Biological effects of exposure to static electric fields in humans and vertebrates: a systematic review

Field perception

Field perception experiments provided evidence that detection thresholds for static EF are much lower for whole-body exposure [29, 31] than limb exposure (e.g., arm and forehand) [30, 34]. Because these effects were confirmed by independent investigators, they can be considered as replicated. Blondin et al. [29] found that under whole-body exposure (static EF strength up to 50 kV/m, 7–11 s/trial) the median detection threshold of seated and grounded male and female subjects was 45.1 kV/m for a static EF. Approximately, 5% of the participants could detect a static EF below 20 kV/m, 33% of the subjects detected a static EF below 40 kV/m and 66% detected fields below 50 kV/m. Co-exposure to air ions with ion current densities of 60 nA/m² did not affect detection thresholds. When air ions in high concentrations (120 nA/m²) were added, the sensitivity was increased, permitting subjects to detect the EF at lower field strengths. Here, the median value was 36.9 kV/m, with some participants being able to perceive weaker fields of 10 kV/m or less. The authors estimated that the detection thresholds reported for seated subjects would be lowered if they were standing. Clairmont et al. [31] made observations under a hybrid power line (AC/DC) and found that when static EF (up to 40 kV/m) were combined with AC EF, detection thresholds were lower than what would be expected for static or AC EF alone, i.e., the combination of both greatly enhanced the perceived sensation. For static EF alone, an average detection threshold of 20 kV/m was estimated from the given data. Furthermore, 20% of the participants rated a static EF alone of 15 kV/m as “just perceptible”. However, this study had some methodological flaws (e.g., no appropriate control for confounders, subjects not grounded and not naive as to the purpose of the study, see Fig. 3).

Two further experiments were conducted under partial-body exposure where only the participants’ arm was exposed to static EF [30, 34]. Odagiri-Shimizu and Shimizu [34] used EF strengths of up to 450 kV/m and showed that the subjects were able to perceive static EF above 250 kV/m on their forearm when the relative humidity was 90%. When the humidity was set to 50%, the detection threshold increased to about 375 kV/m. Furthermore, when the volunteers knew that the field was on (awareness), detection of the static EF was facilitated. When the arm was shaved, the participants were no longer able to perceive a static EF at intensities up to 450 kV/m. This suggests that the perceived sensation is dependent on body hair. A similar study was conducted by Chapman et al. [30]. They exposed only the forearm of the subjects to a static EF (between 30 and 65 kV/m, 7–11 s/trial), but none of the subjects was able to perceive the fields. However, the maximum applied EF strength was much lower (65 kV/m) than in the study by Odagiri-Shimizu and Shimizu [34]. The authors concluded that the applied field strengths were too low to be detected under partial-body exposure and that the exposed body surface area could play a crucial role in the detection of static EF.

The striking differences in detection thresholds under whole-body and partial-body exposure are most parsimoniously explained by the higher EF on some parts of the body with whole-body exposure. The presence of a person in an EF will perturb the uniformity of the field. Field lines then concentrate on body parts closest to the EF source, i.e., the field increases at the top of the body (e.g., head/shoulder) about a factor of 13–18 while it decreases at lower body parts (e.g., arms and legs) relative to upper body parts [36]. Such a field increase in the head/shoulder region should facilitate the perception of the field. The notable field increases may also explain why both Blondin et al. [29] and Clairmont et al. [31] reported that some people are able to detect static EF at field strengths of 10 kV/m and even below that level. Field perturbation occurs much less when only the forearm is exposed because of the comparatively flat shape of the arm. This could explain the much higher EF strengths required for detection performance under partial-body exposure in the studies by Odagiri-Shimizu and Shimizu [34] and Chapman et al. [30]. Other factors that may influence perception of static EF are the density and length of hair on the body. In addition, in human studies, a lowering of detection thresholds in experimental situations might occur where awareness as to possible exposure and lack of distracting/confounding stimuli prevail.

Physiological/health-related effects

In addition to field perception experiments, we identified four other studies which examined physiological and health-related effects in humans upon exposure to static EF. The results of these studies have not been replicated, yet.

