This International Standard specifies a reference method using transmission electron microscopy for the determination of the concentration of asbestosstructures in ambient atmospheres and includes measurement of the lengths, widths and aspect ratios of the asbestosstructures. The method allows determination of the type(s) of asbestosfibres present. The method cannot discriminate between individual fibres of the asbestos and non-asbestos analogues of the same amphibole mineral.