The quartz hazard: a variable entity

An IARC Working Group recently classified crystalline silica (quartz) into IARC's Group 1, i.e. a carcinogen. This classification is based on evidence of carcinogenicity in experimental animals and in humans. However, the evaluation stated that in making the overall evaluation, the Working Group noted that carcinogenicity to humans was not detected in all industrial circumstances studied and that carcinogenicity may be dependent on inherent characteristics of the crystalline silica or on external factors affecting its biological activity. The present review seeks to put the apparently conflicting findings of cancer incidence in quartz-exposed industries into a unifying thesis, based on mechanistic studies. These mechanistic studies have enabled the events leading from deposition of quartz to silicosis and cancer to be partially elucidated and have demonstrated that the biological effects of quartz can be understood in terms of surface reactivity. We particularly emphasise the ability of quartz to generate free radicals and cause oxidative stress and the fact that this could be modified by a range of substances that affect the quartz surface; some of these modifying substances could originate from other minerals. We therefore propose that the hazard posed by quartz is not a constant entity, but one that may vary dramatically depending on the origin of the silica sample or its contact with other chemicals/minerals within its complex constitution. The mechanistic data described here could assist in the interpretation of epidemiological studies and pose further hypotheses that could be tested in order to help resolve the quartz carcinogenesis anomaly. The data suggest that quartz cannot be death with as a single hazard entity, as is the case with most other chemicals.