An international comparison of counts of airborne asbestos fibres sampled on membrane filters

An asbestos fibre counting trial has been conducted between nine countries, based on the postal exchange of mounted and unmounted membrane filter samples between all participating laboratories. The experiment was designed to distinguish differences due to counting technique from those due to the method of clearing the filter. Counting technique differences gave ratios of highest to lowest count of 1.7 for amphibole and 2.8 for chrysotile asbestos. The most widely used clearing techniques (triacetin; or dimethyl phthalate with diethyl oxalate containing dissolved filter material) gave similar results; filters cleared by acetone vapour gave significantly higher counts.