This document specifies a reference method using transmission electron microscopy for the determination of airborne asbestosfibres and structures in in a wide range of ambient air situations, including the interior atmospheres of buildings, and for a detailed evaluation for asbestosstructures in any atmosphere. The method allows determination of the type(s) of asbestosfibres present and also includes measurement of the lengths, widths and aspect ratios of the asbestosstructures. The method cannot discriminate between individual fibres of asbestos and elongate fragments (cleavage fragments and acicular particles) from non-asbestos analogues of the same amphibole mineral^[13].