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Occupational Medicine 2005 55(7):541-548; doi:10.1093/occmed/kqi157
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© The Author 2005. Published by Oxford University Press on behalf of the Society of Occupational Medicine. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Mortality of UK electricity generation and transmission workers, 1973–2002

Linda Nichols and Tom Sorahan

Institute of Occupational and Environmental Medicine, University of Birmingham, Edgbaston, Birmingham, West Midlands B15 2TT, UK

Correspondence to: Tom Sorahan, Institute of Occupational and Environmental Medicine, University of Birmingham, Edgbaston, Birmingham, West Midlands B15 2TT, UK. Tel: +121 414 3644; fax: +121 414 6217.


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conflict of interest
 References
 
Objective To examine mortality from cancer and non-malignant causes among a large cohort of UK electricity generation and transmission workers.

Methods The mortality experienced by a cohort of 83 923 employees of the former Central Electricity Generating Board of England and Wales was investigated for the period 1973–2002. All employees had worked for at least 6 months with some employment between 1973 and 1982. Standardized mortality ratios (SMRs) were used to assess mortality in the total cohort and in three sub-cohorts: power station workers, substation and transmission workers and workers at non-operational locations. These classifications were based on the place of work of the first known job.

Results Overall mortality was significantly below that expected, based on national rates [males: observed (Obs) 18 773, expected (Exp) 22 497.9, SMR 83; females: Obs 1122, Exp 1424.9, SMR 79]. Statistically significant deficits of deaths were also found for most of the major disease groupings. However, significant excesses of deaths were found in male power station workers for cancer of the pleura (Obs 129, Exp 30.3, SMR 426) and in male workers from non-operational locations for cancer of the brain (Obs 55, Exp 36.0, SMR 153). There was also a non-significant excess of deaths from cancer of the breast in male power station workers (Obs 10, Exp 5.3, SMR 190).

Conclusions Mortality was exceptionally low for most causes of death but late health effects from earlier asbestos exposure were still in evidence.

Keywords      Cohort mortality study; electricity supply industry


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conflict of interest
 References
 
A cohort of UK electricity generation and transmission workers was established in the 1970s to investigate whether such workers have elevated mortality; original concerns related to non-malignant respiratory disease. More recently, concerns that electromagnetic field (EMF) exposure may be carcinogenic have been the focus of many epidemiological studies in this industry [1–7]. A number of reviews are available [8,9]; the most common outcomes of interest have been leukaemia and cancer of the brain [1,2,5,6]. The UK cohort has been used previously to investigate mortality from leukaemia, cancer of the brain and cardiovascular disease [5–7]. An analysis of standardized mortality ratios (SMRs) for all major diseases in the UK cohort is now presented with the aim of identifying any other causes of death which merit further investigation.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conflict of interest
 References
 
The study population and computerized data have been described previously [5–7]. The cohort made available for analysis comprised 83 923 employees (72 889 males and 11 034 females) of the former Central Electricity Generating Board of England and Wales for whom computerized information was available. (The cohort previously comprised 83 997 employees [7]. A total of 74 records were found to be duplicates recently and these have now been removed.) All employees were known to have a minimum of 6 months employment with some period of employment between 1973 and 1982.

The total cohort was subdivided into three categories based on the work location of the first known job: power stations (53 265), substation or transmission sites (3223) and non-operational locations (22 327). (There were 5108 individuals for whom no job history was available or whose work history could not be classified under one of these three headings.) For those in employment on the date of computerization of personnel records [7] the first known job was the job held on that date. For later entrants, the first job was available for selection. The study received follow-up particulars from the National Health Service Central Register of the Office for National Statistics (ONS). Underlying cause coding was supplied by the ONS [International Classification of Diseases (ICD)-8 1973–1978, ICD-9 1979–1999, ICD-10 2000–2003]. The vital status of study subjects on the closing date of the study (31 December 2002) is summarized in Table 1.


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  		Table 1. Vital status at closing date of study (31 December 2002)

 
The mortality experience of the cohort was compared with that which might have been expected to occur if mortalities for the general population of England and Wales had been operating on the study cohort, having due regard to the composition of the study cohort by sex, age (5-year age groups) and calendar period (5-year calendar periods). Expectations based on person-years at risk (p.y.r.) were calculated using the PersonYears computer program [10]. Study subjects entered the p.y.r. on completion of 6 months employment or the date of computerization for the region of their employment [7], whichever was later. Individuals ceased to contribute to the p.y.r. on the date of death, date of embarkation, the date they were last confirmed to be alive or the closing date of the study, whichever was the earlier. Study subjects made no contribution to observed or expected numbers after their 85th birthday.

