Coalminers’ mortality in relation to radiological category, lung function and exposure to airborne dust. Final report on CEC Contract 7246-16/8/001

This)report describes mortality among 29 553 British coalminers. They represent 93.5% of 31 611 men who were surveyed radio-logically at 2ff coal mines in the period 1953 through 1958. Vital status for the remaining 6.5% (2 058 men) was not established.Comparisons with age, regional and calendar-year-specific death rates for all men in the areas where the collieries were situated showed that the miners studied had lower mortality than British men generally. There is strong evidence from results for men who had worked in England and Wales that the relatively favourable mortality experience among these miners was less marked during the latter period of the follow-up. This is consistent with other prospective studies of groups composed of persons employed in industry, and it points to the operation of the so-called “”healthy worker”” effect in these data.Death rates among the 5 253 men from the four Scottish collieries considered were also 16% lower on average than the published Scottish statistics but the follow-up rate for this group was lower (91.8%) than that achieved for the English and Welsh miners (93.8%),and there is evidence suggesting that the untraced Scottish miners included a disproportionately high number who have died. Detailed cause-specific analyses of the available material were therefore confined to results for 2736 men who had been employed at the English and Welsh collieries and whose vital status 22 years after the start of the follow-up was established.The main aim of the work was to consider risks of death attributed to various underlying causes, as recorded on death certificates, in relation to radiological signs of pneumoconiosis, measures of lung function, and exposure to respirable coalmine dust. Information regarding these factors was obtained from the records of the National Coal Board’s Pneumoconiosis Field Research. The initial radiological surveys for that prospective study constitute the start of the 22-year mortality follow-up reported here. Estimates of the miners’ exposures to airborne dust before the start of the study were made using information on occupational histories, obtained at interviews with the men, and average concentrations of respirable dust in six broad categories of coalmining activities at the collieries concerned during the ten years after the initial medical surveys.The statistical analyses of contrasts between sub-groups, defined according to different levels of the various factors of interest, were made .by estimating age-specific survival probabilities using life-table methods. Standard errors of these statistics are provided in tabular presentations of results, and the statistical significance of some of the observed trends has been investigated using methods based on Cox’s regression model for survival data.Miners with progressive massive fibrosis at the start of the study, including those with earliest signs of this disease (category A in the International Labour Office’s Classification system) had considerably higher mortality than their colleagues with no large shadows on their radiographs initially. Survival rates for those with category 1 simple pneumoconiosis were also low, for all ages greater than 2k years, in comparison with men whose radiographs had been classified as category 0 initially. There was no consistent trend for mortality to increase with increasing category of simple pneumoconiosis. Results pooled over all three categories (1,2 and 3) for the age-group 25-34 indicated a mortality hazard for non-violent causes about 1.6 times as high as that for similarly aged men with no radiological signs of the disease at the start of the study (P < 0.05), but this effect was not observed in men over 35 years of age.Mortality attributed to pneumoconiosis, to bronchitis and emphysema, and possibly also to other respiratory diseases, was more severe for miners who had relatively high exposures to respirable coalmine dust before the start of the follow-up. These findings are reflected in a tendency for mortality from all internal causes to increase with increasing ranges of estimated dust exposures.Risk of death from cancers of the digestive system also tended to increase with increasing dust exposure, but there was a similar association between mortality due to these diseases and the presence of pneumoconiosis on the radiographs. It is not clear therefore whether the results implicate exposure to dust per se.There is no suggestion of an association between lung cancer and exposure to coalmine dust, nor is there any sign that miners with coalworkers' pneumoconiosis are at greater risk than others of developing this disease - if anything, the contrary appears to be true. Analysis of data from a sub-group, for whom information on smoking habits was available confirmed the latter finding, and indicated that lung cancer mortality among the smokers in this sub-group was about five-and-a-half times higher than among non-smokers.There was a clear and consistent pattern of higher mortality over 12 years for miners with relatively low levels of standardised timed Forced Expiratory Volumes (FEVi). The increased risks for men with relatively low levels of lung function was evident in terms of deaths attributed to lung cancer, to bronchitis and emphysema (for those aged 55 to 64 at start of follow-up), to pneumoconiosis, to other respiratory diseases, and to ischaemic heart diseases.A study of mortality in a sub-group in relation to the quartz content of the coalmine dust to which the men had been exposed showed conflicting patterns. No conclusions specific to the effect of quartz are possible from these results because the variations in the quartz levels were closely associated with different collieries and the geographical regions in which they were situated. ""