The influence of external aerodynamic factors on the measurement of the airborne concentration of asbestos fibres by the membrane filter method

This paper describes experiments carried out in the laboratory to clarify interpretation of the role of sampling flow rate and wind speed on the efficiency of sampling airborne asbestos fibres in workplaces. The experiments were carried out with test clouds of UICC amosite and UICC chrysotile with aerodynamic characteristics sufficiently close to those of workplace dusts to allow direct comparison to be made, at least as far as the immediate aims of this enquiry are concerned. It was shown unambiguously, for conditions of practical interest, that there were no significant effects of sampling flow rate and wind speed on the measurement of fibre mass concentration and fibre number concentration respectively. Thus, any observed bias in the measurement of fibre concentration by manual counting must instead be attributable to psychological effects relating to the density of fibres on the filter.

On the basis of these results, it is suggested that recommended procedures which impose limitations on sampling flow rate or face velocity are unnecessarily restrictive. Rather it is preferable to tighten up the prescribed limits on fibre density on the filter.