SMRs were calculated as the ratio of observed to expected numbers of deaths expressed as a percentage. In calculating P-values and 95% confidence intervals (95% CIs), deaths were assumed to occur as a Poisson process. The significance tests were two tailed.


   Results
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 Abstract
 Introduction
 Methods
 Results
Discussion
 Conflict of interest
 References
 
Table 2 shows observed and expected numbers of deaths from individual sites of cancer and from broad groupings of non-cancer deaths for male and female study subjects. Compared with national rates, all-causes mortality was significantly below expectation for both males [observed (Obs) 18 773, expected (Exp) 22 497.9, SMR 83] and females (Obs 1 122, Exp 1 424.9, SMR 79). Statistically significant deficits of deaths were also found for most non-malignant disease groupings. Mortality from all cancers combined showed statistically significant deficits (males: Obs 6428, Exp 6919.6, SMR 93; females: Obs 533, Exp 548.3, SMR 97). The majority of the SMRs for site-specific cancers were below 100, although mortality from cancer of the pleura in males was significantly in excess (Obs 166, Obs 43.4, SMR 383).


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  		Table 2. Mortality in UK electricity generation and transmission workers, 1973–2002 (n = 83 923; male = 72 889, female = 11 034)

 
Observed and expected numbers of deaths from cancers and non-malignant disease groupings are presented in Table 3 for power station workers. SMRs for all causes combined and for most of the major disease groupings were significantly below expectation. Among males, deaths from cancers of the peritoneum (Obs 14, Exp 7.6, SMR 185) and pleura (Obs 129, Exp 30.3, SMR 426) were significantly in excess. There was also a non-significant excess for cancer of the breast (Obs 10, Exp 5.3, SMR 190). For most other cancers, mortality was either near or below expectation. For female power station employees, mortality from all neoplasms was close to expectation (Obs 223, Exp 216.0, SMR 103). The findings for individual sites of cancer in females were unexceptional; many were based on small numbers of observed deaths.


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  		Table 3. UK study of electricity generation and transmission workers: power station workers, 1973–2002 (n = 53 265; male = 49 424, female = 3841)

 
Table 4 shows observed and expected numbers of deaths for male substation and transmission site workers. Overall mortality was below expectation (Obs 710, Exp 1005.6, SMR 71). There was no excess mortality for cancer of the pleura. Findings for female workers were unexceptional although based on small numbers of deaths (not shown in table). Mortality was also investigated for workers from non-operational locations (16 179 males, 6148 females) (not shown in tables). Overall mortality was significantly below expectation in males and females. Significant excesses were found in males for cancers of the pleura (Obs 17, Exp 8.6, SMR 197) and brain (Obs 55, Exp 36.0, SMR 153). Among female workers, there was an excess of deaths from cancer of the gall-bladder (Obs 5, Exp 1.5, SMR 326).


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  		Table 4. UK study of electricity generation and transmission workers: male substation and transmission workers, 1973–2002 (n = 3036)

 
Table 5 shows all-causes mortality by year of hire, time since first employment and year of death for the total cohort. The SMRs in the most recent period of follow-up (1996–2002) were similar to those in preceding years, indicating that inadequate tracing of deaths was not a problem in this study. The pattern of all-causes mortality with period from first employment indicates that a healthy worker effect may be present in this cohort, particularly in females.


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  		Table 5. Mortality from all causes by year of commencing employment, period from hire and year of death: study of UK electricity generation and transmission workers, 1973–2002 (n = 83 923; male = 72 889, female = 11 034)

 
The excess of deaths from cancer of the pleura among males first employed at power station sites is further investigated in Table 6. This table follows the format of the previous table. A highly significant negative trend in the pattern of SMRs by year of hire was found. Earlier decades of hire were associated with higher SMRs, although mortality rates remained significantly above expectation for more recent decades of hire. A monotonic positive trend was found with time since first employment. A similar analysis was also carried out for asbestosis (ICD-9: 501) in male power station workers (not shown in table) for which there was a highly elevated SMR (Obs 20, Exp 3.5, SMR 574, 95% CI 351–887). Although numbers of deaths were limited, the patterns of SMRs by year of hire, period from first employment and year of death were similar to those shown for cancer of the pleura.


