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The occurrence of cancer in a cohort of New South Wales coal miners

The occurrence of cancer in a cohort of New South Wales coal miners Richard Taylor New South Wales Central Cancer Reptry, Sydney Margaret A. Seccombe Discipline of Environmental and Occupational Health, Univer.rityof Newcastle Marylon S. Coates New South Wales Central Cancer Reptry, Sydney Abstract: To describe the incidence of cancer in coal miners in New South Wales (NSW) between 1973 and 1992, an inception cohort of all male coal industry employees who entered the industry between 1 January 1973 and 31 December 1992 was constructed from the medical examination records of the Joint Coal Board. This cohort was matched with the NSW State Cancer Registry to determine the occurrence and type of cancer. In the cohort of 23 630 men, 297 developed 301 primary cancers in the 20-year period of observation. The standardised incidence ratio (SIR) for all cancers was 0.82. Stomach cancer has been reported to be common in coal miners but the SIR for stomach cancer was not higher than average in this cohort. A cluster of non-Hodgkin’s lymphoma has been reported in a NSW coal mine but an increased risk of this cancer was not evident in the industry as a whole. Similarly a cluster of cases of brain tumour has been reported. In this cohort, the SIR for brain tumour was 1.05 (95 per cent confidence interval (CI) 0.57 to 1.76) and a risk. for brain tumour remains unconfirmed. The SIR for malignant melanomawas 1.13 (CI 0.90 to 1.39) altogether and 2.02 (CI 1.31 to 2.98) for those workers who started in an open-cut mine. Overall, there does not appear to be a general risk of cancer in the NSW coal industry. Open-cut miners have an increased risk of malignant melanoma, which may be related to their exposure to the sun at work. (Aust NZJPubZic Health 1997; 21: 29-32) HE history of coal mining is littered with caveins and explosions-ample evidence that this is a dangerous industry. Accidents and injuries have always been prominent,’-3 and if this were not enough, lung disease, specifically coal workers pneumoconiosis, has been the traditional occupational d i ~ e a s e Conditions have improved over the years. .~ Control of dusts in mines and regular medical surveillance have led to a dramatic reduction in lung disease in coal miners; the mortality from respiratory disease among New South Wales (NSW) coal miners is now similar to that of the general community.5The mortality of NSW coal miners has recently been found to be less than that of the general p o p ulation, reflecting a ‘healthy worker effect’, but there remains an excess of deaths from accidents.’ While the introduction of safe mining practice has reduced the risk of accident and dust diseases, other hazards to the miner may be emerging. Cancer has been suggested as a risk of coal mining and there Correspondence to Dr A.M. Brown, Discipline of Environmental and Occupational Health, University of Newcastle, Newcastle Mater Misericordiae Hospital, Waratah, NSW 2298. Fax (049) 218 677. have been reports of clusters of cancers in various NSW coal The occurrence of these clusters of cancer has raised concern about cancer in the coal industry generally. In NSW, coal production traditionally has been one of our most important industries, both as an employer and as a major export earner. Coal miners, possibly uniquely among Australian industrial workforces, are represented on a database, which has been developed over many years by the Joint Coal Board. The NSW Central Cancer Registry has been population-based since 1972, and reliable data from the beginning of 1973 are available for matching. It was possible to analyse an inception cohort for the occurrence of cancer in those employees of the NSMT coal industry who entered the workforce from January 1973 to December 1992. Methods Since the late 1940s the Joint Coal Board has had a program of medical examinations for workers entering the industry and periodically thereafter. The entry examination has been almost universal, so the list of people who have ever had one should include VOL. AUSTRALIAN AND NEW ZEALAND JOURNAL O PUBLIC HEALTH 1997 F NO. BROWN n AL. all those who have worked in the NSW coal industry since the Joint Coal Board commenced operation. The Joint Coal Board has had computerised records since the early 1970s. From these records, we assembled an inception cohort consisting of all male’coal industry workers who were first registered with the Joint Coal Board on or after 1January 1973, or were recorded as having had their