Measurement of magnetically induced electric fields in conductive media near a 60 Hz current-carrying wire.

Electric fields induced in a conductive body by the magnetic field of a current-carrying wire were analyzed theoretically and experimentally to assess the dosimetric importance of highly nonuniform, field-exposure conditions. Experimentation revealed that a 60-Hz magnetic field was inversely proportional to the radius of a wire bundle carrying 100 A within a 0.5-m2 test area. A miniaturized electric field probe was used to measure the electric fields induced in 5-cm-deep, saline-filled models. In the theoretical analysis, numerical estimates of induced fields were made by a spreadsheet method. The theoretical calculations and the measured values of induced electric fields were generally in good agreement. The induced fields were in a plane perpendicular to a vertically incident magnetic field; the maximally induced fields were in areas nearest the wire bundle. The strength of the induced field increased with model size: from 96 microV/cm in a 10 x 10 cm model to 176 microV/cm in a 40 x 40 cm model. The strength of the field induced in a 20 x 20 cm model decreased with increasing model-to-wire spacing: from 132 microV/cm for a 1-cm spacing (2-mT maximum, incident field) to 50 microV/cm for a 6-cm spacing (0.33-mT maximum). The results indicate that increases in local values of nonuniformly incident fields produce relatively small increases in induced electric fields. This finding may be important in dosimetric consideration of circumstances, such as use of electric blankets, in which fields of low average strength are accompanied by intense local fields.     

Test for the effects of 60-Hz magnetic fields on fecundity and development in Drosophila

Ramirez et al (1983) reported reduced egg laying by Drosophila melanogaster and reduced survival of those eggs to adulthood when adult flies were exposed to magnetic fields. In a similar study, no effects from exposures of Drosophila to 1-mT, 60-Hz magnetic fields were found.     

Current densities measured in human models exposed to 60-Hz electric fields

This paper gives current densities measured in homogeneous grounded human models exposed to vertical, 60-Hz electric fields. The methods used for these measurements were validated by measuring the current densities induced in a grounded hemisphere and in a grounded prolate hemispheroid; agreement between measurement and theory was good. For an unperturbed field strength of 10 kV/m, current densities measured in the human chest were in the range 125-300 nA/cm2. A strong horizontal current-density enhancement was observed in the axillae, with peak values of about 400 nA/cm2. The vertical current density in the arms, when held downward, was in the opposite direction to that in the chest. Current densities in the abdomen, pelvis, and legs were a strong function of whether the body was grounded through one or both feet. With one foot grounded, the horizontal current density in the lower pelvic region, just above the crotch, was 770 nA/cm2. This value was the largest of those measured in the head, arms, or torso of the human model. Scaling factors derived from these data and similar data for animals will provide a quantitative basis for comparing animal and human exposure to 60-Hz electric fields. In addition, current-density data given in this paper can be directly extrapolated to higher frequencies, at least to 1 MHz. These extrapolated data may be useful to individuals and groups involved in the determination of safety standards for the lower radiofrequency region.     

Electric fields induced in chicken eggs by 60-Hz magnetic fields and the dosimetric importance of biological membranes.

Chicken eggs are convenient models for observing the effects of inhomogeneities and variations, such as those found in biological membranes and in cellular conductivities, on the distribution of internal electric fields as induced by exposure to magnetic fields. The vitelline membrane separates the yolk, which has a conductivity of 0.26 S/m, from the white, which has a conductivity of 0.85 S/m. A miniaturized probe with 2.4-mm resolution was used to measure induced fields in eggs placed in a uniform, 1-mT magnetic field at 60 Hz. The E fields induced in eggs with homogenized contents agreed with expectations based on simple theory. Results were similar to intact eggs unless the probe moved the yolk off-center, which greatly perturbed the induced fields. A more reproducible arrangement, which consisted of saline-agar filled dishes with a hole cut for test samples, was developed to enhance definition of electrical parameters. With this test system, the vitelline membrane was found to be responsible for most of the perturbation of the induced field, because it electrically isolates the yolk from the surrounding white. From a theoretical viewpoint, this dosimetry for the macroscopic egg yolk is analogous to the interaction of fields with microscopic cells. These findings may have important implications for research on biological effects of ELF electromagnetic fields, especially for studies of avian embryonic development.     

