Bioelectromagnetics Supplement 1:15-26 (1992)
Experimental Macroscopic Dosimetry for Extremely-Low-Frequency Electric and Magnetic Fields T. Dan Bracken T. Dan Bracken, Inc., 5415 SE Milwaukie Avenue, Portland, Oregon Environmental and laboratory exposure to electric and magnetic fields (EMF) in the extremely-low-frequency range (ELF) produces electrical quantities that interact directly with the exposed biological system on il scale small compared to the size of the human body but large with respect to cellular dimensions. The purpose of this paper is to describe these macroscopic electrical quantities and their characterization through measurements on living systems and experimental models. Electric field exposure results in a total induced current, surface electric fields, internal electric fields, and internal currents. Magnetic field exposure results in internal magnetic field, internal electric fields, and internal currents. Basic properties of fields and matter determine the methods by which these quantities can he measured. Quantification or dosimetry for these parameters on a macroscopic basis can be directed to the whole body, a cross section across the body, a local surface area, or a local volume. Models of varying degrees of sophistication have been used to establish spatial distributions of external fields and internal fields and currents. 0 I Y Y ? Wiley-Lib\, Inc.
Key words: electric field, magnetic field, ELF, dosimetry
INTRODUCTION
The purpose of this paper is to describe the experimental determination of the macroscopic physical parameters that result from exposure to extremely-low-frequency (ELF) electric and magnetic fields (EMF). Time-varying electric and magnetic fields induce surface charge densities, currents, and fields i n the bodies of humans and animals. This paper discusses the measurement of these quantities interior and exterior to the body over dimensions large compared to the dimensions of individual cells. A companion paper discusses theoretical approaches to describing these macroscopic parameters [Hart, 199 11. As understanding of the interactions of fields with biological systems has progressed, so has the sophistication of measurements (and models): from simple systems addressing the whole body to more complex approaches that attempt to determine the interior distribution of induced currents and fields. Extensive reviews of the topics discussed here have appeared elsewhere Address rcprint requests to T. Dan Bracken, 5415 SE Milwaukie Avenue, Portland, OR 97202.
0 1992 Wiley-Liss, Inc.
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[Kaune and Phillips, 1985; Kaune, 1986; Tenforde and Kaune, 1987; Anderson and Kaune, 19891. The author has drawn heavily on these reviews. Characterization of the exposure of biological systems to EL,F electric and magnetic fields can be done in terms of external fields or internal physical parameters. Exposure generally refers to the external agent, i.e., unperturbed electric or magnetic field, while dosimetry refers to the quantification of the physical quantity that actually interacts with the biological system at the surface or inside the body. The unperturbed field outside a body is easier to measure than the induced field or current in a relatively small volume inside the body. Consequently, there is considerable exposure information available but only a limited amount of data for electric and magnetic field dosimetry. The physics of ELF electric and magnetic fields and their interaction with biological systems have been described by Kaune and Gillis [ 1981) and Kaune [ 19861. The basic physical quantities that have been used to characterize exposure are the unperturbed electric field E in volts/meter (V/m) and the magnetic flux density B in Tesla (T). The magnetic field strength H in amperes per meter (A/m) has also been used to describe magnetic field exposures. In a vacuum B and H are directly related by B = pH,where = p = 4 n x lo-’ T/ ( A h ) is the permeability of free space. Since this relationship is valid to a good approximation in all non-magnetic materials, including biological tissues, using one or the other of these vector quantities is sufficient to describe exposures. (Historically, exposures to magnetic flux density have been reported in gauss or milligauss (mG), and the term magnetic field has been substituted for the technically correct magnetic flux density. Here, the term magnetic field will be used interchangeably with magnetic flux density. Kaune [ 19861 provides a detailed discussion of units and terminology for magnetic fields.) Fields can be characterized by their frequency f i n Hertz (Hz) and their magnitude. The ELF range extends from 0 to 300 Hz. Laboratory exposures are typically single frequency sinusoidal waveforms at the power frequency of 50 or 60 Hz. Other ELF frequencies have also been used. Real-world field exposures include fields at many frequencies but are predominately at the power frequency. Field magnitudes are expressed in terms of the root-mean-square (rms) value. In some instances, however, waveform amplitude or peak-to-peak values have been used to describe field magnitudes. Exposure of humans and animals to E and B fields results in the generation of electrical quantities that interact directly with body tissues in a localized area or volume that is small compared to dimensions of the whole body. Characterization of these localized quantities constitutes macroscopic dosimetry. Electric field exposure results i n an induced charge density at the surface of the body, electric fields within the body, and induced currents in the body. The induced charge density produces an electric field which enhances the original field outside the body and attenuates it inside the body. The perturbed electric field at the outer surface of the body is a measure of the local induced surface charge and is one macroscopic dosimetric parameter. This surface field is generally enhanced above the unperturbed field with the greatest enhancement occurring on the upper portions of the body. For applied ELF fields the induced surface charge density is proportional to the external field and will have the same time dependency as the external field. The surface charge is not affected by the internal properties of the body. The varying charge density results in internal electric fields and internal current
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flow. The internal field and current are dependent on the time rate of change of the applied field and, hence, in sinusoidal ELF fields they are proportional to frequency. The induced current is described by a vector current density J = o E , where E is the internal electric field, 0 is the conductivity in siemendmeter (s/m), and the units of current density are A/m2. Both internal electric field and current density are macroscopic dosimetric parameters. The total induced current in a body, or portion of a body, is also a parameter of dosimetric interest. The total current in amperes (A) is defined to be current passing between two volumes of interest. The total current induced between a grounded body and the earth is called the short-circuit current. The interaction of ELF magnetic fields with the bodies of humans and animals is simpler than that for electric fields: at ELF frequencies non-magnetic biological systems do not perturb the magnetic field [Tenforde and Kaune, 19871. At considerably higher frequencies, the magnetically induced currents in the body generate fields that tend to cancel the applied magnetic field. However, for ELF frequencies the unperturbed external magnetic field is a valid indicator of the magnetic field inside the body and constitutes a macroscopic dosimetric parameter. Timevarying ELF magnetic fields induce internal electric fields by Faraday’s law of induction. The presence of an electric field in the conducting body results in small induced currents. In the case of an applied magnetic field, the induced electric field is a function of the magnetic field magnitude and frequency and the size of the body. It is independent of the conductivity. But, the magnitude and distribution of magnetically induced currents will depend on the conductivity of the medium. The magnetically induced fields and currents interact with biological systems i n similar manner to the fields and currents induced by electric fields and consequently require characterization to the same degree. Thus, internal electric fields and current densities are macroscopic dosimetric parameters for magnetic field exposures as well as for electric field exposures. The physical parameters of possible significance in the interaction of electric and magnetic fields with biological systems are listed in Table 1 along with general characteristics such as frequency dependence, scaling properties, and whether measurements of these quantities have been reported. Scaling refers to the dependence of electric and magnetic field dosimetric parameters on body size and shape, that is, different geometric coupling factors exist between external fields and different species. Thus, the magnitude of the applied unperturbed field may have to be adjusted to yield equivalent dosimetric exposure parameters in different species and in animals of different size in the same species. For example, the electric field at the top of the head of an upright human in a given unperturbed field is much greater than the field at a comparable location on a fourlegged animal in the same field. Hence, to produce a comparable field at the head, the animal would have to be placed in a larger unperturbed field than the human. The electrical conductivity and dielectric constant of biological tissue determine the internal distribution of induced currents and fields. They are especially important in analyses of the interactions of fields and currents with biological systems at the cellular and macromolecular level [Schwann, 1985a,b; Foster and Schwann, 19861. Characterization of the electrical properties of tissue has been done over a wide range of frequencies [Schwann, 1985al. On a macroscopic level at ELF frequencies, electrical conduction dominates, capacitive currents are minimal, and induced currents tend to concentrate in regions of high conductivity. In order to
T V/m Aim’
V/m Vlm A/m’ A
Units
(5
Body shape Body shape Body shape Body shape and size
Scaling
None None f, ci Body shape and size f. ci Bodv shaDe and size
f
f, f,
None
Dependence”
’f. frequency; ci, local conductivity. measurements performed; N, no measurements available.
