Australas Phys Eng Sci Med (2014) 37:37–44 DOI 10.1007/s13246-013-0237-6
SCIENTIFIC PAPER
No effects of power line frequency extremely low frequency electromagnetic field exposure on selected neurobehavior tests of workers inspecting transformers and distribution line stations versus controls Li Li • De-fu Xiong • Jia-wen Liu • Zi-xin Li Guang-cheng Zeng • Hua-liang Li
•
Received: 20 June 2013 / Accepted: 16 December 2013 / Published online: 31 December 2013 Ó Australasian College of Physical Scientists and Engineers in Medicine 2013
Abstract We aimed to evaluate the interference of 50 Hz extremely low frequency electromagnetic field (ELF-EMF) occupational exposure on the neurobehavior tests of workers performing tour-inspection close to transformers and distribution power lines. Occupational short-term ‘‘spot’’ measurements were carried out. 310 inspection workers and 300 logistics staff were selected as exposure and control. The neurobehavior tests were performed through computer-based neurobehavior evaluation system, including mental arithmetic, curve coincide, simple visual reaction time, visual retention, auditory digit span and pursuit aiming. In 500 kV areas electric field intensity at 71.98 % of total measured 590 spots were above 5 kV/m (national occupational standard), while in 220 kV areas electric field intensity at 15.69 % of total 701 spots were above 5 kV/m. Magnetic field flux density at all the spots was below 1,000 lT (ICNIRP occupational standard). The neurobehavior score changes showed no statistical significance. Results of neurobehavior tests among different age, seniority groups showed no significant changes. Neurobehavior changes caused by daily repeated ELF-EMF exposure were not observed in the current study. Keywords Extremely low frequency electromagnetic field (ELF-EMF) Occupational exposure Neurobehavior test L. Li (&) J. Liu G. Zeng H. Li Electric Power Research Institute of Guangdong Power Grid Corporation, No. 8 Shuijungang Dongfengdong Road, Guangzhou 510080, Guangdong, China e-mail: [email protected] D. Xiong Z. Li Guangdong Huianhengda Management Consulting Co., Ltd, Guangzhou, China
Introduction In the past two decades, a great number of studies focused on the effects of electromagnetic fields (EMFs) on human physiology, for example, heart rate variability [1] and cognitive performance [2, 3]. Although there was guidances published by ICNIRP (2010) and IEEE (2002), which are based on the avoidance of acute effects in the central nervous system (CNS), there were still many investigations that focused on the relationship between extremely low frequency electromagnetic field (ELF-EMF) exposure and variety of the interferences on neurobehavior. Studies suggesting that magnetic field (MF) (50 Hz, 1,000 lT) exposure could have a subtle delayed but not pathological effect on human behavior [4]. Another previous study recorded that 1 h MF (60 Hz, 1,800 lT) exposure might modulate human involuntary motor control without being detected in the cortical electrical activity [5]. A small negative delayed effect of a 50 Hz, 100 lT MF exposure on memory recognition accuracy although no effect in reaction time and accuracy in the visual discrimination task were observed [3]. There was also study presenting data that suggest detrimental effects of a 50 Hz, 28 lT MF on short-term memory and executive functions [6]. Furthermore, a decreased performance in attention and several working and secondary memory tasks under exposure to a 50 Hz, 0.6 mT MF was found [7]. Additionally, subjects showed decreased accuracy in a choice reaction task. For example, a previous study revealed a small improvement in the accuracy of a visual duration discrimination task under exposure to 50 Hz, 100 lT MF [8]. The result of another study centering on reaction time showed that ELF-MF had a relaxing effect on motor control and resulted in an attenuation of postural tremor intensity [9]. Besides, the postural tremor of subjects were
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affected by MF (50 Hz, 1,000 lT) exposure, but the sensitivity of individual subjects to this effect exerted differentially [10]. Another issue on this topic is whether the effects of ELF-EMF exposure on neurobehavior could be persistent after the exposure ceased. On the other hand, there were studies indicating that ELF-EMF had no impact on neurobehavior performance. In a study, no effects of 50 Hz, 400 lT MF exposure on attention, working memory, integrate information ability and simple reaction time were found [11]. Similar to that, another study investigated the effects of 400 and 20 lT, 50 Hz MF on several tasks completed by subjects on attention, working memory, cognitive flexibility, and time perception also found no meaningful results [12]. There was also a study showing decreased errors and non-affected reaction time in a choice reaction task, but performances in a simple reaction task, an attention task and a memory task were not affected by exposure to a 60 Hz ELF-EMF [13]. The inconsistency of the findings and the failures of replication attempts could be attributed partially to the differences of exposure conditions, such as frequency, duration, flux density, measurements after or during the exposure, individual sensitivity, etc. (see [2, 9, 10, 13] for a review) Also, it had to be noted that interpretation of the results was limited by the diversity of the neurobehavior tests that were applied in the studies mentioned above. At present, little is known about the biological mechanisms underlying the suspected impact of ELF-EMF exposure on neurobehavior abilities. There is some speculation that frontal and parietal cortex activity might be affected by MF exposure, however, this hypothesis has never been verified [14], although many researchers have described redox-related cellular changes following ELFMF exposure [15, 16]. There are possible mechanisms that ELF-EMF might influence the motor coordination, spatial and short-term working memory by intervening synaptic potentials of Purkinje cells, which form the main output of the cerebellum, and pyramidal cells in the hippocampus [13]. Researchers recently demonstrated that ELF-MF exposure could cause significant changes in antioxidant capacity, together with a reduced tolerance towards oxidative challenges and DNA damage in newborn rats [17]. In vitro and in vivo results together might allow a rough interpretation, but in order to develop strategies concerning public health, studies on occupationally exposed population are still in need. So far as we know, the persistence of the effects of ELFEMF exposure is not clearly established. Therefore, in the current study, we conducted a series of computer-based neurobehavior tests in 310 regularly ELF-EMF exposed subjects, compared with 300 control subjects. All of the 310 exposed and 300 control subjects finished the computer-based tests in an ample room where the ELF-EMF
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level could not be detected with the same measure device. We aimed to examine whether there were effects and, if so, which functions of the CNS might be affected and whether the effects could be detectable after long-time repeated ELF-EMF exposure, because the neurobehavior tests were being regularly used to recognize sub-clinical symptoms of CNS dysfunction that might be caused by ELF-EMF exposure.
