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.     

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.     

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.     

Magnetic field characteristics of electric bed-heating devices

Measurements of the flux density and spectra of magnetic fields (MFs) generated by several types of electric bed heaters (EBH) were made in order to characterize the MFs to which the fetus may be exposed in utero from the mother's use of these devices. Data on MFs were gathered from more than 1,300 in-home and laboratory spot measurements. In-home measurements taken at seven different positions 10 cm from the EBHs determined that the mean flux density at the estimated position of the fetus relative to the device was 0.45 μT (4.5 mG) for electric blankets and 0.20 μT (2.0 mG) for electrically heated water beds. A rate-of-change (RC) metric applied to the nighttime segment of 24 h EMDEX-C personal-dosimeter measurements, which were taken next to the bed of volunteers, yielded an approximate fourfold to sixfold higher value for electric blanket users compared to water-bed heater users. These same data records yielded an approximate twofold difference for the same measurements when evaluated by the time-weighted-average (TWA) MF exposure metric. Performance of exposure meters was checked against standard fields generated in the laboratory, and studies of sources of variance in the in-home measurement protocols were carried out. Spectral measurements showed that the EBH's measured produced no appreciable high-frequency MFs. Data gathered during this work will be used in interpreting results from a component of the California Pregnancy Outcome Study, which evaluates the use of EBHs as a possible risk factor in miscarriage. © 1996 Wiley-Liss, Inc.     

Short-circuit currents,surface electric fields,and axial current densities for guinea pigs exposed to ELF electric fields.

Short-circuit currents, surface electric fields, and axial current densities were measured in electrically grounded guinea pigs exposed to a uniform, vertical, ELF electric field. These data are 70–110% of corresponding values obtained in grounded rats exposed to the same electric field.     

Numerical calculation and measurement of 60-Hz current densities induced in an upright grounded cylinder

Power-frequency electric fields are strongly perturbed in the vicinity of human beings and experimental animals. As a consequence, the extrapolation of biological data from laboratory animals to human-exposure situations cannot use the unperturbed exposure field strength as a common exposure parameter. Rather, comparisons between species must be based on the actual electric fields at the outer surfaces of and inside the bodies of the subjects. Experimental data have been published on surface and internal fields for a few exposure situations, but it is not feasible to characterize experimentally more than a small fraction of the diverse types of exposures which occur in the laboratory and in the field. A predictive numerical model is needed, one whose predictions have been verified in situations where experimental data are available, and one whose results can be used with confidence in new exposure situations. This paper describes a numerical technique which can be used to develop such a model, and it carries out this development for a test case, that of a homogeneous right-circular cylinder resting upright on-end on a ground plane and exposed to a vertical, uniform, 60-Hz electric field. The accuracy of the model is tested by comparing short-circuit currents and induced current densities predicted by it to measured values: Agreement is good.     

ELF in vitro exposure systems for inducing uniform electric and magnetic fields in cell culture media.

Many in vitro experiments on the biological effects of extremely low frequency (ELF) electromagnetic fields utilize a uniform external magnetic flux density (B) to expose biological materials. A significant number of researchers do not measure or estimate the resulting electric field strength (E) or current density (J) in the sample medium. The magnitude and spatial distribution of the induced E field are highly dependent on the sample geometry and its relative orientation with respect to the magnetic field. We have studied the E fields induced in several of the most frequently used laboratory culture dishes and flasks under various exposure conditions. Measurements and calculations of the E field distributions in the aqueous sample volume in the containers were performed, and a set of simple, quantitative tables was developed. These tables allow a biological researcher to determine, in a straightforward fashion, the magnitudes and distributions of the electric fields that are induced in the aqueous sample when it is subjected to a uniform, sinusoidal magnetic field of known strength and frequency. In addition, we present a novel exposure technique based on a standard organ culture dish containing two circular, concentric annular rings. Exposure of the organ culture dish to a uniform magnetic field induces different average electric fields in the liquid medium in the inner and outer rings. Results of experiments with this system, which were reported in a separate paper, have shown the dominant role of the magnetically induced E field in producing specific biological effects on cells, in vitro. These results emphasize the need to report data about the induced E field in ELF in-vitro studies, involving magnetic field exposures. Our data tables on E and J in standard containers provide simple means to enable determination of these parameters.     