Electromagnetic field treatment of nerve crush injury in a rat model: effect of signal configuration on functional recovery

Electromagnetic fields (EMFs) have been demonstrated to enhance mammalian peripheral nerve regeneration in vitro and in vivo. Using an EMF signal shown to enhance neurite outgrowth in vitro, we tested this field in vivo using three different amplitudes. The rat sciatic nerve was crushed. Whole body exposure was performed for 4 h/day for 5 days in a 96-turn solenoid coil controlled by a signal generator and power amplifier. The induced electric field at the target tissue consisted of a bipolar rectangular pulse, having 1 and 0.3 ms durations in each polarity, respectively. Pulse repetition rate was 2 per second. By varying the current, the coils produced fields consisting of sham (no current) and peak magnetic fields of 0.03 mT, 0.3 mT, and 3 mT, corresponding to peak induced electric fields of 1, 10, and 100 microV/cm, respectively, at the tissue target. Walking function was assessed over 43 days using video recording and measurement of the 1-5 toe-spread, using an imaging program. Comparing injured to uninjured hind limbs, mean responses were evaluated using a linear mixed statistical model. There was no difference found in recovery of the toe-spread function between any EMF treatments compared to sham.     

RF Exposure During Use of Electrosurgical Units

Electrosurgical units (ESUs) commonly used in operating suites employ radiofrequency (RF) energy for cutting and coagulation, and operate at different frequencies in the range 0.3–5 MHz. Around the electrode and cables, electric and magnetic fields at similar frequencies will be generated, and the surgeon using the ESU will therefore be exposed to these electromagnetic fields. In this study we have measured the levels of RF fields near the lead wires of two electrosurgical units, BARD 3000 operating at a fixed frequency of 0.5 MHz, and ERBE ICC 350 with a frequency range from 0.3 to 1 MHz. Electric fields were measured at distances from 5–30 cm from the lead wire. Measurements were done with the ESU both cutting and coagulating, and power levels ranging from 10–100 W. The magnetic field outside the lead wire was calculated from the measured current through the leads using standard theory. Using those measurements as a base, the calculated local exposure of the surgeon's hand was estimated to exceed 15 kV/m for the electric field and the corresponding value for the magnetic field was 16 µT. These calculations exceed the suggested international reference levels at 0.5 MHz (610 V/m and 4 µT, respectively).     

Activity in torso muscles during relaxed standing

Electromyographic activity of erector spinae, external oblique, and rectus abdominis muscles was studied during relaxed standing compared to lying down. Activity in the forearm extensors and forearm flexors was also studied. Surface electrodes were used. Each of the torso muscles exhibited 0.2 microV of activity and the forearm muscles 0.1 microV while subjects were relaxed and lying down. During quiet standing the erector spinae, external oblique, and rectus abdominis muscles showed a median activity of 1.0 microV, 2.5 microV, and 0.7 microV respectively (for a minimum of ten 10-sec samples per subject). Examination of the integrated records during standing revealed no periods without increased muscle activity in the torso muscles. By contrast, activity in the forearm muscles did not increase during standing. The major superficial muscles of posture in the torso appear to act as guy wires, being continually active during standing. There is no support for hypotheses of passive support for the torso, nor do torso muscles act in either/or fashion; both anterior and posterior muscles are active at once. There is no sign of generally increased muscle tone in all muscles or in extensors; only the postural muscles are continuously active.     

Investigating the effects of external fields polarization on the coupling of pure magnetic waves in the human body in very low frequencies

In this paper we studied the effects of external fields' polarization on the coupling of pure magnetic fields into human body. Finite Difference Time Domain (FDTD) method is used to calculate the current densities induced in a 1 cm resolution anatomically based model with proper tissue conductivities. Twenty different tissues have been considered in this investigation and scaled FDTD technique is used to convert the results of computer code run in 15 MHz to low frequencies which are encountered in the vicinity of industrial induction heating and melting devices. It has been found that external magnetic field's orientation due to human body has a pronounced impact on the level of induced currents in different body tissues. This may potentially help developing protecting strategies to mitigate the situations in which workers are exposed to high levels of external magnetic radiation.     

