Occupational Magnetic Field Exposure, Cardiovascular Disease Mortality, and Potential Confounding by Smoking
Introduction
Cardiovascular disease (CVD) is a significant worldwide public health problem and the leading cause of death in the United States (1). Numerous modifiable and non-modifiable risk factors and contributing factors have been identified. Among the many identified and suspected risk factors of cardiovascular morbidity and mortality are age, dyslipidemia, hypertension, obesity, alcohol and substance abuse, smoking, and lack of physical activity 2, 3, 4, 5, 6, 7, 8. Several of these risk factors may be confounders in the relationship between other risk factors and CVD. For example, obesity is a risk factor for CVD, independently as well as indirectly through hypertension and high serum cholesterol (6).
Occupational exposures have been associated with CVD. Exposures include solvents, particulate products of combustion, and diesel exhaust in a variety of occupations 9, 10, 11, 12, 13, 14. Noise, shift work, long hours in a sitting posture, temperature extremes, and vibrations experienced in the workplace have also been associated with CVD 15, 16, 17.
Recent studies have examined occupational exposure to magnetic fields (MF) as a possible risk factor for cardiovascular morbidity and mortality 18, 19, 20, 21, 22. The hypothesis links exposure to MF with decreased heart rate variability, an indicator of altered autonomic nervous system control of the heart, leading to increased risk of acute cardiac diseases, such as acute myocardial infarction (AMI) and arrhythmia (18). Experimental evidence involving human volunteers suggests that exposure to MF may interact with the autonomic nervous system's control of the heart resulting in decreased heart rate variability (23). Reduction in heart rate variability has been identified as a risk factor for acute cardiac morbidity and mortality 24, 25. This hypothesis was initially examined in an epidemiologic study among electric utility workers, which reported an association between occupational MF exposure and increased mortality due to AMI and arrhythmia, but no increase for chronic coronary heart disease (CCHD) and atherosclerosis (18). These relationships, however, were not observed in a separate electric utility worker study (21). Additional epidemiologic studies also showed conflicting results 19, 20, 22, 26.
One common weakness of many occupational epidemiologic studies is the lack of control for potentially confounding medical and lifestyle-related variables. It may be assumed that some of these cardiovascular risk factors are independent of occupation; therefore, they are probably not confounders in an epidemiologic study. However, some of these factors, mainly those associated with lifestyle, may be associated with occupation. Smoking, for example, has been described as more prevalent in occupations with greater exposure to workplace hazards, and thus the relationship between disease and occupation may be confounded (27). Further, smoking prevalence declined in numerous occupations from the 1980s to the 1990s, though it has remained highest in blue-collar positions (28). To date, a subset of studies of occupational exposure and CVD outcomes have controlled for smoking 13, 14, 15, 19. Smoking has been described as a risk factor to be controlled in occupational studies of MF and CVD 18, 21, but most previous studies have not had enough information to conduct such analyses. The amount of bias that results from not accounting for smoking depends on the strength of the association between smoking and CVD and the correlation between smoking and specific occupational groups. While smoking has frequently been evaluated using indirect adjustment methods 29, 30, 31, we used the National Mortality Followback Study (NMFS) database to directly evaluate the potential for confounding in the relationship between occupational exposure and CVD-specific mortality.
The purpose of the current study was, therefore, to evaluate whether NMFS data suggest a link between occupational MF exposure and CVD, and whether smoking confounds this relationship.
Section snippets
Subjects and Methods
Our analyses made use of data from the National Mortality Followback Survey in 1986 and 1993 32, 33. The 1986 and 1993 surveys were the most recent in this series conducted by the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention. The data sets include information on a probabilistic sample of individuals aged 25 years or older (15 years or older in 1993) who resided and died in the United States in 1986 (excluding Oregon) and in 1993 (excluding South
Results
The NMFS data sets included 18,733 death certificates from 1986 and 22,957 death certificates from 1993, representing a systematic sample of 1,986,869 and 2,215,440 registered deaths in those years in the United States. Table 1 shows the distribution of deaths, for these 2 years, by cause of death, sex, race, and level of education. In addition to the actual number of death certificates included in the surveys, we estimated the number of deaths they represented after taking the surveys'
Discussion
Adjustment for risk factors traditionally used in occupational epidemiologic studies resulted in small changes in the associations between occupational MF exposure and each of the four cardiovascular outcomes as reflected in the NMFS data. Additional adjustment for smoking behavior resulted in only minimal further changes in the associations between these causes of mortality and occupational MF exposure. Similar numbers of cigarettes per day were smoked by the workers in the medium and high MF
References (42)
- et al.
