Prevalence of Cardiac Problems Among Workers Exposed to Magnetic Fields in a High Voltage Electrical Power Station in Egypt

Waheed, Amani

Department of Community Medicine/Faculty of Medicine/ Suez Canal University, Ismailia, Egypt

+20 64 337 02 35/ amaniwaheed@yahoo.com

Gamal, Amira (*)

Department of Community Medicine/Faculty of Medicine/ Suez Canal University, Ismailia, Egypt Phone: +2 010 188 21 24; Fax. +2064 33

285 43 +20 10 188 21 24/ a_rem1@yahoo.com

Nasr, Gamela M.A.

Department of Cardiology/Faculty of Medicine/Suez Canal University, Ismailia, Egypt

+20 10 190 47 02/ gam_nasr@yahoo.com

Moustafa, Hassan M.

Studies and Research Department/Egyptian Electricity Holding Company/Cairo, Egypt

+20 24 192 17 8/ Hassan01_Egypt@lycos.com

ABSTRACT

Background: Extremely low frequency (60 Hz, 20 G) radiation could suppress the increase in heart rate by affecting ventricular repolarization and may have a down-regulatory effect on responses of the cardiovascular system induced by sympathetic agonists. Aim of the work: This work aimed at assessing the cardiac function and associated other cardiovascular parameters of workers occupationally exposed to high voltage electromagnetic fields. Subjects and Methods: A cross-sectional study conducted at a high-voltage electrical power station. A sample of sixty male workers exposed to extremely low-frequency (50-Hz) electromagnetic radiation chosen randomly. A-self administered questionnaire was used. All participant workers were subjected to cardiac, electrocardiographic and echocardiographic examinations. Results: There were statistically significant differences regarding high density lipids and P wave dispersion. Coronary ischemia and arrhythmias in the form of atrial fibrillation were more frequent among the exposed workers. Conclusions: The study concluded that exposure to low frequency electromagnetic field has an influence on the cardiovascular system.

Key words

: Low frequency radiation, Magnetic field, Echocardiography, Electrocardiography, Cardiovascular problems, lipid profile, Egypt

INTRODUCTION

The advent of residential and industrial use of electricity for power, heating, and lighting has brought about increasing exposures over the last 120 years, from the generation, transmission, and use of electricity [1, 2]. These exposures are now a ubiquitous part of modern life, and there has been concern in some quarters that they might have adverse health effects [2]. Electric and magnetic fields (EMF) exist around electrical equipment and wiring throughout industry, as well as in outdoors and indoors environment. Workers who maintain transmission and distribution lines may be exposed to very high electric and magnetic fields. Within generating stations and substations, electric fields inexcess of 25 kV/m and magnetic fields in excess of 2 mT may be found. Also, as a result of technological modern development, there is hardly any place on earth not permeated by electric transmission lines, or a place near which there are no power stations or transmission lines in the high tension stations up to 500 KV and intermediate stations of 66 KV and low tension stations up to 11000 Volts [1]. The earliest research concerning potential occupational health effects of EMF blurred the distinction between "exposed to EMF" and simply "working in an electrical occupation." The modern era of research in occupational EMF exposure began with Milham, who compiled a list of jobs that were presumed, without empirical evidence, to incur elevated exposures to electric and/or magnetic fields, as they were thought to involve frequent or prolonged work in proximity to energized electric equipment [3]. A well-known mechanism of interaction of EMF with biological tissues is the introduction of time-varying electric currents and fields. It can produce direct stimulation of excitable tissues such as nerve and muscle cells. At the cellular level, the interaction induces voltage across the membranes of cells sufficient to stimulate nerves to conduct or muscles to contract. This mechanism accounts for humans and animals perceiving of electric currents in their bodies and experiencing electric shocks. This is the only established mechanism of action of these fields [4]. Living cells are electrically active, which makes them potentially vulnerable to electromagnetic interference. In the heart, electrical activity is crucial in coordinating the contraction of millions of cardiac cells, and disturbances in cardiac electrical activity, also known as arrhythmias, are often life threatening [5]. Over the past two decades, there has been increasing interest in the biological effects and possible health outcomes of weak, low-frequency electric and magnetic fields. Epidemiological studies on magnetic fields and cancer, reproduction and neurobehavioral reactions have been presented. More recently, neurological, degenerative and heart diseases have also been reported to be related to such electromagnetic fields [6]. Although some reports remain unconfirmed about cardiac effects of exposure to EMF [7], several reports indicate that EMF influences the cardiovascular system. Exposure of human volunteers to combined 60 Hz electric and magnetic fields (9 k V/m, 0.02 mT) resulted in small changes in cardiac function and heart rate [8]. While continuous exposure to combined electric and magnetic fields at 9 k V/m, 0.02 mT slows the heart, intermittent exposure can result in both slowing and increasing heart rate. None of the effects on heart beat exceeded the normal range [4].