STIMULATION OF GERMINATION IN RICE (ORYZA SATIVA L.) BY A STATIC MAGNETIC FIELD

This study evaluates the percentage and rate of germination of rice (Oryza sativa L.) seeds, when exposed to magnetic treatment in laboratory conditions. The seeds were exposed to 150 and 250 mT magnetic fields both chronically and for 20 min after seeding. Nonexposed seeds were used as control. Chronic exposure to a 150-mT magnetic field increased (p < 0.05) both the rate and percentage of germination relative to nonexposed seeds (18% at 48 h). Significant differences were also obtained for seeds exposed to a 250-mT magnetic field for 20 min (12% at 48 h). Additionally, seeds were moistened with water magnetically treated by static and dynamic methods. Dynamic and static treatment of water improved the germination of seeds related to the control, but significant differences (p < 0.05) were only obtained for the dynamic method (16% at 48 h).     

Empirical and theoretical dosimetry in support of whole body resonant RF exposure (100 MHz) in human volunteers

This study reports the dosimetry performed to support an experiment that measured physiological responses of volunteer human subjects exposed to the resonant frequency for a seated human adult at 100 MHz. Exposures were performed in an anechoic chamber which was designed to provide uniform fields for frequencies of 100 MHz or greater. A half wave dipole with a 90 degrees reflector was used to optimize the field at the subject location. The dosimetry plan required measurement of transmitter harmonics, stationary probe drift, field strengths as a function of distance, electric and magnetic field maps at 200, 225, and 250 cm from the dipole antenna, and specific absorption rate (SAR) measurements using a human phantom, as well as theoretical predictions of SAR with the finite difference time domain (FDTD) method. On each exposure test day, a measurement was taken at 225 cm on the beam centerline with a NBS E field probe to assure consistently precise exposures. A NBS 10 cm loop antenna was positioned 150 cm to the right, 100 cm above, and 60 cm behind the subject and was read at 5 min intervals during all RF exposures. These dosimetry measurements assured accurate and consistent exposures. FDTD calculations were used to determine SAR distribution in a seated human subject. This study reports the necessary dosimetry for work on physiological consequences of human volunteer exposures to 100 MHz.     

Cancer promotion in a mouse-skin model by a 60-Hz magnetic field: II. Tumor development and immune response.

This paper describes preliminary findings on the influence of 60-Hz (2-mT) magnetic fields on tumor promotion and co-promotion in the skins of mice. The effect of magnetic fields on natural killer (NK) cell activity in spleen and blood was also examined. Groups of 32 juvenile female mice were exposed to the magnetic field as described in part I. The dorsal skin of all animals was treated with a subthreshold dose of the carcinogen 7,12-dimethyl-benz(a)anthracene (DMBA). One week after the treatment, two groups were sham exposed (group A) or field exposed at 2 mT (group B) 6 h/day for 21 weeks, to test whether the field would act as a tumor promoter. No tumors developed in these two groups of mice. To test whether the magnetic field would modify tumor development by directly affecting tumor growth or by suppressing immune surveillance, two additional groups of mice were treated weekly with the tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) and then either sham exposed (group C) or field exposed (group D). The time to appearance of tumors was shorter (but not statistically so) in the group exposed to magnetic fields and TPA. Some differences in NK cell activity and spleen size were observed between the sham- and field-exposed groups.     

Behavioral studies with mice exposed to DC and 60-Hz magnetic fields

Behavioral measures were evaluated in adult CD-1 and LAF-1 mice continuously exposed for 72 h to a 1.5-Tesla (1 T = 10(4) Gauss) homogeneous DC magnetic field, and in LAF-1 mice continuously exposed for 72 h to a sinusoidal 60-Hz, 1.65-mT (rms) homogeneous AC field. Three types of behavioral tests were employed: (1) Memory of an electroshock-motivated passive avoidance task was assessed in animals that had been trained immediately prior to the field exposure. The strength of memory was varied either by altering the strength of the electric footshock during training, or by administering a cerebral protein synthesis inhibitor, anisomycin, at the time of training. (2) General locomotor activity was measured using a quadrant-crossing test immediately after termination of the magnetic field exposure. (3) Sensitivity of the experimental subjects to the seizure-inducing neuropharmacological agent, pentylenetrazole , was assessed immediately after the field exposure on the basis of three criteria: (a) the percentage of subjects exhibiting a generalized seizure, (b) the mean time to seizure, and (c) the mean seizure level. The results of these studies revealed no behavioral alterations in exposed mice relative to controls in any of the experimental tests with the 1.5-T DC field or the 60-Hz, 1.65-mT (rms) AC field.     

