3

COUPLING OF GWEN ELECTROMAGNETIC FIELDS TO THE HUMAN BODY

ELECTROMAGNETIC FIELDS FOR GWEN SITES

Low-Frequency (LF) Transmitter

The carrier frequency of the LF transmitter is 150.625-174.625 kHz. The largest electric (E) and magnetic (B) fields estimated for the GWEN relay nodes (RNs) in public-access areas are at the 4-ft-high perimeter fence typically 333 ft from the base of the LF antenna. Measurements have been conducted by the MITRE Corporation for E and B fields at various distances from the base of the LF antenna for an input power of 50 W, and the results have been scaled to a power of 5,000 W for an operating system. The maximum E and B fields at the GWEN RN perimeter fence obtained by this scaling procedure are 50 V/m and 0.7 mG (0.07 µT), respectively. These values, therefore, have been used as nominal values for the calculations in this chapter. These are the highest E and B fields for areas of general public access, and both diminish fairly rapidly with increasing distance from the LF antenna.

ULTRA-HIGH-FREQUENCY (UHF) TRANSMITTER

The UHF transmitter would typically radiate about 20 W of power at 225-400 MHz. According to calculations by the MITRE Corporation, the maximum power density of exposure at the perimeter fence would be 0.001 mW/cm2, and this would decrease rapidly with increasing distance from the antenna and the perimeter fence.

INDUCED FIELDS AND CURRENTS IN THE HUMAN BODY

We have used an anatomically based model of the human body1,2 to estimate induced fields and currents for both LF and UHF electromagnetic fields. For the LF fields, we have assumed a highest frequency of 174.625 kHz, because the induced currents are known to increase linearly with frequency and this frequency would be the highest used for the LF transmitter; an E field that is vertically polarized; a B field that is oriented from arm to arm of the hypothetically exposed person because these conditions are known to produce the highest internal fields; and a barefoot exposed person, because that is the worst-case condition. For electromagnetic fields due to the UHF transmitter, we have similarly assumed a vertically polarized E field, a B field oriented from arm to arm, and a grounded barefoot person. The salient features of the



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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK 3 COUPLING OF GWEN ELECTROMAGNETIC FIELDS TO THE HUMAN BODY ELECTROMAGNETIC FIELDS FOR GWEN SITES Low-Frequency (LF) Transmitter The carrier frequency of the LF transmitter is 150.625-174.625 kHz. The largest electric (E) and magnetic (B) fields estimated for the GWEN relay nodes (RNs) in public-access areas are at the 4-ft-high perimeter fence typically 333 ft from the base of the LF antenna. Measurements have been conducted by the MITRE Corporation for E and B fields at various distances from the base of the LF antenna for an input power of 50 W, and the results have been scaled to a power of 5,000 W for an operating system. The maximum E and B fields at the GWEN RN perimeter fence obtained by this scaling procedure are 50 V/m and 0.7 mG (0.07 µT), respectively. These values, therefore, have been used as nominal values for the calculations in this chapter. These are the highest E and B fields for areas of general public access, and both diminish fairly rapidly with increasing distance from the LF antenna. ULTRA-HIGH-FREQUENCY (UHF) TRANSMITTER The UHF transmitter would typically radiate about 20 W of power at 225-400 MHz. According to calculations by the MITRE Corporation, the maximum power density of exposure at the perimeter fence would be 0.001 mW/cm2, and this would decrease rapidly with increasing distance from the antenna and the perimeter fence. INDUCED FIELDS AND CURRENTS IN THE HUMAN BODY We have used an anatomically based model of the human body1, 2 to estimate induced fields and currents for both LF and UHF electromagnetic fields. For the LF fields, we have assumed a highest frequency of 174.625 kHz, because the induced currents are known to increase linearly with frequency and this frequency would be the highest used for the LF transmitter; an E field that is vertically polarized; a B field that is oriented from arm to arm of the hypothetically exposed person because these conditions are known to produce the highest internal fields; and a barefoot exposed person, because that is the worst-case condition. For electromagnetic fields due to the UHF transmitter, we have similarly assumed a vertically polarized E field, a B field oriented from arm to arm, and a grounded barefoot person. The salient features of the

