Toxicity of quartz in mixed dusts with special reference to the London Underground

1. A literature review has been carried out into the toxicity (fibrogenicity) of quartz in mixed dusts with special regard to the presence of iron and its likely influence on the harmful effects of quartz.2. The mechanism of injury caused by quartz when it deposits in the lung is partly understood and involves interaction of the ionised, negatively charged groups on the quartz surface with biological structures and molecules. When a negatively charged surface, such as that of quartz, encounters positively charged ions; these can bind and effectively reduce the net negative charge on the surface;aluminium and iron represent naturally occurring elements which give rise to positively charged ions and so could have this effect on the quartzsurface.3. Quartz dusts produced in different ways and from different sources are likely to have different trace element chemistry including elements such as iron and aluminium: different quartz samples are therefore likely to demonstrate different biological activities.4. Any fresh fracturing of the quartz to reveal new surfaces could expose new reactive sites and these may undergo a series of reactions to produce free radicals that could be injurious to tissue. The half life of these radicals may be quite short and so ‘ageing’ of these surfaces may lead to a reduction intheir toxic potential.5. In theory iron could enhance the toxicity of quartz by interacting with some of these free radicals by the Fenton Reaction but there is no evidence that this occurs in vivo.6. Iron therefore has the potential to both enhance and ameliorate the toxicity of quartz.7. Whilst theoretically possible, experimental evidence that iron can ameliorate the toxicity of quartz in vivo is flimsy. Studies with aluminium strongly suggest that treatment or mixture – of quartz with a source of aluminium ions results in amelioration of the toxicity of quartz in the short-term. There is much less evidence that iron has the same type of activity but the fact that the form of the iron could profoundly influence its ionisation could be important. In vitro, however, there is evidence that iron can reduce the injurious effects of quartz in some isolated cell systems whose relevance to events in the lung are questionable. Experimentally the data suggestive of an ameliorating effect of aluminium is far more convincing than that for iron.8. When quartz is delivered to the lung experimentally it can cause inflammation and this can be dramatically reduced by an instillation of aluminium at a later date. It therefore appears that the interaction between quartz and any ameliorating metal could occur either in the mineral during its formation or inthe lung.9. Human epidemiological evidence is equivocal as to whether the presence of cationic metals such as iron or aluminium can ameliorate the toxicity of quartz. There is a suggestion that this is indeed the case but evidence is far from conclusive.10. Very recent evidence suggests that inhaled quartz may be carcinogenic in man but exposure-response relations have not yet been quantified.