Occupational exposure to power-frequency fields was associated with an increased incidence of certain types of cardiovascular disease including arrhythmia-related mortality [9]. Mortality rates of workers employed in an electric company in Quebec, were generally lower than those in the unexposed groups, including overall cardiovascular mortality [10]. Moreover, Savitz et al., (1999) showed an increased risk of death due to cardiac arrhythmias and acute myocardial infarction among utility workers [11]. However, Johansen (2004) has reported that there is no clear evidence that 50-Hz EMF is associated with cardiovascular diseases, and no increased mortality rate from cardiovascular disease was observed among employees exposed to extremely low-frequency (50-Hz) electromagnetic fields (EMF) in the Danish utility industry [12]. Also, Korpinen and Partanen (1994) have demonstrated that analysis of ECG recordings showed that the subjects' pulse rates were the same in and outside the fields [13]. Nonetheless, recent reports suggest that changes in heart rate (HR) variability may be associated with exposure to intermittent magnetic fields (60 Hz, 28.3 microT) in the laboratory. Also, mortality is increased in cardiac disease categories related to altered HR variability for utility workers whose jobs involve longer exposure to elevated magnetic fields [14, 15]. The function of the circulatory system in workers occupationally exposed to medium frequency electromagnetic fields showed that the electrocardiographic abnormalitiesdetected in the resting and/ or 24 h ECG were significantly more frequent in workers exposed to electromagnetic fields than in non-exposed subjects. A clear tendency for a higher number of rhythm disturbances, mostly extraventricular, was observed [16]. Therefore, the effects of occupational exposure to EMF are still debatable, with some controversy among different studies. This is particularly so as related to cardiovascular functions. Hence, there is need to study the relationship between heart disease endpoints and occupational exposure to EMF. This paper, focused upon the effects of electrical power lines and electrical occupations, including the biophysics of power-frequency electromagnetic fields, on the heart.