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK anatomically based model and the numerical procedure used for the calculations are given in Appendix A to this chapter. Even though the exposure is in the “near field” of the LF transmitter, the E and B fields do not vary a great deal over the physical extent of the human body. As shown in the literature, 3 coupling to the human body for such relatively uniform incident fields is no different than that for far-field conditions. Far-field exposure conditions have therefore been used to estimate coupling of the GWEN EM fields to the human body. INDUCED CURRENTS AND E FIELDS AT 174.625 kHz For the calculations, we used an E field of 50 V/m (vertical) and a B field of 0.7 mG (oriented from arm to arm of the model). The value of 50 V/m is the highest electric field intensity at the perimeter fence of any GWEN facility, and a more typical value is 40 V/m at the perimeter fence. The induced current for these exposure conditions is primarily vertical (z). The calculated z-directed current from the grounded anatomically based model is shown in Figure 3-1 as a function of height above ground for the various sections of the body. An induced current of 2.88 mA is calculated to be flowing through the feet of a standing person. The calculated current is in excellent agreement with that estimated with the empirical equation given previously by Gandhi et al.4 In the empirical equation, the foot current Ih for a vertically polarized E field flowing through a standing, barefoot person is given by Ih = 0.108hm2fMHzE, (1) where hm is the height of the human in meters (1.75 m for our model), fMHZ is the frequency (for this case, 0.174625 MHz), and E is the incident electric field in V/m (for this case, 50 V/m). For the assumed exposure conditions, from Equation 1, Ih = 2.89 mA. That is in excellent agreement with the value calculated for the section through the feet of the anatomically based model (Figure 3-1). The calculated current through the feet (2.88 mA) is considerably smaller than the maximum permissible current of 90 mA for the uncontrolled environment in the new radiofrequency (RF) protection guide suggested by the Institute for Electric and Electronic Engineering (IEEE) Standards Coordinating Committee.5

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK FIGURE 3-1. Calculated section currents for grounded anatomically based model of human body for exposure to electromagnetic fields at 174.625 kHz. Fields assumed are at 4-ft perimeter fence. E = 50 V/m (vertical): B = 0.7 mG (from arm to arm model). Table 3-1 shows maximum current densities calculated for some representative sections of the human body. The exposure dimensions are those measured for the public-access points closest to the LF transmitting antenna, i.e., at the 4-ft perimeter fence. For the numbers given in Table 3-1, we have taken E = 50 V/m (vertical) and B = 0.7 mG from arm to arm of the model.

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK TABLE 3-1 Calculated Maximum Values of Current Density (JT,max) and Electric Field (ET,max) for Some Representative Sections of Anatomically Based Human Model (E = 50 V/m; B = 0.7 mG) Section of Body Height above ground, cm JT,max , µA/cm2 ET,max , mV/m Brain 165.7 3.5 170 Neck 155.2 8.1 380 Lung 140.8 7.2 475 Heart 133.0 8.4 250 Kidney 122.5 7.1 220 Liver 117.2 7.5 200 Bladder 89.7 10.4 405 Ankle 16.4 71.4 2,140 SARs FOR UHF ELECTROMAGNETIC FIELDS According to calculations by the MITRE Corporation, the maximum power density for the general public would be at the 4-ft perimeter fence, where the power density is estimated to be 0.001 mW/cm2. We have calculated the whole-body-averaged specific absorption rates (SARs) for some representative frequencies in the UHF band, assuming vertically polarized E fields and both the isolated and grounded anatomically based models of a human. The values calculated for various frequencies are given in Table 3-2. They are considerably lower than the 0.08 W/kg (80,000 µW/kg) suggested in the new RF protection guide issued by IEEE.5 Shown in Figure 3-2 are the section-averaged SAR distributions for grounded and isolated models of the human body. The highest SARs calculated for points in the body are smaller by a factor of several thousand than the local SARs of 1.6 W/kg permissible in the new RF protection guide5 For all of the preceding calculations in this chapter, the effect of the ground plane has been considered. Reflections from any structures have, however, been ignored since they are so dependent on the size and shape of these structures. These reflections can cause interference with the incident EM fields resulting in a slight enhancement or decrease of the fields depending upon the physical location on the ground.

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK TABLE 3-2. Whole-Body-Averaged SARs Calculated for Anatomically Based Model of Human Body (Incident Power Density, 0.001 mW/cm2; Vertically Polarized E Field) Frequency MHz SAR µW/kg Isolated Model SAR µW/kg Grounded Model 225 94.5 85.1 250 89.3 96.7 300 79.9 97.9 350 80.4 84.4

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK FIGURE 3-2. Calculated section-averaged SAR distributions for anatomically based model of human body at various ultra-high frequencies. Both isolated and grounded conditions of model are considered. Calculations assume incident power density of 0.001 mW/cm2 that is estimated for 4-ft perimeter fence around GWEN antennae.

