Occupational health aspects of quartz in bituminous coal fly ash in Israel 2009

Quartz (α-quartz), is the most common crystalline silica polymorph in nature and is the only polymorph found in significant concentrations in coal and in coal ash. Since exposure to quartz is liable to cause lung diseases, it is very important to know the concentration of biologically active quartz in respirable particles in order to evaluate their effect on the workers in contact with them. This is necessary since only these particles can cause silica-induced respiratory diseases. Similarly to what was found in the Netherlands, we found that the quartz concentration in the respirable fraction of the fly ash produced in Israel is considerably lower than in the bulk ash. While X-ray diffraction is the most accurate way to measure quartz concentrations in mineral samples, this method cannot distinguish between coated and uncoated particles. Scanning electron microscopy, though less accurate, is able to differentiate between coated and uncoated particles. Our experiments, confirming those of other researchers, show that most of the quartz particles in the respirable fraction are coated by amorphous aluminosilicate layers. Coated quartz particles are not considered to have pneumoconiotic effects. Nevertheless, the occupational safety and health regulations in Israel still classify fly ash as toxic dust due to the presence of crystalline silica. Accordingly, workers in coal power stations are considered as exposed to free crystalline silica, and their exposure to airborne silica is monitored twice a year. The summary of the results obtained during four consecutive years (2006-2009) shows that in any way exposure levels to silica are very low. The results of recent studies, and the present one, strongly suggest that the inclusion of fly ash in the category of toxic dust in occupational safety and health regulations in Israel should be reconsidered.