Coalminers’ mortality in relation to low-level exposures to radon and thoron daughters. Final report on NRPB Contract (EMR 7/87) 90214000 and CEC Contract No.7270-ZZ-003

Epidemiological studies of many groups of miners, of various ores, have shown that exposure to radon daughters is associated with increased lung cancer risk. These results, taken in conjunction with surveys of domestic radon levels, suggested thatindoor exposure might also present a hazard for some segments of the generalpopulation. However, the magnitude of risk at the comparatively low levels found indoors was uncertain, since the exposure of mining groups from whom risk estimates were derived, was in general higher. Surveys of radon gas in British coalmines had shown levels of a similar order of magnitude to those of indoors,and it therefore appeared that coalminers might be a suitable occupational group in which to study the effects of low-level exposure to radon daughters. The resources of the Pneumoconiosis Field Research (PFR) database presented the opportunity of carrying out such a study, which was begun in November 1988, with support from the National Radiological Protection Board and also, subsequently, from the Commission of the European Communities. The objective was to study relationships between exposure to low levels of radon and thoron daughters at 10 British collieries, and subsequent mortality, in particular from lung cancer.The study group consisted of 19418 male industrial coalmine workers who attended either or both of two medical surveys carried out at 10 British collieries during the 1950s and early 1960s as part of the PFR. Deaths in this group (up to 31December 1989) were notified to the Institute of Occupational Medicine (IOM) by the Office of Population Censuses and Surveys. Cumulative exposures to radon and thoron daughters acquired during working time, were calculated for a subgroupof 14956 men using data from two sources: a set of 146 measurements of radon and thoron daughter levels made at the 10 collieries during the 1970s; and extensive information held in the PFR database on time worked underground andon the surface. Indoor exposure acquired outside working hours was not estimated. An overlapping subgroup of 14145 men were categorized by smoking habit, using questionnaire information obtained at a sequence of medical surveys carried out during the 1950s, 60s and 70s.Analyses of mortality from two causes (lung and stomach cancer) used the person-years-at-risk method, and also matched case-referent comparisons.further eight causes of death (malignant neoplasm of: oral cavity, oesophagus, larynx, bone, prostate, kidney; malignant melanoma, leukaemia – excluding chronic lymphoid) were analysed using case-referent methods only. All analyses of exposure-response were carried out entirely within the study group. External comparisons of mortality were carried out for lung cancer and all-causes only; exposure-response was not examined.Only men with reliable data on vital status, exposure and smoking were included in statistical analyses – a subgroup of 12361 men. Of the complete study group (19418 men), 3.3% were excluded because of unknown vital status. The median date of entry to the study was 1960, the median age at entry was 44; the average complete cumulative working time exposure to radon daughters was 250 WL hour (sd 291), and to thoron daughters, 162 WL hour (sd 121); the proportion of ‘never-smokers’ among men examined at PFR surveys varied from 12% to 14%. The SMR for all causes was 96, based on 5822 deaths in the study group of 12361men. For lung cancer, the SMR was 87, based on 521 deaths. Person-years analysis did not show any relationship between lagged cumulative exposure to radon daughters and lung cancer death rate.Case-referent analysis showed a joint effect of smoking and radon daughter exposure upon mortality: relative risks were elevated for non-smokers and smokers of up to 10 cigarettes per day, but declined to less than unity for heavier smokers. No relationship between thoron daughterexposure and mortality was identified in either statistical analysis. However the same pattern of interaction between smoking and exposure was evident, but not statistically significant, in the case-referent study. Case-referent analysis of acombined dose measure showed a joint effect of smoking and dose upon mortality. The pattern of interaction was similar to that observed for radon daughter exposure. Neither the person-years analysis, nor the case-referent study, showed any relationship between stomach cancer and radon daughter, or thoron daughter, exposure. For oesophageal cancer, an apparently negative association between exposure (both radon and thoron daughter) and mortality was observed, whichvaried with age. No associations were found for any other causes of death analysed.The pattern of the joint effect of radon daughter exposure and smoking upon lung cancer mortality presents difficulties of interpretation; the negative association among heavy smokers is biologically implausible, and may be due to the operation of a bias, as yet unidentified. However, it would be premature to dismiss the positive association between lung cancer mortality and radon daughter exposure in light smokers because of the problematical negative association in heavy smokers. Finally, the joint influence of age and both exposures upon oesophageal cancer mortality may be due to health-related movements between jobs. “”