first Joint Coal Board medical examination on or after 1 January 1973 and before the end of the follow-up period. The date of entry to the cohort was taken as the first of these two dates, and only those men who were 60 years of age or under on 1 January 1973 were eligible. The start date of 1973 was chosen because from this date information on the incidence of cancer was available from the NSW Central Cancer Registry and because mining practices from that time are relevant to current conditions. The cohort was followed until 31 December 1992. The coal industry employs a small number of women, mostly in office positions (although this is changing); however, there were too few women (405) for meaningful analysis. The cohort consisted only of men, and the final membership was 23 630. We matched the data file from the Joint Coal Board with data from the NSW Central Cancer Registry, using the computer program Automatch,8 to identify members of the cohort who had developed cancer. The completeness of the Cancer Registry is good, and its reliability has been accepted by the International Agency for Research on Cancer.9 Initial matching results were verified by referring to the paper records of the Joint Coal Board and the Cancer Registry. For this study, ‘occurrence of a cancer’ was defined as the first appearance on the NSW Central Cancer Registry of the name of a member of the inception cohort. Men who had had a cancer registered before they started in the industry were excluded as new cases of cancer, unless they developed another cancer. Skin cancers, apart from melanoma, and benign tumours are not recorded on the register and therefore cannot be included in the cohort’s cancer experience. Complete occupational histories were not available from the database. It was not possible to identify for certain that a man had left the coal industry or to determine his length of time in the industry. The most available and reliable data concerned the employment location at the time of first contact with the Joint Coal Board. Each person was therefore classified by his job location at the time of entry to the cohort. Some men examined by the Joint Coal Board did not work in coal a mine because they did not pass the medical examination, had the examination in the hope of employment that did not eventuate or worked for another company (such as a transport or maintenance company) that used the Joint Coal Board for medical services. Men who started in a coal mine were classifed by their ‘mine of start’ being underground or open-cut, but the ‘mine of start’ might not have been the mine in which they spent most time or have been the mine of last employment. Nevertheless, industry and union representatives confirmed that over the period studied, movement of a miner from open-cut to underground work was unusual. For this reason, occurrence of cancer in men who worked in opencut mines and those who worked underground can be compared. Movement of people from one type of mine to the other would tend to dilute any differences. Comparisons of one underground mine and another, or one open-cut mine and another would not be valid. The data were analysed using SAS.‘O For indirect age-standardisation, expected cancers were calculated from the Cancer Registry’s cancer incidence rates for all of NSW from 1973 to 1992. Standardised incidence ratios (SIRs) were calculated and 95 per cent confidence intervals (CIS)for SIRS were calculated, assuming a Poisson distribution and using a method described by Morris and Gardner.” Table 1 : Age-standardised cancer incidence ratios (SIRs) in a cohort of N e w South Wales coal miners Incidence Cancer or site All cancers lip Other mouth and pharynx Stomach Colon Rectum Other gastrointestinal Larynx Lung Pleura Nasal cavities, sinuses Bone Connective tissue Melanoma Prostate Testis Bladder Brain and other nervous system Lymphomas, etc. Multiple myeloma Leukaemia Other cancers Observed Expected SIRa 0.82 1.02 0.49 0.70 1 .OO 0.83 0.57 1.02 0.74 1.05 0.54 1.67 0.51 1.13 0.43 0.95 0.80 1.05 0.62 0.28 0.42 0.98 CI 302 10 8 7 27 15 9 6 29 2 1 A 3 85 6 21 10 14 18 1 366.35 9.85 16.3 10.01 27.04 18.13 15.71 5.9 39.16 1.9 1.85 2.4 5.92 75.52 13.91 22.18 12.45 13.36 29.01 3.59 11.96 22.A 0.73 to 0.92 0.49 to 1.87 0.21 to0.97 0.28 to 1.44 0.66 to 1.45 0.46 to 1.36 0.26 to 1.09 0.37 to 2.21 0.50 to 1.06 -c -c -c -c 0.90 to 1.39 0.16 to 0.94 0.59 to 1.45 0.39 to 1.48 0.57 to 1.76 0.37 to 0.98 _ c 0.14 to 0.98 0.58 to 1.43 Notes: (a) SIR = incidence ratios indirectly standardised, based on specific incidence of cancer in males in New South Wales, 