Electromagnetic absorption in an inhomogeneous model of man for realistic exposure conditions

The advances made in quantifying electromagnetic absorption and its distribution for various exposure profiles are described. The conditions that have been studied extensively are: free-space irradiation and its variations, such as the presence of ground and reflecting surfaces and other humans in close proximity. Using an inhomogeneous block model of man, work has recently been extended to leakage-type near-fields such as those from RF heat sealers and other electronic equipment. Projections are made for the extension of this work to evaluate coupled near-fields, design of multielement near-field applicators to obtain physician-prescribed uniform or nonuniform rates of regional heating, and for the inverse scattering problem necessary for electromagnetic biomedical imaging. Accurate information about the dielectric properties of various tissues becomes increasingly important for proper inhomogeneous modeling of man.     

Specific absorption rate and electric field measurements in the near field of six mobile phone base station antennas

In this article, the exposure to radio frequency electromagnetic fields was studied in close proximity (distances of 10, 100, 300, and 600 mm) to six base station antennas. The specific absorption rate (SAR) in 800 mm × 500 mm × 200 mm box phantom as well as unperturbed electric field (E) in air was measured. The results were used to determine whether the measurement of local maximum of unperturbed electric field can be used as a compliance check for local exposure. Also, the conservativeness of this assessment method compared to the ICNIRP basic restriction was studied. Moreover, the assessment of whole‐body exposure was discussed and the distance ranges presented in which the ICNIRP limit for local exposure could be exceeded before the limit for whole‐body SAR. These results show that the electric field measurement alone can be used for easy compliance check for the local exposure at all distances and for all antenna types studied. However, in some cases when the local peak value of E was compared directly to the ICNIRP reference level for unperturbed E, the exposure was overestimated only very slightly (by factor 1.1) compared to the basic restriction for localized SAR in a human, and hence these results can not be generalized to all antenna types. Moreover, it was shown that the limit for localized exposure could be exceeded before the limit for the whole‐body average SAR, if the distance to the antenna was less than 240 mm. Bioelectromagnetics 30:307–312, 2009. © 2009 Wiley‐Liss, Inc.     

Reevaluation and improved design of the TEM cell in vitro exposure unit for replication studies

The transverse electromagnetic (TEM) cell system developed by Litovitz et al. and utilized by Penafiel et al. for the exposure of cells in T25 flasks at 835 MHz has been reevaluated for the purpose of replicating the studies published by Penafiel. The original setup has been reconstructed as closely as possible, with improvements enabling blinded exposures, forced cooling and better repeatable positioning of the flasks, as well as tight exposure and environmental parameter control. The signal unit can simulate the original signal but also enables various other exposure schemes. The setup has been evaluated for four T25 flasks filled with 5 and 10 ml of cell medium by experimental and numerical means. Comparing E field, SAR and temperature measurements resulted in good agreement: <0.4 dB (4.5%) for E field and 0.48 dB (10.5%) for SAR. The overall average SAR within the medium is 6.0 W/kg at 1 W input power with a standard deviation of less than 52%. The temperature increase was determined to be 0.13 degrees C/(W/kg). This can be reduced to 0.045 degrees C/(W/kg) by applying active air flow cooling. The comparison of SAR values from temperature measurements with the corresponding simulated values resulted in excellent agreement. These results do not correspond to the previous study reporting an average SAR within the medium of 2.5 W/kg at an input power of 0.96 W.     

Outdoor measurement of SAR in a full-sized human model exposed to 29.9 MHz in the near field

Localized and averaged specific absorption rates (SARs) were obtained in a full-size muscle-equivalent human model exposed to near-field 29.9 MHz irradiation at an outdoor facility. The model was positioned erect on a metallic groundplane 1.22 m (4 ft) from the base of a 10.8-m (35 ft) whip antenna with an input power of 1.0 kW. For whole-body SAR, a mean value of 0.83 W/kg was determined using two gradient-layer calorimeters in a twin-well configuration. The localized SARs at 12 body locations were measured using nonperturbing temperature probes and were highest in the ankle region. We conclude that averaged SAR measurements in a full-size phantom are feasible using a twin-calorimeter approach; measurements in the field are practical when human-size (183 x 61 x 46 cm) calorimeters are used.     