Obesity, physical inactivity, and risk for cardiovascular disease
Am J Med Sci
(2002) - et al.
Risk stratification of obesity as a coronary risk factor
Am J Cardiol
(2002) - et al.
Reproductive hormones and cardiovascular disease: Mechanisms of action and clinical implications
Obstet Gynecol Clin N Am
(2002) - et al.
Changes in cardiovascular disease risk factors among American Indians: The Strong Heart Study
Ann Epidemiol
(2002) - et al.
Decreased heart rate variability and its association with increased mortality after acute myocardial infarction
Am J Cardiol
(1987) - et al.
The confounding of occupation and smoking and its consequences
Soc Sci Med
(1990) Deaths: Leading causes for 2000. National vital statistics report. v. 50, no. 16
(2002)- et al.
Primary prevention of coronary heart disease: Guidance from Framingham. A statement for healthcare professionals from the AHA task force on risk reduction
Circulation
(1998) - et al.
Risk factors for congestive heart failure in US men and women. NHANES I epidemiologic follow-up study
Arch Intern Med
(2001) - et al.
Smoking, blood pressure, and serum cholesterol—Effects on 20-year mortality
Epidemiology
(2003)
Solvent exposure and cardiovascular disease
Am J Ind Med
Cardiovascular disease and work place exposures
Arch Environ Health
Mortality pattern in the cohort of workers exposed to carbon disulfide
Int J Occup Med Environ Health
Industrial noise exposure and risk factors for cardiovascular disease: Findings from the CORDIS study
Noise Health
Risk factors of coronary heart disease among personnel in a bus company
Int Arch Occup Environ Health
A population-based case–referent study of myocardial infarction and occupational exposure to motor exhaust, other combustion products, organic solvents, lead, and dynamite
Epidemiology
Cardiovascular dysfunction due to shift work
J Occup Environ Med
Chronic noise exposure and the cardiovascular system in aircraft pilots
Med Lav
Municipal employees' cardiovascular diseases and occupational stress factors in Finland
Int Arch Occup Environ Health
Magnetic field exposure and cardiovascular disease mortality among electric utility workers
Am J Epidemiol
Occupational exposure to extremely low-frequency magnetic fields and mortality from cardiovascular disease
Am J Epidemiol
Cited by (8)
A Population-Based Cohort Study of Occupational Exposure to Magnetic Fields and Cardiovascular Disease Mortality
2009, Annals of EpidemiologyCitation Excerpt :Subsequent studies have attempted to reproduce this finding but without success. This inconsistency has been attributed to potential confounding and other biases (2–5). The majority of studies investigated industry-based cohorts and thus had limited ability to adjust for potentially important demographic factors and CVD risk factors such as cigarette smoking.
Occupational exposure to extremely low-frequency magnetic fields and cardiovascular disease mortality in a prospective cohort study
2013, Occupational and Environmental MedicineAssessment of occupational exposure to extremely low frequency magnetic fields in hospital personnel
2011, BioelectromagneticsIntracellular Ca<sup>2+</sup> levels in rat ventricle cells exposed to extremely low frequency magnetic field
2011, Electromagnetic Biology and MedicineCell-phone use and self-reported hypertension: National health interview survey 2008
2011, International Journal of HypertensionCardiovascular mortality and exposure to extremely low frequency magnetic fields: A cohort study of Swiss railway workers
2008, Environmental Health: A Global Access Science Source
This work was supported by the Electric Power Research Institute (EPRI). The analyses were conducted and the conclusions were reached by the authors, and not by the National Center for Health Statistics, which is responsible only for the initial data.