Oxidative stress associated with long term occupational exposure to extremely low frequency electric and magnetic fields

2.1 Electric and magnetic fields measurements

In order to measure the magnetic fields, the Tes-1394 device (TES Electrical Electronic, Taipei, Taiwan) was used. This device measured the magnetic fields flux density in three axes (X, Y, and Z). In order to measure the electric fields, the HI-3604 ELF device (Holaday Industries, Inc., Eden Prairie, MN, USA) was used. The level of electromagnetic fields was measured based on the IEEE Std C95.3.1 standard [22] and the method proposed in a previous study was conducted [23].

Initially, participants were asked to specify their working and non-working spots (e.g. their places of resting and eating lunch) and the time they spend in each spot on the presented map. The pre-determined points were considered as spot measurement places. In order to measure the magnetic fields, the device probe was placed in a 1 m height and the displayed value was recorded as the magnetic flux density in the measured spot. In order to measure the electric fields, the device probe was placed in a 1 m height and 2.5 m distance by using a remote-control system to prevent the human body’s own electric fields interfere with measuring. In addition, the probe was used for measuring the vertical electrical fields, because this value is often used for describing the effects induced on objects close to ground surface. The measurement of both spots took about two minutes and due to the effect of weather on the results, all measurements were performed between 4 to 9 pm in midday summer and on sunny days. The dose of exposure to electromagnetic fields was calculated as the intensity of electromagnetic flux in an 8-hour time-weighted average. The following equation was used for the calculation: TWA = [ B 1 × t 1 + … + B n × t n ] 8(1)

In this equation, B is the intensity of the magnetic flux density (μT) or the electric fields strength (V/m) in the measured spot and t is the time spent in the desired spot.

A questionnaire was given to the participants including questions about age, height, weight, marital status, shift work, and work experience. Before completing the questionnaire, 5cc peripheral blood was taken by venepuncture from all participants from 6 to 10 am. Samples were taken by a trained nurse. The samples were allowed to clot, then serums were separated by centrifuge to measure oxidative stress. The collected serum samples were kept at –20°C before analysing.

Oxidative stress indicators were measured in the serum samples. The measured indicators were Malondialdehyde (MDA), superoxide dismutase (SOD), Catalase (Cat) and total antioxidant capacity (TAC). MDA is the final sustainable product of the lipid peroxidation process and an index for lipid oxidative damage. SOD is a large family of enzymes participating in the process of producing active oxygen species by catalysing the superoxide anion radical to oxygen and hydrogen peroxide. Cat is a key enzyme in the human body’s antioxidant defence against oxidative stress, and exists in the peroxisome of almost all aerobic cells. Cat converts hydrogen peroxide to water and oxygen and reduces the toxicant effect of hydrogen peroxide. In addition, TAC is an biomarker which is frequently used for evaluating the antioxidant status of biologic samples and can evaluate the antioxidant response to free radicals [24, 25]. Measuring oxidative stress indicators was done by using Hangzhou Eastbiopharm kits through the Double Antibody Sandwich (DAS) ELISA method. The mean value of three replicates was reported for each sample.

2.3 Ethical considerations

This study was conducted after approved by the Ethic in Research committee of Shahroud University of Medical Sciences (IR.SHMU.REC.1396.186). A letter about seeking cooperation, was sent to the power station, and the study began after receiving the necessary permissions. Before data collection, the research objectives were explained to all participants and written consent was inquired. Participants were ensured that they can leave the study whenever, without facing trouble.

2.4 Statistical analysis

Descriptive statistics (frequency, percent, and standard deviation) were used to present the results. The difference between demographic variables and oxidative stress levels in the two groups was evaluated by chi-square and Mann-Whitney U tests. The weighted average of exposure to magnetic fields in the exposed group was divided into three categories of low (<10.42μT), medium (10.42–17.82μT,), and high (>17.82μT,); and exposure to electric fields into three categories of low (<14.87 V/m), medium (14.87–25.18 V/m), and high (>25.18 V/m). These categories were respectively lower than the 33rd percentile, from the 33rd to 66th percentiles and more than the 66th percentile of the magnetic or electric fields exposure. The mean oxidative parameters among the different levels of exposure were compared by the Kruskal-Wallis test. All statistical tests were performed by SPSS version 24.0 at a p < 0.05 significance level.