SAR and efficiency evaluation of a 900 MHz waveguide chamber for cell exposure

In this work we present the results of numerical and experimental dosimetry carried out for an in vitro exposure device to irradiate sample groups at 900 MHz. The cells are kept in 8 and 15 ml cell cultures, contained, respectively in T25 and T75 rectangular flasks. The dosimetric assessment of the distribution of the specific absorption rate (SAR) is performed for both the bottom of the flask and the whole volume of the sample to provide results for experiments on either the cell layer or the cell suspension. The irradiating chamber is a rectangular waveguide (WG). Different configurations are considered to assess the optimum orientation and positioning of the cell cultures inside the WG. The system performance is optimal when the electric field is parallel to the sample and the WG is terminated by a matched load. In this condition two 15 or four 8 ml cells cultures can be exposed. The efficiency (ratio between the power absorbed by the sample and the incident power) and the non-uniformity degree (ratio between the standard deviation of SAR values and the average SAR over the sample) are calculated and successfully verified through measurements of the scattering parameters and local temperature increases. In the chosen exposure configuration, the efficiency is 0.40 and the non-uniformity degree is 39% for the 15 ml samples. For the 8 ml samples, the efficiency is 0.19 and a low non-uniformity degree (15%) is found.     

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.     

Large scale in vitro experiment system for 2 GHz exposure

A beam formed radiofrequency (RF) exposure-incubator employing a horn antenna, a dielectric lens, and a culture case in an anechoic chamber is developed for large scale in vitro studies. The combination of an open type RF exposure source and a culture case through which RF is transmitted realizes a uniform electric field (+/-1.5 dB) in a 300 x 300 mm area that accommodates 49 35 mm diameter culture dishes. This large culture dish area enables simultaneous RF exposure of a large number of cells or various cell lines. The RF exposure source operates at 2142.5 MHz corresponding to the middle frequency of the downlink band of the International Mobile Telecommunication 2000 (IMT-2000) cellular system. The dielectric lens, which has a gain of 7 dB, focuses RF energy in the direction of the culture case and provides a uniform electric field. The culture case is sealed and connected to the main unit for environmental control, located outside the anechoic chamber, via ducts. The temperature at the center of the tray, which contains the culture dishes in the culture room, is maintained at 37.0 +/- 0.2 degrees C by air circulation. In addition, the appropriate CO2 density and humidity supplied to the culture case realizes stable long-term culture conditions. Specific absorption rate (SAR) dosimetry is performed using an electric field measurement technique and the Finite Difference Time Domain (FDTD) calculation method. The results indicate that the mean SAR of the culture fluid at the bottom of the 49 (7 x 7 array) culture dishes used in the in vitro experiments is 0.175 W/kg for an antenna input power of 1 W and the standard deviation of the SAR distribution is 59%. When only 25 culture dishes (5 x 5 array) are evaluated, the mean SAR is 0.139 W/kg for the same antenna input power and the standard deviation of the SAR distribution is 47%. The proliferation of the H4 cell line in 72 h in a pair of RF exposure-incubators reveals that the culture conditions are equivalent to those of a common CO2 incubator.     