Materials and methods Subjects Subjects with CNS diseases like epilepsy (epilepsy family history), cerebritis (cerebritis history), severe brain trauma history, visual and auditory disturbance, upper limb dyskinesia were not qualified to be involved in the current study. A total of 710 subjects with good state of health were recruited and designated into exposure (n = 364) and control (n = 346) group depending on whether there was an occupational history of ELF-EMF exposure. The research protocol was approved by the Institutional Review Board (The Ethics Committees of Electric Power Research Institute of Guangdong Power Grid Corporation, Guangzhou, China). Informed consent and Health Insurance Portability and Accountability consent were obtained from each subject. All the 25 investigators received standard and unified training before the investigation started. All subjects were required to complete a series of neurobehavior tests (Table 1) unless they were not willing to, including tests on mental arithmetic, visual retention, auditory digit span and visual simple reaction time on the same laptops, which mainly included memory recognition accuracy, visual discrimination, short-term memory, cognitive performance and executive functions. Six control and four exposed workers were not willing to take the neurobehavior tests for unknown reasons. All the subjects (except the six control and four exposed workers) were also required to complete a questionnaire that were finished in
Table 1 Testing items of NES neurobehavioral capacity Item (neurobehavioral capacity)
Name
Intelligence
Mental arithmetic
Learning and memory
Visual retention
Perception and number-involved memory capacity
Auditory digit span
Mental campaign and hand-eye coordination
Visual simple reaction time Curve coincide Pursuit aiming
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no \15 min, which included questions about age, sex, nationality, smoking, alcohol, green tea drinking, medicine intake, medical inspections like chest perspective, X-ray based imagery and barium meal (in recent half year), emotional shock or trauma like being victim of a violent crime or losing a family member (in recent half year). After each of the 700 subjects finished the questionnaire in 2 months, 610 of the total 700 questionnaires were selected and were input with Epidata software (version 3.1). Other 90 subjects did not meet the inclusion criteria in part because of incomplete data collection or because they had past medical histories of mental diseases. 300 subjects comprised 276 males and 24 females aged 23–50 years (mean = 31.6 years) were finally selected as control. This group included administrative staff and logistical personnel who were not authorized to enter the 500 and 220 kV zone during workday. The exposure group consisted 310 workers who performed tour-inspection close to voltage transformers and distribution power lines, including 284 males and 26 females aged between 25 and 49 years (mean = 30.5 years). All of the 310 exposed workers had an occupational history of long-time ELF-EMF exposure varying from 3 to 25 years. The inspectors worked in the similar office environment with the control subjects while the inspectors were not performing tour-inspection. Both the inspectors and the control subjects exposed constantly to ELF-EMF produced by computers and household appliances and circuits. A pre-enrollment survey showed no disease-related symptoms among control and exposure subjects. None of the 610 subjects reported family history or past medical history of mental and cardiovascular diseases. All subjects denied having history of hepatitis, liver or kidney failure.
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Three-axis ELF-EF intensity and ELF-MF flux density outdoor in the 500, 220 kV areas of voltage transformers and distribution power lines respectively were measured with PMM 8053A portable field meter with an EHP50 three-axial probe (PMM Construction Center for Electronic Radio Measurements, Cisano Sul Neva, Italy) at spots located in the routes where workers regularly perform tour-inspections. The spots started 2 m away from the enclosing wall and dotted in the route of tourinspection, which surround 500 or 220 kV areas respectively, with 5 m apart from each other and 1.5 m high from the ground. Besides, it is necessary to mention that the investigated frequency in current study is 50 Hz.
Computer-based neurobehavior tests Neurobehavior tests were performed on 25 specified laptops through computer neurobehavioral evaluation system (NES) software, in a quiet room with soft light. All of the 310 exposed and 300 control subjects finished the computer-based tests in a large room where the ELF-EMF was below the level of detection using the survey meter described in section ‘‘ELF-MF measurement protocols’’. All the qualified subjects were told not to take alcohol, nor did take any depressant or stimulant medicine 12 h before the tests. Items including mental arithmetic, curve coincide, simple visual reaction time, visual retention, auditory digit span and pursuit aiming were set in NES software for the tests of subjects (Table 1). A standard procedure of the tests and the algorithms of the tests score or index in the NES software were based on the WHO neurobehavioral core test battery (NCTB) [18].
ELF-MF measurement protocols Statistical analysis The 500 kV areas were grouped into three types depending on the placement of equipments. Type A: The electrical equipment was set outdoor, circuit breaker, switch, current transformer (CT) and busbar potential transformer (PT) were fixed independently in an open-plan. Type B: The electrical equipment was set outdoor, circuit breaker, switch, CT and PT were fixed assembly in combined arrangement, called ‘‘Half GIS’’. Type C: The electrical equipment was set in a room that was built with concrete, with a gas insulation closed switch, called ‘‘GIS’’. 220 kV areas were also grouped into type A: The electrical equipment was set out door, circuit breaker, switch, CT and PT were fixed independently in an open-plan. Type B: The electrical equipment was set outdoor, with a gas insulation closed switch, called out door ‘‘GIS’’. Type C: The electrical equipment was set indoor, with a gas insulation closed switch, called indoor ‘‘GIS’’.
The group average of neurobehavior tests score and/or match capability index were calculated and presented in form of mean ± standard deviation (SD), and range for 95 % confidence interval was included. Neurobehavioral test score and index between control and exposure were compared by student t test, with green tea drinking and alcohol consumption being treated as covariant through covariance analysis. The total group averages and averages in different age (\30, 30–40, 40 years) or seniority groups (control, \5, [5 year) of neurobehavior evaluation score and index were compared using one-way ANOVA to determine if any statistical significance existed. All P values \0.05 were considered as statistically significant. All the statistical analysis was performed using SPSS 11.0 software (SPSS Inc., Chicago, USA).
