A comparison of fibre counting stopping rules in the application of the membrane filter method

The European Reference Method (ERM) specified in the CEC Directive on Asbestos requires fibre counting to cease after 100 fibres have been counted or 100 fields of view have been evaluated, whichever is reached first, with a minimum of 20 fields being examined. With this stopping rule, more of the filter area is evaluated at lower concentrations than is examined when the concentration is around the compliance limit. This paper describes an investigation carried out to identify and test alternatives to the ERM stopping rule which would reduce the counting effort at low concentrations whilst retaining an acceptable degree of accuracy. Simulation studies were performed on available data, involving random selections of fibres and fields. This, together with theoretical considerations, led to the definition of two candidate stopping rules applicable to assessing compliance with the European control limit for chrysotile and amosite of 1.0 fibre ml −1 . These candidate rules are: (a) count until at least 135 fibres have been counted or until 30 fields have been examined; a minimum of 20 fields must be examined; (b) examine 20 fields; if fewer than 75 fibres, or more than or equal to 105 fibres have been counted, then stop counting. Otherwise, continue counting until at least 180 fibres have been counted or until 40 fields have been examined. The two selected candidate rules were compared with the ERM rule in a small experimental trial. Evaluated fibre densities obtained by the candidate and ERM rules did not differ systematically. For concentrations below about 0.8 fibres ml −1 , the time involved in applying the candidate rules progressively decreased as concentration decreased; at low concentrations, application of the candidate rules took as little as one-fifth of the time of the ERM rule.