1972 to 1991. (b) CI = 95% confidence interval, assuming Poisson distribution. ” ( c ) Where the observed number of cancers was less than 5,the 95% confidence interval was not calculated because the numbers were too small for a meaningful result. AUSTRALIAN AND NEW ZEALAND JOURNAL O PUBLIC HEALTH 1997 vot. 21 NO. 1 F CANCER IN COAL MINERS Results The average age of men in the cohort at the end of the follow-up period was between 40 and 50 years, and although there were many older men who were born before 1944, essentially this was a young cohort. Most men (76.8 per cent) were employed in a colliery at the time they joined the industry, 13.5 per cent were in the coal industry but not working in a mine, 7.2 per cent were not in the industry and 2.5 per cent could not be classified. A total of 297 men were identified as appearing on the Cancer Registry between the beginning of 1973 and the end of 1992. Of these, five people appeared twice, that is, they developed a second primary cancer. The total number of cancers was 302. The SIRS for all cancers and site-specific cancers are given in Table 1. The SIR for 'all cancers' was 20 per cent lower than expected from the rates applying in the reference population (NSW). Ratios for cancer of the stomach and lung cancer were not higher than expected. Both have been reported in association with coal mining. Cancers of the brain and nervous system, lip, colon and larynx had SIRS of 1 or slightly more, but the confidence intervals were wide and included 1. Similarly, while the SIRS for cancers of the pleura and bone exceeded 1, the numbers of cases were so low that meaningful confidence limits could not be calculated. There were 85 cases of melanoma (with 75.5 expected). The SIR was 1.13, but the confidence interval included 1. To explore possible mine-related factors, further analysis was restricted to those men recorded as starting work in a coal mine. Again, the SIR for all cancers was not significantly high and there was no appreciable difference in cancer occurrence between those starting in underground and those starting in open-cut mines (Table 2). Three specific cancer groups (lymphoma, brain tumour and melanoma) were considered further. Lymphoma and Hodgkin's disease were the subject of the Huntley Colliery cancer cluster.6A cluster of brain tumours from a NSW mine has been reported.' It has been suggested that melanoma may be more common in those working in coal mines." Table 3: Age-standardised incidence ratios (SIRs) for selected cancers in a cohort of N e w South Wales coal miners, by type of mine of start Incidence Type of cancer and type of mine Observed Lymphoma and Hodgkin's disease Underground Open cut All mines Brain tumour Underground Open cut All mines Melanoma Underground Open cut All mines Expected SIRa Clb 0.75 0.21 0.63 1.37 1.08 0.97 2.02' 1.20 0.40 to 1.28 -c 0.35 to 1.06 0.68 to 2.45 -c 0.54 to 1.94 0.71 to 1.30 1.31 to 2.98 0.93 to 1.51 Notes: (a) SIR = incidence ratios indirectly standardised, based on specific incidence of cancer in males in New South Wales, 1972 to 1991 . (b) CI = 95% confidence interval, assuming Poisson distribution. (c) Where the observed number of cancers was less than 5 , the 95% confidence interval was not calculated because the numbers were too small for a meaningful result. 'Pc 0.05 In this study, melanoma had an elevated SIR and a large number of cases. Table 3 shows the SIRS for these cancers for the group that started in a coal mine. The SIR for lymphoma and Hodgkin's disease was not elevated overall, nor was it elevated for those working in either of the mine types. For brain tumours, the SIR for all mines was marginally elevated, with all the cases occurring in underground s miners. A a consequence, the SIR for the underground miners was 1.37, but with only 11 cases, the CI still included 1. For melanoma, the all-mines SIR was 1.20. Most of the cases were in men who worked in underground mines, for whom the SIR was 0.97, but 25 occurred in men from open-cut mines, for whom the SIR was 2.02 (CI 1.31 to 2.98). Table 2: Age-standardised cancer incidence ratios (SIRs) in a cohort of N e w South Wales coal miners, by type of mine of start ~ ~ ~~ Type of mine Underground Open cut All coal mines Incidence Observed Expected SIRa CI 0.73 to 0.98 0.74 to 1.94 0.77 to 0.99 Notes: (a) SIR = incidence ratios indirectly standardised, based on specific incidence of cancer in males in New South Wales, 1972 to 1991, (b) CI = 95% confidence interval, assuming Poisson distribution. I' Discussion This cohort had a lower-than-expected SIR for all cancers (0.82). Such a finding has been reported from other occupational cohorts, including one from the Australian petroleum industry, for whom the SIR for all cancers was 0.9 (CI 0.7 to l.l)."' It seems likely that this finding results from an inappropriate reference population, because, as Miettinen pointed out, different selection forces were operating in the study group and the general p~pulation.'