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  		Table 6. Mortality from cancer of the pleura by year of commencing employment, period from hire and year of death: power station workers, males only, 1973–2002 (n = 49 424)

 
In the time periods covered by the 8th and 9th revisions of the ICD, death certificates recording mesothelioma (without mention of pleura or peritoneum) were classified in national statistics under the heading ‘malignant neoplasm without specification of site’ (ICD-9: 199). In this study there were 532 deaths classified to the latter rubric. Following a review of these death certificates, 109 (20.5%) were found to be mesotheliomas; 85 occurred in power station workers.


   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conflict of interest
 References
 
All-causes mortality among this cohort of UK electricity generation and transmission workers was considerably below that which might be expected, based on national mortality rates. Indeed, the majority of SMRs for the main disease groupings were significantly below 100. These findings are consistent with those of other cohorts of electrical utility workers [2,3,11], and are likely to be due, at least in part, to the ‘healthy worker’ effect. In addition, the workforce would be described as skilled rather than semi-skilled or unskilled and the influence of socio-economic status is likely to have had a beneficial effect on overall mortality. It has to be accepted that relying on comparisons with national rates can lead to health effects being missed.

Cancer of the pleura was significantly elevated in male workers in two of the three occupational subgroups examined. For men first employed in power stations, the patterns of SMRs with time since first employment and decade of hire suggest that occupational exposure in previous decades has influenced the risk of this disease. Asbestos exposure is a well-established risk factor for mesothelioma and asbestos was widely used in power stations for lagging pipes and boilers. It should not be thought that the SMRs for cancer of the pleura have been underestimated by the non-inclusion of unspecified mesotheliomas; such cancers are excluded from both the observed and expected numbers. Expected numbers for unspecified mesotheliomas will not be trivial; national mortality statistics for England and Wales in 2001 make use of ICD-10 (a classification system providing more detail on mesothelioma deaths) and indicate that some 61% of all mesotheliomas are reported without specification of site. Most of the large cohorts of electric utility workers do not report pleural cancers separately from other respiratory cancers, but the study that did also found elevated risks of this disease [4]. It seems reasonable to conclude, therefore, that most of the excess for cancer of the pleura in power station workers is a consequence of asbestos exposure.

Initially, it was more difficult to conclude that the excess of mesothelioma deaths among men employed in non-operational locations was a result of asbestos exposure received while working in the industry. Use of first known job, rather than first job, may, however, have given rise to misclassification of employment group. Further details of job histories were sought, therefore, from company records for the 17 male workers from non-operational locations who died from mesothelioma. Additional information was found for 11 of these workers; this information indicated that for all 11 workers there was a potential for asbestos exposure at other sites and facilities. Four workers were found to have had potential asbestos exposure from being involved with power station construction and commissioning, two from occasional work at power stations, four from earlier periods of employment at power stations and one further worker may have been exposed from earlier employment in other industries.

Exposure to asbestos, either while working in the industry or in previous employment, is also the most probable explanation for the excess of deaths from cancer of the peritoneum in male power station workers. The underlying cause of death for 13 of the 14 subjects in this group was described as peritoneal malignant mesothelioma. Asbestos exposure is also an established risk factor for mesothelioma of the peritoneum [12], and such mesotheliomas have been associated with heavier exposures than pleural cancers [13]. It is not possible to compare the UK findings for peritoneal cancer with corresponding findings from other large cohort studies of electric utility workers as the latter do not report cancers of the peritoneum separately from other digestive cancers [1–4].

Elevated mortality rates of cancer of the breast were found in male power station workers. Whilst several studies of workers potentially exposed to EMFs have reported increased risks of male breast cancer [14–17], three cohort studies of electric utility workers reported no overall excess for this disease [2–4], and it would be unwise to attach too much importance to the non-significant excess shown in the present study.

An excess of brain cancers was found among male staff employed in non-operational locations. The relationship between occupational EMF exposure and brain cancer has been investigated in many studies although a causal relationship has not been established. Brain cancer risks in this cohort have been examined in detail previously in relation to cumulative magnetic field exposure; no association was found [6]. There was, however, a much higher percentage of professional, administrative and clerical workers at non-operational locations (80.9%) than in other sectors of the industry (24.2%), and an elevated rate of brain cancer in UK professional and administrative workers was highlighted in the 1995 Decennial Supplement on Occupational Health [18].