Testing the effectiveness of small radiation shields for mobile phones

Nine small radiation shields made to adhere to the case of mobile phones were tested at 914 and 1880 MHz. Five popular products were tested because advertisements typically claim they are up to 99% effective in blocking radio frequency (RF) radiation emitted from mobile phones. Also, four other conceptually unusual products were tested because advertisements typically claim they emit oscillations that counteract the RF radiation from mobile phones. Each shield was tested on the same mobile phone, and measurements were made to compare the absorption of RF energy in the head with and without each shield attached to the phone. The phone was positioned against a head model, and an automated measurement process was used to determine specific absorption rate (SAR) in the same way it is used at Motorola to test the compliance of mobile phones with respect to human exposure limits. The location of the peak SAR was not observed to change with any of the shields attached to the phone, and the 1 g, peak spatial average SAR did not change by any statistically significant amount. These results indicate the small shields are ineffective in reducing the exposure of the head to RF energy emitted by a mobile phone.     

Design and Dosimetric Characterization of a Broadband Exposure Facility for In Vitro Experiments in the Frequency Range 18–40.5 GHz

A novel exposure facility for exposing cell monolayers to centimeter and millimeter waves (18–40.5 GHz) used by future 5G mobile communication technology and similar applications has been developed. A detailed dosimetric characterization of the apparatus for frequencies of 27 and 40.5 GHz and 60 mm petri dishes, used in a presently ongoing study on human dermal fibroblasts and keratinocytes, was carried out. The exposure facility enables a well-defined, randomized, and blinded application of sham exposure and exposure with selectable values of incident power flux density, and additionally provides the possibility of continuous monitoring of the sample temperature during exposure while it does not require significant deviations from routine in vitro handling procedures, i.e. petri dishes are not required to be placed inside waveguides or TEM cells. Mean specific absorption rate (SAR) values inside the cell monolayer of 115 W/kg (27 GHz) and 160 W/kg (40.5 GHz) per watt antenna input power and corresponding transmitted power density (S_t) values at the bottom of the cell monolayer of 65 W/m² (27 GHz) and 70 W/m² (40.5 GHz) per watt antenna input power can be achieved, respectively. For reasonable amounts of harvested cells (80% of petri dish bottom area), the variation (max/min) of SAR and S_t over the cell monolayer remains below 3.7 dB (27 GHz) and 3.0 dB (40.5 GHz), respectively. © 2021 Bioelectromagnetics Society.     

Far-field dosimetric measurements in a full-sized man model at 2.0 GHz

Electromagnetic dosimetry was conducted in a tissue-equivalent full-sized model of man irradiated at 2 GHz inside a microwave-anechoic chamber. A nonperturbing temperature probe and a gradient-layer calorimeter were used to determine local and whole-body specific absorption rate (SAR), respectively. Relatively high SAR values were found in the limbs compared to the axis of the trunk of the model. The calorimeter experiments yielded an average SAR about three times higher than that estimated theoretically for a prolate spheroidal model of man. It is suggested that resonant interactions involving the limbs may be responsible for the disparity between theory and experiment.     

Near-field absorption in a circular cylinder from electric and magnetic line sources

A homogeneous, lossy circular cylinder is used as a simple model of a biological object in which interior heating is produced by the absorption of electromagnetic waves. For this model, we determined the optimum frequency, polarization, orientation and shape of applicators. Analytical and numerical results are given for both electric and magnetic line sources, with three different polarizations relative to the cylinder. Coupling efficiencies and contour plots are presented for a range of parameters. One particularly interesting result is the production of maximum energy deposition at the center of a cylinder of muscle tissue when exposed in the 100-MHz frequency range by the use of four applicators surrounding the cylinder.     

Finite difference calculations of current densities in a homogeneous model of a man exposed to extremely low frequency electric fields