~ Alternative reference groups were hard to find. Because the subjects were followed after leaving the industry a survivor bias cannot operate. Cancer has been noted in epidemiological studies of coal miners. A study of the mortality of coal miners in the United States in the 1970s noted an increase in cancer of the stomach.I5 A case-control VOL. AUSTRALIAN AND NEW ZEALAND JOURNAL O PUBLIC HEALTH 1997 F NO. BROWN ~l AL. study of stomach cancer in the Netherlands in 1985 failed to confirm this association,I6 yet more recently, Gonzalez et al. in Spain found an odds ratio for stomach cancer in coal miners of 11.8, although the numbers were small and the CI wide (1.36 to 103).” There was no suggestion of an increased risk of gastrointestinal cancer in the present cohort. There have been reports of lung cancer in workers exposed to diesel fume^.'^,'^ In this cohort, the SIR for lung cancer was not elevated. In 1988, Corbett and O’Neill reported on a cluster of cases of lymphoma in the workforce of the Huntley Colliery on the NSW South Coast.‘ In their historical cohort study, based on NSW cancer incidence, the SIR for non-Hodgkin’s lymphoma was 3.27 (CI 1.31 to 6.74) and for Hodgkin’s disease 7.27 (CI 1.98 to 18.59). There was no apparent cause for this cluster of cancers and this generated considerable concern. In our statewide cohort, the SIRS for lymphoma and Hodgkin’s disease were not significantly elevated. This suggests that the excess lymphoma found in the Huntley Colliery formed a cluster related to local phenomena rather than being a feature of the industry (if the occurrence was not due to chance). Brown et al. investigated a cluster of cases of brain tumour in another NSW coal mine.’ In a cohort of miners from that mine the SIR for brain tumour (using Australian incidence rates) was 5.30 (CI 1.08 to 14.04) based on three cases, while the SIRS in the two neighbouring mines were 1.72 (CI 0.04 to 8.06) and 0.81 (CI 0.02 to 3.80). These mines were compared for a range of possible relevant exposures, and it was found that the index mine had ordered, and presumably used, substantially more organic solvents than the reference mines. Solvent exposure was tentatively suggested as the cause of the brain tumours, but it was recognised that the number of cases was small. The present cohort covers all mines in the state. The SIR for brain tumour was elevated for underground mines, but it did not reach statistical significance. Hersey et al. reported that miners were overrepresented in the patients reporting to the Newcastle Melanoma Unit.12These authors did not appear to differentiate between underground and opencut miners. In the current study, the data are consistent with an elevated risk of melanoma, which seems to be related entirely to opencut mining. The obvious connection between open-cut mining and melanoma is exposure to the sun, and although the relationship is not fully understood, there is little doubt that exposure to the sun is associated with the development of melanoma. This is not unique to coal mining and illustrates again that sun-related skin disease is an occupational hazard for outdoor workers. Strategies for prevention currently exist, and in an educational program, the Joint Coal Board is actively promoting sun-protection in opencut mines. It appears that there is no evidence of serious hazard for cancer in modern coal mines, and in particular, no support is given to an association of mining and haematological cancer. A risk of brain tumour still cannot be discounted, although any such risk would be small. It is likely that open-cut miners run excess risk of melanoma related to exposure to ultraviolet radiation. Work is being done to maintain and extend the database and it is hoped that it can be analysed again in the future to provide ongoing surveillance for cancer in this important industry. Acknowledgments This study was funded by the Joint Coal Board and we thank Mr M. Hicks and Ms M. James for their invaluable contributions. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Australian and New Zealand Journal of Public Health Wiley

The occurrence of cancer in a cohort of New South Wales coal miners

