Indoor air - Part 27: Determination of settled fibrous dust on surfaces by SEM (scanning electron microscopy) (direct method) (ISO 16000-27:2014, IDT)
This part of ISO 16000 specifies a method giving an index for the numerical concentration of fibrous structures with fibres equal or greater than 0,2 µm in diameter in settled dust on surfaces and their classification into specific substance groups (e.g. chrysotile, amphibole asbestos, other inorganic fibres). It is primarily applicable to indoor areas, but it is also suitable for certain outdoor situations. A sampling technique for collection of settled dust using adhesive tape is described. The method incorporates an analytical method for evaluation of the collected samples by scanning electron microscopy. The result can be specified in asbestos structures per unit area and/or classified into four different loading classes. The analytical sensitivity depends on the area examined and can be as low as 10 structures/cm2.
For the purpose of this part of ISO 16000, an asbestos or fibrous structure is defined as an asbestos or (other inorganic/organic) fibre-containing particle regardless of its diameter.
(minor roughness and curvature) and the thickness of dust layer. If the dust layer is too thick, the dust layer can be sampled by other means and eventually analysed as powder sample. 
It is assumed that the settled dust has particle diameters mostly below 1 mm.