A. Electric Field Surface electric field Internal electric field Internal current density Induced whole body current B. Magnetic field Internal magnetic flux density Induced electric field Induced current densitv
Quantity
TABLE 1. Dosimetric Parameters for E L F Electric a n d Magnetic Fields
N N N
Y N N Y
Animals
Y
Y
N Y
Y
Y Y
Homogeneous
N N N
Y
N N
Y
Heterogeneous
Models
Status of measurements”
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approximate such regions, theoretical and physical modeling of biological systems needs to include inhomogeneities in conductivity. Determination of the electrical properties of tissue represents a separate topic beyond the scope of this discussion. For purposes of extending the results of biological studies to health effects and/or risk assessment, specific dosimetric quantities should be preferably related to biologic responses. In the case of EMF the relationship between physical agent and biologic response is not necessarily known. Therefore the choice of which physical quantities to include in a discussion of macroscopic dosimetry is somewhat arbitrary. For this discussion, macroscopic dosimetry has been expanded to include characterization of fields outside the body and short-circuit currents as well as internal field and current distributions. This expanded scope for macroscopic dosimetry has the advantage of including the known short-term effects of contact currents and field perception while still providing a framework that encompasses the historical development and increasing sophistication of the subject. ELECTRIC FIELDS The basic nature of the physical interaction of a slowly varying electric field with an animal or man has been analyzed by Kaune and Gillis [ 19811. The results given in their basic description of the phenomena have far reaching consequences in electric field dosimetry. Their analysis showed that: 1 ) electric fields induced in humans or animals are very small: generally less than 10” of the external field and probably never more than 10 of the applied field in air. 2) the field outside the body and the induced surface charge are determined only by the external shape of the body and by its location relative to other bodies. 3) the total current passing through any section of a body is independent of its internal structure. 4) the surface field and induced charge density are independent of ELF frequency but dependent on body shape. 5 ) the total induced currents are proportional to frequency (f) and body size, leading to a fW2“ dependence, where W is body weight. These observations emphasized the value of conducting models as a first approximation for investigating ELF electric field dosimetry both external and internal to the body and, consequently, provided impetus for much experimental work. First, since the external fields are dependent only on the charge distributions on the surface, conducting hollow models can be used to replicate external fields near exposed bodies. Second, since the induced surface charge is the source of internal currents and since the induced charge is dependent only on external quantities, certain properties of the internal currents can be determined from external characteristics only, i.e., hollow conducting surface models can be used to provide information about average internal currents. Such models have therefore been utilized to quantify basic electrical quantities. The fields at the surface of humans, and a conducting model of a human, were measured by Den0 [ 19771 utilizing a small copper foil current sensor. The maximum field enhancement observed at the forehead of the subject was a factor of 20. Measurements of surface fields at the surface of conducting animal models have been reported by Kaune and Phillips [1980].
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Measurements of localized surface fields have also been employed in electric field exposure meters for humans [Deno, 1979; Den0 and Silva, 1984; Chartier et al., 1985; Heroux, 19911. Generally, the surface field is measured with a small electrode that is an integral part of an instrument with data recording capabilities. The surface field measurement is recorded at specified time intervals and serves as a measure of exposure. The surface field is very dependent on the placement of the meter on the body, the posture of the wearer, and the position of the meter relative to field sources. Consequently, there is considerable uncertainty in the determination of real human exposure to electric fields with these devices. To overcome the large variability inherent in small surface area sensors Den0 and Silva [ 19841 utilized a conducting vest with a much larger area as the sensor. The total current, called the short circuit current, induced between a grounded animal and ground is a measure of the coupling of the whole animal to the electric field. However, it also has value in the context of macroscopic dosimetry: first, total current is directly associated with perception and more severe responses to electrical current; and, second, the induced short-circuit current also can be used to estimate the average electric field acting o n the surface of the body. Kaune [I9811 showed that the average electric field, Ee,g,on the surface of a body with surface area A, is given by
where I,c is the short-circuit current, f is frequency, and &, is the permitivity of free space. Extending the concept of induced current to various subsurfaces of the body allows estimates of the average current density through cross sections of the body. Deno [ 19771 used a copper-foil covered human mannequin with separated body parts to estimate the induced currents through various sections of the body. Kaune and Phillips [ 19801 performed similar measurements with grounded models of rats and pigs, again taking advantage of the fact that the average current density through a body cross section is equal to the total induced current flowing between the two body parts divided by the area of the cross section. Takuma et al. 119901have reported similar measurements with baboon models. Results of the measurements of Kaune and Phillips [ 19801 are shown in Figure 1. Figure 1 is one of the most commonly used graphics to illustrate the concept of electric field exposure scaling between species. The surface electric fields and axial current densities averaged across a cross section are shown in Figure I for a human, pig, and rat exposed to an unperturbed vertical electric field of 10 kV/m. Both the maximum surface electric field and the internal current densities for equivalent cross sections are highest for the human. For example, the current density through the ankle scales between species as 2,000 nA/cm‘: 1,100 nA/cm’ or I .8:1 for man:pig and as 1.4: 1 for man:rat. Thus, if equal magnitudes of the current density through the ankle were required to stimulate equivalent responses in the three species, different unperturbed field levels would be required for the three species. A summary of the scaling factors for the pig and rat compared to man are given in Table 2. As can be seen in Table 2, the scaling factors between man and different species are not the same nor are scaling factors for different parameters within the same pair of species. For example, the scaling factor between man and pig for
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180 kVlm
31 kVlm
Fig. 1. Surface electric fields and estimated average axial current densities for grounded human, pig, and rat exposed to vertical, 60 Hz, 10 kVlm electric field. Reproduced from Kaune and Philllps [ 19801.
