Cross-sectional study of risks of respiratory disease in relation to exposures of airborne quartz in the heavy clay industry

Aims of the Study: The National Federation of Clay Industries commissioned the Institute of Occupational Medicine to conduct an independent study to determine whether airborne dust caused a significant health risk in the non-refractory heavy clay industry, and to obtain information on the degree of risk, if present, which would help to inform decisions on dust control measures.Field Studies: Eighteen factories in England and Scotland were studied between 1990 and 1991. 1407, mostly full shift, dust samples were collected using lapel-mounted samplers, and the mass of mixed fine dust and quartz was measured. Usual exposures to dust and quartz were characterised according to broad occupational groups. Dust concentrations in the past were assumed to be similar to current measurements, with the exception of plants where changes of kiln type had occurred, for which the assumed past dust concentrations were adjusted.1925 workers were medically examined and provided detailed occupational histories. Chest X-rays were taken and interpreted to identify pneumoconiosis (dust in the lungs) according to standard procedures. Chronic bronchitis (persistent cough and phlegm) was identified by questionnaire. Each man’s lifetime history of cumulative exposure to fine mixed dust and quartz was calculated by combining the dust concentrations for occupational groups with the occupational history.The data were processed, analysed and reported at the IOM headquarters in Edinburgh.Results:A small number of men (eight) had category two or greater small spots (opacities), or large opacities in the lungs visible on the chest X-ray. These categories correspond to established pneumoconiosis, and diagnostic confusion with other, non-occupational disease, is rare at this level. The men had all worked for prolonged periods in the heavy clay industry, and only two had worked in other industries where exposure to harmful dusts had occurred.Another seventeen men had category 1 small spots in the lungs visible on the X-ray, consistent with definite but slight abnormality or, possibly, in some cases, other disease. Thus in total twenty five men had small spots of category 1 or greater. This frequency (1.4%) is less than those found in recent studies of opencast coalminers (4.4%), hard rock quarry workers (4.7%), and non-dust-exposed postal and telecommunication workers (2.7%).3.7% of the heavy clay workers had X-ray appearances on the borderline between normality and abnormality (category 0/1) or worse. These low categories of spots can also be caused by age, smoking and other diseases.Men were statistically significantly more likely to have small spots of category 0/1 or greater on their X-ray if they were older, or had experienced higher lifetime exposures to dust and quartz than other men. Smoking habit was taken into account, but had little influence in this population.Men with small spots of category 1 or greater were similarly more likely to be older and to have had higher dust exposures, though the smaller numbers of men reduced the statistical significance. The statistical analyses estimated, for example, that the risk of having category 1 or greater for a 40 year old non-smoker after 20 years work in jobs with the lowest dust concentrations would be 0.6%. He would have a 1.4% risk, if he had worked for 20 years exposed to a mixed respirable dust concentration of 2.5 mgm~3 (half the accepted occupational exposure standard for dusts not assigned a specific occupational exposure limit). If he had been exposed to 0.4 mgm-3 of quartz for 20 years, he would have a 2.1% risk.For comparison a recent study of underground coalminers exposed to coalmine dusts (for which the health risk is well recognised but which usually contain less than 10% quartz) indicated a 2.8% risk of category 1 or greater small spots after 20 years exposure to 2.5 mgm-3 of mixed dust on average.Thus the risks in proportion to degree of exposure to mixed dust appear to be no greater, and are possibly less, in heavy clay workers than in coalminers, even though the amount of quartz in the dust is higher, in some cases substantially, in the heavy clay workers. Free quartz particles were present in the airborne dust, and these only rarely appeared to be freshly fractured. Possibly the very ancient nature of the quartz particles reduces their harmfulness.Nevertheless the estimates of risk in heavy clay workers still suggest small health risks even at quartz concentrations as low as 0.1 mgm-3 (over a long period), a much lower concentration than the 0.4 mgm-3 Maximum Exposure Limit. If confirmed, this has implications for the dust control strategy, and some further statistical analyses would be desirable to clarify the evidence for health risks in this low exposure range.Chronic bronchitis was also found to be more frequent in those with high exposure to mixed dust than in those with low exposure. In general this effect was small in relation to the effects of smoking and non-occupational causes, but was particularly severe in kiln demolition workers.Recommendations: It is recommended that the industry focusses on reducing respirable quartz concentrations in specific jobs and occupations where levels exceeding the Maximum Exposure Limit have been shown to occur, particularly kiln demolition workers, clean up squads, sand users and some pan mill operators and tile moulders. The most cost-effective methods of control could be implemented based on well-established principles.The extent to which the target quartz concentrations should be below the MaximumExposure Limit will be influenced by practicability and by the degree of andseverity of health risks at low quartz concentrations. Some further statisticalanalysis is recommended to clarify the evidence for risks at concentrations belowthe exposure limit.Medical surveillance, comprising chest radiography, questionnaire of respiratory symptoms and, preferably, simple lung function tests, is indicated for all dust exposed workers in the industry. The lung function of the kiln demolition workers and other very highly exposed groups in particular should be assessed. The interval recommended by the Health and Safety Executive for silica exposed workers is two years for chest radiography. Medical surveillance should also include the collation and reporting of health statistics on a national basis.However, a case could be made for a lesser frequency of medical surveillance, including chest X-rays, for workers in the heavy clay industry, in view of the relatively small risks and expected slow progression of disease. A lesser frequency should be subject to review. “”