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK MICROSCOPIC FIELD INTERACTIONS AT THE MOLECULAR, CELLULAR, AND TISSUE LEVELS As described in the preceding section of this report, the coupling of both LF and UHF signals from the GWEN system at locations beyond the site perimeter is too weak to produce measurable tissue heating. However, it has been reported that electromagnetic fields can produce biological effects through nonthermal interactions.6, 7 As discussed in Chapter 4, Chapter 5, Chapter 6, Chapter 7 and Chapter 8, effects of nonthermal fields have been reported to occur in the nervous, cardiovascular, endocrine, immune, and reproductive systems

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK (for general reviews, see Tenforde and Budinger8 and Michaelson and Lin9). Possibly the most widely discussed nonthermal effect of electromagnetic fields is the reported change in Ca2+ binding to nerve cell surfaces as a result of exposure to RF radiation with amplitude modulation at extremely low frequencies (ELFs), reviewed in detail in Chapters 6 and Chapters 7. The most effective band of modulation frequencies has been found at 6-20 Hz, although some higher frequencies are also effective10 In none of the studies on Ca2+ binding to cell surfaces has the unmodulated RF carrier wave itself been found effective. Modulation of GWEN signals by the minimum key shifting procedure produces a waveform very different from that of the sinusoidally amplitude-modulated RF signals that were used in most of the Ca2+ experiments. Specifically, the GWEN fields have the general characteristics of pulsed on-off RF signals during message transmission (see Chapter 2 ). It is therefore unlikely that the Ca2+ effects observed with sinusoidally amplitude-modulated RF fields have direct implications for biological or human health effects of the GWEN transmitter. That conclusion is supported by the inability of Merritt et al.11 to alter Ca2+ binding to brain tissue using RF fields that were pulse modulated at 16 Hz. RF fields from sources outside the body induce E and B fields in tissue that can be calculated with the procedures described earlier in this chapter. At frequencies below approximately 105 Hz, the induced fields can alter the electrical properties of cellular membranes and, at sufficiently high intensities, stimulate excitable tissues. At these frequencies, the impedance of the cell membrane is high and induced currents flow primarily in the extracellular fluids. At higher frequencies, the cell membrane poses less of a dielectric barrier to current flow and the electromagnetic field is more strongly absorbed by tissue. As described in Chapter 4, there is a distinct difference in human perception of fields with frequencies below and above 105 Hz. In the lower-frequency range, the perception is of tingling (typical of nerve stimulation); at higher frequencies, the sensation is of warmth. For the field intensities encountered at the perimeter of a GWEN site, the LF and UHF fields are not expected to produce either of those effects (see Chapter 4). Extensive studies on the interactions of ELF fields with cells and tissues are yielding a growing body of evidence that membrane interactions of these fields can trigger intracellular responses, such as alterations in the transcription and translation of macromolecules (see Chapter 7 for a detailed discussion). The exact molecular mechanisms by which the cellular responses occur are poorly understood, but there is evidence of effects of ELF fields on the transmembrane flow of ions, such as Ca2+, and on second-messenger signaling mechanisms that involve membrane receptors.12, 13, 14, 15 Several of these reported effects could result from tangential electric field and currents acting on components of the cell surface (e.g., the extracellular portion of a transmembrane receptor protein), whereas other effects may be attributable to electric fields induced across the cell membrane. The threshold induced current density for reproducible cellular responses to ELF fields14 appears to be in the range of 0.1-1.0 µA/cm2, which is comparable with the intrinsic current densities flowing in the body as

