-mixtures which flow through defined cross-sections, e. g. in chimneys, pipes or ducts. Using this method inorganic fibres of the length of L > 5 ?m, the width of D < 3 ?m and the aspect ratio of L:D < 3:1 are detected. The requirements of guideline VDI 2066 Part 1 are valid unless exceptions are explicitly indicated.