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Results Analysis of neurobehavior influencing factors between control and exposure group Both the control and exposure group were exposed constantly to ELF-EMF produced by household appliances and circuits. The levels of ELF-MF produced by household appliances varied tremendously and could reach 1,500–2,000 lT, like electric shaver or hairdryer [19, 20]. Table 2 Equilibrium analysis of neurobehavioral influencing factors Items
Control
Exposure
P
276
284
0.906
24
26
296 4
307 3
0.704
84
66
0.054
216
244
Sex Male Female Nationality Han Other Smoking Smoker Non smoker Alcohol Never
26
52
244
249
Frequently
28
9
Not clear
2
0
Occasionally
0.000
Green tea Never
5
18
Occasionally
158
199
Frequently
134
90
3
3
Medicine (within 2 weeks) No 294
301
Not clear
Ever
6
0.000
Ever
Ever
Table 3 The ELF-EMF level of the spots dotted at 500 kV 276
281
24
29
0.553
Ever
297
8
13
0.301
299
309
1
1
1.0
Ever
283
295
17
15
0.718
Body stress (within half year) No Ever
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295 5
A 360
B 180
C 50
Maximum (kV/m)
14.00
11.91
4.0 9 10-4
Minimum (kV/m)
0.651
1.648
9.0 9 10-5
Median(kV/m)
7.310
6.517
1.4 9 10-4
C5 kV/m spots number
296
133
0
Maximum (lT)
30.19
19.57
32.02
Minimum (lT)
0.620
0.791
1.426
Median (lT))
17.83
11.99
16.52
C1,000 lT spots number
0
0
0
2–10
2–10
2–15
ELF-MF intensity
Mental stress (within half year) No
Type Total measured spots number ELF-EF intensity
292
Barium meal (within half year) No
The ELF-EMF level at the spots dotted in the route of tourinspections nearby 500 and 220 kV transformers and distribution lines were measured respectively. Both magnetic and electric filed intensity at type A, B and C 500 kV areas were different (Table 3). The ELF-EF intensity of more than 50 % of the spots measured was above 5 kV/m at type A, B 500 kV areas, which is the national ELF-EF exposure limit of China. However the ELF-EF intensity was not found higher than 5 kV/m at the spots in type C 500 kV areas. The ELF-EF intensity at 110 spots in type A 220 kV areas was above 5 kV/m (Table 4). It was reported by the workers performing tour-inspection that the duration time spent at particular locations range from 2 to 15 min.
0.471
Computerized tomography (within half year) No
The ELF-EMF level of 500 and 220 kV transformers and distribution lines
9
X-ray (within half year) No
In the current study, the total levels of ELF-EMF produced by household appliances that both the control and exposure group were exposed to were not assumed statistically different. Each subject was required to complete a questionnaire, in order to investigate the equilibrium of neurobehavior influencing factors between control and exposure group. The equilibrium of the items between control and exposure group were analyzed with v2 test. All the distribution of the influencing factors between control and exposure group were not found significantly different, except alcohol and green tea (P \ 0.05). The distribution proportion of subjects who were accustomed to alcohol and green tea drinking were in disequilibrium between control and exposure, which should be taken into account while doing further statistical analysis of neurobehavior score and index (Table 2).
306 4
0.748
Duration time ranges at particular locations (min)
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Table 4 The ELF-EMF level of the spots dotted at 220 kV Type Total measured spots number
A 605
B 48
C 48
Maximum (kV/m)
9.52
3.623
6.0 9 10-4
Minimum (kV/m)
0.068
0.053
Median(kV/m)
6.862
C5 kV/m spots number
110
Table 5 Neurobehavioral tests score and index between control and exposure Item
Statistical index
Control
Exposure
Total mental arithmetic
8.0 9 10-5
Number Average
300 26.90
310 27.27
1.331
2.659
SD
7.37
7.35
0
0
P
0.55
Number
300
ELF-EF intensity
ELF-MF intensity Maximum (lT) Minimum (lT)
Accurate mental arithmetic 60.11 0.509
44.54 0.396
16.23 1.337
Median (lT))
36.680
20.371
6.833
C1,000 lT spots number
0
0
0
2–10
2–10
2–15
Duration time ranges at particular locations (min)
Visual reaction
Auditory digit span
Neurobehavior evaluation score and index were not different between control and exposure group The neurobehavior capacity tests including mental arithmetic, curve coincide, simple visual reaction time, visual retention, auditory digit span and pursuit aiming test, were carried out for each individual. The items of neurobehavior tests were treated as dependent, ELF-EMF exposure as independent. No significant differences were found between control and exposure group, even after green tea and alcohol were treated as covariant through covariance analysis (Table 5). Neurobehavior evaluation score and index were not significantly different among different age and seniority groups 560 male volunteers were grouped according to the age (\30, 30–40, 40 years) and length of service (control, \5, [5 year). 50 female volunteers were parted into control and exposure group. Neurobehavior evaluation were carried out for each individual and compared in different groups mentioned above with one-way ANOVA. No statistical differences in neurobehavior evaluation score and index were found among different age and seniority groups (Table 6).
Discussion Among the environmental risk factors that affect human health, ELF-EMF might play an role in neurological diseases and neurobehavior performance in adults [21–23]. Studies showed that ELF-EMF exposure could influence the motor control [9], cognitive performance [11] and
Simple visual reaction time
Curve coincide
Total pursuit aiming number
Accurate pursuit aiming number
310
Average
25.92
26.02
SD
7.56
7.45
P
0.79
Number
300
310
Average
7.35
7.45
SD
1.92
1.78
P
0.57
Number
300
Average
17.36
17.08
SD
6.03
6.07
P Number
0.35 300
310
Average
0.421
0.414
SD
0.099
0.092
P
0.27
Number
300
310
310
Average
370.09
365.37
SD
99.67
111.58
P
0.32
Number
300
310
Average
109.87
111.15
SD
25.17
26.59
P
0.67
Number
300
310
Average
99.51
100.47
SD P
20.89 066
21.62
Each P value was obtained while tea and alcohol were treated as covariant through covariance analysis
cognitive function [12] in humans. Therefore, we assumed that abnormal neurobehavior tests results were signs of the alteration of CNS function. However, there are also studies indicating that ELF-EMF have no impact on neurobehavior activities [24, 25]. In the current study, we measured the ELF-EMF exposure level of workers performing tourinspection close to 500 and 220 kV transformers and distribution power lines, and conducted a series of tests on the computer system to examine whether there were effects and, if so, which neurobehavior activity of the CNS might be affected.
