American Journal of Epidemiology Copyright © 2002 by the Johns Hopkins Bloomberg School of Public Health All rights reserved
Vol. 156, No. 9 Printed in U.S.A. DOI: 10.1093/aje/kwf137
Risk of Severe Cardiac Arrhythmia in Male Utility Workers: A Nationwide Danish Cohort Study
Christoffer Johansen1, Maria Feychting2, Mogens Møller3, Per Arnsbo3, Anders Ahlbom2, and Jørgen H. Olsen1 1
Institute of Cancer Epidemiology, Danish Cancer Society, DK-2100 Copenhagen, Denmark. Institute of Environmental Medicine, Karolinska Institutet, S-171 77 Stockholm, Sweden. 3 Danish Pacemaker and ICD Register, Department of Cardiology, Odense University Hospital, 5000 Odense C, Denmark. 2
Received for publication March 18, 2002; accepted for publication July 16, 2002.
arrhythmia; electricity; electromagnetic fields; occupational exposure; pacemaker, artificial
In a cohort study of 138,903 workers at five large electric utility companies in the United States, Savitz et al. (1) found an increased rate of mortality from arrhythmia-related conditions and acute myocardial infarction among workers with a long duration of work (rate ratios were 1.4–1.5 for the longest employment intervals) and workers with high exposure to magnetic fields (rate ratios were 1.6–2.4 in the highest exposure category). The study hypothesis was based on observations made in a double-blind laboratory investigation in which exposure to 20 µT of intermittent 60-Hz magnetic fields was found to reduce the normal variation of the heart rate reversibly (2). Several prospective cohort studies have indicated that reductions in some components of the variation in heart rate increase the risk of heart disease (3–6), increase the overall mortality rate among survivors of myocardial infarction (7–10), and increase the risk of sudden cardiovascular death (11). Contrary to the results of Savitz et al., in a nationwide Danish cohort study of male utility
employees, Johansen and Olsen (12) observed no increase in the rate of mortality from cardiovascular disorders, including acute myocardial infarction and cardiac arteriosclerosis, by duration of employment or by estimated average exposure to 50-Hz magnetic fields at work. A permanent pacemaker is implanted for the treatment of severe symptomatic bradycardias such as atrioventricular block, sinus node dysfunction, and atrial fibrillation with bradycardia. Using data from the Danish Pacemaker Register, we determined the frequency of pacemaker implantation in a large cohort of male electric utility employees to investigate their incidence of severe cardiac arrhythmia and to compare it with rates of this procedure in the general population. The frequency of implantation was examined with regard to duration of employment and estimated average level of exposure to electromagnetic fields at work.
Reprint requests to Dr. Christoffer Johansen, Institute of Cancer Epidemiology, Danish Cancer Society, Strandboulevarden 49, DK-2100 Copenhagen, Denmark (e-mail:
[email protected]).
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Am J Epidemiol 2002;156:857–861
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To address concern about the potential cardiovascular effects of occupational exposure to electromagnetic fields in the 50- to 60-Hz frequency band, the authors launched an epidemiologic study of the incidence of severe cardiac arrhythmia, as indicated by the need for a pacemaker, in a nationwide cohort of Danish utility workers. The cohort of 24,056 men employed at utility companies between 1900 and 1993 was linked to the nationwide, population-based Danish Pacemaker Register, and the numbers of persons who had undergone pacemaker implantation between 1982 and 2000 were compared with corresponding numbers in the general population. In addition, the data on the utility workers were fitted to a multiplicative Poisson regression model in relation to estimated levels of exposure to 50-Hz electromagnetic fields. Overall, based on 135 men with pacemakers (140 expected), there was no increased risk of severe cardiac arrhythmia among the utility employees; the risk estimate was 0.96 (95% confidence interval: 0.81, 1.14). No clear dose-response pattern emerged with increasing levels of exposure to electromagnetic fields or with duration of employment. These results are largely reassuring, since they do not support the hypothesis of a link between occupational exposure to electromagnetic fields and an excess risk of severe cardiovascular arrhythmia leading to permanent implantation of a pacemaker.