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Publisher
Wiley
Copyright
Copyright © 1997 Wiley Subscription Services, Inc., A Wiley Company
ISSN
1326-0200
eISSN
1753-6405
DOI
10.1111/j.1467-842X.1997.tb01649.x
Publisher site
See Article on Publisher Site

Abstract

Richard Taylor New South Wales Central Cancer Reptry, Sydney Margaret A. Seccombe Discipline of Environmental and Occupational Health, Univer.rityof Newcastle Marylon S. Coates New South Wales Central Cancer Reptry, Sydney Abstract: To describe the incidence of cancer in coal miners in New South Wales (NSW) between 1973 and 1992, an inception cohort of all male coal industry employees who entered the industry between 1 January 1973 and 31 December 1992 was constructed from the medical examination records of the Joint Coal Board. This cohort was matched with the NSW State Cancer Registry to determine the occurrence and type of cancer. In the cohort of 23 630 men, 297 developed 301 primary cancers in the 20-year period of observation. The standardised incidence ratio (SIR) for all cancers was 0.82. Stomach cancer has been reported to be common in coal miners but the SIR for stomach cancer was not higher than average in this cohort. A cluster of non-Hodgkin’s lymphoma has been reported in a NSW coal mine but an increased risk of this cancer was not evident in the industry as a whole. Similarly a cluster of cases of brain tumour has been reported. In this cohort, the SIR for brain tumour was 1.05 (95 per cent confidence interval (CI) 0.57 to 1.76) and a risk. for brain tumour remains unconfirmed. The SIR for malignant melanomawas 1.13 (CI 0.90 to 1.39) altogether and 2.02 (CI 1.31 to 2.98) for those workers who started in an open-cut mine. Overall, there does not appear to be a general risk of cancer in the NSW coal industry. Open-cut miners have an increased risk of malignant melanoma, which may be related to their exposure to the sun at work. (Aust NZJPubZic Health 1997; 21: 29-32) HE history of coal mining is littered with caveins and explosions-ample evidence that this is a dangerous industry. Accidents and injuries have always been prominent,’-3 and if this were not enough, lung disease, specifically coal workers pneumoconiosis, has been the traditional occupational d i ~ e a s e Conditions have improved over the years. .~ Control of dusts in mines and regular medical surveillance have led to a dramatic reduction in lung disease in coal miners; the mortality from respiratory disease among New South Wales (NSW) coal miners is now similar to that of the general community.5The mortality of NSW coal miners has recently been found to be less than that of the general p o p ulation, reflecting a ‘healthy worker effect’, but there remains an excess of deaths from accidents.’ While the introduction of safe mining practice has reduced the risk of accident and dust diseases, other hazards to the miner may be emerging. Cancer has been suggested as a risk of coal mining and there Correspondence to Dr A.M. Brown, Discipline of Environmental and Occupational Health, University of Newcastle, Newcastle Mater Misericordiae Hospital, Waratah, NSW 2298. Fax (049) 218 677. have been reports of clusters of cancers in various NSW coal The occurrence of these clusters of cancer has raised concern about cancer in the coal industry generally. In NSW, coal production traditionally has been one of our most important industries, both as an employer and as a major export earner. Coal miners, possibly uniquely among Australian industrial workforces, are represented on a database, which has been developed over many years by the Joint Coal Board. The NSW Central Cancer Registry has been population-based since 1972, and reliable data from the beginning of 1973 are available for matching. It was possible to analyse an inception cohort for the occurrence of cancer in those employees of the NSMT coal industry who entered the workforce from January 1973 to December 1992. Methods Since the late 1940s the Joint Coal Board has had a program of medical examinations for workers entering the industry and periodically thereafter. The entry examination has been almost universal, so the list of people who have ever had one should include VOL. AUSTRALIAN AND NEW ZEALAND JOURNAL O PUBLIC HEALTH 1997 F NO. BROWN n AL. all those who have worked in the NSW coal industry since the Joint Coal Board commenced operation. The Joint Coal Board has had computerised records since the early 1970s. From these records, we assembled an inception cohort consisting of all male’coal industry workers who were first registered with the Joint Coal Board on or