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK a result of endogenous electrical activity in excitable tissues16 As described earlier in this chapter, the largest current density induced in most parts of the human body by the LF fields at the perimeter of a GWEN site is in the range of 3-10 µA/cm2. Those LF current densities are therefore about 10 times the threshold of ELF current densities for eliciting membrane and cellular responses. However, the effective E field induced within the cell membrane will be substantially lower, by a factor of about 1,000, at low frequencies than at extremely low frequencies, because of the lower membrane impedance.17, 18 Consequently, the maximum currents induced in body tissues by the LF fields of a GWEN transmitter would be expected to establish electrical signals in cell membranes that are approximately one-hundredth the magnitude of the threshold values found to elicit reproducible cellular responses to ELF fields. The shift in transmembrane potential associated with a 10-µA/cm2 current density is expected to be about 10 µV at GWEN frequencies (150-175 kHz), where the effective membrane impedance is about 1 ohm/cm2. On the basis of both theoretical predictions and experimental measurements, approximately 0.01% of a transmembrane LF potential is rectified to a DC signal.19, 20 It can therefore be predicted that a 10-µV LF signal induced by GWEN fields would produce a DC transmembrane potential shift of about 1 nV, which is less by a factor of about 107 to 108 than the resting potential of eukaryotic cell membranes. A consideration of factors such as cell size, shape, and orientation relative to the field will not alter the conclusion that the transmembrane potential shift induced by GWEN fields is negligible. Many possible interaction mechanisms that could lead to membrane transduction and amplification of weak electromagnetic signals have been discussed in theoretical terms.6, 7 The proposed mechanisms include cooperative phenomena (e.g., phase transitions) triggered by weak field interactions, strong dipolar oscillations induced in large membrane proteins, nonlinear wave excitations (e.g., solutions), and resonance interactions induced by the simultaneous presence of the geomagnetic field and a time-varying field with an appropriate frequency (e.g., nuclear magnetic resonance or ion-cyclotron resonance effects). Although such phenomena have been observed in model systems, they have not been demonstrated conclusively to produce substantial functional perturbations in biological systems. In addition, there are strong theoretical arguments against the existence of several classes of proposed interactions, especially the ion-cyclotron resonance model. INDIRECT COUPLING--SHOCK AND BURNS Commonly encountered ungrounded metallic objects--such as cars, vans, and buses--can develop open-circuit voltages from incident electric fields. Large currents can flow through a person who touches such an object, depending on the magnitude of the incident electric fields. If the incident fields are large enough, currents larger than those which produce perception, pain, or even burns can occur.

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK Gandhi and Chatterjee21 and Guy and Chou22 have studied the problem of shock hazard from ungrounded objects in RF fields for frequencies of 10-3,000 kHz. The body impedance and threshold currents needed to produce perception and pain were measured with 367 human subjects (197 male and 170 female; ages, 18-70 years).23, 24 Various types of contact with metallic electrodes were used to simulate situations in which a person would be in contact with a large ungrounded metallic object. It was found that the sensation is of tingling or pricking at frequencies below about 70-100 kHz and of warmth at frequencies higher than 100 kHz. For LF fields at 150.625-174.625 kHz, the sensation due to contact with ungrounded bodies would therefore be of warmth in the region of the contact. Both small-area contacts (25 and 144 mm2), such as finger contact, and larger-area contacts (cylindrical rod 1.5 cm in diameter and 14 cm long), such as a grasped handlebar of a vehicle, have been studied to establish the average threshold currents needed for perception and pain.23, 24 Measurements were performed only on adult subjects, but it was possible to predict thresholds for 10-year-old children by scaling the physical dimensions. Gandhi et al.23 and Chatterjee et al.24 have estimated the threshold E fields that produce various sensations upon contact with various commonly metallic objects. Because somewhat larger currents would flow at 174.625 kHz than at 150.625 kHz and the threshold currents for various sensations are generally independent of frequency for frequencies higher than 100 kHz, somewhat smaller E fields can cause sensations at 174.625 kHz than at 150.625 kHz. Table 3-3 lists the estimated E fields for various sensations at 174.625 kHz related to contact with ungrounded metallic objects. The data could not be taken as threshold currents for pain under conditions of grasping contact because of the unavailability of high-power sources, but the currents are likely to be about 20% higher than those needed only for perception, as is the case for finger contact. The threshold E fields needed for pain under conditions of grasping contact would therefore be about 20% higher than the numbers given in the right-hand column of Table 3-3. Table 3-3 shows that the vertical E fields needed for perception or pain are considerably larger than the 50 V/m measured for LF transmitters close to the 4-ft perimeter fence around the GWEN RNs. Inasmuch as both E and B fields diminish rapidly with increasing distance from the LF antenna, it is clear that the potential for shock and burns is low. An exception would be the erection of ungrounded metal towers or guy wires parallel to the E field within a few hundred meters of the antenna. The phenomenon of perception, pain, or burns is peculiar to LF transmissions (less than about 50-100 MHz) and is relatively unimportant at UHF frequencies, where the dimensions of the ungrounded objects are larger than the wavelength and the concept of open-circuit voltage as a source of contact currents is irrelevant.