maximum surface field is 2.7:l; for axial current density through the neck it is 14: 1; and for current density through the ankle it is 1 .ti:1. Consequently, it is necessary to identify both the physical parameter of interest and the site of interaction before comparisons of modeled doses between species can be made accurately. The results depicted in Figure 1 are for grounded humans and animals with current flow between the body and the earth. When an animal or person is not grounded, the surface charge distribution, external field distribution, and internal current distribution are different than for the grounded case. with no current flowing to the grounded surface, the maximum total axial currents occur at the midplane of the body rather than at the feet. (The maximum current density, of course, will depend on the area of the axial cross section.) Axial current densities in a human model as a function of its height above ground are shown in Figure 2 which is taken from Kaune et al. [ 19871. The ratio of average axial current densities through the ankle for a grounded human and an ungrounded human 1.23 m above the ground are 2.5: 1. At ELF frequencies, a person is essentially grounded when the contact resistance to ground is less than about 1 MQ [Kaune, 19861. TABLE 2. Scaling Factors for Grounded Animals Compared to a Grounded Human Standing in a Vertical Electric Field* Scaling factors Parameter
Standing human (1.7 m)
Surface field: Peak, kV1m Average, kVlm Current density: Neck, nAlcm’ Torso (axial) nA/cm’ Torso (total, nA/cm2 Anklelleg, nAlcm2 Short circuit current. uA
Human:pig (60 kg)
Human:rat (500 g)
I80 27
2.7: 1 1.9: 1
4.9: 1 2.2: I
550 246 249 2,000 160
14:l 13:l 1.3 1.8:l 2.3: 1
20: I 49: 1 12:l I .4:1
~
~
~
101:l
~~~~~
*Dosimetric parameters are given for a 1.7 m tall man standing in a I0 kV/m field. [Adapted from Kaune and Phillips, 1980; Kaune and Forsythe, 1988.1
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Experimental dosimetry performed with hollow conducting models is a technique that has provided information on surface fields, short-circuit current, and average current densities. Empirically determined surface fields and short-circuit currents are valid for quantifying the levels of these parameters i n humans and animals. Measurements with hollow conducting model have also provided essential data for validating and understanding theoretical models of field coupling. However, the average current densities derived from conducting models provide only an approximation of internal currents. As previously indicated, the results depicted in Figures 1 and 2 show only the average axial current across the cross section of interest. In order to capture information on the total current at a location and the distribution of currents across a plane, measurements in solid three-dimensional models have been performed. Guy et al. [ I9821 used thermographic techniques to measure energy deposition in solid conducting human and animal models exposed to 57 MHz electric fields. The measured specific absorption rate (SAR) was then scaled to calculate electric field strength and current density at 60 Hz for full scale models. The ability of this technique to determine current densities in areas of low current density such as the chest and head was limited by intense heating in the extremities where current densities were greatest. Kaune and Forsythe [ 1985, 19881 used small spatially separated potential probes to measure the localized electric field in saline-filled scale models of humans and animals. A two-axis probe was used in the measurements in the human models and a three-axis probe was employed in the measurements in pig and rat models. The spacing between the chlorided-silver potential electrodes was 5 mm for the twoaxis probe and 8 mm for the three-axis probe. These dimensions were from I0 to 25 times less than the width of the models, providing considerable spatial resolution with respect to the total volume. The estimated overall accuracy of the measurements for the animals with the three-axis probe was f 15%. The distributions of current densities measured in grounded saline-filled human models are shown in Figure 3 [Kaune and Forsythe, 19851. The vertical currents dominate and are fairly uniform across cross sections of the body. However there are small horizontal current components, and in the axillae (armpit) region the horizontal currents dominate. Similar measurements in the pig and rat models indicated increased horiId)
Fig. 2. Average vertical current densities in nAlrn' induced in human exposed to vertical, 10 kVlni, 60 H L electric field as a function of position relative to ground plane: a) electrically grounded; b) electrically insulated, feet 1 . 1 cm above ground; c) electrically insulated, feet 12.8 cm above ground; and d ) electrically insulated, feet 124 cm above ground. Reproduced from Kaune et al. [1Y87j.
Electric Field Dosimetry
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zontal components arising from the horizontal orientation of the animal bodies as opposed to the vertical orientation for the human model [Kaune and Forsythe, 19881. The current distributions in the animal models were also very dependent on the grounding configuration: the number and location of the grounded feet.
29
27
296
19
7
2
280 277
7
28
38
284 296
38
Fig. 3. Induced current densities in nA/cm2 measured in the midfrontal plane of a saline model of a standing human with both feet grounded exposed to a 60 Hz, 10 kV/m electric field. Reproduced from Tenforde and Kaune [ 19871, from the journal Hrulth Physics with permission from the Health Physics Society.
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Based on the measurements with the saline-filled models, Kaune and Forsythe [ 19881 estimated scaling factors between humans and animals for total current densities induced i n the torso as 7.3: 1 and 12.5: 1 for human:pig and human:rat,
respectively. As shown in Table 2, these values are considerably different than the scaling factors determined from hollow conducting models that only take into account the axial current (Kaune and Phillips, 19851. The results from the saline-filled models clearly provide a better representation of the actual current distributions than do the average current densities derived from hollow conducting models. Figure 3 indicates the spatial resolution that is needed to characterize even a homogeneous model and emphasizes that the resolution required to scale these results between species is dependent on what location of the body is of interest. Just as the results from hollow conducting models pointed out the need for internal measurements in solid conducting models, the measurements in homogeneous conducting models emphasize the need to pursue the next level of sophistication: the investigation of heterogeneous models and measurements in whole animals. Measurements in heterogeneous systems have not been reported. However, the techniques employed by Kaune and Forsythe [1985, 19881 have been refined by Miller [1991] and possibly could be adapted and/or enhanced to perform field characterization at the same or smaller scales in more complex models and finally animals. As with the initial measurements great care must be taken to ensure that the fields are not perturbed by the probes. The difficult and time consuming nature of field measurements of this type emphasizes the desirability of developing computational models for field and current distributions in heterogeneous media. However, as such theoretical models are developed, there will still be a definite need for the measurement capabilities to test and validate the models. MAGNETIC FIELDS
Internal measurements and modeling are not necessary if the magnetic field is the physical quantity of interest. The internal magnetic field is the same as the external unperturbed magnetic field, given the absence of magnetic materials in the body and the small magnitude of the induced currents in human and animal bodies [Tenforde and Kame, 19871. Because magnetic fields couple internally to the body, there is no simple whole body modeling approach analogous to the use of hollow conducting models for electric field induction. Measurements of magnetically induced fields and currents in homogeneous conductive agar models of a human and a rat have been reported by Miller [1991]. He employed a smaller version of the probe used previously by Kaune and Forsythe [ 19881. By reducing the electrode spacing and size, improk ing the shielding, and optimizing the location of the sensitive electronics, the sensitivity of the field probe was improved by a factor of 30 over the previous version. Measurements using this improved probe demonstrated the radial dependence of magnetically induced fields and currents. Based on the measurements Miller [ 19911 indicated that an overall scaling factor of 5: 1 for rat:human exposures to magnetic fields may be appropriate for experimental design and that a 1 mT magnetic field exposure was equivalent in terms of induced current density in the torso to 60 and 28 kV/m electric field exposures for rats and humans, respectively. There have also been theoretical approaches to modeling the coupling of magnetic fields to simple homogeneous geometric models [Spiegel, 1976; Hart and Marino, 19821.
