Thoracic sampling of fibres

Status:

completed 02/2001

Aims:

The objective of the project was to develop a procedure for the fibrous dust sampling which conforms to the technique for dust sampling at the workplace in accordance with EN 481, "Workplace atmospheres: size fraction definitions for measurement of airborne particles". The harmonized sampling procedure was to reduce the variability and to improve and facilitate the fibre counting in comparison with the European reference method recommended at the present time.

Activities/Methods:

A sampler was modified in order to meet the requirements for sampling the thoracic fraction. Fulfilment of the requirement was verified by the use of spherical particles covering the size range for the thoracic fraction. The homogeneity of the particle deposition on the filter was checked by means of isometric particles and by counting under an optical microscope. Suitable fibrous aerosols had to be generated for determining the sampling efficiency under well defined laboratory conditions. Laboratory tests with fibrous aerosols were performed for determining the fibre sampling efficiency; the evaluation was done by counting and sizing the fibres by means of a scanning electron microscope. Eight field trials on real-life workplaces provided the basis for comparison of the results obtained by means of the thoracic method with those obtained by the conventional method.

Results:

Two (GK2.69 and modified Simpeds) of the four thoracic samplers tested satisfy the requirements of the thoracic selection curve very well. As anticipated, however, the homogeneity of the filter deposition was less satisfactory for these two samplers, which are equipped with cyclone preseparators: the concentration of coarser particles was higher in the centre of the filters. Nevertheless, the tests performed with fibres yielded a sufficiently homogeneous fibre deposition for all four samplers. Following conversion of the fibre dimensions into aerodynamic diameters, the dependence of the sampling efficiency on the aerodynamic diameter was also found to hold true for fibres. The field trials showed that the thoracic samplers yielded results comparable to those of the conventional method when the same microscopic counting rules (WHO fibres) were applied. For non-respirable fibres, slightly lower concentrations were measured by the thoracic samplers, as anticipated. Evaluation of the thoracic samples is easier and somewhat faster than that of conventional samples when the coarse dust component at the sampling was high. Size-selective sampling could in principle reduce or even eliminate the need for fibres to be discriminated according to their diameter. The results obtained in this way, however, would not necessarily be directly comparable to those obtained so far: depending on the workplace situation, the resulting increase in the measured fibre concentration would lie between <1% and over 100%.

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