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK TABLE 3-3. Incident Vertical Threshold E Fields Needed to Produce Various Sensations in Contact with Ungrounded Metallic Objects   Finger contact, contact area = 144 mm2   Object Person E | perception , V/m E | pain , V/m Grasping contact, E | perception , V/m Compact car Man, 280 340 1,490   Woman 300 400 1,200   10-year-old child 205 255 950 Van Man 155 180 735   Woman 155 220 580   10-year-old child 120 140 430 Bus Man 160 200 480   Woman 165 230 425   10-year-old child 130 155 360 Fork-lift truck Man 95 120 395   Woman 90 125 330   10-year-old child 65 80 260 50-ft fence Man 420 450 2,700   Woman 390 450 2,250   10-year-old child 250 300 1,680 Source: O. P. Gandhi et al.23; I. Chatterjee et al.24

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK APPENDIX A: ANATOMICALLY BASED MODEL AND NUMERICAL PROCEDURE USED FOR CALCULATIONS ANATOMICALLY BASED MODEL As described by Sullivan et al.,1, 2 the inhomogeneous model of the human body is taken from A Cross-Section Anatomy25 which contains cross-sectional diagrams of the human body that were obtained by making cross-sectional cuts at intervals of about 1 inch in human cadavers. The process for creating the data base of the man model was as follows: A 0.25 inch grid was taken for each cross-sectional diagram. Each cell on the grid was assigned a number corresponding to air or one of 16 tissue types--muscle, fat, bone, blood, intestine, cartilage, liver, kidney, pancreas, spleen, lung, heart, nerve, brain, skin, and eye. The data associated with a particular layer consisted of three numbers for each square cell: x and y positions relative to some anatomical reference point in the layer, usually the center of the spinal cord and an integer indicating which tissue the cell contained. Because the cross-sectional diagrams available in Eychleshymer and Schoemaker25 are for variable separations, typically 2.3-2.7 cm, a new set of equispaced layers was defined at 0.25-inch (0.635-cm) intervals by interpolating the data onto the layers. Because the 0.25-inch cell size is too small for the memory space of readily accessible computers, the proportion of each tissue type was calculated for somewhat larger cells, 0.5 inch (1.27 cm) and the data on 2 × 2 × 2 = 8 cells of the smaller dimension were combined. Without changes in the anatomy, the process allows some variability in the height and weight of the body. We have taken the final cell size of 1.31 cm (rather than 0.5 in.) to obtain the total height and body weight of 176.85 cm and 70 kg, respectively. The electrical properties measured for the various tissues at both LF and UHF frequencies are shown in Table A-1. FINITE-DIFFERENCE TIME-DOMAIN METHOD The finite-difference time-domain (FDTD) method was proposed by Yee 28 and developed by Umashankar and Taflove,29 Holland,30 and Kunz and Lee.31 It has been extended for calculations of the distribution of electromagnetic (EM) fields in a human model for incident plane waves,1, 2, 32, 33, 34 for pulsed exposures,35 and for exposures in near fields.36, 37 In the method, described in detail elsewhere,1, 28, 29, 32 the coupled Maxwell's equations in the differential form are solved for various cubic subvolumes (cells) of the model and its surroundings in a time-stepped manner. To ensure stability of the solution, the time step δt is taken to be ∆/2ν, where ∆ is cell size and ν is maximum velocity of the EM wave encountered anywhere in the modeled space. For our calculations, ν = c, is the velocity of EM waves in air. Because we have taken ∆ = 1.31 cm, the time step δt = 0.02183 ns.