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Given that the empirical results for electric field induced currents in homogeneous human and animal models emphasize the importance of investigating heterogeneous models and live animals, it seems appropriate to proceed to these systems in the magnetic induction case as well. As indicated in calculations by Polk and Song [ 19901, the deviation from the simple circular induced electric field lines of homogeneous media can be quite striking when inhomogeneities in conductivity are introduced even in simple models. Measurements of magnetically induced fields and currents in homogeneous simple geometric volumes are useful for validation of measurement techniques that are used in more complex settings. The technique developed by Kaune and Forsythe [ 19881 and refined by Miller [ 199 1 ] appears to be appropriate to perform initial measurements in inhomogeneous media. However, as with electric field induction, the time consuming and difficult nature of measurements dictates that if possible the empirical results should be used to guide the development of and validate models rather than as a means of mapping field and current distributions. Whereas measurement of human exposures to electric field is difficult and uncertain, the determination of magnetic field exposure is straightforward with the use of microprocessor based data loggers [Enertech 1989; Deadman et a]., 1988; Bracken, 1990; Heroux, 19911.Magnetic field exposure meters utilize three orthogonal sensing coils to measure the magnetic field components. The outputs from the three transducers are recorded by a datalogger on a periodic basis and then combined during analysis to estimate the magnitude of the field exposure over time. because the magnetic field is not perturbed by the wearer, the exposure meter measures the relevant field exposure directly. CONCLUSIONS
Experimental dosimetry has progressed from simple whole body models to measurements of internal current distributions in conducting models. The next logical step is to extend the internal measurements to heterogeneous models and whole animals. Techniques are available to extend dosimetric measurements to the next level of interest. However, additional efforts devoted to miniaturization and increased measurement efficiency would be of benefit. Experimental dosimetry has highlighted the requirements and difficulties of scaling between species. The value of experimental dosimetry lies not only in its ability to measure macroscopic fields and currents, but also in its usefulness for evaluating theoretical models. This latter capability will take on more importance as the modeled systems become more complex and the models more sophisticated. REFERENCES Anderson LE, Kaune WT (1989): Electric and magnetic fields at extremely low frequencies. In “Non ionizing radiation protection.” 2nd Ed. Copenhagen, Denmark: World Health Organization, pp 175-243. Bracken TD (1990): The EMDEX Project: Technology Transfer and Occupational Measurements, Volumes 1-3 Interim Report. (EPRI Research Project 2966-1) Electric Power Research Institute, Palo Alto, CA. (Report EN-7048) Chartier VL, Bracken TD, Capon AS (1985): BPA study of occupational exposure to 60-Hz electric fields. IEEE Trans. on Power App. and Sys. 104:733.
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Deadinan JE, Camus M, Armstrong BG, Heroux P, Cyr D. Plante M, Theriault G (1988): Occupational and residential 60-Hz electromagnetic fields and high frequency electric transients: Exposure assessment using a new dosimeter. Am Ind Hyg Assoc J 49 8:409-419. Deno DW ( 1 977): Currents induced i n the human body by high voltage transmission line electric field: Measurement and calculation of distribution and dose. IEEE Trans. Power App. Sys. 96: 15 17. Deno DW (1979): Monitoring of personnel exposed to a 60-Hz electric field. In: Biological effects of extremely low frequency electromagnetic fields. ( 18th Hanford Life Sciences Symposium, Richland, Wash., 1978) Technical Information Center, U S Dept of Energy. Springfield, VA. Deno DW, Silva M ( 1984): Method for evaluating human exposure to 60 Hz electric fields. IEEE Trans. on Power App and Sys: 103, 1699. Enertech (1989): EMDEX system manuals. Volume I : User’s manual. (EPRI EN-65 18. Rescarch Project 799-16) Electric Power Research Institute, Palo Alto, CA. Foster KR, Schwan HP (1986): Dielectric properties of tissues. I n eds: Polk E, Postrow C: Handbook of biological effects of electromagnetic fields. CRC Press, Inc., Boca Raton, FL, pp 27-96. Guy AW, Davidow S, Yang GY, Chou CK (1982): Determination of electric current distributions in animals and humans exposed to a uniform 60-Hz high-intensity electric field. Bioelectromagnetics 3:47-7 I . Hart FX. Marino AA (1982): ELF dosage in ellipsoidal models of man due to high voltage transrnission lines. J Bioelectricity 1: 129. Hart FX (1991): Scaling with a spreadsheet. Paper presented at EPRI Dosimetry Workshop. Carmel. CA. March 19-23. Heroux P ( I99 I ) : A dosimeter for assessmen1 of exposures to ELF fields. Bioelectroinagnelics 12:241-257. Kaune WT ( 198 I ): Power-frequcncy electric fields averaged over the body surfaces of grounded humans and animals. Bioelectromagnetics 2:403. Kaune WT (1986): Physical interaction of I-HL to 100 kHz electric and magnctic fields with living organisms. I n “Proceedings of 23rd Annual Meeting 2-3 April, 1986.” Bethesda, MD: National Council of a Radiation and Protection. pp 129-161, Kaune WT, Forsythe WC ( 1985):Current densities measured i n human niodels exposed to 60-Hz electric fields. Bioelectromagnetics 6 :13. Kaune WT, Forsythe WC ( 1988): Current densities induced in swine and rat models by power-frcquency electric fields. Bioelectroinagnetics 9: 1-24. Kaune WT, Gillis MF ( 198 1 ): General properties of the interaction between animals and ELF electric fields. Bioelectromagnetics 2: I . Kaune WT, Phillips RD (1980): Comparison of the coupling of grounded humans, swine and rats to vcrtical, 6 0 - H L electric fields. Rioelectromagnetics I : 1 17. Kaune WT, Phillips RD ( 1985): Dosimetry for extremely low-frcqucncy electric fields. I n Grandolfo. M: Michaelson, SM: Rindi, A (eds): Biological Effects and Dosimetry of Non-IoniLing Radiation: Static and ELF Electromagnetic Fields. New York: Plenum, pp 145-1 65. Kaune WT, Kistler LM, Miller MC (1987): Comparison ofthe coupling of grounded and ungrounded humans to vertical 60-Hz electric fields. I n Anderson LE. Wcigel RJ, Kelman BJ (eds): “lnteraction of Biological Systems with Static and ELF Electric and Magnetic Fields (Proceedings of the 23rd Annual Hanford Life Sciences Symposium) (DOE Symposium Series). Springfield, V A : NTIS. pp 185-196. Millcr DL (199 I ) : Miniature probe measurements of electric fields and currents induced by 60-Hr. magnetic field i n rat and human models. Bioclcctrornagnetics. 12: 157-1 7 I , Polk C, Song JH ( 1990):Electric fields induced by low frequency magnetic fields in inhomogeneous biological structures that are surrounded by an electric insulator. Bioelectromagnetics I 1235-249. Schwann HP ( 198Sa): Biophysical principlcs of the interaction of ELF lields with living matter: I. Propcrties and mechanism. In: cds: Grandolfo M, Michaelson SM, Rindi A: “Biological clfkcts and dosimetry of static and ELF electromagnetic fields.” Plenum Press, New York. NY, 221-241. Schwann HP (1985b): Biophysical principles of the interaction of ELF fields with living matter: 11. Coupling considerations and forces. In Eds: Grandolfo M, Michaelson SM, Rindi, A : “Biological effects and dosimetry of static and ELF electromagnetic fields.” Plenum Press, New York, N Y . 243-27 I . Spiegel RJ (1976): ELF coupling to spherical models of man and animals. IEEE Trans Biorned Eng 23:387. Takuina T, Kawamoto T, Isaka K, Yokoi Y (1990): A three-dimensional method for calculating current\ induced in bodies by extremely low-frequency electric fields. Bioelectromagnetics 1 I :7 1-89. Teiiforde TS, Kaune WT ( 1987): Interaction of extremely low frequency electric and magnetic fields with humans. Health Phys. 53:585-606. (From: Special Section: Non-ionizing Radiation)
Experimental Macroscopic Dosimetry for Extremely-Low-Frequency Electric and Magnetic Fields T. Dan Bracken T. Dan Bracken, Inc., 5415 SE Milwaukie Avenue, Portland, Oregon Environmental and laboratory exposure to electric and magnetic fields (EMF) in the extremely-low-frequency range (ELF) produces electrical quantities that interact directly with the exposed biological system on il scale small compared to the size of the human body but large with respect to cellular dimensions. The purpose of this paper is to describe these macroscopic electrical quantities and their characterization through measurements on living systems and experimental models. Electric field exposure results in a total induced current, surface electric fields, internal electric fields, and internal currents. Magnetic field exposure results in internal magnetic field, internal electric fields, and internal currents. Basic properties of fields and matter determine the methods by which these quantities can he measured. Quantification or dosimetry for these parameters on a macroscopic basis can be directed to the whole body, a cross section across the body, a local surface area, or a local volume. Models of varying degrees of sophistication have been used to establish spatial distributions of external fields and internal fields and currents. 0 I Y Y ? Wiley-Lib\, Inc.
