Inter-laboratory comparisons of the counting of asbestos fibres sampled on membrane filters

Two series of exchanges of slide-mounted asbestos samples have been conducted between British laboratories, to determine the degree of agreement between different laboratories counting the same slides, the effectiveness of written compared with personal instruction, and the reasons for disagreements. It was found that disagreements between laboratories that were isolated or new to fibre counting were large, but were reduced by direct personal consultation. The main causes of these differences were uncertainties in the counting of irregular fibres and fibre masses, misalignment of the microscope phase rings, and failure to scan the full depth of focus in which fibres were present. Results of an exchange between several experienced laboratories showed that counting over the whole field of view of the microscope consistently returned substantially lower count densities than when only the central graticule area was evaluated. All disagreements were more serious for UICC asbestos samples than for samples collected from factory air. It is concluded that there is a need for closer definition and standardisation of counting procedures.