Current densities induced in swine and rat models by power-frequency electric fields

Measurements have been made of vector current densities induced by vertical, uniform, 60-Hz electric fields in the torsos of homogeneous models of swine and rats. The observed data were a strong function of the five grounding configurations invested: all four feet grounded, only front feet grounded, only rear feet grounded, left front and right rear feet grounded, and right front and left rear feet grounded. In the first configuration and with an exposure field strength of 10 kV/m, average total current densities induced in the torsos of pigs and rats were 34 nA/cm2 and 20 nA/cm2, respectively. The corresponding value for human exposure is about 250 nA/cm2, 7.3 and 12.5 times larger than for swine and rats, respectively. Current densities measured at 60 Hz can be linearly extrapolated to frequencies in a range extending from at least 1 Hz to 1 MHz. Human and animal current-density data can provide an improved rationale for extrapolating biological data across species. In addition, these data can be used to validate the predictions of numerical models.     

Preliminary report: modification of cardiac contraction rate by pulsed magnetic fields.

Isolated rat hearts and excised canine cardiac tissues were subjected to pulsed magnetic fields. The fields excited in coils by tandem pairings of sinusoidal pulses were presented at various inter-pair delays and repetition rates. The waveform of the magnetic field was a single or multiple sinusoid followed after a variable delay by another single or multiple sinusoid. Small but reliable increases in the beating rate of rat heart were observed. Similar increases occurred in contraction rates of canine tissues. Both preparations exhibited a contraction-rate dependency on the repetition rate of the paired magnetic pulses: 4.5-6 rep/s for canine tissue, and 20-25 and 40-55 reps/s for rat heart. Flux-density thresholds for both preparations approximated 10 mT (100 gauss) rms.     

Low-voltage ELF electric field measurements in ionic media

Low-voltage electric fields were measured in conductive tissue culture media using three techniques: voltage slope, current density-conductivity, and dipole methods. All three methods tested yielded comparable results. However, all three techniques have associated errors. These errors fall into three major categories: those associated with the measurement equipment, those associated with electrodes, and errors in cross-sectional area measurements. Each source of error is discussed so that all can be taken into account during construction and/or testing of exposure equipment.     

Electric fields induced in rat and human models by 60-Hz magnetic fields: comparison of calculated and measured values.

The calculated distribution of electric fields induced in homogeneous human and rat models by a 60-Hz magnetic field is compared with values measured in instrumented mannequins. The calculated values agree well with measured values.     

Spreadsheet method for calculating the induced currents in bone-fracture healing by a low-frequency magnetic field

A commercially available spreadsheet program is used on a microcomputer to calculate the induced current density and electric field patterns produced in a nonhomogeneous, anisotropic model of tissue by a localized, low-frequency magnetic field source. Specific application is made to coils used to promote the healing of bone fractures in limbs. The variation of the conductivity of the fracture gap during healing causes the induced current density pattern to change correspondingly, whereas the induced electric field remains relatively unchanged. Use of more simplified, isotropic models for the bone and for the soft tissue leads to results that differ significantly from those obtained from the full model. The magnetic field beyond the region of the coils contributes little to the induced currents in the fracture gap if the gap is located near the center of the coils. © 1994 Wiley-Liss, Inc.     

The influence of temperature during electric- and magnetic-field-induced alteration of calcium-ion release from in vitro brain tissue

A technique based on release of calcium ions from in vitro preparations of avian brain tissues has been used by several investigators to demonstrate a biological effect of weak electric and magnetic fields. When the tissues have been exposed to ELF-modulated, VHF or UHF fields, enhanced release of calcium ions has resulted. In contrast, when the tissues have been exposed directly to an ELF field, outcomes have differed. Both inhibition and enhancement in release of calcium ions have been reported. We now find that either outcome--or a null result--is possible, depending on the temperature of tissue samples before and during exposure. Avian-brain tissues were exposed to 16-Hz sinusoidal electromagnetic fields at 14.1 Vrms/m (in air) and 64 nTrms. During 20-min exposures, as tissue-sample temperature rose by 0.7 to 2.5 degrees C to a final temperature of 35, 36, or 37, but not of 38 or 39 degrees C, an enhanced release of ions was observed. When the temperature was stable during exposure (i.e., constant within +/- 0.3 degrees C) at a final value of 36 or 37, but not of 35 or 38 degrees C, the quantity of ions released was reduced. And when descending by 0.7 to 1.5 degrees C to any final temperature from 35 to 38 degrees C, a null result occurred. These findings may reconcile the apparent disagreement in the direction of a field-induced response, and they may explain why experimental outcomes have been difficult to confirm in some laboratories. Of greater importance, the findings may also provide insight into the mechanism of the field-induced phenomenon.     