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK TABLE A-1. Tissue Properties for Human Modela       225 MHz 250 MHz 350 MHz 300 MHz Tissue Type Mass Density, ρ, kg/m3 174.625 kHz σ, S/m σ, S/m εr σ, S/m εr σ, S/m εr σ, S/m εr Air 1.2 0.0 0.0 1.0 0.0 1.0 0.0 1.0 0.0 1.0 Muscle 1,050 0.40 1.22 57 1.18 58 1.05 59 1.03 61 Fat, bone 1,020 0.05 0.075 6.6 0.08 7.2 0.09 8.3 0.10 9.5 Blood 1,000 0.68 1.16 65 1.17 65 1.19 65 1.20 65 Intestine 1,000 0.30 0.61 29 0.62 29 0.64 27 0.66 26 Cartilage 1,000 0.05 0.075 6.6 0.08 7.3 0.09 8.4 0.10 9.5 Liver 1,030 0.20 0.73 55 0.75 54 0.78 52 0.82 50 Kidney 1,020 0.34 1.09 64 1.11 62 1.14 58 1.16 53 Pancreas 1,030 0.45 1.09 67 1.11 64 1.14 58 1.16 53 Spleen 1,030 0.45 0.88 94 0.89 93 0.89 92 0.90 90 Lungb 330 0.05 0.21 12 0.22 12 0.22 12 0.22 12 Heart 1,030 0.40 0.87 59 0.90 58 0.90 57 1.0 56 Nerve, brain 1,050 0.18 0.60 60 0.61 58 0.61 55 0.62 51 Skin 1,000 0.34 0.79 56 0.78 54 0.77 51 0.76 48 Eye 1,000 0.45 1.85 80 1.80 78 1.70 74 1.60 70 Source: Johnson and Guy;26 Stuchly and Stuchly.27 aσ, electrical conductivity; εr, dielectric constant (= relative permittivity). bWe have used 33% lung tissue and 67% air for calculating the electrical properties of the lung.

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK Both sinusoidal and prescribed time-varying incident fields can be used with the FDTD method. For sinusoidally varying fields, the solution is completed when a sinusoidal steady-state behavior is observed for each. For lossy biological bodies, this typically takes a stepped time on the order of 3 to 4 time periods of oscillation. The method is relatively straightforward to use at UHF frequencies 2, 34 because three to four periods of oscillation are not an inordinate lengths of time to cover with iterations of time step δt = 0.02183 ns. For calculations of mass-normalized rates of energy absorption (specific absorption rates, SARs), both maximum and minimum values of components Ex, Ey, and Ez in time are obtained for each cell. The SAR for the (i, j, k) cell in the body can then be calculated from where σ(i, j, k) and ρ(i, j, k) are the volume-averaged electrical conductivity and mass density for cell (i, j, k). From the individual SARs thus calculated, one can obtain the section-averaged SARs for each of the sections of the body that are 1.31 cm from each other and can obtain the whole-body-averaged SAR. At low frequencies, use of the FDTD method is not as straightforward: a horrendous number of iterations would be needed to cover three to four periods of oscillation for converged results. A scaling procedure was recently developed that recognizes the quasistatic nature of coupling at lower frequencies, as previously pointed out by Kaune and Gillis38 and Guy et al.39 According to a logic similar to that of those authors, the fields outside the body depend not on the internal tissue properties, but only on the shape of the body, as long as the quasistatic approximation is valid; that is, the size of the body is smaller by a factor of 10 or more than the wavelength, and | σ + jωε | > > ωεo where σ and ε are the conductivity and the permittivity of the tissues, respectively, ω = 2πf is the radian frequency, and εo is the permittivity of the free space outside the body. Under those conditions, the fields in air are normal to the body surface and the internal tissue fields are given from the boundary conditions in terms of the fields outside:

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK A higher quasistatic frequency f′ may therefore be used for irradiation of the model, and the internal fields E′ thus calculated may be scaled back to the frequency f of interest, e.g., 174.625 kHz. From Equation A-2, we can write assuming that σ + jωε α σ at both f′ and f, which is a close approximation. For our calculations, we have used a full-scale anatomically based model of the human body and a frequency f′ of 10 MHz to reduce the computation time by orders of magnitude. Because in the FDTD method one needs to calculate in the time domain until convergence is obtained (typically three to four periods), frequency scaling to 10 MHz for f′ reduces the needed number of iterations by almost a factor of 60. At the higher irradiation frequency f′, we have taken σ′ = σ, i.e., conductivities of the various tissues at 174.625 kHz (see Table A-1). Furthermore, we have taken the incident E field Ei(f′) = fEi(f)/f′ to obtain Etissue(f) at, say, Ei(f) = 50 V/m. The incident B field Hi(f′) has similarly been taken to be considerably lower, fHi(f)/f′, in recognition that currents induced are proportional to the frequency of the incident fields. Quantities of interest for the LF band are the internal total electric fields and current densities for the various regions of the body. We have used the calculated internal fields to obtain the total electric field ET (i, j, k) and current density JT (i, j, k) for (i, j, k) cell in the body with the following equations: JT(i,j,k) = σ(i,j,k)ET(i,j,k) (A-5)

