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Specifies a refernce method using transmission electron microscopy (TEM) for determination of the concentration of asbestos structures in ambient atmospheres. The specimen preparation procedure incorperates ashing and dispersion of the collected particulate, so that all asbestos is measured, including the asbestos originally incorperated in particle aggregates or particles of composite materials. The lenths, widths and aspect ratios of asbestos fibres and bundles are measured, and these, together with the density of the type of asbestos, also allow the total mass concentration of airborne asbestos to be calculated. The method allows determination of the type(s) of asbestos fibre present.