after 1January 1973, or were recorded as having had their first Joint Coal Board medical examination on or after 1 January 1973 and before the end of the follow-up period. The date of entry to the cohort was taken as the first of these two dates, and only those men who were 60 years of age or under on 1 January 1973 were eligible. The start date of 1973 was chosen because from this date information on the incidence of cancer was available from the NSW Central Cancer Registry and because mining practices from that time are relevant to current conditions. The cohort was followed until 31 December 1992. The coal industry employs a small number of women, mostly in office positions (although this is changing); however, there were too few women (405) for meaningful analysis. The cohort consisted only of men, and the final membership was 23 630. We matched the data file from the Joint Coal Board with data from the NSW Central Cancer Registry, using the computer program Automatch,8 to identify members of the cohort who had developed cancer. The completeness of the Cancer Registry is good, and its reliability has been accepted by the International Agency for Research on Cancer.9 Initial matching results were verified by referring to the paper records of the Joint Coal Board and the Cancer Registry. For this study, ‘occurrence of a cancer’ was defined as the first appearance on the NSW Central Cancer Registry of the name of a member of the inception cohort. Men who had had a cancer registered before they started in the industry were excluded as new cases of cancer, unless they developed another cancer. Skin cancers, apart from melanoma, and benign tumours are not recorded on the register and therefore cannot be included in the cohort’s cancer experience. Complete occupational histories were not available from the database. It was not possible to identify for certain that a man had left the coal industry or to determine his length of time in the industry. The most available and reliable data concerned the employment location at the time of first contact with the Joint Coal Board. Each person was therefore classified by his job location at the time of entry to the cohort. Some men examined by the Joint Coal Board did not work in coal a mine because they did not pass the medical examination, had the examination in the hope of employment that did not eventuate or worked for another company (such as a transport or maintenance company) that used the Joint Coal Board for medical services. Men who started in a coal mine were classifed by their ‘mine of start’ being underground or open-cut, but the ‘mine of start’ might not have been the mine in which they spent most time or have been the mine of last employment. Nevertheless, industry and union representatives confirmed that over the period studied, movement of a miner from open-cut to underground work was unusual. For this reason, occurrence of cancer in men who worked in opencut mines and those who worked underground can be compared. Movement of people from one type of mine to the other would tend to dilute any differences. Comparisons of one underground mine and another, or one open-cut mine and another would not be valid. The data were analysed using SAS.‘O For indirect age-standardisation, expected cancers were calculated from the Cancer Registry’s cancer incidence rates for all of NSW from 1973 to 1992. Standardised incidence ratios (SIRs) were calculated and 95 per cent confidence intervals (CIS)for SIRS were calculated, assuming a Poisson distribution and using a method described by Morris and Gardner.” Table 1 : Age-standardised cancer incidence ratios (SIRs) in a cohort of N e w South Wales coal miners Incidence Cancer or site All cancers lip Other mouth and pharynx Stomach Colon Rectum Other gastrointestinal Larynx Lung Pleura Nasal cavities, sinuses Bone Connective tissue Melanoma Prostate Testis Bladder Brain and other nervous system Lymphomas, etc. Multiple myeloma Leukaemia Other cancers Observed Expected SIRa 0.82 1.02 0.49 0.70 1 .OO 0.83 0.57 1.02 0.74 1.05 0.54 1.67 0.51 1.13 0.43 0.95 0.80 1.05 0.62 0.28 0.42 0.98 CI 302 10 8 7 27 15 9 6 29 2 1 A 3 85 6 21 10 14 18 1 366.35 9.85 16.3 10.01 27.04 18.13 15.71 5.9 39.16 1.9 1.85 2.4 5.92 75.52 13.91 22.18 12.45 13.36 29.01 3.59 11.96 22.A 0.73 to 0.92 0.49 to 1.87 0.21 to0.97 0.28 to 1.44 0.66 to 1.45 0.46 to 1.36 0.26 to 1.09 0.37 to 2.21 0.50 to 1.06 -c -c -c -c 0.90 to 1.39 0.16 to 0.94 0.59 to 1.45 0.39 to 1.48 0.57 to 1.76 0.37 to 0.98 _ c 0.14 to 0.98 0.58 to 1.43 Notes: (a) SIR = incidence ratios indirectly standardised, based