Key words: electric field, magnetic field, ELF, dosimetry
INTRODUCTION
The purpose of this paper is to describe the experimental determination of the macroscopic physical parameters that result from exposure to extremely-low-frequency (ELF) electric and magnetic fields (EMF). Time-varying electric and magnetic fields induce surface charge densities, currents, and fields i n the bodies of humans and animals. This paper discusses the measurement of these quantities interior and exterior to the body over dimensions large compared to the dimensions of individual cells. A companion paper discusses theoretical approaches to describing these macroscopic parameters [Hart, 199 11. As understanding of the interactions of fields with biological systems has progressed, so has the sophistication of measurements (and models): from simple systems addressing the whole body to more complex approaches that attempt to determine the interior distribution of induced currents and fields. Extensive reviews of the topics discussed here have appeared elsewhere Address rcprint requests to T. Dan Bracken, 5415 SE Milwaukie Avenue, Portland, OR 97202.
0 1992 Wiley-Liss, Inc.
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Bracken
[Kaune and Phillips, 1985; Kaune, 1986; Tenforde and Kaune, 1987; Anderson and Kaune, 19891. The author has drawn heavily on these reviews. Characterization of the exposure of biological systems to EL,F electric and magnetic fields can be done in terms of external fields or internal physical parameters. Exposure generally refers to the external agent, i.e., unperturbed electric or magnetic field, while dosimetry refers to the quantification of the physical quantity that actually interacts with the biological system at the surface or inside the body. The unperturbed field outside a body is easier to measure than the induced field or current in a relatively small volume inside the body. Consequently, there is considerable exposure information available but only a limited amount of data for electric and magnetic field dosimetry. The physics of ELF electric and magnetic fields and their interaction with biological systems have been described by Kaune and Gillis [ 1981) and Kaune [ 19861. The basic physical quantities that have been used to characterize exposure are the unperturbed electric field E in volts/meter (V/m) and the magnetic flux density B in Tesla (T). The magnetic field strength H in amperes per meter (A/m) has also been used to describe magnetic field exposures. In a vacuum B and H are directly related by B = pH,where = p = 4 n x lo-’ T/ ( A h ) is the permeability of free space. Since this relationship is valid to a good approximation in all non-magnetic materials, including biological tissues, using one or the other of these vector quantities is sufficient to describe exposures. (Historically, exposures to magnetic flux density have been reported in gauss or milligauss (mG), and the term magnetic field has been substituted for the technically correct magnetic flux density. Here, the term magnetic field will be used interchangeably with magnetic flux density. Kaune [ 19861 provides a detailed discussion of units and terminology for magnetic fields.) Fields can be characterized by their frequency f i n Hertz (Hz) and their magnitude. The ELF range extends from 0 to 300 Hz. Laboratory exposures are typically single frequency sinusoidal waveforms at the power frequency of 50 or 60 Hz. Other ELF frequencies have also been used. Real-world field exposures include fields at many frequencies but are predominately at the power frequency. Field magnitudes are expressed in terms of the root-mean-square (rms) value. In some instances, however, waveform amplitude or peak-to-peak values have been used to describe field magnitudes. Exposure of humans and animals to E and B fields results in the generation of electrical quantities that interact directly with body tissues in a localized area or volume that is small compared to dimensions of the whole body. Characterization of these localized quantities constitutes macroscopic dosimetry. Electric field exposure results i n an induced charge density at the surface of the body, electric fields within the body, and induced currents in the body. The induced charge density produces an electric field which enhances the original field outside the body and attenuates it inside the body. The perturbed electric field at the outer surface of the body is a measure of the local induced surface charge and is one macroscopic dosimetric parameter. This surface field is generally enhanced above the unperturbed field with the greatest enhancement occurring on the upper portions of the body. For applied ELF fields the induced surface charge density is proportional to the external field and will have the same time dependency as the external field. The surface charge is not affected by the internal properties of the body. The varying charge density results in internal electric fields and internal current
Electric Field Dosimetry
17
flow. The internal field and current are dependent on the time rate of change of the applied field and, hence, in sinusoidal ELF fields they are proportional to frequency. The induced current is described by a vector current density J = o E , where E is the internal electric field, 0 is the conductivity in siemendmeter (s/m), and the units of current density are A/m2. Both internal electric field and current density are macroscopic dosimetric parameters. The total induced current in a body, or portion of a body, is also a parameter of dosimetric interest. The total current in amperes (A) is defined to be current passing between two volumes of interest. The total current induced between a grounded body and the earth is called the short-circuit current. The interaction of ELF magnetic fields with the bodies of humans and animals is simpler than that for electric fields: at ELF frequencies non-magnetic biological systems do not perturb the magnetic field [Tenforde and Kaune, 19871. At considerably higher frequencies, the magnetically induced currents in the body generate fields that tend to cancel the applied magnetic field. However, for ELF frequencies the unperturbed external magnetic field is a valid indicator of the magnetic field inside the body and constitutes a macroscopic dosimetric parameter. Timevarying ELF magnetic fields induce internal electric fields by Faraday’s law of induction. The presence of an electric field in the conducting body results in small induced currents. In the case of an applied magnetic field, the induced electric field is a function of the magnetic field magnitude and frequency and the size of the body. It is independent of the conductivity. But, the magnitude and distribution of magnetically induced currents will depend on the conductivity of the medium. The magnetically induced fields and currents interact with biological systems i n similar manner to the fields and currents induced by electric fields and consequently require characterization to the same degree. Thus, internal electric fields and current densities are macroscopic dosimetric parameters for magnetic field exposures as well as for electric field exposures. The physical parameters of possible significance in the interaction of electric and magnetic fields with biological systems are listed in Table 1 along with general characteristics such as frequency dependence, scaling properties, and whether measurements of these quantities have been reported. Scaling refers to the dependence of electric and magnetic field dosimetric parameters on body size and shape, that is, different geometric coupling factors exist between external fields and different species. Thus, the magnitude of the applied unperturbed field may have to be adjusted to yield equivalent dosimetric exposure parameters in different species and in animals of different size in the same species. For example, the electric field at the top of the head of an upright human in a given unperturbed field is much greater than the field at a comparable location on a fourlegged animal in the same field. Hence, to produce a comparable field at the head, the animal would have to be placed in a larger unperturbed field than the human. The electrical conductivity and dielectric constant of biological tissue determine the internal distribution of induced currents and fields. They are especially important in analyses of the interactions of fields and currents with biological systems at the cellular and macromolecular level [Schwann, 1985a,b; Foster and Schwann, 19861. Characterization of the electrical properties of tissue has been done over a wide range of frequencies [Schwann, 1985al. On a macroscopic level at ELF frequencies, electrical conduction dominates, capacitive currents are minimal, and induced currents tend to concentrate in regions of high conductivity. In order to
T V/m Aim’
V/m Vlm A/m’ A
Units
(5
Body shape Body shape Body shape Body shape and size
Scaling
None None f, ci Body shape and size f. ci Bodv shaDe and size
f
f, f,
None
Dependence”
’f. frequency; ci, local conductivity. measurements performed; N, no measurements available.