SUBJECTS AND METHODS

A cross-sectional analytic research design with an exposure and control groups (ex-post-facto design) was used in carrying out the study. The assessment of exposure and outcome were done at the same point in time, with no prospective or retrospective follow-up. The study was conducted in a high- voltage power station in Cairo, Egypt. Workers eligible for the study were those who were non-smokers, working at the station for at least one year, and not suffering from cardiac or chest diseases. All were males living in Cairo. A sample of sixty workers was chosen randomly. The duration of exposure of the exposed workers ranged from 3–20 years. Another sixty male non-smoker workers and administrative staff working away from electric siege were chosen randomly as a control group. They were also free from cardiac and chest diseases. The mean age of the exposed group was 39±7.2 years and that of the non-exposed group was 40±7.8 years, with no statistically significant difference. Induced Current Density by Magnetic Field: Electric power transmission lines operate at very high voltages (usually >=50,000 volts, abbreviated 50 kilovolts or 50 kV) and may carry currents of many hundreds of amperes. Thus, these lines can produce strong electric and magnetic fields. The exterior walls and roofs of most homes are fairly effective shields for electric fields, but they have little, if any, effect on the magnetic fields produced by power lines. The magnetic field produced by a three-phase transmission line outside its right-of-way is where most human exposure occurs. The most common transmission line configuration has all three conductors arrayed in either a horizontal or a vertical plane. The relation between the induced current density and the external field is complex. It depends not only on the field, but also on its frequency, its orientation with respect to the body, the size and shape of the body, and the position within it at which the current density is of concern [17, 18]. Since the basic restriction on current density was derived as an order of magnitude value, simple models are often used to relate the induced current density, JB (A/m2), to the external magnetic field B(T), JB=П fBr ơ, where f(Hz) is the frequency, and R(m) is the effective radial position within the body. Ơ is the effective tissue conductivity, typically 0.2 s/m [19]. Electrical Gride Configuration: The Structure of the electric power or energy system is very large and complex. Nevertheless, it can be divided into basically five main stages, or components, or subsystems. These are the energy source (fuel), the energy converter (generation system), the transmission system, the distribution system, and the load (energy sink). Workers under electric siege: these are workers who have become surrounded by electromagnetic waves everywhere, such as power lines, overhead transmission lines, and electrical power substation. In this work, this has been defined as a certain area called Canal area in the Egyptian Unified Network. Worker’s exposure time approximated 8 hours per day for 6 days per week. A self administered questionnaire used to collect data about personal characteristics, occupational history, and occurrence and frequency of the cardiac and chest symptoms, if present. The form also included a section for complete general and cardiologic examination. Detailed electrocardiographic studies were done with resting 12-lead electrocardiography. Echocardiographicevaluation was performed using a Hewlett-Packard phased array (sons 1800, USA made, model: DR 53 15) ultrasonoscope with a 2.5 and 3.5 MHz transducer. Measurements of ventricular septum, posterior wall, and left ventricular cavity, left ventricular ejection fraction and left atrium dimensions were performed according to the American Society of Echocardiography criteria [20]. Left ventricular mass was calculated from left ventricular end-diastolic cavity, and septal and posterior wall thickness using the Penn convention and the American Society of Echocardiography guidelines [20,21]. Left ventricular mass index (LVMI) was determined as the ratio of left ventricular mass in grams to the body surface area in square meters. The relative wall thickness (RWT) was measured at the end-diastole as the ratio of posterior wall thickness plus septal thickness divided by left ventricular internal dimension. The transmitral flow velocity profile was recorded from the apical four-chamber view with the pulsed wave Doppler sample volume positioned at the tips of mitral leaflets during diastole. The left ventricular outflow velocity pattern was recorded from the apical long axis view with the pulsed wave Doppler sample volume positioned just below the aortic valve. Five consecutive beats were measured and averaged for each measurement. From the mitral inflow signal we measured the E velocity (E), the A velocity (A), and the E/A ratio.

Carotid intima media thickness was measured. It is a non-invasive quantitative measurement of the presence of premature atherosclerotic disease [22]. Left common carotid artery diameter was scanned by using B-mode ultrasound [23, 24]. The image was focused on the posterior (far) wall, and images of the distal 10 mm of the common carotid artery were recorded from the angle showing the greatest distance between the lumen-intima interface and the media-adventitia interface. Several end-diastolic frames (at the R-wave in the ECG recording) were selected for mean IMT. P wave dispersion constitutes a recent contribution to the field of noninvasive electrocardiology and is defined as the difference between the longest and the shortest P-wave duration recorded from multiple different surfaces ECG leads. This parameter was adopted assuming that surface P waves in different locations could be affected to a different extent by regional changes in atrial activation times. P-wave dispersion has proved to be a sensitive and specific ECG predictor of atrial fibrillation in the various clinical settings. A cutoff value of 40 msec seems to furnish the best predictive accuracy [25]. QT dispersion was defined as the difference between the maximum and minimum QT intervals. The QT dispersion was calculated at rest. A cutoff resting value of 60 msec seems to be accurate [26]. Lipid profile was also done for the studied sample. Levels of total cholesterol, high density lipids, and triglycerides were measured by a standard multichannel analyzer on all fasting specimens. Low density lipoproteins levels were calculated using other lipid fractions. All exposed workers and non-exposed subjects at work were informed about the aim of the study. Their verbal consent for participation was obtained. Complete confidentiality of any obtained information was secured. Professional medical services were provided to workers in case of need. Initial comparisons between exposed workers and that comparison group were done using the student’s t-test for continuous variables and Pearson’s Chi square test for categorical variables. The adjusted risk factors for cardiovascular problems were obtained using the logistic regression analysis. The dependent variable was the presence and absence of exposure at work to electromagnetic field in all the subjects. All variables described previously were considered as possible candidates for the final model. The initial multivariable model construction consisted in the preliminary selection of variables using a manual purposeful selection method and a relatively large significance level (alpha approximately 0.25). Subsequently the resulting model was reduced using a likelihood ratio test with a significance level of 0.05. Before accepting a final model, the interactions as well as confounding were evaluated. All statistical analyses were performed using the Statistical Package for Social Science (SPSS) version 11.0.