on specific incidence of cancer in males in New South Wales, 1972 to 1991. (b) CI = 95% confidence interval, assuming Poisson distribution. ” ( c ) Where the observed number of cancers was less than 5,the 95% confidence interval was not calculated because the numbers were too small for a meaningful result. AUSTRALIAN AND NEW ZEALAND JOURNAL O PUBLIC HEALTH 1997 vot. 21 NO. 1 F CANCER IN COAL MINERS Results The average age of men in the cohort at the end of the follow-up period was between 40 and 50 years, and although there were many older men who were born before 1944, essentially this was a young cohort. Most men (76.8 per cent) were employed in a colliery at the time they joined the industry, 13.5 per cent were in the coal industry but not working in a mine, 7.2 per cent were not in the industry and 2.5 per cent could not be classified. A total of 297 men were identified as appearing on the Cancer Registry between the beginning of 1973 and the end of 1992. Of these, five people appeared twice, that is, they developed a second primary cancer. The total number of cancers was 302. The SIRS for all cancers and site-specific cancers are given in Table 1. The SIR for 'all cancers' was 20 per cent lower than expected from the rates applying in the reference population (NSW). Ratios for cancer of the stomach and lung cancer were not higher than expected. Both have been reported in association with coal mining. Cancers of the brain and nervous system, lip, colon and larynx had SIRS of 1 or slightly more, but the confidence intervals were wide and included 1. Similarly, while the SIRS for cancers of the pleura and bone exceeded 1, the numbers of cases were so low that meaningful confidence limits could not be calculated. There were 85 cases of melanoma (with 75.5 expected). The SIR was 1.13, but the confidence interval included 1. To explore possible mine-related factors, further analysis was restricted to those men recorded as starting work in a coal mine. Again, the SIR for all cancers was not significantly high and there was no appreciable difference in cancer occurrence between those starting in underground and those starting in open-cut mines (Table 2). Three specific cancer groups (lymphoma, brain tumour and melanoma) were considered further. Lymphoma and Hodgkin's disease were the subject of the Huntley Colliery cancer cluster.6A cluster of brain tumours from a NSW mine has been reported.' It has been suggested that melanoma may be more common in those working in coal mines." Table 3: Age-standardised incidence ratios (SIRs) for selected cancers in a cohort of N e w South Wales coal miners, by type of mine of start Incidence Type of cancer and type of mine Observed Lymphoma and Hodgkin's disease Underground Open cut All mines Brain tumour Underground Open cut All mines Melanoma Underground Open cut All mines Expected SIRa Clb 0.75 0.21 0.63 1.37 1.08 0.97 2.02' 1.20 0.40 to 1.28 -c 0.35 to 1.06 0.68 to 2.45 -c 0.54 to 1.94 0.71 to 1.30 1.31 to 2.98 0.93 to 1.51 Notes: (a) SIR = incidence ratios indirectly standardised, based on specific incidence of cancer in males in New South Wales, 1972 to 1991 . (b) CI = 95% confidence interval, assuming Poisson distribution. (c) Where the observed number of cancers was less than 5 , the 95% confidence interval was not calculated because the numbers were too small for a meaningful result. 'Pc 0.05 In this study, melanoma had an elevated SIR and a large number of cases. Table 3 shows the SIRS for these cancers for the group that started in a coal mine. The SIR for lymphoma and Hodgkin's disease was not elevated overall, nor was it elevated for those working in either of the mine types. For brain tumours, the SIR for all mines was marginally elevated, with all the cases occurring in underground s miners. A a consequence, the SIR for the underground miners was 1.37, but with only 11 cases, the CI still included 1. For melanoma, the all-mines SIR was 1.20. Most of the cases were in men who worked in underground mines, for whom the SIR was 0.97, but 25 occurred in men from open-cut mines, for whom the SIR was 2.02 (CI 1.31 to 2.98). Table 2: Age-standardised cancer incidence ratios (SIRs) in a cohort of N e w South Wales coal miners, by type of mine of start ~ ~ ~~ Type of mine Underground Open cut All coal mines Incidence Observed Expected SIRa CI 0.73 to 0.98 0.74 to 1.94 0.77 to 0.99 Notes: (a) SIR = incidence ratios indirectly standardised, based on specific incidence of cancer in males in New South Wales, 1972 to 1991, (b) CI = 95% confidence interval, assuming Poisson distribution. I' Discussion This cohort had a lower-than-expected SIR for all cancers (0.82). Such a finding has been reported from other occupational cohorts, including one from the Australian petroleum industry, for whom the SIR for all cancers was 0.9 (CI 0.7 to l.l)."' It seems likely that this finding results from an inappropriate reference population, because, as Miettinen pointed out, different selection forces were operating in the study group and the general p~pulation.'