A. Electric Field Surface electric field Internal electric field Internal current density Induced whole body current B. Magnetic field Internal magnetic flux density Induced electric field Induced current densitv
Quantity
TABLE 1. Dosimetric Parameters for E L F Electric a n d Magnetic Fields
N N N
Y N N Y
Animals
Y
Y
N Y
Y
Y Y
Homogeneous
N N N
Y
N N
Y
Heterogeneous
Models
Status of measurements”
Electric Field Dosimetry
19
approximate such regions, theoretical and physical modeling of biological systems needs to include inhomogeneities in conductivity. Determination of the electrical properties of tissue represents a separate topic beyond the scope of this discussion. For purposes of extending the results of biological studies to health effects and/or risk assessment, specific dosimetric quantities should be preferably related to biologic responses. In the case of EMF the relationship between physical agent and biologic response is not necessarily known. Therefore the choice of which physical quantities to include in a discussion of macroscopic dosimetry is somewhat arbitrary. For this discussion, macroscopic dosimetry has been expanded to include characterization of fields outside the body and short-circuit currents as well as internal field and current distributions. This expanded scope for macroscopic dosimetry has the advantage of including the known short-term effects of contact currents and field perception while still providing a framework that encompasses the historical development and increasing sophistication of the subject. ELECTRIC FIELDS The basic nature of the physical interaction of a slowly varying electric field with an animal or man has been analyzed by Kaune and Gillis [ 19811. The results given in their basic description of the phenomena have far reaching consequences in electric field dosimetry. Their analysis showed that: 1 ) electric fields induced in humans or animals are very small: generally less than 10” of the external field and probably never more than 10 of the applied field in air. 2) the field outside the body and the induced surface charge are determined only by the external shape of the body and by its location relative to other bodies. 3) the total current passing through any section of a body is independent of its internal structure. 4) the surface field and induced charge density are independent of ELF frequency but dependent on body shape. 5 ) the total induced currents are proportional to frequency (f) and body size, leading to a fW2“ dependence, where W is body weight. These observations emphasized the value of conducting models as a first approximation for investigating ELF electric field dosimetry both external and internal to the body and, consequently, provided impetus for much experimental work. First, since the external fields are dependent only on the charge distributions on the surface, conducting hollow models can be used to replicate external fields near exposed bodies. Second, since the induced surface charge is the source of internal currents and since the induced charge is dependent only on external quantities, certain properties of the internal currents can be determined from external characteristics only, i.e., hollow conducting surface models can be used to provide information about average internal currents. Such models have therefore been utilized to quantify basic electrical quantities. The fields at the surface of humans, and a conducting model of a human, were measured by Den0 [ 19771 utilizing a small copper foil current sensor. The maximum field enhancement observed at the forehead of the subject was a factor of 20. Measurements of surface fields at the surface of conducting animal models have been reported by Kaune and Phillips [1980].
20
Bracken
Measurements of localized surface fields have also been employed in electric field exposure meters for humans [Deno, 1979; Den0 and Silva, 1984; Chartier et al., 1985; Heroux, 19911. Generally, the surface field is measured with a small electrode that is an integral part of an instrument with data recording capabilities. The surface field measurement is recorded at specified time intervals and serves as a measure of exposure. The surface field is very dependent on the placement of the meter on the body, the posture of the wearer, and the position of the meter relative to field sources. Consequently, there is considerable uncertainty in the determination of real human exposure to electric fields with these devices. To overcome the large variability inherent in small surface area sensors Den0 and Silva [ 19841 utilized a conducting vest with a much larger area as the sensor. The total current, called the short circuit current, induced between a grounded animal and ground is a measure of the coupling of the whole animal to the electric field. However, it also has value in the context of macroscopic dosimetry: first, total current is directly associated with perception and more severe responses to electrical current; and, second, the induced short-circuit current also can be used to estimate the average electric field acting o n the surface of the body. Kaune [I9811 showed that the average electric field, Ee,g,on the surface of a body with surface area A, is given by
where I,c is the short-circuit current, f is frequency, and &, is the permitivity of free space. Extending the concept of induced current to various subsurfaces of the body allows estimates of the average current density through cross sections of the body. Deno [ 19771 used a copper-foil covered human mannequin with separated body parts to estimate the induced currents through various sections of the body. Kaune and Phillips [ 19801 performed similar measurements with grounded models of rats and pigs, again taking advantage of the fact that the average current density through a body cross section is equal to the total induced current flowing between the two body parts divided by the area of the cross section. Takuma et al. 119901have reported similar measurements with baboon models. Results of the measurements of Kaune and Phillips [ 19801 are shown in Figure 1. Figure 1 is one of the most commonly used graphics to illustrate the concept of electric field exposure scaling between species. The surface electric fields and axial current densities averaged across a cross section are shown in Figure I for a human, pig, and rat exposed to an unperturbed vertical electric field of 10 kV/m. Both the maximum surface electric field and the internal current densities for equivalent cross sections are highest for the human. For example, the current density through the ankle scales between species as 2,000 nA/cm‘: 1,100 nA/cm’ or I .8:1 for man:pig and as 1.4: 1 for man:rat. Thus, if equal magnitudes of the current density through the ankle were required to stimulate equivalent responses in the three species, different unperturbed field levels would be required for the three species. A summary of the scaling factors for the pig and rat compared to man are given in Table 2. As can be seen in Table 2, the scaling factors between man and different species are not the same nor are scaling factors for different parameters within the same pair of species. For example, the scaling factor between man and pig for
Electric Field Dosimetry
21
180 kVlm
31 kVlm
Fig. 1. Surface electric fields and estimated average axial current densities for grounded human, pig, and rat exposed to vertical, 60 Hz, 10 kVlm electric field. Reproduced from Kaune and Philllps [ 19801.