Effects of a 30 kV/m, 60 Hz electric field on the social behavior of baboons: a crossover experiment.

Using a crossover experimental design, we evaluated our earlier findings that exposure to a 30 kV/m, 60 Hz electric field for 12 hours per day, 7 days per week for 6 weeks produced significant changes in the performance rates of social behaviors among young adult male baboons. In the crossover experiment, the former control group was exposed to a 30 kV/m, 60 Hz electric field for 3 weeks. Only an extremely small, incidental magnetic field was generated by the exposure apparatus. We found that electric-field exposure again produced increases in the performance rates that index Passive Affinity, Tension, and Stereotypy. These findings, combined with results from our other electric-field experiments, indicate that exposure to strong electric fields, in the absence of associated magnetic fields, consistently produces effects that are expressed as increases in rates of performance of social behaviors in young adult male baboons.     

Power-frequency electric fields averaged over the body surfaces of grounded humans and animals

Calculated electric-field strengths averaged over the body surfaces of grounded humans, swine, rats, horses, and cattle exposed to vertical, uniform, power-frequency electric fields are presented. To produce the same average fields over the body surfaces of grounded animals, as that experienced by a grounded man exposed to an unperturbed vertical field of 10 kV/m, the following unperturbed fields are required: swine, 19 kV/m; rat, 37 kV/m; horse, 18 kV/m; cow, 18 kV/m.     

Calculation of electric fields and currents induced in a millimeter-resolution human model at 60 Hz using the FDTD method

The finite-difference time-domain (FDTD) method has previously been used to calculate induced currents in anatomically based models of the human body at frequencies ranging from 20 to 915 MHz and resolutions down to about 1.25 cm. Calculations at lower frequencies and higher resolutions have been precluded by the huge number of time steps that would be needed in these simulations. This paper describes a method used to overcome this problem and efficiently calculate induced currents in an MRI-based, 6-mm-resolution model of the human under a high-voltage transmission line. This model is significantly higher resolution than the 1.31-cm-resolution model previously used; therefore, it can be used to pinpoint locations of peak current densities in the body. Proposed safety guidelines would allow external electric fields of 10 kV/m and 25 kV/m for exposure to 60 Hz fields of the general public and workers, respectively. For this external electric field exposure of 10 kV/m, local induced current densities as high as 20 mA/m² are found in the head and trunk with even higher values (above 150 mA/m²) in the legs. These currents are considerably higher than the 4 or even 10 mA/m² that have been suggested in the various safety guidelines, thus indicating an inconsistency in the proposed guidelines. In addition, several ratios of E/H typical of power line exposures were examined, and it was found that the vertical electric field couples strongly to the body, whereas the horizontal magnetic field does not. Bioelectromagnetics 19:293–299, 1998. © 1998 Wiley-Liss, Inc.     

Pulsed Low-Frequency Magnetic Fields Induce Tumor Membrane Disruption and Altered Cell Viability

Tumor cells express a unique cell surface glycocalyx with upregulation of sulfated glycosaminoglycans and charged glycoproteins. Little is known about how electromagnetic fields interact with this layer, particularly with regard to harnessing unique properties for therapeutic benefit. We applied a pulsed 20-millitesla (mT) magnetic field with rate of rise (dB/dt) in the msec range to cultured tumor cells to assess whether this affects membrane integrity as measured using cytolytic assays. A 10-min exposure of A549 human lung cancer cells to sequential 50- and 385-Hz oscillating magnetic fields was sufficient to induce intracellular protease release, suggesting altered membrane integrity after the field exposure. Heparinase treatment, which digests anionic sulfated glycan polymers, before exposure rendered cells insensitive to this effect. We further examined a non-neoplastic human primary cell line (lung lymphatic endothelial cells) as a typical normal host cell from the lung cancer microenvironment and found no effect of field exposure on membrane integrity. The field exposure was also sufficient to alter proliferation of tumor cells in culture, but not that of normal lymphatic cells. Pulsed magnetic field exposure of human breast cancer cells that express a sialic-acid rich glycocalyx also induced protease release, and this was partially abrogated by sialidase pretreatment, which removes cell surface anionic sialic acid. Scanning electron microscopy showed that field exposure may induce unique membrane “rippling” along with nanoscale pores on A549 cells. These effects were caused by a short exposure to pulsed 20-mT magnetic fields, and future work may examine greater magnitude effects. The proof of concept herein points to a mechanistic basis for possible applications of pulsed magnetic fields in novel anticancer strategies.     