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Table 6 Neurobehavioral evaluation score and index among different age or seniority group Group Sex
Age
Male \30
30–40
C40
Accurate mental arithmetic
Visual retention
Seniority
Number
Average
Number
Average
Female
Average
P
Number
Average
141
0.412
P
141
28.13
141
7.78
141
18.91
123
27.97
0.98
123
7.80
1.00
123
18.41
0.70
123
0.394
0.22
C5 years
28
26.07
0.31
28
8.07
0.60
28
19.18
0.96
28
0.428
0.65
99
0.429
Control
99
25.31
99
7.07
99
16.91
\5 years
25
26.04
0.87
25
7.08
1.00
25
17.84
0.70
25
0.432
0.99
C5 years
78
25.03
0.95
78
7.26
0.77
78
17.05
0.98
78
0.419
0.77
Control
36
19.72
36
6.50
36
11.94
\5 years
7
19.71
1.00
7
6.57
1.00
7
8.29
0.21
0.15
0.90
0.42
23
22.87
Control
276
26.03
\5 years C5 years
155 129
27.28 24.87
Control
26
24.27
Exposure
24
24.54
0.17 0.25
23
6.26
276
7.36
155 129
7.63 7.26
26
7.38
24
7.25
0.91
0.27 0.83 0.80
23
10.22
276
17.28
155 129
17.86 16.29
26
16.42
24
18.04
Accurate pursuit aiming
Sex
Age
Seniority
Number
Average
Male
\30
Control
141
105.08
\5 years
123
105.41
C5 years
28
106.07
Control
99
97.88
\5 years
25
95.24
C5 years
78
98.64
30–40
C40
Total
0.54 0.22 0.36
36
0.421
7
0.445
0.75 0.50
23
0.445
276
0.419
155 129
0.403 0.425
26
0.422
24
0.440
0.17 0.79 0.32
Curve coincide P
Number
Average
P
141
374.08
0.99
123
378.28
0.93
0.96
28
343.11
0.25
99
373.31
0.80
25
382.76
0.89
0.96
78
369.09
0.95
Control
36
83.44
36
326.78
\5 years
7
87.29
0.87
7
368.14
0.66
C5 years
23
90.70
0.33
23
316.70
0.94
Control
276
99.67
276
367.63
\5 years
155
102.95
0.21
155
378.55
0.50
C5 years
129
98.84
0.91
129
354.11
0.39
Control
26
94.00
26
348.77
Exposure
24
98.17
0.47
24
400.96
The exposure level in the current study is higher than that measured in other industries in China, like automotive industry [26]. Besides, the ELF-MF level in the current study is also higher than that measured in residential apartments close to transformers or distribution power lines in other countries, like Hungary [27], Finland [28] and Israel [29]. The exposure group in the current study is exposed to relative higher levels of ELF-EMF than the ELF-EMF exposure groups reported in the researches mentioned above. However, uncertainty evaluation needs to be considered in the measurement of ELF-EMF [30], which would increase the comparability of different measurement results. A recent study showed that ELF-EMF might have effects on the nervous, cardiovascular, liver and
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Number
Control
Group
Female
P
Simple visual reaction time
\5 years
C5 years Total
P
Auditory digit span
0.09
hematology system of workers in automotive industry [26]. However, in current study, the items of neurobehavior tests were treated as dependent, ELF-EMF exposure as independent, no significant differences were found between control and exposure group, even after green tea and alcohol were treated as covariant through covariance analysis. Further analysis was carried out in different age, seniority groups, but no statistical significant results were found either. Obviously, our results do not seem to support our assumption, but there are always inconsistencies among results of in vivo, in vitro and epidemic studies. The inconsistencies might be attributed mainly to the differences of the demographic characteristics of the study subjects and experimental conditions, etc. [9, 10, 13]. On the other hand, in a recent research, a considerable nocebo
Australas Phys Eng Sci Med (2014) 37:37–44
effect in symptoms related to 50 Hz EMF exposure was reported, because idiopathic environmental intolerance to EMF seemed to be formed through a vicious circle of psychosocial factors, such as enhanced perception of risk and expectations, self-monitoring, somatization and somatosensory amplification and misattribution [25]. On this basis, previous studies that reported ELF-EMF exposure influenced the neurobehavior activities might not take the nocebo effect into account when the conclusions were made. Besides, despite all that mentioned above, there are possibilities that ELF-EMF exposure might induce or promote effects (probably damages) on CNS. In the last few years, both in vitro and in vivo studies on the health effects of ELF-EMF exposure described redox-related changes in CNS following ELF-EMF exposure [16, 31]. In fact, abnormal neurobehavior might emerge after but not before the alteration of CNS function, which means that we should have chosen biomarkers that emerge in earlier state of the alteration of CNS function. Such as nerve growth factor (NGF) which is widely recognized to play a crucial role in the process of free radicals scavenging and neuronal systems protection [32]. Therefore, more reliable and earlier biomarkers would be applied in following studies. On the other side, the effects of ELF-EMF exposure on CNS could only be detected during the exposure, and the effects might be disappeared as soon as the exposures stops [9, 33]. This indicated that the effects of ELF-EMF on humans CNS function, if any, exerted in a on and off way, but the cumulative effects of long-term intermittent ELF-EMF exposure were not observed in the current study. That means ELF-EMF might have transient effects without interfering with the CNS in a way measured here. Nerve cells are spontaneously active in awake animals, soma and dendrite membrane potentials continually fluctuate. So there is a possibility that thresholds for impulse initiation by externally ELF-EMF would be lowered than those necessary for the direct stimulation of peripheral nerves [34], however, the overall effect could be either excitatory or inhibitory, depending on the functional and brain tissue properties of the neuron(s) involved [13, 14, 35]. In all, despite the shortcomings mentioned in the current study, it seemed that long-term intermittent ELF-EMF exposure do not appear to disrupt normal neurobehavior like learning and memory capacity, perception and number-involved memory capacity, mental campaign and handeye coordination. The acute effects of ELF-EMF exposure were not explored and discussed in the current study, and the possible cumulative effects of ELF-EMF exposure on the CNS functions mentioned above were not observed in the current study. In the future, researches on this issue must include subjects that exposed to higher level of ELFEMF, in order to reveal whether there is the latent dose–
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response relation between ELF-EMF exposure and the alteration of CNS function. Acknowledgments This work was supported by the Guangdong Power Grid Corporation, Guangzhou, China. Conflict of interest The authors declare they have no competing financial interests.