One of the experimental studies on skin symptoms among visual display unit users found that facial skin complaints might be caused by a combination of exposure to static EF (0.23 kV/m on average for 6 h/day) and high dust concentrations [33], while a study by Oftedal et al. [32] could not find a relation between skin symptoms and exposure to static EF (2 kV/m on average for 2 h/day). Furthermore, it could not be shown that static EF (1 kV/m) alter psychomotor and physiological functions in a group of pilots [28]. In the only epidemiological study in this field, Haupt and Nolfi [35] considered potential health effects in relation to residential proximity to a HVDC transmission line. Examined endpoints were symptoms of discomfort (e.g., headache, depression, eye irritation), health status, number of physician visits, and illness days. People who had lived in close proximity (less than 225 m) to the 400 kV Pacific Intertie HVDC transmission line in California for at least 5 years were included in this study (n = 438). Static EF strengths were approximately 21 kV/m under the positive pole and −16 kV/m under the negative pole at ground level according to measurements on a similar test line. Static magnetic fields and air ions were also present. The results showed no statistically significant association between exposure to the HVDC transmission line and perceived health problems among adjacent residents.

Perception mechanism/Behavioral responses

Similar to what has been shown in humans, Kato et al. [48] found evidence that body hair is involved in the perception of static EF by cats. The authors recorded afferent impulse discharges of hair receptors when the anesthetized cats were exposed to static EF (180–310 kV/m). The stronger the EF, the wider was the angle of the hair movement. In addition, more action potentials were triggered with increasing EF strength. Deeper skin receptors were not affected. This effect, therefore, is consistent with electrostatic forces causing hair movement that leads to sensory stimulation and detection of static EF.

A further eight studies investigated behavioral responses of vertebrates to static EF exposure [39, 45–47, 49–52], but the results have not been clearly replicated by separate laboratories. Locomotor activity, avoidance behavior and food and water intake were mainly examined in mammals. Birds were studied besides mice in one study [49]. The studies differed greatly regarding the applied EF strengths (1–340 kV/m), exposure duration (1 h to several months) and the numbers of treated animals (10 to 360). Additionally, the provided documentation often did not allow us to appropriately assess the quality of the experimental setup and methods e.g., [49, 51, 52]. Despite these methodological limitations and the limited data available, there is good evidence that static EF can be detected and elicit behavioral responses in vertebrates probably due to sensory stimulation of the skin and body hair. In rodents and some other animals, the vibrissae are important mechanosensory receptors that are sensitive to tactile stimulation, which modulate a wide variety of behaviors, and this helps explain why secondary physiological responses to tactile stimuli, including static EF should be expected [53]. Besides hair movement as a physical mechanism for the detection of static EF, it can be further hypothesized that high EF strengths may lead to an ionization of air ions and ozone production, known as the corona effect. The well-developed sense of smell in animals also may help them perceive the simultaneous presence of ozone and initiate a response to the static EF.

Three of these studies reported that static EF (between 1 kV/m and 23.8 kV/m, between 1 h and 20 days) have a stimulating effect on the locomotor activity [49, 51, 52]. Studies by Altmann and Möse on locomotor activity [49, 52] were motivated by previously reported results of positive and stimulating effects of both static EF and air ions on humans and animals (e.g., improvement in cognitive performance in humans, general health promotion of human and cows [54, 55]). These findings were not confirmed by Bailey and Charry [39]. As part of a study of air ions in which groups of animals were exposed to static EF alone (3 kV/m or 12 kV/m for 2, 18 or 66 h), the authors found no influence of static EF on two continuous measures of motor activity in rats.

Two studies investigated avoidance behavior in rats [46, 47]. Creim et al. [47] showed that rats avoided static EF (between 55 and 80 kV/m for 1 h), regardless of the presence of air ions. This behavior was found to be dose dependent with higher field strengths inducing greater field avoidance. In a later study, Creim et al. [46] failed to induce taste-aversion learning in rats in exposed environments (75 kV/m, 4 h/day for 5 days). The authors speculated that avoidance behavior observed in the earlier study was likely prompted by a response to external sensory stimulation, i.e., the perception of the static EF on the fur. The second study, however, indicated that internal stimulation such as gastrointestinal distress did not occur as a consequence of exposure.

Exposure effects on food and water intake were investigated in three studies [39, 50, 52]. Bailey and Charry [39] (with exposures at 3 kV/m or 12 kV/m for 2, 18 or 66 h) did not report any effect in rats, but the other two studies by Fam [50] (exposure at 340 kV/m for 18–22 h/day for 30 weeks) and Möse and Fischer [52] (exposure at 23.8 kV/m for 15 to 20 days) found altered food and water intake in mice.

