A comparison of fibre stopping rules in the application of the membrane filter method – Phase II

f view have been examined, whichever is reached first, with a minimum of 20 fields being evaluated. This ERM stopping rule directs more counting effort than is necessary towards concentrations well below present control limits for asbestos fibres. A preliminary investigation confirmed that alternative stopping rules could be devised to give comparable results to the ERM rule, whilst yielding substantial savings in evaluation time for low density slides. Their applicability, however, was limited to a single control limit value and specific sampling conditions. This report describes an extension to this study to identify and validate a more robust candidate stopping rule of generally wider applicability.The candidate rule developed has a similar form to the ERM rule, except that the upper limit on the number of fields evaluated is 50 fields. The rule was devised to have a 95 per cent chance of detecting non-compliance when the actual fibre concentration is 20 per cent above the control limit, and is applicable to control limits of 0.5 fibre/ml and 1.0 fibre/ml. The candidate rule was compared experimentally with the ERM rule in eight laboratories from within the European Community countries and Canada, using workplace samples selected by the laboratories to be representative of. the density range they normally encounter.Evaluated fibre densities obtained by each laboratory for each of the two rules were compared, together with the evaluation times required. Analysis of these data indicates that there is no evidence that the candidate rule gives fibre density estimates different from those obtained by the ERM rule, when account is taken of intra-observer variation. Within each laboratory, the total evaluation times with the candidate rule were 52-74 per cent of those for the ERM rule. The greatest absolute time saving by a laboratory in evaluating 12 samples was about 3.7 hours.