~ Alternative reference groups were hard to find. Because the subjects were followed after leaving the industry a survivor bias cannot operate. Cancer has been noted in epidemiological studies of coal miners. A study of the mortality of coal miners in the United States in the 1970s noted an increase in cancer of the stomach.I5 A case-control VOL. AUSTRALIAN AND NEW ZEALAND JOURNAL O PUBLIC HEALTH 1997 F NO. BROWN ~l AL. study of stomach cancer in the Netherlands in 1985 failed to confirm this association,I6 yet more recently, Gonzalez et al. in Spain found an odds ratio for stomach cancer in coal miners of 11.8, although the numbers were small and the CI wide (1.36 to 103).” There was no suggestion of an increased risk of gastrointestinal cancer in the present cohort. There have been reports of lung cancer in workers exposed to diesel fume^.'^,'^ In this cohort, the SIR for lung cancer was not elevated. In 1988, Corbett and O’Neill reported on a cluster of cases of lymphoma in the workforce of the Huntley Colliery on the NSW South Coast.‘ In their historical cohort study, based on NSW cancer incidence, the SIR for non-Hodgkin’s lymphoma was 3.27 (CI 1.31 to 6.74) and for Hodgkin’s disease 7.27 (CI 1.98 to 18.59). There was no apparent cause for this cluster of cancers and this generated considerable concern. In our statewide cohort, the SIRS for lymphoma and Hodgkin’s disease were not significantly elevated. This suggests that the excess lymphoma found in the Huntley Colliery formed a cluster related to local phenomena rather than being a feature of the industry (if the occurrence was not due to chance). Brown et al. investigated a cluster of cases of brain tumour in another NSW coal mine.’ In a cohort of miners from that mine the SIR for brain tumour (using Australian incidence rates) was 5.30 (CI 1.08 to 14.04) based on three cases, while the SIRS in the two neighbouring mines were 1.72 (CI 0.04 to 8.06) and 0.81 (CI 0.02 to 3.80). These mines were compared for a range of possible relevant exposures, and it was found that the index mine had ordered, and presumably used, substantially more organic solvents than the reference mines. Solvent exposure was tentatively suggested as the cause of the brain tumours, but it was recognised that the number of cases was small. The present cohort covers all mines in the state. The SIR for brain tumour was elevated for underground mines, but it did not reach statistical significance. Hersey et al. reported that miners were overrepresented in the patients reporting to the Newcastle Melanoma Unit.12These authors did not appear to differentiate between underground and opencut miners. In the current study, the data are consistent with an elevated risk of melanoma, which seems to be related entirely to opencut mining. The obvious connection between open-cut mining and melanoma is exposure to the sun, and although the relationship is not fully understood, there is little doubt that exposure to the sun is associated with the development of melanoma. This is not unique to coal mining and illustrates again that sun-related skin disease is an occupational hazard for outdoor workers. Strategies for prevention currently exist, and in an educational program, the Joint Coal Board is actively promoting sun-protection in opencut mines. It appears that there is no evidence of serious hazard for cancer in modern coal mines, and in particular, no support is given to an association of mining and haematological cancer. A risk of brain tumour still cannot be discounted, although any such risk would be small. It is likely that open-cut miners run excess risk of melanoma related to exposure to ultraviolet radiation. Work is being done to maintain and extend the database and it is hoped that it can be analysed again in the future to provide ongoing surveillance for cancer in this important industry. Acknowledgments This study was funded by the Joint Coal Board and we thank Mr M. Hicks and Ms M. James for their invaluable contributions.

Journal

Australian and New Zealand Journal of Public HealthWiley

Published: Feb 1, 1997

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