RESULTS

Sixty work exposed workers were exposed to low frequency magnetic fields and sixty work not exposed workers from administrative areas in the same plant considered as comparison group were included in our study. All work exposed workers and their comparison group were males. Table 1 shows the characteristics of the studied groups, the mean age in years of work exposed workers was 41.5±9.2 compared to 40.6±8.4 in work not exposed workers. Mean of heart rate among work exposed workers was 66.9±6.5 beats per minute compared to 71.9±6.9 beats per minute among work not exposed workers with p= 0.000. among work exposed workers, 26.7% have a high systolic blood pressure, while among work not exposed workers 21.7% have high systolic blood pressure with p= 0.67. Table (2) revealed ECG findings, P wave dispersion in mesc among work exposed workers 8 (13.3%) with a mean of 37.8±3.3 compared to 34.2±2.4 among comparison group. Also, atrial fibrillation and ischemia were found in both groups, while QT dispersion in mesc was normal among both groups. P wave dispersion was found among 8 (13.3%) work exposed workers. Echocardiography consists of several aspects, one of them is systolic function which is measured by left ventricular ejection fraction, exposed group were normal, while among control 1 (1.7%) was found. The second aspect, is diastolic function which measured by isovolumic relaxation time and E/A ratio, the former one was normal among both groups, while the later is shown in table (3). Dimensions are one of the parameters measured by echocardiography represented by left ventricular diastolic diameter, right ventricular end diastolic dimension, left ventricular systolic diameter, left atrium and aortic dimension, they were normal in both groups. Regarding measuring hypertrophy, left ventricular mass index, posterior wall and septum were normal among the studied population, while relative wall thickness showed abnormalities among the two studied groups with no statistical significant difference as shown in table (3). Carotid intima media thickness was normal among the studied groups. Lipid profile findings are presented in table (4), low density lipoproteins and triglycerides levels were high among both groups. The multivariate logistic regression model has identified cholesterol and high density lipoproteins as the independent risk factors of cardiac problems (table 5).

DISCUSSION

The possible influence of electromagnetic field on the heart has been a subject to a lot of research. The exposure is complex and multifaceted because of the cyclical patterns. Although one can readily determine an individual's job title, or even the environment in which the worker spends time, determining the actual exposure to various forms of EMF is a major challenge [27]. The current study results have revealed statistically significant difference between the work exposed workers and work not exposed workers regarding heart rate (HR) though it was within normal range. Meanwhile, Graham and co workers (2000) have claimed that a biologic mechanism that could provide the necessary link between exposure to power-frequency magnetic fields and alterations in human physiology is not known. Cellular activity or function, however, may be modulated by the electric fields induced in the body by exposure to the ambient magnetic field. They added that the biophysical issue is whether the intensity of the generated magnetic field presented in the studies examined is of sufficient strength to alter the endogenous electric fields generated by the heart [2]. According to Sait et al, (2005) the reported effects on heart rate (HR) are so small, procedures which can provoke changes in the sympathovagal balance in a controlled manner may have a greater capacity for identifying subtle field-related changes, if they do exist [28]. Investigation of HR and heart rate variability (HRV) spectral indices in 20 volunteers subjected to a tilt from the supine position to 60 degrees was performed, although the anticipated significant changes in HR and the high frequency (HF), low frequency (LF) and LF/HF ratiooccur with tilting, there were no significant differences between corresponding measures with and without exposure to magnetic fields [28]. In support of these findings, Gadzicka et al, (1997) explained the decreases in heart rates by the occurrence of disorders in the neurovegetative regulation [29]. Korpinen et al. (1993) also found a small decrease in heart rate, which was observed after field exposure [30].