Various aspects of cattle behavior were investigated in an experimental field study by Ganskopp et al. [45]. They tracked the animals’ activity and distribution under exposure to the static EF of a 500 kV HVDC transmission line and concluded from the data that they do not provide evidence that a static EF or other aspects of the HVDC electrical environment altered the behavior of cattle.

Effects on the brain and nervous system

Five studies were identified that investigated the effects of static EF on the nervous system of rats and mice [38, 40, 56–58], but the results of these studies have not been replicated thus far. Exposure durations were between 50 min and 35 days and the applied EF strengths varied between 3 kV/m and 23.8 kV/m. Study populations had a size of 5 to 30 animals per group.

Four of these studies examined various neurotransmitter concentrations in the brains of rodents, but the results were inconsistent [38, 40, 56, 57]. Möse et al. [56] reported significantly reduced serotonin levels in the brain of guinea pigs that had been exposed to a static EF (23.8 kV/m for 6 days). They hypothesized an association between metabolic changes – possibly triggered by an activating action of static EF and air ions – and the decrease in serotonin level (see section Histological and biochemical organ parameters). In contrast, three other studies found no changes in neurotransmitter concentrations. Bailey and Charry [38] and Charry and Bailey [40] reported that norepinephrine, dopamine and serotonin concentrations were not affected in rats’ brains after the animals were exposed to a static EF (3 kV/m for 2,18 or 66 h). Xu et al. [57] tested spatial learning and memory abilities of mice previously exposed to a static EF (between 2.3 and 21.85 kV/m for 35 days) beneath a HVDC line in the ambient environment. They did not find changes in glutamate and GABA levels which have been associated with learning and memory abilities in some other studies. However, the authors found that mice which were exposed at the highest field strengths showed behavior suggestive of impaired memory ability in a water-filled maze. Because changes in neurotransmitter concentrations did not account for the differences in performance between exposed and control mice, Xu and his co-workers hypothesized that static EF might suppress the expression of receptors which are involved in memory formation.

Lott and McCain [58] found changes in electro-encephalographic (EEG) recordings of rats under the influence of static EF (10 kV/m for 50 min). They showed that the EEG was modified (increase in cortical brain activity and reduced hypothalamic activity) when switching the static EF on and returned back to baseline values when the field was turned off again. The authors suggested that the increased general brain activity under exposure conditions lowered the activity of the hypothalamus. They interpreted their data showing a neuronal correlate for the rats’ ability to detect static EF, with the hypothalamus being a putative electro-sensitive region. Potential confounding due to coupling of the external field to the electrode, especially when the field was turned on or off during recording the electrical activity of the brain, or that the EEG recording reflected sensory stimulation of the skin or fur was not considered or discussed.

Histological and biochemical organ parameters

In total, 18 studies examined various histological and biochemical parameters (metabolic activity, histological effects, collagen synthesis, oxidative stress and bone density) in different organs in rodents. No studies by independent investigators attempted to replicate the reported results. Organ parameters were the main focus in ten studies [59–68], whereas in other more comprehensive studies, organ parameters were only one of the endpoints evaluated among others (e.g., [43, 49, 50, 52, 56, 69–71]). The number of animals per group differed between 5 and 32. The applied field strengths ranged from 0.42 to 340 kV/m and exposure durations varied between 3 days and 2 years. A good number of studies reported effects on several histological and biochemical parameters upon exposure to static EF, but most of these studies had several methodological flaws (see Fig. 4). Some of the evaluated studies also lacked clear hypotheses as to the choice of examined endpoints or a discussion on the relevance of their results for possible health effects. The reported effects on metabolic functions and collagen synthesis were mainly discussed in terms of direct cell-field interactions. Some studies emphasized the beneficial effects of static EF on metabolism compared to animals held in an environment shielded by a Faraday cage.

Five studies – all conducted by Altmann and Möse – reported that static EF have a stimulating effect on metabolic activity in rodents [49, 56, 60, 62, 68]. Static EF strengths in the studies by Altmann were 0.42 kV/m [62] and 1 kV/m [49], respectively, while in the studies by Möse and colleagues the animals were exposed at a field strength of 23.8 kV/m [56, 60, 68]. Only one study with exposures at 23.8 kV/m did not find such a stimulating effect on metabolism [52]. It was speculated that altered metabolic functions may be the result of direct effects of static EF and air ions. Altmann [49] and Möse et al. [60] suggested a mechanism through which static EF act on cell functions by modifying bioelectrical potentials which in turn lead to increased cellular respiration. Möse and colleagues discussed that absorbed air ions may induce a serotonin release in the brain [56] or a shift in the metabolic activity of organs [68]. However, the authors did not consider the possibility that the responses reported also could have been indirect effects resulting from external sensory stimulation by the static field.