Determination of the induced ELF electric field distribution in a two layer in vitro system simulating biological cells in nutrient solution

In-vitro studies of biological effects of electromagnetic fields are often conducted with cultured cells either in suspension or grown in a monolayer. In the former case, the exposed medium can be assumed to be homogeneous; however, eventually the cells settle to the bottom of the container forming a two layer system with different dielectric and conductive properties. In the present work the effect of this separation on the electric field distribution is calculated and experimentally measured at selected positions for a commonly used exposure configuration. The settled cell suspension is modeled by a well-defined two layer system placed in a rectangular container with the base of the container parallel to the direction of the magnetic field. Theoretical calculations based on numerical techniques are done for various two layer systems with different conductivities in each layer. The agreement between the theoretical calculations and the experimental measurements is within ± 1.5 mV/m, or 10% of the maximum induced field when the conductivity of the lower layer is ten times that of the upper layer. This result is well within experimental error. When the thickness of one of the layers is small compared to the thickness of the other layer, it is found that the electric field distribution is essentially that of the homogeneous case. The latter situation corresponds to a typical cell exposure condition. © 1993 Wiley-Liss, Inc.     

Residential magnetic and electric fields

A magnetic flux density (MFD) and electric-field (E-field) data-acquisition system was built for characterizing extremely low-frequency fields in residences. Every 2 min during 24-h periods, MFD and E-field measurements were made in 43 homes in King, Pierce, and Snohomish counties of Washington State. The total electrical energy used in each residence during the 24-h measurement period was also recorded, and maps were drawn to scale of the distribution wiring within 43 m (140 ft) of these homes. Finally, on a separate date, field measurements were made in each home during an epidemiological interview. The results of this study can be summarized as follows: 1) 24-h-average MFD measured at two separate points in the family room were correlated, as were a 24-h-average bedroom measurement and the mean of the two family-room measurements. 2) The 24-h-average family-room MFD and E-field measurements were uncorrelated. 3) The 24-h-average total harmonic distortions of family-room MFD and E-fields were less than about 24% and 7%, respectively. 4) Residential MFD exhibited a definite 24-h (diurnal) cycle. 5) The 24-h-average and interviewer-measured MFD were correlated. 6) Residential 24-h-average MFD were correlated with the wiring code developed by Wertheimer and Leeper. 7) An improved prediction of 24-h-average residential MFD was obtained using the total number of service drops, the distance to neighboring transmission lines, and the number of primary phase conductors.     

Residential exposure to 60-Hz magnetic fields from appliances.

A model has been developed that permits assessment of residential exposure to 60-Hz magnetic fields emitted by appliances. It is based on volume- and time-averaging of magnetic-dipole fields. The model enables the contribution of appliances in the total residential exposure to be compared with that of other sources in any residence under study. Calculations based on measurements reported in the literature on 98 appliances revealed that appliances are not a significant source of whole-body exposure, but that they may be the dominant source of exposure of the body's extremities.     

Early pregnancy loss and exposure to 50-Hz magnetic fields

The possibility of an association of early pregnancy loss (EPL) with residential exposure to ELF magnetic fields was investigated in a case-control study. Eighty-nine cases and 102 controls were obtained from the data of an earlier study aimed at investigating the occurrence of EPL in a group of women attempting to get pregnant. Magnetic-field exposure was characterized by measurements in residences. Strong magnetic fields were measured more often in case than in control residences. In an analysis based on fields measured at the front door, a cutoff score of 0.5 A/m (0.63 μT) resulted in an odds ratio of 5.1 (95% confidence interval 1.0–25). The results should be interpreted cautiously due to the small number of highly exposed subjects and other limitations of the data. © 1993 Wiley-Liss. Inc.