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK REFERENCES 1. Sullivan, D. M., D.T. Borup, and O. P. Gandhi. 1987. Use of the finite-difference time-domain method in calculating EM absorption in human tissues. IEEE Trans. Biomedical Engineering BME 34 : 148-157. 2. Sullivan, D. M., O. P. Gandhi, and A. Taflove. 1988. Use of the finite-difference time-domain method in calculating EM absorption in man models. IEEE Trans. Biomedical Engineering BME 35 : 179-186. 3. Chatterjee, I., M. J. Hagmann, O. P. Gandhi. 1981. An Empirical Relationship for Electromagnetic Absorption in Man for Near-Field Exposure Conditions. IEEE Trans. on Microwave Theory and Techniques MTT 29 : 1235-1238. 4. Gandhi, O. P., J. Y. Chen, and A. Riazi. 1986. Currents induced in a human being for plane-wave exposure conditions 0-50 MHz and for RF sealers. IEEE Trans. Biomedical Engineering BME 33 : 757-767. 5. Institute for Electric and Electronic Engineers Standards Coordinating Committee (SCC) 28, IEEE C95.1, 1991. IEEE Standard for Safety Levels with Respect to Human Exposure to Radiofrequency Electromagnetic Fields, 3 kHz to 300 GHz. Approved September 26, 1991. 6. Taylor, L. S. 1981. The mechanisms of athermal microwave biological effects. Bioelectromagnetics 2 : 259-267. 7. Postow, E., and M. L. Swicord. 1986. Modulated fields and “window” effects. Pp. 425-460 in Handbook of Biological Effects of Electromagnetic Fields, C. Polk and E. Postow, eds. Boca Raton, FL : CRC Press. 8. Tenforde, T. S., and T. F. Budinger. 1986. Biological effects and physical safety aspects of NMR imaging and in vivo spectroscopy. Pp. 493-548 in NMR in Medicine: Instrumentation and Clinical Applications, S. R. Thomas, and R. L. Dixon, eds. Medical Physics Monograph No. 14. New York, NY : American Association of Physicists in Medicine. 9. Michaelson, S. M., and J. C. Lin. 1987. Biological Effects and Health Implications of Radiofrequency Radiation . New York, NY : Plenum Press. 10. Blackman, C.F. 1990. ELF effects on calcium homeostasis. Pp. 187-208 in Extremely Low Frequency Electromagnetic Fields: The Question of Cancer , B. W. Wilson, R. G. Stevens and L. E. Anderson, eds. Columbus, OH : Battelle Press.

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK 11. Merritt, J. H., W. W. Shelton, and A. F. Chamness. 1982. Attempts to alter 45Ca2+ binding to brain tissue with pulse-modulated microwave energy. Bioelectromagnetics 3 : 475-478. 12. Adey, W. R. 1981. Tissue interactions with nonionizing electromagnetic fields. Physiol. Rev. 61 : 435-514. 13. Tenforde, T. S., and W. T. Kaune. 1987. Interaction of extremely-low-frequency electric and magnetic fields with humans. Health Phys. 53 : 585-606. 14. Tenforde, T. S. 1991. Biological interactions of extremely-low-frequency electric and magnetic fields. Bioelectrochem. Bioenerget. 25 : 1-17 [ J. Electroanalyt. Chem. 320 : 1-17 ]. 15. Tenforde, T. S. 1992. Biological interactions and potential health effects of extremely-low-frequency magnetic fields from power lines and other common sources. Annu. Rev. Publ. Health 13 : 173-196. 16. Bernhardt, J. 1979. The direct influence of electromagnetic fields on nerve and muscle cells of man within the frequency range of 1 Hz to 30 MHz. Radiat. Environ. Biophys. 16 : 309-323. 17. Cole, K. S. 1968. Membranes, ions, and impulses. Berkeley, CA : University of California Press. 18. Foster, K. R., and H. P. Schwan. 1986. Dielectric properties of tissues. Pp. 27-96 in Handbook of Biological Effects of Electromagnetic Fields, C. Polk and E. Postow, eds. Boca Raton, FL : CRC Press. 19. Pickard, W. F., and F. J. Rosenbaum. 1978. Biological effects of microwaves at the membrane level: Two possible athermal electrophysiological mechanisms and a proposed experimental test. Math. Biosci. 39 : 235-253. 20. Montaigne, K., and W. F. Pickard. 1984. Offset of the vacuolar potential of Characean cells in response to electromagnetic radiation over the range 250 Hz - 250 kHz. Bioelectromagnetics 5 : 31-38. 21. Gandhi, O. P., and I. Chatterjee. 1982. Radio-frequency hazards in the VLF to MF band. Proc. IEEE 70 : 1462-1464. 22. Guy, A. W., and C.-K. Chou. 1985. Very low frequency hazard study -- Part I. Bioelectromagnetics Research Laboratory, University of Washington, Seattle.