maximum surface field is 2.7:l; for axial current density through the neck it is 14: 1; and for current density through the ankle it is 1 .ti:1. Consequently, it is necessary to identify both the physical parameter of interest and the site of interaction before comparisons of modeled doses between species can be made accurately. The results depicted in Figure 1 are for grounded humans and animals with current flow between the body and the earth. When an animal or person is not grounded, the surface charge distribution, external field distribution, and internal current distribution are different than for the grounded case. with no current flowing to the grounded surface, the maximum total axial currents occur at the midplane of the body rather than at the feet. (The maximum current density, of course, will depend on the area of the axial cross section.) Axial current densities in a human model as a function of its height above ground are shown in Figure 2 which is taken from Kaune et al. [ 19871. The ratio of average axial current densities through the ankle for a grounded human and an ungrounded human 1.23 m above the ground are 2.5: 1. At ELF frequencies, a person is essentially grounded when the contact resistance to ground is less than about 1 MQ [Kaune, 19861. TABLE 2. Scaling Factors for Grounded Animals Compared to a Grounded Human Standing in a Vertical Electric Field* Scaling factors Parameter
Standing human (1.7 m)
Surface field: Peak, kV1m Average, kVlm Current density: Neck, nAlcm’ Torso (axial) nA/cm’ Torso (total, nA/cm2 Anklelleg, nAlcm2 Short circuit current. uA
Human:pig (60 kg)
Human:rat (500 g)
I80 27
2.7: 1 1.9: 1
4.9: 1 2.2: I
550 246 249 2,000 160
14:l 13:l 1.3 1.8:l 2.3: 1
20: I 49: 1 12:l I .4:1
~
~
~
101:l
~~~~~
*Dosimetric parameters are given for a 1.7 m tall man standing in a I0 kV/m field. [Adapted from Kaune and Phillips, 1980; Kaune and Forsythe, 1988.1
22
Bracken
Experimental dosimetry performed with hollow conducting models is a technique that has provided information on surface fields, short-circuit current, and average current densities. Empirically determined surface fields and short-circuit currents are valid for quantifying the levels of these parameters i n humans and animals. Measurements with hollow conducting model have also provided essential data for validating and understanding theoretical models of field coupling. However, the average current densities derived from conducting models provide only an approximation of internal currents. As previously indicated, the results depicted in Figures 1 and 2 show only the average axial current across the cross section of interest. In order to capture information on the total current at a location and the distribution of currents across a plane, measurements in solid three-dimensional models have been performed. Guy et al. [ I9821 used thermographic techniques to measure energy deposition in solid conducting human and animal models exposed to 57 MHz electric fields. The measured specific absorption rate (SAR) was then scaled to calculate electric field strength and current density at 60 Hz for full scale models. The ability of this technique to determine current densities in areas of low current density such as the chest and head was limited by intense heating in the extremities where current densities were greatest. Kaune and Forsythe [ 1985, 19881 used small spatially separated potential probes to measure the localized electric field in saline-filled scale models of humans and animals. A two-axis probe was used in the measurements in the human models and a three-axis probe was employed in the measurements in pig and rat models. The spacing between the chlorided-silver potential electrodes was 5 mm for the twoaxis probe and 8 mm for the three-axis probe. These dimensions were from I0 to 25 times less than the width of the models, providing considerable spatial resolution with respect to the total volume. The estimated overall accuracy of the measurements for the animals with the three-axis probe was f 15%. The distributions of current densities measured in grounded saline-filled human models are shown in Figure 3 [Kaune and Forsythe, 19851. The vertical currents dominate and are fairly uniform across cross sections of the body. However there are small horizontal current components, and in the axillae (armpit) region the horizontal currents dominate. Similar measurements in the pig and rat models indicated increased horiId)
Fig. 2. Average vertical current densities in nAlrn' induced in human exposed to vertical, 10 kVlni, 60 H L electric field as a function of position relative to ground plane: a) electrically grounded; b) electrically insulated, feet 1 . 1 cm above ground; c) electrically insulated, feet 12.8 cm above ground; and d ) electrically insulated, feet 124 cm above ground. Reproduced from Kaune et al. [1Y87j.
Electric Field Dosimetry
23
zontal components arising from the horizontal orientation of the animal bodies as opposed to the vertical orientation for the human model [Kaune and Forsythe, 19881. The current distributions in the animal models were also very dependent on the grounding configuration: the number and location of the grounded feet.
29
27
296
19
7
2
280 277
7
28
38
284 296
38
Fig. 3. Induced current densities in nA/cm2 measured in the midfrontal plane of a saline model of a standing human with both feet grounded exposed to a 60 Hz, 10 kV/m electric field. Reproduced from Tenforde and Kaune [ 19871, from the journal Hrulth Physics with permission from the Health Physics Society.
24
Bracken
Based on the measurements with the saline-filled models, Kaune and Forsythe [ 19881 estimated scaling factors between humans and animals for total current densities induced i n the torso as 7.3: 1 and 12.5: 1 for human:pig and human:rat,
respectively. As shown in Table 2, these values are considerably different than the scaling factors determined from hollow conducting models that only take into account the axial current (Kaune and Phillips, 19851. The results from the saline-filled models clearly provide a better representation of the actual current distributions than do the average current densities derived from hollow conducting models. Figure 3 indicates the spatial resolution that is needed to characterize even a homogeneous model and emphasizes that the resolution required to scale these results between species is dependent on what location of the body is of interest. Just as the results from hollow conducting models pointed out the need for internal measurements in solid conducting models, the measurements in homogeneous conducting models emphasize the need to pursue the next level of sophistication: the investigation of heterogeneous models and measurements in whole animals. Measurements in heterogeneous systems have not been reported. However, the techniques employed by Kaune and Forsythe [1985, 19881 have been refined by Miller [1991] and possibly could be adapted and/or enhanced to perform field characterization at the same or smaller scales in more complex models and finally animals. As with the initial measurements great care must be taken to ensure that the fields are not perturbed by the probes. The difficult and time consuming nature of field measurements of this type emphasizes the desirability of developing computational models for field and current distributions in heterogeneous media. However, as such theoretical models are developed, there will still be a definite need for the measurement capabilities to test and validate the models. MAGNETIC FIELDS
Internal measurements and modeling are not necessary if the magnetic field is the physical quantity of interest. The internal magnetic field is the same as the external unperturbed magnetic field, given the absence of magnetic materials in the body and the small magnitude of the induced currents in human and animal bodies [Tenforde and Kame, 19871. Because magnetic fields couple internally to the body, there is no simple whole body modeling approach analogous to the use of hollow conducting models for electric field induction. Measurements of magnetically induced fields and currents in homogeneous conductive agar models of a human and a rat have been reported by Miller [1991]. He employed a smaller version of the probe used previously by Kaune and Forsythe [ 19881. By reducing the electrode spacing and size, improk ing the shielding, and optimizing the location of the sensitive electronics, the sensitivity of the field probe was improved by a factor of 30 over the previous version. Measurements using this improved probe demonstrated the radial dependence of magnetically induced fields and currents. Based on the measurements Miller [ 19911 indicated that an overall scaling factor of 5: 1 for rat:human exposures to magnetic fields may be appropriate for experimental design and that a 1 mT magnetic field exposure was equivalent in terms of induced current density in the torso to 60 and 28 kV/m electric field exposures for rats and humans, respectively. There have also been theoretical approaches to modeling the coupling of magnetic fields to simple homogeneous geometric models [Spiegel, 1976; Hart and Marino, 19821.
