The toxic effects of silica. A report prepared for the Health and Safety Executive

Quartz, a crystalline form of silicon dioxide, is one of the most abundant minerals in the earth’s crust. Other minerals grouped under the term free silica include cristobalite, tridymite, coesite, keatite and stishovite (which are also crystalline) and amorphous silicas. Man may be exposed to quartz or, occasionally, other forms of silica, in many trades and industries, including mining, tunnelling, quarrying, masonry, metal foundry, pottery and production of silica flour and diatomaceous earths.e there is some evidence that amorphous silicas may occasionally cause radiological changes in man, most silica-related disease described in human populations has been the result of inhalation of crystalline silica, usually quartz. Quartz has been shown in experimental studies to be toxic to phagocytic cells and to initiate, when inhaled, a process of nodular lung fibrosis. High doses in animals may also provoke the rapidly fatal condition of alveolar proteinosis. These effects are analogous to classical and acute silicosis occurring in human populations. Acute silicosis has been described relatively infrequently, in response to very high, uncontrolled exposures to quartz or cristobalite. It is always fatal. Classical silicosis ranges from small nodular opacities on the chest radiograph, unassociated with clinical disablement, to progressive massive fibrosis which may lead to premature death. The disease, once initiated, is usually progressive and it may occur for the first time after exposure has ceased.There is evidence from studies of human populations and of Individual patients that some diseases other than silicosis may result from the inhalation of quartz. In particular, an adverse effect on lung function leading to airflow obstruction has been related to quartz exposure. The mean intensity of this effect at levels of dust exposure insufficient to cause clinically significant silicosis is likely to be small. High exposures to quartz, usually sufficient to cause silicosis as well, have been associated with an increased risk to the worker of tuberculosis, other mycobacterial infections, systemic sclerosis and renal disease.Studies of the relation between exposure to quartz dust and the risk of radiological evidence of silicosis all suffer from deficiencies either in determining exposures or in measuring radiological response. Nevertheless, it is clear that a relation exists between exposure and disease. It is not possible to determine from the published literature whether or not there is a safe dose at which no disease occurs. However, at working lifetime exposures to respirable quartz dust of 100 ng.m~3 or less (or its equivalent in particle counts) clinically significant silicosis appears to be rare or possibly absent, Nevertheless, exposures to lower levels of quartz in several industries have been associated with the development of radiological changes in a proportion of the workforce. The clinical significance of these changes remains undetermined. The variations in types, size distribution and properties of silica in mixed dusts in industry make it unlikely that a single standard would be appropriate to all circumstances in which silica exposures may occur.The geographical distribution of rapid progression of CWSP was very different from that of PMF. Highest risks were observed in Western and North Derbyshire Areas whereas risks were relatively low in South Wales. Risk also varied with starting category of CWSP but in a very different manner from PMF. The attack rate peaked at category 1/2 but, although there were sufficient higher categories available for progression of two or more steps, risks were much lower from categories 2/1, 2/2 and 2/3. On the other hand risks of PMF were substantially higher from categories 1/2 and above suggesting that if a rapid deterioration occurs from the more advanced stages of CWSP it is likely to be to PMF rather than appear as rapid progression of CWSP.The risks of rapid progression increased with age up to around 45 in all UK coalfields. There were clear differences between areas. In particular, in South Wales there is a substantial reduction in risk amongst older men. It is noticeable again that this decline in risk of rapid progression coincides with a substantial increase in risk of PMF.The most important finding in this part of the study was that a high proportion of men who showed first signs of PMF late in the risk period had previously displayed rapid progression. This clearly suggested that rapid progression of CWSP predisposes men to PMF.The case-control studies of rapid progression of CWSP and PMF comprised the largest part of the research. These case-control studies concentrated on collieries where there were substantial numbers of cases. Cases, who were selected from the cases identified in the general study of distributions of rapid progression and PMF outlined above, were included only from collieries where there were either 10 or more PMF cases or 8 or more rapid progression cases. 260 cases of rapid progression and 310 of PMF were selected using these criteria. Where possible, each case was matched with four controls. Controls had worked at the same collieries, had neither exhibited rapid progression nor developed PMF and their dates of birth were within three years of those of the cases with which they were matched. Since the 1950s the NCB has routinely monitored respirable dust in coalmines to assess compliance with dust standards. This information was available for most of the collieries in the study from around 1960 onwards and for all collieries from 1970. Past exposures to respirable dust were estimated from working histories obtained by interview and the NCB routine dust measurements. Estimates of times worked in occupations with different potential quartz exposures were made using the working histories together with estimates of likely quartz contents of mined dusts made from seam geology.ause of both a low response to requests for interviews and a policy of deliberate exclusions of cases and controls for whom little dust and quartz information could be obtained the numbers of men actually studied were substantially lower than intended. Nevertheless, the studies remained broad-based, covering the majority of UK collieries where rapid progression or PMF was prevalent. 79 cases of rapid progression together with 228 matched controls from 12 collieries were investigated in the rapid progression studies while 94 PMF cases and 248 controls from 14 collieries were included from the PMF study.In the PMF case-control study cases were found to have much higher starting category of coalworkers simple pneumoconiosis, to have worked longer in dusty occupations and to have higher recent measured dust exposures than controls. No differences between cases and controls were found in any quartz-related exposure index but about 70% of cases came from South Wales where quartz levels are normally low. No attempt was made to investigate separately the PMF cases from other Areas who were included in the study.In the case-control studies of rapid progression it was found that cases had worked longer in the dustier occupations and had higher recent dust exposures than their matched controls. Quartz was also implicated in rapid progression. Cases had worked longer in occupations where quartz exposures were potentially relatively high. Logistic analyses of risk also suggested that the times worked in the different quartz categories may be influential in the development of rapid progression. In addition, an apparently disproportionately high proportion of collieries, where cases of rapid progression were identified for inclusion in the case control study, had mined seams with a potential to produce dust with high proportions of quartz.This is consistent with the earlier work of SEATON.et al. (1981), HURLEY et al. (1982) and JACOBSEN & MACLAREN (1982). Those studies were based on observations at only a few, but intensively studied collieries. They showed an association between rapid progression of CWSP and exposure to quartz. The work reported now, on the other hand, refers to a much wider sample of British coalmines, including most of those where there was some evidence of rapid progression during the 1970s. The larger scale of the study resulted in considerable non-response, and this implies greater possibilities of bias in results. The less reliable estimates of exposures and of radiological responses used for the work will have increased random fluctuations, but there is no reason to suppose that this will have biased results one way or the other. In our view, the results reported here suggest that the earlier findings on quartz and rapid progression, based on selected collieries, apply generally in coalmines in Britain.