The mortality of tar workers

The mortality of a group of 259 male industrial workers employed at four Thomas Ness plants on 1 January 1967 was studied by following up the group from 1 January 1967 to 31 December 1983. Two of the four plants were attached to coke works operated by National Smokeless Fuels Limited, and were engaged in tar distillation. The other two were self-contained tar distillation plants. The aims of the study were twofold: first, to compare cause-specific death rates in the study group with those occurring in the general male population of the same geographical regions, and secondly, to investigate any possible associations between occupation and mortality from those causes of death suspected of being linked to tar distillation work either on a priori grounds, or as a result of the external comparisons in this study. Investigation of such associations was undertaken in two stages. The influence of time worked on death rates was assessed using regression techniques; links between job-type and mortality were investigated by case-control methods.The mortality occurring in a group of 28 male industrial workers employed on 1 January 1967 at a Thomas Ness refinery primarily involved in the distillation of benzene was also studied over the same period. Only external comparisons of mortality were undertaken for this group of men; results are reported separately in an appendix.The mortality occurring over the same period among maintenance workers at the National Smokeless Fuels coke works associated with one of the tar distillation plants is also reported in an appendix. Although these men were not employees of Thomas Ness, some of them had undertaken maintenance work in the tar distillation plant, and consequently might have been exposed to tar.Follow-up: All deaths occurring in the entire group of 287 workers (i.e. 259 tar workers and 28 refinery workers) during the follow-up period were notified to the Institute of Occupational Medicine by the Office of Population Censuses and Surveys. A copy of the death certificate, with the primary cause of death coded according to the eighth revision of the International Classification of Diseases, accompanied the notification.Occupational histories: A history of all jobs done since joining Ness by each of the 287 men involved in the study, and the duration of these jobs, was obtained from Ness clerical staff during 1975 and 1976. Personnel records formed the main source of information. Additional data were obtained at interview, both of current employees, and of former colleagues of men no longer employed. For the men at the four tar distillation plants, all job-names appearing in the histories were classified into one of five categories for the purposes of examining associations between mortality and job-type.External comparisons of mortality: The number of deaths from all causes in the group of 259 tar workers was slightly in excess of that expected on the basis of regional rates (75 compared with 73.3). Similarly, deaths due to malignant neoplasms exceeded those expected by only a small margin (19 compared with 18.2). Notable cause-specific excesses were: 12 deaths from lung cancer compared with 7.5 expected (P – 0.08), 3 deaths from bladder cancer compared with 0.7 expected (P = 0.03), and 5 deaths from diseases of arteries, arterioles and capillaries, diseases of veins and lymphatics, and other diseases of the circulatory system (ICD 440-458) compared with 2.1 expected (P = 0.06).The 12 lung cancer deaths occurred in men aged over 55, the three bladder cancer deaths in men aged over 65. Eight of 29 deaths from ischaemic heart disease were in men younger than 55, compared with 3.9 expected (P = 0.04).There were seven deaths from all causes compared with 6.8 expected in the group of 28 refinery workers. Of these, four were due to ischaemic heart disease, compared with 2.4 expected (P= 0.22).Observed mortality in a group of National Smokeless Fuels maintenance workers: There were 28 deaths from all causes in this group of 73 workers. Of these, seven (25%) were from lung cancer and 13 (46%) from ischaemic heart disease.Mortality and occupation:An adverse effect of total time elapsed between joining and leaving the tar distillation industry upon the death rate from four causes considered jointly (viz., lung cancer, bladder cancer, ischaemic heart disease and diseases of arteries and veins) was established for tar workers aged under 45 at the start of follow-up, allowance being made for age. For men aged between 45 and 55 and for men aged over 55, there was no evidence of any relationship between time elapsed and death rate.To investigate links between job-type and mortality, men at the four tar plants who had died from one of these four causes (cases) were compared to men from the same plant, matched on date of birth and date of joining the industry, who had survived to 31 December 1983 (controls). For ischaemic heart disease, only men aged under 55 at death were considered as cases.The number of controls per case varied according to the cause of death being considered. Each of 12 men dying from lung cancer and eight men dying from ischaemic heart disease was matched to two controls; three men dying from bladder cancer were each compared with four controls and five men dying from diseases of arteries and veins were each matched to three controls. The comparisons were in respect of time worked in each of five job categories.Only for diseases of arteries and veins was there any suggestion of differences in time worked between cases and controls. Men who died had spent on average 12.6 years longer in part-time work on tar distillation or with by-products than their matched controls, and 4.4 years less in other jobs not directly connected with the distillation process. Only the latter of these differences was statistically significant at the 5% level, and furthermore the quality of the matching of cases to controls for this cause of death was unsatisfactory.Conclusions: The results of the comparisons of mortality with that occurring regionally, when considered in conjunction with results from other studies, indicate that tar distillation workers suffer increased risks of lung and bladder cancer as a consequence of their occupation.The presence of an increased death rate from ischaemic heart disease in men under 55, which resembles a result previously obtained in a study of British coke workers, points to the possible existence of an occupational hazard. The selection out of the industry on grounds of health of the most susceptible men may be a possible explanation for the age-related pattern of excess mortality from this cause.A further indication of links between tar work and increased mortality rates due to lung cancer and ischaemic heart disease is provided by the proportionately high mortality from these causes found in a group of National Smokeless Fuels maintenance workers.Finally, the results of the case control studies have not shed any light on possible associations between excess mortality and job-type.