Characteristics, Dosimetry & Measurement Of

CHARACTERISTICS, DOSIMETRY & MEASUREMENT OF EMF Masao Taki 1), Soichi Watanabe 2), and Kanako Wake 2) Department of Electrical Engineering, Tokyo Metropolitan University 1) EMC Research Group, Communications Research Laboratories (CRL) 2) Tel: +81 426 77 2763, FAX: +81 426 77 2756, Email: [email protected] ?? Abstract In this paper we will describe these aspects of engineering and physics related to the safety of EMFs to This paper describes characteristics of electromagnetic clarify the physical concepts as well as their important fields and their interaction with body as a fundamentals roles in this issue. of electromagnetic field (EMF) safety issue. It also presents how engineering and physics contribute to this. ?? Characterization of EMF [1] EMF should be appropriately and precisely characterized when we consider its interaction with body. Dosimetry, 1 .Variables to be determined which is the quantification of substance interacting with the exposed body, is of quite importance in the exposure The electromagnetic field is characterized by a pair of assessment for both compliance testing with guidelines vector fields of electric field E [V/m] and magnetic field H and the quality control of researches on EMF effects. The [A/m] (or B [T]). Each vector field has three time varying basic concept of these issues is summarized, and some components in space; hence we need to determine six researches mainly performed by the authors’ group are functions of time at each point in space to characterize the presented as exa mples of researches in this field. field completely. It is obvious that the characterization in this complete sense is far from practically achievable. Fortunately the components of these vectors are not ?? Introduction independent as they are governed by Maxwell’s equations. Hence we just need to specify fewer variables that are When we discuss the biological and health effects of practically achievable. However it is worth recognizing the electromagnetic fields (EMF), it is often the case that fact that the EMF in general can not be characterized biology and health science is of the primary concern, and completely in a simple manner in the strict sense. engineering and physics are of the second concern. It is important, however, to remember that we are dealing with 2. Frequency and waveform interaction between electromagnetic field and body. The physical interaction should first be characterized and Any functions can be decomposed to a summation of quantified. Then substantial phenomena that should occur sinusoidal functions of arbitrary frequencies with in tissue or cells are better realized. With all these appropriate phase. This fact is the basis for the frequency quantitative description we could better define the domain approach. Thus biological effects are often biological and health effects that are subsequently discussed in frequency region. observed. The aspects of engineering and physics are no Power frequency electromagnetic fields are almost pure less important than biology. sinusoidal waves at 50/60 Hz, and waveforms of In the practical situations it is required to establish reliable radiofrequency (RF) communications are relatively narrow and reproducible measurement procedures to perform the band relative to their carrier frequencies. However we assessment of compliance of electromagnetic environment should note that the waveforms of actual electromagnetic with the guidelines properly. It is also important to fields are not pure sinusoidal waves. understand electromagnetic phenomena to avoid artifacts due to unwanted coupling between objects and measuring 3. Polarization instruments. We sometimes encounter situations where the measurement of external field is not appropriate to The orientation of field vectors E and H are another determine the compliance of exposure with the guidelines. important characteristic. It is called “polarization”. It is well Near-field exposures are such the cases. In these cases we known that RF electromagnetic field with E-field parallel to need to quantify electromagnetic phenomena in the body. the body axis shows resonant characteristics at around the This technique is called “dosimetry”. Exposure to EMF frequency where the body height is the half-wavelength. from mobile telephone is the typical case where dosimetry The current density induced by time varying magnetic field should be applied in the hazard assessment. is also dependent on the direction of H field. The larger the Engineering plays a crucial role in experiments to cross sections of body perpendicular to B, the larger the investigate biological effects of EMF. Electromagnetic induced current density. The orientation of field relative to fields must be characterized as precisely as possible. We the exposed object is very important when we consider the must characterize not only the incident fields but also coupling of electromagnetic fields with biological bodies. internal fields in the biological specimen. Dosimetry of Figure 1 shows the directions of fields that shows largest exposure setup is one of the key