858 Johansen et al. MATERIALS AND METHODS Study population
Work area (division of utility company)
Job title(s)
High exposure (≥1.0 µT) Overhead cable department (production and transmission)
Cable splicer Engine mechanic Engineer
Medium exposure (0.1–0.99 µT) Cables and overhead lines (distribution)
Worker Cable splicer, master Lineman, master
Workshop (distribution)
Engine mechanic Electrical mechanic Technical serviceman
Background exposure (≤0.09 µT) Administration (distribution)
Cleaning service worker Administration, manager Engineer, manager
The Danish Pacemaker Register
The population-based Danish Pacemaker Register was established in 1982. Since its inception, all of the data in the register have been validated and are considered 100 percent complete (15). All 12 medical centers that implant pacemakers in Denmark report new implantations and renewal of pacemakers or pacemaker electrodes to the Pacemaker Register on a continuous basis. Each Pacemaker Register record contains the date of implantation, the name of the hospital and cardiology department in which the implantation was performed, the type of pacemaker and lead implanted, and the patient’s Central Population Register number. Analyses
We calculated age- and calendar year-specific national rates of pacemaker implantation by dividing the number of men in each 5-year age group and calendar year in Denmark who had undergone this procedure (first known pacemaker implantation) by the corresponding mean population. Pacemaker implantation rates provide a measure of the frequency of pacemaker implantation in Denmark. We measured the association between exposure to magnetic fields and rate of pacemaker implantation as a standardized incidence ratio and assessed random variation through 95 percent confidence intervals based on the Poisson distribution (16). We calculated expected numbers of pacemaker implantations in the cohort by multiplying the number of person-years of follow-up during the period January 1, 1982–December 31, Am J Epidemiol 2002;156:857–861
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The study population comprised all male employees of the 99 utility companies that supply Denmark with electricity. The construction of the cohort has been described in detail elsewhere (13) and is summarized here. The utility companies were established between 1900 and 1979, and data on the study subjects were retrieved from 1) employment records at each company, 2) the files of the Danish Supplementary Pension Fund, which has retained employment information on all wage-earners (aged 18–66 years) in Denmark since April 1, 1964, and 3) the files of the public payroll administration of all Danish counties. With these procedures, we identified a total of 32,006 employees (26,135 men and 5,871 women) who had been employed for at least 3 months at a Danish electric utility company between 1900 and 1993. Because there were few women in the data set, the analyses were restricted to men. The information available on each employee included name, date of birth or personal identification number (a unique 10-digit number introduced in Denmark on April 1, 1968, that incorporates sex and date of birth), job task(s), date of first employment, and subsequent period(s) of employment. A job-exposure matrix specific to electromagnetic fields was designed that distinguished among 25 job titles held by workers in utility companies and 19 work areas within the utility industry. The job-exposure matrix has been described in detail previously (13). We assigned to each of the 475 combinations of job title and work area an average level of exposure to 50-Hz electromagnetic fields during a working day, which in turn was grouped into three categories of exposure to extremely low frequency electromagnetic fields— background exposure (≤0.09 µT), medium exposure (0.1– 0.99 µT), and high exposure (≥1.0 µT)—and a category with unknown exposure (14). We assigned exposure to individuals on the basis of the characteristics of the first employment held; according to the company files, only 323 men changed their job title during their employment. We assumed that category-specific levels of exposure were unchanged over the study period. Table 1 gives the estimated average levels of exposure to extremely low frequency electromagnetic fields over a full working day for various combinations of job titles and work areas. The Danish personal identification number permits accurate linkage of information between registries. Linkage of these data with the Central Population Register allowed us to verify the personal identification numbers of 26,135 male workers and provided us with information on their vital status and migration through the year 2000. With this procedure, we excluded 2,075 workers (8 percent) who had died before January 1, 1982, and four workers (