Additionally, one of these studies reported that mice which were kept in a Faraday cage (which blocks both ambient static and low frequency EF), had a lower oxygen consumption compared to the control group under ambient conditions [60]. According to the authors, lowered oxygen consumption, i.e., decreased metabolic activity, of rodents held in a Faraday cage indicates that these animals were disadvantaged by the absence of both static EF and air ions (see also section Hematology and immunology, Möse et al. [72]). The authors speculated that shielding from the natural EF, as occurs in most buildings, may have adverse effects on health.

A direct interaction between static EF and tissue proteins was proposed in several studies, all conducted by the same research group, which examined collagen synthesis in guinea pigs based on measurements of hydroxyproline levels in various organs [59, 63–65]. Güler, Atalay and colleagues chose to examine collagen, being the most abundant protein in vertebrates. Low EF strengths (between 0.58 and 0.9 kV/m with exposures of 9 h/day for 3 days) [63, 65] led to a reduction in the tissue hydroxyproline concentration, while exposure at 1.9 kV/m for the same exposure durations [59, 64, 65] led to an increase in hydroxyproline levels. The authors suggested that static EF influences on protein biosynthesis may be the result of penetration of static EF into the tissue. However, there was no attempt by the authors to explain why decreases and increases of hydroxyproline levels vary unpredictably as a function of EF strength. It was merely suggested that there could be a threshold below and above which decreases and increases of hydroxyproline concentration are triggered, respectively. In all four trials, the vertical field resulted in a stronger effect than the horizontal EF and this finding was confirmed in an additional histological examination of the liver with decrease and increase in collagen fibers being only observed under vertical static EF exposure [65].

Four more recent studies on rodents by some of the same investigators who proposed static EF effects on proteins also reported that exposure to static EF can induce oxidative stress in various organs [66, 67, 70, 71]. The authors of these studies did not discuss the potential mechanisms of action by which oxidative stress could be induced and it remains unclear how or why static EF could cause this response.

The study by Okudan et al. [61] provided some evidence for the influence of static EF (10 kV/m for 28 days) on bone density and mineral content after exposure of fetal and newborn rats, although the basis for this finding is unclear.

Finally, studies in three separate laboratories investigated the possible effects of static EF (with exposures between 0.6 and 340 kV/m for at least 30 days up to 30 weeks) on the histological appearance of diverse organ systems of rats and mice [43, 50, 69]. None of these studies found histological abnormalities in organs such as lungs, liver, kidney or testis. However, Marino et al. [69] reported that some of the animals developed secondary glaucoma (an eye disease). This unexpected effect was only observed in rats exposed to vertical static EF, but not in those exposed to horizontal fields or in the control group. The authors considered it likely that glaucoma was induced by static EF. However, no other study in the evaluated literature examined or reported any effect of static EF on eyes.

Hematology and immunology

Fourteen studies evaluated hematologic and/or immunologic parameters. Again, the results of these studies have not been replicated by independent investigators. Four studies focused on the effect of air ions [41–43, 73], the remaining ten studies examined whether the static EF itself affected these parameters [50, 69–72, 74–78]. The applied static EF field strengths varied between 0.04 kV/m and 340 kV/m and animals were exposed between 1 h and 30 weeks. The number of animals ranged from 5 to 60 per group. All but one study [73] reported variations in hematologic and/or immunologic parameters upon exposure of the animals to static EF. Direct and indirect mechanisms of the influences of static EF were considered to explain altered hematologic and immunologic parameters. Most of these studies had methodological limitations (see Fig. 4, e.g., allocation of animals to study groups not concealed, no verification of static EF strength, missing control for possible confounders) and it was often not clear from the interpretation of the data what significance they might have for health, i.e., whether static EF have beneficial or detrimental effects on the investigated hematological and immunological parameters in animals.

Möse et al. [72] reported an increased immune response in mice under static EF exposure (static EF strengths between 0.04 and 24 kV/m for 15 days), whereas the immune response was decreased in animals kept in a Faraday cage (zero field). The authors cited these results in support of their hypothesis that exposure to static EF was beneficial and shielding animals from static EF had a negative impact [60].