4.1 The effect of electric fields on oxidative stress indicators

Occupational exposure of power plant workers to electric fields is generally more than the public, because these workers are exposed to high-voltage transmission lines or distribution and production equipment. In this study, MDA and SOD was significantly higher in people with high exposure to extremely low frequency magnetic fields; but no significant difference in TAC and Cat levels were found among exposure groups. Studies on the effect of extremely low frequency electric fields on guinea pigs in the range of 0.3 to 1.8 kV/m indicated that increased exposure to electric fields, increases lipid peroxidation indicators (MDA and SOD) in different tissues such as plasma, spleen, liver, kidneys, lungs, and testis [27]. Furthermore, the increase in SOD and MDA in different tissues of guinea pigs depended on the direction of the 1.35 kV/m and 50 Hz electric fields, and exposure duration [28]. Güler et al. showed that exposure to 12 kV/m and 50 Hz electric fields, for 8 hours a day and for 7 days a week, significantly increased hydroxyproline, MDA, NO, and oxygenase among guinea pigs in the exposed group [20]. Some researchers have reported that exposure to extremely low frequency electric fields disrupts the function of the brain, hormones, and enzyme activity in humans, and their effect depends on the frequency and duration of exposure [29]. A study by Aslankoc et al. indicated that exposure to 10 kV/m electric fields for 23 hours per day and for 30 days can damage the testicular tissue of rats. They also found that exposure to electric fields can significantly increase MDA, and body weight, cause DNA damage, significantly reduce the number and motility of sperms, and decrease SOD, and GPx [30]. However, in Aslankoc et al.’s study, the activity of antioxidant enzymes and MDA was measured in testis tissue, which can be the reason for the difference between their results and the present study.

In our study, workers were considerably exposed to a lower level of electric fields than the above-mentioned studies, and this makes it difficult to compare the results. Moreover, unlike other studies, in our study, occupational exposure among workers employed for more than 2 years (longer exposure) has been investigated. Electric fields may cause adverse health effects by increasing the negative charge on the cell surface and inducing cellular stress [31].

4.2 The effect of magnetic fields on oxidative stress indicators

The results of the present study showed a significant relation between increased exposure to magnetic fields and increased MDA and all antioxidant enzymes. Similar to the present study, in Tiwari et al.’s study, MDA, Cat, SOD, and GPx were measured; and in people exposed to extremely low frequency magnetic fields, MDA increased with increased exposure. But in their study, Cat was significantly lower in people with moderate exposure than subjects with high exposure [32]. Chu et al. found that in the cerebellum of rats exposed to 2.3μT for 3 hours, MDA, hydroxyl radicals, and SOD increased. But, no change was observed in Glutathione and GPx levels [33]. Canseven et al. found that MDA significantly increased in the liver of rats exposed to 1 and 2μT for 4 hours. However, rats exposed to 3μT for 8 hours had less MDA in their liver and heart tissue than the control group. These authors suggested that the intensity and duration of exposure to magnetic fields can be effective in the production of free radicals and the behavior of antioxidant enzymes [34]. Unlike the present study, Li et al. found no significant relation between exposure to magnetic fields and MDA, TAC, SOD, and GPx levels [17]. In Li’s study, exposed subjects were divided into three groups of <1.56μT, 1.56–7.3μT and >7.3μT; and logistic and office subjects were used as the control group. This may be the reason for their different results.

In a study done by Rageh et al., an increase in the average of tail moment (as an indicator of DNA damage) and increased MDA and SOD levels were reported in new-born rats after 30-days exposure to 0.5 mT magnetic fields. But, exposure to magnetic fields caused no change in Glutathione activity [35]. Seifirad et al. reported that exposure to ELF magnetic fields increased lipid peroxidation (MDA and conjugated dienes) as well as serum antioxidant activity (HDL, paraoxonase activity, and serum TAC). They also suggested that chronic exposure caused irreversible, while acute exposure caused reversible changes [36].

The RPM is a generally accepted physical mechanism for describing how applied magnetic flux densities as low as 0.1–1 mT influence specific types of chemical and biochemical reactions nonthermally, by generally increasing the concentration of reactive free radicals in low fields [12]. Thus, the increase in antioxidant enzymes such as SOD and Cat in our study, can be explained by the overproduction of free radicals.

4.3 The effect of electric and magnetic fields on oxidative stress indicators

Our results indicated that MDA and SOD levels were significantly higher in subjects with medium and high exposure to magnetic fields as well as subjects with high exposure to electric fields. In addition, subjects with high exposure to magnetic fields and medium exposure to electric fields had higher Cat levels. The results indicated that the changes in exposure to magnetic fields had a stronger effect on MDA and antioxidant enzyme activity than exposure to electric fields. But, exposure to electric fields had no effect on TAC. El-Helaly and Abu-Hashem showed that electric repairmen with high exposure to electromagnetic fields, had higher MDA and lower Melatonin than controls [37]. Some researchers think that both occupational and residential exposure to ELF-EMFs can be a possible carcinogen. Most components, systems, and cell processes such as proliferation, apoptosis, gene expression, and differentiation may be affected by exposure to ELF-EMFs [38]. Meanwhile, it has been suggested recently that the initial biologic response of exposure to electromagnetic fields, may be the increase in production and lifespan of reactive oxygen species and other free radicals [39, 40].

This study had some strengths and weaknesses. One of the strengths was that the effects of electric and magnetic fields were assessed separately. Furthermore, electric and magnetic fields exposures were quantified through measuring them in different workstations, during normal work shifts. However, this cross-sectional study was conducted in only one power plant that makes it difficult to generalize the results. Moreover, all participants were male and we could not consider gender differences in the effects.