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SCIENTIFIC PAPER
No effects of power line frequency extremely low frequency electromagnetic field exposure on selected neurobehavior tests of workers inspecting transformers and distribution line stations versus controls Li Li • De-fu Xiong • Jia-wen Liu • Zi-xin Li Guang-cheng Zeng • Hua-liang Li
•
Received: 20 June 2013 / Accepted: 16 December 2013 / Published online: 31 December 2013 Ó Australasian College of Physical Scientists and Engineers in Medicine 2013
Abstract We aimed to evaluate the interference of 50 Hz extremely low frequency electromagnetic field (ELF-EMF) occupational exposure on the neurobehavior tests of workers performing tour-inspection close to transformers and distribution power lines. Occupational short-term ‘‘spot’’ measurements were carried out. 310 inspection workers and 300 logistics staff were selected as exposure and control. The neurobehavior tests were performed through computer-based neurobehavior evaluation system, including mental arithmetic, curve coincide, simple visual reaction time, visual retention, auditory digit span and pursuit aiming. In 500 kV areas electric field intensity at 71.98 % of total measured 590 spots were above 5 kV/m (national occupational standard), while in 220 kV areas electric field intensity at 15.69 % of total 701 spots were above 5 kV/m. Magnetic field flux density at all the spots was below 1,000 lT (ICNIRP occupational standard). The neurobehavior score changes showed no statistical significance. Results of neurobehavior tests among different age, seniority groups showed no significant changes. Neurobehavior changes caused by daily repeated ELF-EMF exposure were not observed in the current study. Keywords Extremely low frequency electromagnetic field (ELF-EMF) Occupational exposure Neurobehavior test L. Li (&) J. Liu G. Zeng H. Li Electric Power Research Institute of Guangdong Power Grid Corporation, No. 8 Shuijungang Dongfengdong Road, Guangzhou 510080, Guangdong, China e-mail: [email protected] D. Xiong Z. Li Guangdong Huianhengda Management Consulting Co., Ltd, Guangzhou, China
Introduction In the past two decades, a great number of studies focused on the effects of electromagnetic fields (EMFs) on human physiology, for example, heart rate variability [1] and cognitive performance [2, 3]. Although there was guidances published by ICNIRP (2010) and IEEE (2002), which are based on the avoidance of acute effects in the central nervous system (CNS), there were still many investigations that focused on the relationship between extremely low frequency electromagnetic field (ELF-EMF) exposure and variety of the interferences on neurobehavior. Studies suggesting that magnetic field (MF) (50 Hz, 1,000 lT) exposure could have a subtle delayed but not pathological effect on human behavior [4]. Another previous study recorded that 1 h MF (60 Hz, 1,800 lT) exposure might modulate human involuntary motor control without being detected in the cortical electrical activity [5]. A small negative delayed effect of a 50 Hz, 100 lT MF exposure on memory recognition accuracy although no effect in reaction time and accuracy in the visual discrimination task were observed [3]. There was also study presenting data that suggest detrimental effects of a 50 Hz, 28 lT MF on short-term memory and executive functions [6]. Furthermore, a decreased performance in attention and several working and secondary memory tasks under exposure to a 50 Hz, 0.6 mT MF was found [7]. Additionally, subjects showed decreased accuracy in a choice reaction task. For example, a previous study revealed a small improvement in the accuracy of a visual duration discrimination task under exposure to 50 Hz, 100 lT MF [8]. The result of another study centering on reaction time showed that ELF-MF had a relaxing effect on motor control and resulted in an attenuation of postural tremor intensity [9]. Besides, the postural tremor of subjects were
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affected by MF (50 Hz, 1,000 lT) exposure, but the sensitivity of individual subjects to this effect exerted differentially [10]. Another issue on this topic is whether the effects of ELF-EMF exposure on neurobehavior could be persistent after the exposure ceased. On the other hand, there were studies indicating that ELF-EMF had no impact on neurobehavior performance. In a study, no effects of 50 Hz, 400 lT MF exposure on attention, working memory, integrate information ability and simple reaction time were found [11]. Similar to that, another study investigated the effects of 400 and 20 lT, 50 Hz MF on several tasks completed by subjects on attention, working memory, cognitive flexibility, and time perception also found no meaningful results [12]. There was also a study showing decreased errors and non-affected reaction time in a choice reaction task, but performances in a simple reaction task, an attention task and a memory task were not affected by exposure to a 60 Hz ELF-EMF [13]. The inconsistency of the findings and the failures of replication attempts could be attributed partially to the differences of exposure conditions, such as frequency, duration, flux density, measurements after or during the exposure, individual sensitivity, etc. (see [2, 9, 10, 13] for a review) Also, it had to be noted that interpretation of the results was limited by the diversity of the neurobehavior tests that were applied in the studies mentioned above. At present, little is known about the biological mechanisms underlying the suspected impact of ELF-EMF exposure on neurobehavior abilities. There is some speculation that frontal and parietal cortex activity might be affected by MF exposure, however, this hypothesis has never been verified [14], although many researchers have described redox-related cellular changes following ELFMF exposure [15, 16]. There are possible mechanisms that ELF-EMF might influence the motor coordination, spatial and short-term working memory by intervening synaptic potentials of Purkinje cells, which form the main output of the cerebellum, and pyramidal cells in the hippocampus [13]. Researchers recently demonstrated that ELF-MF exposure could cause significant changes in antioxidant capacity, together with a reduced tolerance towards oxidative challenges and DNA damage in newborn rats [17]. In vitro and in vivo results together might allow a rough interpretation, but in order to develop strategies concerning public health, studies on occupationally exposed population are still in need. So far as we know, the persistence of the effects of ELFEMF exposure is not clearly established. Therefore, in the current study, we conducted a series of computer-based neurobehavior tests in 310 regularly ELF-EMF exposed subjects, compared with 300 control subjects. All of the 310 exposed and 300 control subjects finished the computer-based tests in an ample room where the ELF-EMF
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level could not be detected with the same measure device. We aimed to examine whether there were effects and, if so, which functions of the CNS might be affected and whether the effects could be detectable after long-time repeated ELF-EMF exposure, because the neurobehavior tests were being regularly used to recognize sub-clinical symptoms of CNS dysfunction that might be caused by ELF-EMF exposure.