This paper presents three-dimensional finite difference calculations of induced current densities in a grounded, homogeneous, realistically human-shaped phantom. Comparison is made with published experimental values of current density at 60 Hz, measured in conducting saline manikins with their arms down by the side. The congruence between calculation and experiment gives confidence in the applicability of the numerical method and phantom shape to other configurations. The effect of raising both arms above the head is to reduce the current densities in the head and neck by approximately 50% and to increase those from the thorax downwards by 20-30%. A sensitivity analysis was performed on the shape and dimensions of the phantom, from a 45-kg, 1.5-m-tall person to a 140-kg, 1.9-m-tall person. When the phantom is grounded through both feet the current densities range from 50 to 90 microAm-2 in the head (all values for a 60-Hz, 1-kVm-1, vertical applied field), 70 to 140 microAm-2 in the thorax, 150 to 440 microAm-2 at the crotch, and 500 to 2,230 microAm-2 in the ankle. When grounded through only one foot the current densities at the crotch range from 400 to 1,000 microAm-2 and from 1,000 to 4,400 microAm-2 in the ankle of the grounded leg. Scale transformations of the short-circuit current with phantom height, weight, and surface area are confirmed.     

Off-center spherical model for dosimetry calculations in chick brain tissue

This paper presents calculations for the electric field and absorbed power density distribution in chick brain tissue inside a test tube, using an off-center spherical model. It is shown that the off-center spherical model overcomes many of the limitations of the concentric spherical model, and permits a more realistic modeling of the brain tissue as it sits in the bottom of the test tube surrounded by buffer solution. The effect of the unequal amount of buffer solution above the upper and below the lower surfaces of the brain is analyzed. The field distribution is obtained in terms of a rapidly converging series of zonal harmonics. A method that permits the expansion of spherical harmonics about an off-center origin in terms of spherical harmonics at the origin is developed to calculate in closed form the electric field distribution. Numerical results are presented for the absorbed power density distribution at a carrier frequency of 147 MHz. It is shown that the absorbed power density increases toward the bottom of the brain surface. Scaling relations are developed by keeping the electric field intensity in the brain tissue the same at two different frequencies. Scaling relations inside, as well as outside, the brain surface are given. The scaling relation distribution is calculated as a function of position, and compared to the scaling relations obtained in the concentric spherical model. It is shown that the off-center spherical model yields scaling ratios in the brain tissue that lie between the extreme values predicted by the concentric and isolated spherical models.     

Determination of safety distance limits for a human near a cellular base station antenna,adopting the IEEE standard or ICNIRP guidelines

This paper investigates the minimum distance for a human body in the near field of a cellular telephone base station antenna for which there is compliance with the IEEE or ICNIRP threshold values for radio frequency electromagnetic energy absorption in the human body. First, local maximum specific absorption rates (SARs), measured and averaged over volumes equivalent to 1 and to 10 g tissue within the trunk region of a physical, liquid filled shell phantom facing and irradiated by a typical GSM 900 base station antenna, were compared to corresponding calculated SAR values. The calculation used a homogeneous Visible Human body model in front of a simulated base station antenna of the same type. Both real and simulated base station antennas operated at 935 MHz. Antenna-body distances were between 1 and 65 cm. The agreement between measurements and calculations was excellent. This gave confidence in the subsequent calculated SAR values for the heterogeneous Visible Human model, for which each tissue was assigned the currently accepted values for permittivity and conductivity at 935 MHz. Calculated SAR values within the trunk of the body were found to be about double those for the homogeneous case. When the IEEE standard and the ICNIRP guidelines are both to be complied with, the local SAR averaged over 1 g tissue was found to be the determining parameter. Emitted power values from the antenna that produced the maximum SAR value over 1 g specified in the IEEE standard at the base station are less than those needed to reach the ICNIRP threshold specified for the local SAR averaged over 10 g. For the GSM base station antenna investigated here operating at 935 MHz with 40 W emitted power, the model indicates that the human body should not be closer to the antenna than 18 cm for controlled environment exposure, or about 95 cm for uncontrolled environment exposure. These safe distance limits are for SARs averaged over 1 g tissue. The corresponding safety distance limits under the ICNIRP guidelines for SAR taken over 10 g tissue are 5 cm for occupational exposure and about 75 cm for general-public exposure.     