To the contrary, some previous studies concluded that there were no differences between the exposed and the control groups [13, 31]. Also, Baroncelli et al. (1986) has added that workers exposed to electromagnetic fields of moderate strength do not show the presence of clear effects on their state of health, including cardiovascular status [7]. The foregoing present study finding might be explained by Kurokawa et al (2003) suggestion that the ELF-MF to which humans are exposed have no acute influence on the activity of the cardiovascular autonomic nervous system that modulates the heart rate [31]. Regarding blood pressure (BP), no statistically significant differences could be detected between work exposed workers and work not exposed workers. This is in agreement with Gadzicka et al. (1997) who revealed that the mean arterial blood pressure and the day/night blood pressure variability indicator showed no significant differences between the exposed and unexposed groups [29]. On the same line, Korpinen and Partanen (1994) could not show that the fields (< 4.3 kV/m and < 6.6 microT) affects diastolic or systolic blood pressure [13]. More recently, Ghione et al. (2005) reported that there were no effects related to BP or HR among workers exposed to EMF [32]. Since cardiovascular regulation is functionally related to pain modulation, they suggested that these results might be explained by a modulation of sensory gating processes through the opioidergic system, which in turn is influenced by magnetic exposure. Conversely, Mitrov et al, (1990) have reported a tendency to hypotension in exposed workers [33].

Regarding arrhythmias in the form of atrial fibrillation, there was a trend towards statistically significant difference between work exposed workers and work not exposed workers, with a higher frequency among the former group (p=0.061), although it did not reach statistical significance, the difference was of clinical significance, results could need more clarification and bigger sample size to find out the cause of these findings and its relation with the magnetic field exposure. Another study found a clear tendency for a higher number of rhythm disturbances, mostly extra-ventricular, in an AM broadcast station workers [34]. Moreover, a study by Dyachenko (1970) found that exposed subjects experienced more heart rhythm disturbances, impaired conduction, lowered ECG amplitude, particularly of the T wave peak and arterial blood pressure abnormalities [35]. Hence, it has been reported that in workers of electromagnetic stations, under conditions of exposure to electric fields, the risk of electrocardiographic disturbances was increased by 10% [36]. The finding is in agreement with Korpinen et al. (1993) who have also demonstrated from analysis of ECG recordings that extrasystoles or arrhythmias were as frequent outside the field as in the field among transmission-line exposed workers [30]. Also, the electrocardiographic abnormalities detected in the resting and/or 24- hour ECG were statistically significantly more frequent (p=0.006) in workers exposed to electromagnetic fields, compared to unexposed subjects, 75% and 25%, respectively.

A recent parameter of early atrial fibrillation, P-wave dispersion, was shown to be higher among work exposed workers. This parameter was adopted with the assumption that surface P-wave in different locations could be affected to a different extent by regional changes in atrial activation. Thus, it could be a sensitive and specific new ECG predictor of AF. To-date, the most extensive clinical evaluation of P-wave dispersion has been performed in the assessment of the risk for AF in patients without apparent heart disease [37, 38]. Meanwhile,as regards QT dispersion it was normal among the two groups. Increased QT dispersion on the surface electrocardiogram (ECG) is generally attributed to heterogeneity of ventricular repolarization [39]. Nevertheless, P wave and QT dispersion and carotid intima media thickness have not been studied till now, up to our knowledge, in workers exposed to electromagnetic field. They may denote early changes in arrhythmic tendency and premature atherosclerosis. The current study showed that coronary ischemia was more frequent among the exposed workers. The difference was of clinical significance, although it did not reach statistical significance. The findings are in congruence with previous studies. Thus, on studying occupational exposure to extremely low frequency magnetic fields and mortality from cardiovascular disease, Hakansson and co workers (2003) found that there was no association with ELF magnetic fields and arrhythmia-related death, ischemic heart disease other than AMI (acute myocardial infarction), and atherosclerosis [40]. They found a low-level increase in AMI risk in the highest exposure group and suggestions of an exposure- response relation. Similarly, a more recent study could not support the hypothesis that occupational EMF exposure increases the risk of myocardial infarction [41]. This could be due to alterations in coronary microvasculature in response to exposure to magnetic field. Regarding echocadiographic parameters, statistically significant differences were revealed in the present study between work exposed workers and work not exposed workers. These were mainly in Relative wall thickness and E/A ratio, which indicate left ventricular dysfunction. This might be related to the possible ischemic changes associated with exposure. In this respect, Bortkiewicz et al. (1996) concluded that exposure to medium frequency magnetic fields may affect the function of autonomic nervous system [16]. Similarly, ischemic changes were confirmed in other studies [42, 9].