Materials and methods Subjects Subjects with CNS diseases like epilepsy (epilepsy family history), cerebritis (cerebritis history), severe brain trauma history, visual and auditory disturbance, upper limb dyskinesia were not qualified to be involved in the current study. A total of 710 subjects with good state of health were recruited and designated into exposure (n = 364) and control (n = 346) group depending on whether there was an occupational history of ELF-EMF exposure. The research protocol was approved by the Institutional Review Board (The Ethics Committees of Electric Power Research Institute of Guangdong Power Grid Corporation, Guangzhou, China). Informed consent and Health Insurance Portability and Accountability consent were obtained from each subject. All the 25 investigators received standard and unified training before the investigation started. All subjects were required to complete a series of neurobehavior tests (Table 1) unless they were not willing to, including tests on mental arithmetic, visual retention, auditory digit span and visual simple reaction time on the same laptops, which mainly included memory recognition accuracy, visual discrimination, short-term memory, cognitive performance and executive functions. Six control and four exposed workers were not willing to take the neurobehavior tests for unknown reasons. All the subjects (except the six control and four exposed workers) were also required to complete a questionnaire that were finished in
Table 1 Testing items of NES neurobehavioral capacity Item (neurobehavioral capacity)
Name
Intelligence
Mental arithmetic
Learning and memory
Visual retention
Perception and number-involved memory capacity
Auditory digit span
Mental campaign and hand-eye coordination
Visual simple reaction time Curve coincide Pursuit aiming
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no \15 min, which included questions about age, sex, nationality, smoking, alcohol, green tea drinking, medicine intake, medical inspections like chest perspective, X-ray based imagery and barium meal (in recent half year), emotional shock or trauma like being victim of a violent crime or losing a family member (in recent half year). After each of the 700 subjects finished the questionnaire in 2 months, 610 of the total 700 questionnaires were selected and were input with Epidata software (version 3.1). Other 90 subjects did not meet the inclusion criteria in part because of incomplete data collection or because they had past medical histories of mental diseases. 300 subjects comprised 276 males and 24 females aged 23–50 years (mean = 31.6 years) were finally selected as control. This group included administrative staff and logistical personnel who were not authorized to enter the 500 and 220 kV zone during workday. The exposure group consisted 310 workers who performed tour-inspection close to voltage transformers and distribution power lines, including 284 males and 26 females aged between 25 and 49 years (mean = 30.5 years). All of the 310 exposed workers had an occupational history of long-time ELF-EMF exposure varying from 3 to 25 years. The inspectors worked in the similar office environment with the control subjects while the inspectors were not performing tour-inspection. Both the inspectors and the control subjects exposed constantly to ELF-EMF produced by computers and household appliances and circuits. A pre-enrollment survey showed no disease-related symptoms among control and exposure subjects. None of the 610 subjects reported family history or past medical history of mental and cardiovascular diseases. All subjects denied having history of hepatitis, liver or kidney failure.
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Three-axis ELF-EF intensity and ELF-MF flux density outdoor in the 500, 220 kV areas of voltage transformers and distribution power lines respectively were measured with PMM 8053A portable field meter with an EHP50 three-axial probe (PMM Construction Center for Electronic Radio Measurements, Cisano Sul Neva, Italy) at spots located in the routes where workers regularly perform tour-inspections. The spots started 2 m away from the enclosing wall and dotted in the route of tourinspection, which surround 500 or 220 kV areas respectively, with 5 m apart from each other and 1.5 m high from the ground. Besides, it is necessary to mention that the investigated frequency in current study is 50 Hz.
Computer-based neurobehavior tests Neurobehavior tests were performed on 25 specified laptops through computer neurobehavioral evaluation system (NES) software, in a quiet room with soft light. All of the 310 exposed and 300 control subjects finished the computer-based tests in a large room where the ELF-EMF was below the level of detection using the survey meter described in section ‘‘ELF-MF measurement protocols’’. All the qualified subjects were told not to take alcohol, nor did take any depressant or stimulant medicine 12 h before the tests. Items including mental arithmetic, curve coincide, simple visual reaction time, visual retention, auditory digit span and pursuit aiming were set in NES software for the tests of subjects (Table 1). A standard procedure of the tests and the algorithms of the tests score or index in the NES software were based on the WHO neurobehavioral core test battery (NCTB) [18].
ELF-MF measurement protocols Statistical analysis The 500 kV areas were grouped into three types depending on the placement of equipments. Type A: The electrical equipment was set outdoor, circuit breaker, switch, current transformer (CT) and busbar potential transformer (PT) were fixed independently in an open-plan. Type B: The electrical equipment was set outdoor, circuit breaker, switch, CT and PT were fixed assembly in combined arrangement, called ‘‘Half GIS’’. Type C: The electrical equipment was set in a room that was built with concrete, with a gas insulation closed switch, called ‘‘GIS’’. 220 kV areas were also grouped into type A: The electrical equipment was set out door, circuit breaker, switch, CT and PT were fixed independently in an open-plan. Type B: The electrical equipment was set outdoor, with a gas insulation closed switch, called out door ‘‘GIS’’. Type C: The electrical equipment was set indoor, with a gas insulation closed switch, called indoor ‘‘GIS’’.
The group average of neurobehavior tests score and/or match capability index were calculated and presented in form of mean ± standard deviation (SD), and range for 95 % confidence interval was included. Neurobehavioral test score and index between control and exposure were compared by student t test, with green tea drinking and alcohol consumption being treated as covariant through covariance analysis. The total group averages and averages in different age (\30, 30–40, 40 years) or seniority groups (control, \5, [5 year) of neurobehavior evaluation score and index were compared using one-way ANOVA to determine if any statistical significance existed. All P values \0.05 were considered as statistically significant. All the statistical analysis was performed using SPSS 11.0 software (SPSS Inc., Chicago, USA).