issues that determine the coupling with body and the direction of induced current quality of experimental studies. densities. We should note that the polarization is not necessarily is otropic E-field sensor. constant with time. Two field vectors with different 3.3 Measurement method directions superpose to make a total field as a result of the A number of instruments are commercially available vector summation. If the orientations of these vectors are which are specially designed for measuring electric and different and have different phases, the orientation of the magnetic fields for hazard assessment. These instruments total vector is not constant but moves elliptically. provide measured values directly. However it is important Elliptically polarized fields are found in the magnetic field to note that the figures on the display are not always beneath the three-phase power transmission lines. Satellite correct values of interest. communications also employ circularly polarized microwaves. Fig. 1. Orientation of E and B fields which have the maximum coupling with body and induced current. Fig. 2 A picture of isotropic implantable probe ?? Measurement of EMF The artifacts can arise from inappropriate orientation of 1. Basic principle the cable, disturbance of the field by the presence of body, coupling of probes with metallic objects, and so on. Electric and magnetic fields can be measured by using Magnetic field probes could respond to electric field antennas that respond to the field of concern. Electrical erroneously, and vice versa. There are a lot of items to be gap or infinitesimal electric dipole detects the electric field considered in the field measurements. The practical of the component parallel to the dipole. A small loop or an procedures of the measurement have been issued as [2][3] infinitesimal magnetic dipole responds to the magnetic field standard procedures elsewhere. of the component parallel to the dipole moment or perpendicular to the loop face. Larger elements can have ?? Dosimetry higher sensitivity, but the response could be dependent on frequency. A parallel plate is one of the commonly used 1. Dose metric sensors in electric field measurement, which is equivalent to an array of electrical gaps. It should be noted that Dosimetry means the metrology of dose. “Dose” is existence of metal can disturb fields due to the scattering defined by “the amount of a substance we are exposed to of field by the metal. or come in contact with”. Here a problem arises; what is the Devices to detect electric and magnetic fields without “substance” to be determined in the dosimetry of metallic antenna have also been developed. Pockels effect electromagnetic fields? [5][6] is used for electric field sensors. Hall effect is used for There is a consensus as follows . Stimulation effect magnetic field sensors. These sensors are not sufficiently prevails over other possible effects in the low frequency sensitive to the fields for the purpose of hazard region (up to 100 kHz), where the induced current density J assessment in common environment, however. [A/m2] in tissue is the dosimetric quantity. In higher frequencies above 10 MHz up to 300 GHz, thermal effects 2. Polarization are prevailing. Specific absorption rate (SAR), which is defined by the absorbed power per unit mass at The sensor elements respond only to one direction of infinitesimal volume of tissue [W/kg], is the quantity to field components. Combined elements with orthogonal define the dose at frequencies from 100 kHz to about 10 directions are often used to detect three dimensional GHz. Incident power density [W/m] is more appropriate components simultaneously. This type of sensors is called than SAR at frequencies above 10 GHz, where the an “isotropic sensor”. Isotropic probes for the purpose of penetration of EMF into skin is very short. Thermal effects hazard assessment of EMF are commercially available both and stimulation effects could overlap each other in the for E and H fields. Figure 2 shows an example of the intermediate frequencies between 100 kHz and 10 MHz. The basic restrictions of guidelines on electromagnetic resolution is a few milliliters up to sub-millimeters. Figure 3 field exposures are therefore given by these quantities. The shows the examples of whole-body male model and female induced current density in the low frequencies and the model developed by Kitasato University and CRL in Japan. SAR in high frequencies are the quantities to be Experimental approaches are also attempted to measure determined in the electromagnetic dosimetry when we induced current densities by electric fields. On the other perform the exposure assessment based on the exposure hand experimental approach is rather minor in magnetic guidelines. field induced currents. The difference in induced current However we have not been fully convinced that the between E and B (Fig. 1) may be the reason. stimulation effect and the thermal effect are the only mechanisms that can threat human health. Further SAR investigations are still ongoing to explore possible other effects.

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