Physical characterization of MMVF for risk assessment

Size and other physical properties of MMVF play a central role in risk assessment. They can be affected by, for example, conditions during manufacturing, and preparation prior to administration to experimental animals or cells. It is suggested that a necessary requirement for stating that an experiment has been repeated should be documented evidence that: (i) the administered fibres are alike when analysed with not less than a minimum number of well-established methods on single fibre and on bulk sample level; and (ii) doses must be equal, and the fibre concentrations and sizes at the primary target tissue must be documented. Assessment of fibre toxicity and assessment of potential for exposure are equally important in the risk assessment process. The physics of fibrous dust release is described and methods for a priori assessment of exposure are classified into three levels depending on the analytical effort involved: (1) direct analysis of the fibrous bulk material; (2) bench-scale test; and (3) full-scale tests of prefabricated insulation material in a test room during standardized insulation work. For the first method, two approaches are investigated. One relates the size-dependent measures (the mass of fibres with D < 3 microns, the total length of fibres with D < 3 microns and the total number of WHO fibres per unit mass of fibres) and the other considers fractions (the fraction of total fibre length with D < 3 microns, the fraction of WHO fibre number, and the fraction of fibre number with D < 3 microns). The values of the size-dependent measures that are quoted span 2-4 orders of magnitude for the ranges of nominal diameter and GSD(D) considered, and show strong and similar dependence on GSD(D). The second group of measures, based on fractions only range up to 2 orders of magnitude and are almost independent on GSD(D) for nominal diameters ranging 2-4 microns. The combined effect on exposure of bulk fibre size, and of addition of oil and binder, should be assessed by the rotating drum dustiness test. The gold standard test of prefabricated MMVF products should be a full-scale simulation. The slope of the curve relating overall average product nominal diameter and airborne fibre concentrations on a log-log scale [Esmen et al., Am. ind. Hyg. Ass.J. 40, 108-117 (1979)] has been confirmed on a qualitative basis using model calculations, so that this relation can be used as a first estimate of changes in exposure due to changes in nominal diameter of bulk material.