In a long-term experiment, in which mice were continuously exposed to static EF (2 kV/m) during a period of two years, Kellogg and co-workers found increased values in serum glucose and decreased urea nitrogen levels [41–43]. Furthermore, the mice exposed to the static EF alone lived longest. The authors saw a connection between serum glucose level and lifespan which lent support to their hypothesis that bioelectric processes are involved in mortality and aging rate.

Other studies also consistently reported variations in some blood parameters in rodents upon exposure to static EF. The investigated parameters varied considerably and regarded the serum concentration of various proteins such as albumins and globulins [69, 78], content of hemoglobin and lymphocyte number [50], indicators of oxidative stress [70, 71], Ca²⁺-dependent enzyme activities in the membranes of erythrocytes and mitochondria [74], serum lysozyme activity [75], changes in the surface charge of erythrocytes [77] and number of erythrocytes [76].

Possible mechanisms for the observed alterations of blood parameters were discussed by several authors. Fam [50] discussed his findings in terms of indirect effects of static EF. Living systems are well shielded from the direct influence of EF but the field can act on the skin and fur and thus provide sensory stimulation. Any such interactions may then be transmitted through the blood or the nervous system to deeper body layers. Yet, there was no concrete evidence for this hypothesis in this study. Changes in functional states of enzyme activities [74] and modifications of the surface charge of erythrocytes [77] were discussed to be induced by influences of the static EF on the cell membrane, such as polarization or conformational changes of membrane proteins as well as the modification of the distribution of electric charges. Whether this impact is direct or indirect (for example, via a metabolic cascade) is put up for discussion by the authors [77].

Reproduction and development

Three studies examined the reproduction and development of mammals under the influence of a static EF [44, 50, 52]. A replication of the results has not been reported by now. The animals were exposed to static EF between 5.6 kV/m and 340 kV/m and for durations between 4 and 30 months. The size of the study population was between 12 and 50 animals per group.

The data from two extensive laboratory studies on mice were not consistent. Möse and Fischer [52] reported fewer litters in the exposed groups with increasing exposure duration (static EF of 23.8 kV/m for at least 15 days up to 4 month). They did not provide an explanation for this finding because they could not exclude the possibility that this result was raised by chance; the authors therefore suggest that the effect should be verified in upcoming studies. The data on reproduction and development contrasts with the otherwise postulated positive and stimulating effect of the static EF posed by the study authors (see sections Perception mechanism/ Behavioral responses, Histological and biochemical organ parameters and Hematology and immunology). Fam [50], however, did not find an effect of static EF (340 kV/m for 30 weeks) on the number of progenies.

The extensive experimental field study by Angell et al. [44] (observation period of 30 months) provided no evidence for an effect of a HVDC transmission line (mean static EF strength of 5.6 kV/m) on the reproduction and development of cattle (e.g., pregnancy rate, weaning weight) in comparison to a herd kept away from the power line.

Genotoxicity

The two studies on genotoxicity - conducted by the same research group - implanted Ehrlich ascites tumor cells in mice [69, 79]. They found chromosomal abnormalities in these tumor cells after a 14-day static EF exposure (8–16 kV/m). Prolongation of exposure and observation period in the second study showed that the effects in exposed mice were transient and disappeared with continued exposure (15 weeks) [79], but these effects have not been confirmed in independent replication studies. The authors assumed that the cells with chromosomal abnormalities died and that only those cells with intact chromosomes survived and proliferated. They further noted that the energy from the applied static EF would have been too low to cause direct effects on biological systems (i.e., a cell-field interaction); thus, the observed effect had to be transmitted via a as of yet unexplained kind of “information”.

Therapeutic approaches

The study by Gray et al. [37] points to an improved effect of a chemotherapeutic agent in mice, when it is combined with exposure to static EF (450 kV/m, 4 h/day for 13 days). A significantly greater tumor regression of an implanted mammary adenocarcinoma was observed in the group exposed to the static EF and the chemotherapeutic agent compared to mice that received only the chemotherapeutic agent. This effect has not been replicated as yet. The authors speculated on possibilities how static EF may act on cell functions inside the body: Both the inhomogeneous electrical conductor characteristics of the body and the continuous field variations in and around cells due to its dynamic functioning could entail that static EF are not entirely attenuated (i.e., drop to zero) when reaching the body surface.