Specific absorption rate (SAR) in models of the human head exposed to hand-held UHF portable radios

Specific absorption rate (SAR) was measured in models of the human head exposed to hand-held portable radios ("transceivers") transmitting at frequencies in the 800-MHz band. An isotropic implantable electric-field probe was used to measure internal fields induced in the head models, and SARs were determined by calculation. As well as determining representative values and distributions for SARs under various conditions, it was shown that antenna type and orientation with respect to the head are important factors affecting energy absorption.     

Effect of the meniscus at the solid/liquid interface on the SAR distribution in Petri dishes and flasks

Several authors have reported that the meniscus as occurring at the walls of Petri dishes and flasks (solid-liquid interface) can significantly affect the SAR distribution in the entire dish and flask, while others have ignored this effect. In this study, this effect has been comprehensively analyzed by numerical means supported by theoretical considerations. The focus is on E polarization, which results in the most homogeneous exposure for monolayer cell cultures and therefore is the most often applied polarization in exposure setups for risk assessment studies. This includes setups based on TEM cells, waveguides, radial transmission lines (RTL), and HF chambers. The conclusion of this study is that the meniscus and its size have a significant effect on the strength and homogeneity of the induced SAR distribution. Hence, the meniscus needs to be accounted for in the determination of the averaged SAR as well as in the uncertainty assessment. It has also been demonstrated that a voxel size of less than 0.2 mm is needed to predict the SAR values for a monolayer of cells accurately.     

Space efficient system for whole-body exposure of unrestrained rats to 900 MHz electromagnetic fields

The aim of this study was to design, implement and analyze a space-efficient setup for the whole-body exposure of unrestrained Wistar rats to radiofrequency (RF) electromagnetic fields at 900 MHz. The setup was used for 2 years in a cocarcinogenesis study and part of it for 5 weeks in a central nervous system (CNS) study. Up to 216 rats could be placed in separate cages in nine different exposure chambers on three racks requiring only 9 m2 of floor area (24 rats per m2). Chambers were radial transmission lines (RTL), where the rats could freely move in their cages where food and drinking water was provided ad libitum except during RF exposure periods. Dosimetrical analysis was based on FDTD computations with heterogeneous rat models and was validated with calorimetrical measurements carried out with homogeneous phantoms. The estimated whole-body average specific absorption rates (SAR) of rats were 0 (sham), 0.4, and 1.3 W/kg in the cocarcinogenesis study and 0 (sham), 0.27, and 2.7 W/kg in the CNS study with an estimated uncertainty of 3 dB (K = 2). The instantaneous and lifetime variations of whole-body average SAR due to the movement of rats were estimated to be 2.3 and 1.3 dB (K = 1), respectively.     

Current density in a model of a human body with a conductive implant exposed to ELF electric and magnetic fields

A numerical model of a human body with an intramedullary nail in the femur was built to evaluate the effects of the implant on the current density distribution in extremely low frequency electric and magnetic fields. The intramedullary nail was chosen because it is one of the longest high conductive implants used in the human body. As such it is expected to alter the electric and magnetic fields significantly. The exposure was a simultaneous combination of inferior to superior electric field and posterior to anterior magnetic field both alternating at 50 Hz with the values corresponding to the ICNIRP reference levels: 5000 V m^?1 for electric field and 100 µT for magnetic flux density. The calculated current density distribution inside the model was compared to the ICNIRP basic restrictions for general public (2 mA m^?2). The results show that the implant significantly increases the current density up to 9.5 mA m^?2 in the region where it is in contact with soft tissue in the model with the implant in comparison to 0.9 mA m^?2 in the model without the implant. As demonstrated the ICNIRP basic restrictions are exceeded in a limited volume of the tissue in spite of the compliance with the ICNIRP reference levels for general public, meaning that the existing safety limits do not necessarily protect implanted persons to the same extent as they protect people without implants. Bioelectromagnetics 30:591–599, 2009. © 2009 Wiley‐Liss, Inc.