ABSTRACT
In order to define the need for removal versus conservative interventions regarding asbestos-cement roofs and the possible priorities, visual judgement of the state of deterioration or assessments based on the weight of the superficial friable material that can be removed with adhesives are not sufficient. Since in similar deterioration conditions the dispersion of fibres may vary considerably from case to case, only counting of the dispersed fibres will ensure a correct assessment of the pollution potential of an asbestos-cement weathered surface. The paper describes a simple method for measuring the atmospheric dispersion of fibres based on the well demonstrated fact that primary dispersion from the covering, as from many other sources of asbestos pollution, consists of course bundles that are subject to rapid sedimentation. By means of aluminium boxes containing object-holder slides installed under the edge of the covering for about one month, it is possible, using optical phase-contrast microscopy, to count the fibres that have sedimented and calculate the number of fibres/m2/day. If the method is used with the standardized criteria described, it gives reproducible results, involves low costs and has the advantage of balancing the possible variations in primary dispersion intensity due to changes in the local weather conditions over the relatively long sedimentation sampling period.
