A review of present-day automated asbestos fibre counting instruments and some future proposals

This report has been compiled in order to examine the current status of automated asbestos fibre counting, to compare the relative merits of the various existing devices or those which have been previously proposed, and to look at possible new techniques. The existing instruments are divided into two separate categories; fibrous aerosol monitors (FAMs) and filter analysis devices (FADs). FAMs produce ‘real time’ estimations of the concentrations of airborne fibres by discriminating between fibres and particles during sampling. FADs essentially replace the human operative in identifying and counting the number of fibres collected on a filter.Details of the principles of operation and a summary of the existing test results are given for four FAMs and three FADs. In all but one of these devices – the Magiscan Image Analyser – the basis of the fibre-particle discrimination method is the analysis of the intensity and/or the directional dependency of scattered light. It is noted that of the seven devices considered, the two which have been most successful are the only two which do not rely on the directional dependence of scattered signals, i.e., the Royco Optical Particle Counter (which is used in environments where the fibre/isometric particle ratio is high) and the Magiscan.Three specific proposals for future developments are presented. These are:-(1) The development of devices for the separation of fibres and isometric particles prior to the counting of fibres by a non-discriminating optical counter such as the Royco.(2) Comparison of the form of the light scattering signal obtained individual entities when the aerosol is made to pass through the fringe system produced by the crossing of two laser beams (as used in Laser Doppler Anemometry), the fibres being aligned parallel to the fringes and perpendicular to the air flow direction by an a.c. electric field.(3) Comparison of the rates of cooling of individual entities after heating by a laser beam or other suitable source, on the basis that fibres, because of their larger surface to volume ratio, will cool more quickly than more isometric particles raised initially to the same temperature. Temperature would be measured by the intensity of emission of blackbody radiation.