Regulation of Crystalline Silica: Where Next?

Abstract

It has become widely recognised that the biological activities of the various forms of silica are very different. Indeed most regulatory regimes recognise the difference between amorphous and crystalline silica and most em body the belief that one crystalline form - cristobalite - is more active than the others. Although the isomorphs are well-defined mineralogically the names may be used ambiguously with some regimes using the name'quartz' as synonymous with 'crystalline silica'. Even materials with the same names may have very different biological activities whether in laboratory tests or in human popu lations exposed in different industries. This is explicitly accepted in the IARC assessment of any carcinogenic effect of silica but is equally true of other pathogenic activities. Despite this there is pressure on regulators to set simple, or even single, exposure limits. In this paper some of the factors affecting the activity of some forms of silica in a simple in vitro test are described. It is a limi tation that as there are no samples of the various crystal line forms of this compound with matched granulomet ric properties (size distributions, surface areas etc.) there is little objective research justifying the various explana tions put forward for differences in activity.