A statistically significant excess of deaths from cancer of the gall-bladder, based on only five deaths, was found among females from non-operational locations. A non-significant excess was also found among males in the same work locations. These may well be chance findings, given the large number of significance tests carried out for this report. No causative occupational factors have been established for this disease.

There are limitations to be attached to this work. First known job has been used to categorize individuals on their working location, and more than half of the cohort (55%) had some employment within the industry before personnel records were computerized. It is almost certain that misclassification will have occurred for some employees. The aim of this report was, however, to examine whether mortality rates for any cause of death needed to be investigated further; mortality was much lower than expected for most causes. The elevated mortality found for pleural and peritoneal cancers (close to 2% of all deaths in power station workers when unspecified mesotheliomas are included) almost certainly reflects, at least in part, the late health effects of earlier incidental asbestos exposure. More sophisticated analyses of the overall mesothelioma excess, involving retrospective quantitative assessments of asbestos exposure, would probably make a negligible contribution to what is known about mesothelioma risks in relation to asbestos exposure, and such analyses are not planned.


   Conflict of interest
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflict of interest
References
 
None declared.


   Acknowledgements
 
We thank Peter Brodie, formerly of British Energy, for supplying copies of the study databases, Gerry Harte for her earlier involvement in this study, the ONS for follow-up details, Gordon Neale and Tony Lounsbach for classification of work histories, the pensions and human resources departments of the participating companies for solving data queries and Margaret Williams for word processing. Survey costs were defrayed by a research award from companies in the electricity industry.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflict of interest
 References
 

  1. Thériault G, Goldberg M, Miller AB et al. Cancer risks associated with occupational exposure to magnetic fields among electrical utility workers in Ontario and Quebec, Canada and France: 1970–1989. Am J Epidemiol 1994;139:550–572.[Abstract/Free Full Text]

  2. Savitz DA, Loomis DP. Magnetic field exposure in relation to leukaemia and brain cancer mortality among electric utility workers. Am J Epidemiol 1994;141:123–134.

  3. Kelsh MA, Sahl JD. Mortality among a cohort of electric utility workers, 1960–1991. Am J Ind Med 1997;31:534–544.[CrossRef][Web of Science][Medline]

  4. Johansen C, Olsen JH. Risk of cancer among Danish utility workers—a nationwide cohort study. Am J Epidemiol 1998;147:548–555.[Abstract/Free Full Text]

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  6. Sorahan T, Nichols L, van Tongeren M, Harrington JM. Occupational exposure to magnetic fields relative to mortality from brain tumours: updated and revised findings from a study of United Kingdom electricity generation and transmission workers, 1973–1997. Occup Environ Med 2001;58:626–630.[Abstract/Free Full Text]

  7. Sorahan T, Nichols L. Mortality from cardiovascular disease in relation to magnetic field exposure: findings from a study of UK electricity generation and transmission workers, 1973–1997. Am J Ind Med 2004;45:93–102.[CrossRef][Web of Science][Medline]

  8. National Radiological Protection Board. ELF Electromagnetic Fields and the Risk of Cancer: Report of an Advisory Group on Non-Ionising Radiation. Chilton, Oxfordshire: NRPB, 2001.

  9. International Agency for Research on Cancer. IARC Monographs on the Evaluation of the Carcinogenic Risks to Humans. Non-Ionising Radiation, Part 1: Static and Extremely Low Frequency (ELF) Electric and Magnetic Fields. Vol. 80. Lyon: IARC, 2002.

  10. Coleman M, Douglas A, Hermon C, Peto J. Cohort study analysis with a Fortran computer program. Int J Epidemiol 1986;15:134–137.[Abstract/Free Full Text]

  11. Baris D, Armstrong BG, Deadman J, Thériault G. A mortality study of electrical utility workers in Québec. Occup Environ Med 1996;53:25–31.[Abstract/Free Full Text]

  12. International Agency for Research on Cancer. IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Man. Asbestos. Vol. 14. Lyon: IARC, 1977.

  13. Britton M. The epidemiology of mesothelioma. Semin Oncol 2002;29:18–25.

  14. Tynes T, Andersen A, Langmark F. Incidence of cancer in Norwegian workers potentially exposed to electromagnetic fields. Am J Epidemiol 1992;136:81–88.[Abstract/Free Full Text]

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  18. Office of Population Censuses and Surveys. Occupational Health Decennial Supplement. The Registrar General's Decennial Supplement for England and Wales. Series DS No. 10. London: HMSO, 1995.


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