In the current study there was a tendency towards a better lipid profile in the work exposed workers than in the work not-exposed workers which could be attributed to having a better lifestyle habits and being having less sedentary life. Lipid changes with dyslipidemias were confirmed in a study by Chernysheva (1990) in agreement with the current study [43]. Additionally, in the early stages of exposure to the effect of extreme factors the change in lipid metabolism was monotypic in character and was manifested by hypercholesterolemia at the cost of the free fraction, increase of the level of triglycerides, free fatty acids and activity of lecithin-cholesterol acyltransferase, and reduction of the content of total phospholipids and disorders of their composition. These changes are associated with disturbed neurohumoral regulation and reflect the unspecific response of the organism to damage in the form of mobilization of lipids from the depot and switching metabolism from the carbohydrate to the lipid type [44]. Contrary to our results, Radhakrishnan and McConnell (2000) it is reported here that an externally applied electric field of the appropriate sign can destabilize these complexes, resulting in their dissociation [45]. Nevertheless, in international guidelines, limits for restrictions of field exposure are several orders the magnitude above what can be measured from overhead power lines, and those found in "electrical" occupations. These guidelines emphasize that the state of scientific knowledge today does not warrant limiting exposure levels for the public and the workforce, and that further data are required to confirm whether health hazards are present. In some countries, however, the "principle of caution" or "prudent avoidance" has been adopted; meaning the low-cost avoidance of unnecessary exposure as long as there is scientific uncertainty about its health effects [6].

The study had some limitations. Given the cross sectional design used, causation of detected heart problems cannot be claimed. Presence of other risk factors for ischemic heart disease as obesity and overweight were not considered in this study, in addition to the small sample size of our study. Moreover, the exposure of interest is imperceptible, ubiquitous, originates from multiple sources, and can vary greatly over time and over relatively short distances. So,a follow-up study is planned, and further studies should be invited. The present study has shown that exposure to EMF from power lines may affect the heart. There are increased frequencies of premature atherosclerosis (altered lipid profile), arrhythmia mainly atrial (increased P-wave dispersion and frequent atrial fibrillation), and ischemic changes. Therefore, there is a great need to increase clinical awareness of heart disease among exposed electromagnetic fields workers. Identification and definition of those who are at risk are key issues. Regular monitoring of the workplace environment is mandatory to adopt strict health-based EMF exposure limits. Pre-placement and periodic examinations should be done regularly for workers in high voltage electrical power lines stations. Information and training of workers should be provided. Since electricity plays a major role in the development of countries, its use cannot be avoided. It is thus most important to improve electrical safety standards and to observe all preventive measures in order to save lives, to avoid or reduce hazards, and hence to raise productivity. Practical mitigation solutions should be considered in relation to application of principles of prudent avoidance, correct grounding and wire-loop problems and use of dielectric couplers on water service lines to eliminate plumbing currents, magnetic shielding using aluminum, low carbon steel, silicon-iron steel, and active magnetic field cancellation.

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Table (2): Electrocardiography findings among the studied groups

Table (3): Significant echocardiography changes among the studied groups

Table (5): Factors associated with occurrence of cardiac problems in the multivariable analysis.