Induced electric field and current density patterns in bone fractures

We have used the low frequency solver of the computer program SEMCAD‐X to model the induced electric field and current density patterns in simple models of a fractured femur embedded off‐center in cylindrical muscle tissue; a 1 cm fracture gap is filled with callus. The model is exposed to a 1 kHz, 1 mT sinusoidal magnetic field. The frequency chosen is typical of the major Fourier components of many waveforms used to stimulate fracture healing using pulsed magnetic fields; the intensity is also a typical level. Models include fractures perpendicular to the bone and at an angle from the perpendicular, each exposed to a field applied parallel to the bone or parallel to either of the two axes perpendicular to it. We find that all directions of applied magnetic fields produce essentially parallel induced electric fields and current densities through the plane of the callus, but that a magnetic field applied parallel to the bone induces considerably higher fields and currents than the same strength field applied in either perpendicular direction. Because investigations of pulsed‐field devices, including modeling of induced fields and currents, peaked more than a decade ago, this is the first application to our knowledge of the current capabilities of computer modeling systems to biological systems at low frequencies. Bioelectromagnetics 33:585–593, 2012. © 2012 Wiley Periodicals, Inc.     

Comparison of cardiac and 60 Hz magnetically induced electric fields measured in anesthetized rats

Extremely low frequency magnetic fields interact with an animal by inducing internal electric fields, which are in addition to the normal endogenous fields present in living animals. Male rats weighing about 560 g each were anesthetized with ketamine and xylazine. Small incisions were made in the ventral body wall at the chest and upper abdomen to position a miniature probe for measuring internal electric fields. The calibration constant for the probe size was 5.7 mm, with a flat response from at least 12 Hz to 20 kHz. A cardiac signal, similar to the normal electrocardiogram with a heart rate of about 250 bpm, was readily obtained at the chest. Upon analysis of its spectrum, the cardiac field detected by the probe had a broad maximum at 32–95 Hz. When the rats were exposed to a 1 mT, 60 Hz magnetic field, a spike appeared in the spectrum at 60 Hz. The peak-to-peak magnitudes of electric fields associated with normal heart function were comparable to fields induced by a 1 mT magnetic field at 60 Hz for those positions measured on the body surface (where induced fields were maximal). Within the body, or in different directions relative to the applied field, the induced fields were reduced (reaching zero at the center of the animal). The cardiac field increased near the heart, becoming much larger than the induced field. Thus, the cardiac electric field, together with the other endogenous fields, combine with induced electric fields and help to provide reference levels for the induced-field dosimetry of ELF magnetic field exposures of living animals. Bioelectromagnetics 18:317–323, 1997. © 1997 Wiley-Liss, Inc.     

Miniature-probe measurements of electric fields induced by 60 Hz magnetic fields in rats

Extremely-low-frequency (ELF) magnetic fields interact with an animal by inducing internal electric fields, which represent the internal dose from an external exposure. In this study, an electric field probe of approximately 2 mm resolution was used to measure fields induced in rat carcasses by a 60 Hz magnetic field at 1 mT. With the rat lying on its side, the probe was inserted through a small hole in the body wall, and scanned at 5 mm increments from the side with frontal and axial exposure (field horizontal) and from the front with lateral exposure (field vertical). The induced electric field declined from a maximum at the entrance to the abdomen and crossed zero to negative (180° phase shift) values within the body as expected. In general, the magnitudes of the measurements inside the abdomen were less than expected from whole-body calculations that used homogeneous-ellipsoidal models of a rat in the three orientations. The low measurements did not appear to be explained by perpendicular field components, by conductivity differences between the tissue and the probe path, or by air in the lungs. The low measurements probably result from inhomogeneities in actual rats that include conductivity differences between tissues and biological membranes. For example, an alternative model considered the abdominal cavity to be electrically isolated from the body by the diaphragm and the peritoneum and calculations from this model were in better agreement with the measurements inside the abdomen (than were the whole-body calculations). Therefore, inhomogeneities in conductivity and biomembranes such as the peritoneum should be considered in order to fully understand ELF-induced field dosimetry. © 1996 Wiley-Liss, Inc.