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Results Analysis of neurobehavior influencing factors between control and exposure group Both the control and exposure group were exposed constantly to ELF-EMF produced by household appliances and circuits. The levels of ELF-MF produced by household appliances varied tremendously and could reach 1,500–2,000 lT, like electric shaver or hairdryer [19, 20]. Table 2 Equilibrium analysis of neurobehavioral influencing factors Items
Control
Exposure
P
276
284
0.906
24
26
296 4
307 3
0.704
84
66
0.054
216
244
Sex Male Female Nationality Han Other Smoking Smoker Non smoker Alcohol Never
26
52
244
249
Frequently
28
9
Not clear
2
0
Occasionally
0.000
Green tea Never
5
18
Occasionally
158
199
Frequently
134
90
3
3
Medicine (within 2 weeks) No 294
301
Not clear
Ever
6
0.000
Ever
Ever
Table 3 The ELF-EMF level of the spots dotted at 500 kV 276
281
24
29
0.553
Ever
297
8
13
0.301
299
309
1
1
1.0
Ever
283
295
17
15
0.718
Body stress (within half year) No Ever
123
295 5
A 360
B 180
C 50
Maximum (kV/m)
14.00
11.91
4.0 9 10-4
Minimum (kV/m)
0.651
1.648
9.0 9 10-5
Median(kV/m)
7.310
6.517
1.4 9 10-4
C5 kV/m spots number
296
133
0
Maximum (lT)
30.19
19.57
32.02
Minimum (lT)
0.620
0.791
1.426
Median (lT))
17.83
11.99
16.52
C1,000 lT spots number
0
0
0
2–10
2–10
2–15
ELF-MF intensity
Mental stress (within half year) No
Type Total measured spots number ELF-EF intensity
292
Barium meal (within half year) No
The ELF-EMF level at the spots dotted in the route of tourinspections nearby 500 and 220 kV transformers and distribution lines were measured respectively. Both magnetic and electric filed intensity at type A, B and C 500 kV areas were different (Table 3). The ELF-EF intensity of more than 50 % of the spots measured was above 5 kV/m at type A, B 500 kV areas, which is the national ELF-EF exposure limit of China. However the ELF-EF intensity was not found higher than 5 kV/m at the spots in type C 500 kV areas. The ELF-EF intensity at 110 spots in type A 220 kV areas was above 5 kV/m (Table 4). It was reported by the workers performing tour-inspection that the duration time spent at particular locations range from 2 to 15 min.
0.471
Computerized tomography (within half year) No
The ELF-EMF level of 500 and 220 kV transformers and distribution lines
9
X-ray (within half year) No
In the current study, the total levels of ELF-EMF produced by household appliances that both the control and exposure group were exposed to were not assumed statistically different. Each subject was required to complete a questionnaire, in order to investigate the equilibrium of neurobehavior influencing factors between control and exposure group. The equilibrium of the items between control and exposure group were analyzed with v2 test. All the distribution of the influencing factors between control and exposure group were not found significantly different, except alcohol and green tea (P \ 0.05). The distribution proportion of subjects who were accustomed to alcohol and green tea drinking were in disequilibrium between control and exposure, which should be taken into account while doing further statistical analysis of neurobehavior score and index (Table 2).
306 4
0.748
Duration time ranges at particular locations (min)
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Table 4 The ELF-EMF level of the spots dotted at 220 kV Type Total measured spots number
A 605
B 48
C 48
Maximum (kV/m)
9.52
3.623
6.0 9 10-4
Minimum (kV/m)
0.068
0.053
Median(kV/m)
6.862
C5 kV/m spots number
110
Table 5 Neurobehavioral tests score and index between control and exposure Item
Statistical index
Control
Exposure
Total mental arithmetic
8.0 9 10-5
Number Average
300 26.90
310 27.27
1.331
2.659
SD
7.37
7.35
0
0
P
0.55
Number
300
ELF-EF intensity
ELF-MF intensity Maximum (lT) Minimum (lT)
Accurate mental arithmetic 60.11 0.509
44.54 0.396
16.23 1.337
Median (lT))
36.680
20.371
6.833
C1,000 lT spots number
0
0
0
2–10
2–10
2–15
Duration time ranges at particular locations (min)
Visual reaction
Auditory digit span
Neurobehavior evaluation score and index were not different between control and exposure group The neurobehavior capacity tests including mental arithmetic, curve coincide, simple visual reaction time, visual retention, auditory digit span and pursuit aiming test, were carried out for each individual. The items of neurobehavior tests were treated as dependent, ELF-EMF exposure as independent. No significant differences were found between control and exposure group, even after green tea and alcohol were treated as covariant through covariance analysis (Table 5). Neurobehavior evaluation score and index were not significantly different among different age and seniority groups 560 male volunteers were grouped according to the age (\30, 30–40, 40 years) and length of service (control, \5, [5 year). 50 female volunteers were parted into control and exposure group. Neurobehavior evaluation were carried out for each individual and compared in different groups mentioned above with one-way ANOVA. No statistical differences in neurobehavior evaluation score and index were found among different age and seniority groups (Table 6).
Discussion Among the environmental risk factors that affect human health, ELF-EMF might play an role in neurological diseases and neurobehavior performance in adults [21–23]. Studies showed that ELF-EMF exposure could influence the motor control [9], cognitive performance [11] and
Simple visual reaction time
Curve coincide
Total pursuit aiming number
Accurate pursuit aiming number
310
Average
25.92
26.02
SD
7.56
7.45
P
0.79
Number
300
310
Average
7.35
7.45
SD
1.92
1.78
P
0.57
Number
300
Average
17.36
17.08
SD
6.03
6.07
P Number
0.35 300
310
Average
0.421
0.414
SD
0.099
0.092
P
0.27
Number
300
310
310
Average
370.09
365.37
SD
99.67
111.58
P
0.32
Number
300
310
Average
109.87
111.15
SD
25.17
26.59
P
0.67
Number
300
310
Average
99.51
100.47
SD P
20.89 066
21.62
Each P value was obtained while tea and alcohol were treated as covariant through covariance analysis
cognitive function [12] in humans. Therefore, we assumed that abnormal neurobehavior tests results were signs of the alteration of CNS function. However, there are also studies indicating that ELF-EMF have no impact on neurobehavior activities [24, 25]. In the current study, we measured the ELF-EMF exposure level of workers performing tourinspection close to 500 and 220 kV transformers and distribution power lines, and conducted a series of tests on the computer system to examine whether there were effects and, if so, which neurobehavior activity of the CNS might be affected.