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK 23. Gandhi, O. P., I. Chatterjee, D. Wu, J. A. D'Andrea, and K. Sakamoto. 1985. Very low frequency hazard study. USAFSAM-TR-84, USAF School of Aerospace Medicine, Brooks AFB, Texas. 24. Chatterjee, I., D. Wu, and O. P. Gandhi. 1986. Human body impedance and threshold currents for perception and pain for contact hazard analysis in the VLF-MF band. IEEE Trans. Biomedical Engineering BME 33 : 486-494. 25. Eycleshymer, A. C., and D. M. Schoemaker. 1970. A Cross-Section Anatomy. New York, NY : Appleton-Century-Crofts. 26. Johnson, C. C., and A. W. Guy. 1972. Nonionizing electromagnetic wave effects in biological materials and systems. Proc. IEEE 60 : 692-718. 27. Stuchly, M. A., and S. S. Stuchly. 1980. Dielectric properties of biological substances -- Tabulated. J. Microwave Power 15 : 19-26. 28. Yee, K. S. 1966. Numerical solution of initial boundary value problems involving Maxwell 's equations of isotropic media. IEEE Trans. Antennas and Propagation AP 14 : 302-307. 29. Umashankar, K., and A. Taflove. 1982. A novel method to analyze electromagnetic scattering of complex objects . IEEE Trans. Electromagnetic Compatibility EMC 24 : 397-405. 30. Holland, R. 1977. THREDE: A free-field EMP coupling and scattering code. IEEE Trans. Nuclear Science NS 24 : 2416-2421. 31. Kunz, K. S., and K. M. Lee. 1978. A three-dimensional finite-difference solution of the external response of an aircraft to a complex transient EM environment: Part 1 - The method and its implementation. IEEE Trans. Electromagnetic Compatibility EMC 20 : 328-332. 32. Spiegel, R. J. 1984. A review of numerical models for predicting the energy deposition and resultant thermal response of humans exposed to electromagnetic fields. IEEE Trans. Microwave Theory and Tech MTT 32 : 730-746. 33. Spiegel, R. J., M. B. E. Fatmi, and K. S. Kunz. 1985. Application of a finite-difference technique to the human radio-frequency dosimetry problem. J. Microwave Power 20 : 241-254. 34. Chen, J. Y., and O. P. Gandhi. 1989. RF currents induced in an anatomically-based model of a human for plane-wave exposures (20-100 MHz). Health Phys. 57 : 89-98.

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ASSESSMENT OF THE POSSIBLE HEALTH EFFECTS OF GROUND WAVE EMERGENCY NETWORK 35. Chen, J. Y., and O. P. Gandhi. 1991. Currents induced in an anatomically-based model of a human for exposure to vertically polarized electromagnetic pulse. IEEE Trans. Microwave Theory and Tech MTT 39 : 31-39. 36. Wang, C. Q., and O. P. Gandhi. 1989. Numerical simulation of annular phased arrays for anatomically based models using the FDTD method. IEEE Trans. Microwave Theory and Tech MTT 37 : 118-126. 37. Chen, J. Y., and O. P. Gandhi. 1989. Electromagnetic deposition in an anatomically based model of man for leakage fields of a parallel-plate dielectric heater. IEEE Trans. Microwave Theory and Tech MTT 37 : 174-180. 38. Kaune, W. T., and M. F. Gillis. 1981. General properties of the interaction between animals and ELF electric fields. Bioelectromagnetics 2 : 1-11. 39. Guy, A. W., S. Davidow, G. Y. Yang, and C. K. Chou. 1982. Determination of electric current distributions in animals and humans exposed to a uniform 60-Hz high intensity electric field. Bioelectromagnetics 3 : 47-71.