Electric Field Dosimetry
25
Given that the empirical results for electric field induced currents in homogeneous human and animal models emphasize the importance of investigating heterogeneous models and live animals, it seems appropriate to proceed to these systems in the magnetic induction case as well. As indicated in calculations by Polk and Song [ 19901, the deviation from the simple circular induced electric field lines of homogeneous media can be quite striking when inhomogeneities in conductivity are introduced even in simple models. Measurements of magnetically induced fields and currents in homogeneous simple geometric volumes are useful for validation of measurement techniques that are used in more complex settings. The technique developed by Kaune and Forsythe [ 19881 and refined by Miller [ 199 1 ] appears to be appropriate to perform initial measurements in inhomogeneous media. However, as with electric field induction, the time consuming and difficult nature of measurements dictates that if possible the empirical results should be used to guide the development of and validate models rather than as a means of mapping field and current distributions. Whereas measurement of human exposures to electric field is difficult and uncertain, the determination of magnetic field exposure is straightforward with the use of microprocessor based data loggers [Enertech 1989; Deadman et a]., 1988; Bracken, 1990; Heroux, 19911.Magnetic field exposure meters utilize three orthogonal sensing coils to measure the magnetic field components. The outputs from the three transducers are recorded by a datalogger on a periodic basis and then combined during analysis to estimate the magnitude of the field exposure over time. because the magnetic field is not perturbed by the wearer, the exposure meter measures the relevant field exposure directly. CONCLUSIONS
Experimental dosimetry has progressed from simple whole body models to measurements of internal current distributions in conducting models. The next logical step is to extend the internal measurements to heterogeneous models and whole animals. Techniques are available to extend dosimetric measurements to the next level of interest. However, additional efforts devoted to miniaturization and increased measurement efficiency would be of benefit. Experimental dosimetry has highlighted the requirements and difficulties of scaling between species. The value of experimental dosimetry lies not only in its ability to measure macroscopic fields and currents, but also in its usefulness for evaluating theoretical models. This latter capability will take on more importance as the modeled systems become more complex and the models more sophisticated. REFERENCES Anderson LE, Kaune WT (1989): Electric and magnetic fields at extremely low frequencies. In “Non ionizing radiation protection.” 2nd Ed. Copenhagen, Denmark: World Health Organization, pp 175-243. Bracken TD (1990): The EMDEX Project: Technology Transfer and Occupational Measurements, Volumes 1-3 Interim Report. (EPRI Research Project 2966-1) Electric Power Research Institute, Palo Alto, CA. (Report EN-7048) Chartier VL, Bracken TD, Capon AS (1985): BPA study of occupational exposure to 60-Hz electric fields. IEEE Trans. on Power App. and Sys. 104:733.
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Deadinan JE, Camus M, Armstrong BG, Heroux P, Cyr D. Plante M, Theriault G (1988): Occupational and residential 60-Hz electromagnetic fields and high frequency electric transients: Exposure assessment using a new dosimeter. Am Ind Hyg Assoc J 49 8:409-419. Deno DW ( 1 977): Currents induced i n the human body by high voltage transmission line electric field: Measurement and calculation of distribution and dose. IEEE Trans. Power App. Sys. 96: 15 17. Deno DW (1979): Monitoring of personnel exposed to a 60-Hz electric field. In: Biological effects of extremely low frequency electromagnetic fields. ( 18th Hanford Life Sciences Symposium, Richland, Wash., 1978) Technical Information Center, U S Dept of Energy. Springfield, VA. Deno DW, Silva M ( 1984): Method for evaluating human exposure to 60 Hz electric fields. IEEE Trans. on Power App and Sys: 103, 1699. Enertech (1989): EMDEX system manuals. Volume I : User’s manual. (EPRI EN-65 18. Rescarch Project 799-16) Electric Power Research Institute, Palo Alto, CA. Foster KR, Schwan HP (1986): Dielectric properties of tissues. I n eds: Polk E, Postrow C: Handbook of biological effects of electromagnetic fields. CRC Press, Inc., Boca Raton, FL, pp 27-96. Guy AW, Davidow S, Yang GY, Chou CK (1982): Determination of electric current distributions in animals and humans exposed to a uniform 60-Hz high-intensity electric field. Bioelectromagnetics 3:47-7 I . Hart FX. Marino AA (1982): ELF dosage in ellipsoidal models of man due to high voltage transrnission lines. J Bioelectricity 1: 129. Hart FX (1991): Scaling with a spreadsheet. Paper presented at EPRI Dosimetry Workshop. Carmel. CA. March 19-23. Heroux P ( I99 I ) : A dosimeter for assessmen1 of exposures to ELF fields. Bioelectroinagnelics 12:241-257. Kaune WT ( 198 I ): Power-frequcncy electric fields averaged over the body surfaces of grounded humans and animals. Bioelectromagnetics 2:403. Kaune WT (1986): Physical interaction of I-HL to 100 kHz electric and magnctic fields with living organisms. I n “Proceedings of 23rd Annual Meeting 2-3 April, 1986.” Bethesda, MD: National Council of a Radiation and Protection. pp 129-161, Kaune WT, Forsythe WC ( 1985):Current densities measured i n human niodels exposed to 60-Hz electric fields. Bioelectromagnetics 6 :13. Kaune WT, Forsythe WC ( 1988): Current densities induced in swine and rat models by power-frcquency electric fields. Bioelectroinagnetics 9: 1-24. Kaune WT, Gillis MF ( 198 1 ): General properties of the interaction between animals and ELF electric fields. Bioelectromagnetics 2: I . Kaune WT, Phillips RD (1980): Comparison of the coupling of grounded humans, swine and rats to vcrtical, 6 0 - H L electric fields. Rioelectromagnetics I : 1 17. Kaune WT, Phillips RD ( 1985): Dosimetry for extremely low-frcqucncy electric fields. I n Grandolfo. M: Michaelson, SM: Rindi, A (eds): Biological Effects and Dosimetry of Non-IoniLing Radiation: Static and ELF Electromagnetic Fields. New York: Plenum, pp 145-1 65. Kaune WT, Kistler LM, Miller MC (1987): Comparison ofthe coupling of grounded and ungrounded humans to vertical 60-Hz electric fields. I n Anderson LE. Wcigel RJ, Kelman BJ (eds): “lnteraction of Biological Systems with Static and ELF Electric and Magnetic Fields (Proceedings of the 23rd Annual Hanford Life Sciences Symposium) (DOE Symposium Series). Springfield, V A : NTIS. pp 185-196. Millcr DL (199 I ) : Miniature probe measurements of electric fields and currents induced by 60-Hr. magnetic field i n rat and human models. Bioclcctrornagnetics. 12: 157-1 7 I , Polk C, Song JH ( 1990):Electric fields induced by low frequency magnetic fields in inhomogeneous biological structures that are surrounded by an electric insulator. Bioelectromagnetics I 1235-249. Schwann HP ( 198Sa): Biophysical principlcs of the interaction of ELF lields with living matter: I. Propcrties and mechanism. In: cds: Grandolfo M, Michaelson SM, Rindi A: “Biological clfkcts and dosimetry of static and ELF electromagnetic fields.” Plenum Press, New York. NY, 221-241. Schwann HP (1985b): Biophysical principles of the interaction of ELF fields with living matter: 11. Coupling considerations and forces. In Eds: Grandolfo M, Michaelson SM, Rindi, A : “Biological effects and dosimetry of static and ELF electromagnetic fields.” Plenum Press, New York, N Y . 243-27 I . Spiegel RJ (1976): ELF coupling to spherical models of man and animals. IEEE Trans Biorned Eng 23:387. Takuina T, Kawamoto T, Isaka K, Yokoi Y (1990): A three-dimensional method for calculating current\ induced in bodies by extremely low-frequency electric fields. Bioelectromagnetics 1 I :7 1-89. Teiiforde TS, Kaune WT ( 1987): Interaction of extremely low frequency electric and magnetic fields with humans. Health Phys. 53:585-606. (From: Special Section: Non-ionizing Radiation)