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Table 6 Neurobehavioral evaluation score and index among different age or seniority group Group Sex
Age
Male \30
30–40
C40
Accurate mental arithmetic
Visual retention
Seniority
Number
Average
Number
Average
Female
Average
P
Number
Average
141
0.412
P
141
28.13
141
7.78
141
18.91
123
27.97
0.98
123
7.80
1.00
123
18.41
0.70
123
0.394
0.22
C5 years
28
26.07
0.31
28
8.07
0.60
28
19.18
0.96
28
0.428
0.65
99
0.429
Control
99
25.31
99
7.07
99
16.91
\5 years
25
26.04
0.87
25
7.08
1.00
25
17.84
0.70
25
0.432
0.99
C5 years
78
25.03
0.95
78
7.26
0.77
78
17.05
0.98
78
0.419
0.77
Control
36
19.72
36
6.50
36
11.94
\5 years
7
19.71
1.00
7
6.57
1.00
7
8.29
0.21
0.15
0.90
0.42
23
22.87
Control
276
26.03
\5 years C5 years
155 129
27.28 24.87
Control
26
24.27
Exposure
24
24.54
0.17 0.25
23
6.26
276
7.36
155 129
7.63 7.26
26
7.38
24
7.25
0.91
0.27 0.83 0.80
23
10.22
276
17.28
155 129
17.86 16.29
26
16.42
24
18.04
Accurate pursuit aiming
Sex
Age
Seniority
Number
Average
Male
\30
Control
141
105.08
\5 years
123
105.41
C5 years
28
106.07
Control
99
97.88
\5 years
25
95.24
C5 years
78
98.64
30–40
C40
Total
0.54 0.22 0.36
36
0.421
7
0.445
0.75 0.50
23
0.445
276
0.419
155 129
0.403 0.425
26
0.422
24
0.440
0.17 0.79 0.32
Curve coincide P
Number
Average
P
141
374.08
0.99
123
378.28
0.93
0.96
28
343.11
0.25
99
373.31
0.80
25
382.76
0.89
0.96
78
369.09
0.95
Control
36
83.44
36
326.78
\5 years
7
87.29
0.87
7
368.14
0.66
C5 years
23
90.70
0.33
23
316.70
0.94
Control
276
99.67
276
367.63
\5 years
155
102.95
0.21
155
378.55
0.50
C5 years
129
98.84
0.91
129
354.11
0.39
Control
26
94.00
26
348.77
Exposure
24
98.17
0.47
24
400.96
The exposure level in the current study is higher than that measured in other industries in China, like automotive industry [26]. Besides, the ELF-MF level in the current study is also higher than that measured in residential apartments close to transformers or distribution power lines in other countries, like Hungary [27], Finland [28] and Israel [29]. The exposure group in the current study is exposed to relative higher levels of ELF-EMF than the ELF-EMF exposure groups reported in the researches mentioned above. However, uncertainty evaluation needs to be considered in the measurement of ELF-EMF [30], which would increase the comparability of different measurement results. A recent study showed that ELF-EMF might have effects on the nervous, cardiovascular, liver and
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Number
Control
Group
Female
P
Simple visual reaction time
\5 years
C5 years Total
P
Auditory digit span
0.09
hematology system of workers in automotive industry [26]. However, in current study, the items of neurobehavior tests were treated as dependent, ELF-EMF exposure as independent, no significant differences were found between control and exposure group, even after green tea and alcohol were treated as covariant through covariance analysis. Further analysis was carried out in different age, seniority groups, but no statistical significant results were found either. Obviously, our results do not seem to support our assumption, but there are always inconsistencies among results of in vivo, in vitro and epidemic studies. The inconsistencies might be attributed mainly to the differences of the demographic characteristics of the study subjects and experimental conditions, etc. [9, 10, 13]. On the other hand, in a recent research, a considerable nocebo
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effect in symptoms related to 50 Hz EMF exposure was reported, because idiopathic environmental intolerance to EMF seemed to be formed through a vicious circle of psychosocial factors, such as enhanced perception of risk and expectations, self-monitoring, somatization and somatosensory amplification and misattribution [25]. On this basis, previous studies that reported ELF-EMF exposure influenced the neurobehavior activities might not take the nocebo effect into account when the conclusions were made. Besides, despite all that mentioned above, there are possibilities that ELF-EMF exposure might induce or promote effects (probably damages) on CNS. In the last few years, both in vitro and in vivo studies on the health effects of ELF-EMF exposure described redox-related changes in CNS following ELF-EMF exposure [16, 31]. In fact, abnormal neurobehavior might emerge after but not before the alteration of CNS function, which means that we should have chosen biomarkers that emerge in earlier state of the alteration of CNS function. Such as nerve growth factor (NGF) which is widely recognized to play a crucial role in the process of free radicals scavenging and neuronal systems protection [32]. Therefore, more reliable and earlier biomarkers would be applied in following studies. On the other side, the effects of ELF-EMF exposure on CNS could only be detected during the exposure, and the effects might be disappeared as soon as the exposures stops [9, 33]. This indicated that the effects of ELF-EMF on humans CNS function, if any, exerted in a on and off way, but the cumulative effects of long-term intermittent ELF-EMF exposure were not observed in the current study. That means ELF-EMF might have transient effects without interfering with the CNS in a way measured here. Nerve cells are spontaneously active in awake animals, soma and dendrite membrane potentials continually fluctuate. So there is a possibility that thresholds for impulse initiation by externally ELF-EMF would be lowered than those necessary for the direct stimulation of peripheral nerves [34], however, the overall effect could be either excitatory or inhibitory, depending on the functional and brain tissue properties of the neuron(s) involved [13, 14, 35]. In all, despite the shortcomings mentioned in the current study, it seemed that long-term intermittent ELF-EMF exposure do not appear to disrupt normal neurobehavior like learning and memory capacity, perception and number-involved memory capacity, mental campaign and handeye coordination. The acute effects of ELF-EMF exposure were not explored and discussed in the current study, and the possible cumulative effects of ELF-EMF exposure on the CNS functions mentioned above were not observed in the current study. In the future, researches on this issue must include subjects that exposed to higher level of ELFEMF, in order to reveal whether there is the latent dose–
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response relation between ELF-EMF exposure and the alteration of CNS function. Acknowledgments This work was supported by the Guangdong Power Grid Corporation, Guangzhou, China. Conflict of interest The authors declare they have no competing financial interests.
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