EMR Defined

EMR is a form of radiation that ranges from extremely high-energy cosmic and gamma rays at frequencies above 1018 Hz down through the visible spectrum (frequencies near 1015 Hz) to the relatively low-energy microwave (1010 Hz or 10 GHz) and radio frequencies (108 Hz or 100 MHz) (Fig. 1.1). The part of the spectrum used for mobile phone communications is in the frequency range from 800 MHz to 2.5 GHz, labelled Global System for Mobile Communications (GSM) in Fig. 1.1. EMR may be considered to comprise alternating electric, E, and magnetic, B, fields. The E and B fi elds both generate forces on charged particles in materials, but the forces due to the electric fields are normally much larger, except in magnetic materials. However, in the context of mobile phone exposure, the magnetic component of the radiation may be more significant due to its considerable penetrative ability inside not only in human body, but also in buildings.

As will be seen in the discussion below on mobile phone communications, the EM wave may contain components oscillating at a range of different frequencies. This is the case with modulated EM waves where the amplitude or frequency of

Fig. 1.1 Electromagnetic spectrum. The blue box indicates the mobile phone frequency spectrum which begins to enter the microwave spectrum. The energy of mobile phone frequency radiation is far lower than that of ionising radiation (adapted from Physics Central (http://www.physicscentral. com/experiment/askaphysicist/physics-answer.cfm?uid=20110119110703))

Fig. 1.1 Electromagnetic spectrum. The blue box indicates the mobile phone frequency spectrum which begins to enter the microwave spectrum. The energy of mobile phone frequency radiation is far lower than that of ionising radiation (adapted from Physics Central (http://www.physicscentral. com/experiment/askaphysicist/physics-answer.cfm?uid=20110119110703))

the so-called carrier wave is varied in time, in order to carry information. The modulated EM wave contains a component oscillating with the carrier frequency f as well as sideband frequencies f± Df. The challenge for communications engineers is to maximise the information carried while minimising the frequency spectrum, 2Df, used.

Most of the radiation studies related to mobile phone communications are done at frequencies used by the GSM phone network. GSM is a digital standard first offered commercially in 1991 and is currently the world's most popular standard for mobile telephony systems, with over 80% of the global mobile market using the standard. GSM networks operate in a number of different carrier frequency ranges with most 2G GSM networks operating in the 900 or 1,800 MHz bands. Where these bands were already allocated, the 850 and 1,900 MHz bands were used instead (e.g. in Canada and the United States). The more recent 3G networks operate in the 2,100 MHz frequency band in Europe. Although the technology is rapidly evolving, with the incorporation of data transmission and the progression to 4G protocols, the basics of EM radiation emission by GSM mobile phones are relatively unchanged. During a GSM call, speech is converted from analogue sound waves to digital data by the phone itself and transmitted through the mobile phone network by digital means. The EM wave emitted by the phone comprises a range of frequencies. As an example, for a GSM-900 phone, the frequency band 890-915 MHz is used for transmission from the mobile phone to the base station and the band 935-960 MHz from the base station to the mobile phone. In each band, there are 124 separate carrier frequencies spaced 200 kHz apart, starting in the above example at 890.2 MHz. Each 200 kHz frequency is segmented in time, so eight separate channels of information can be sent on each carrier. The digitally encoded information from the codec for all channels together is then used to modulate the frequency of the carrier at a digital rate of 270 kbit s-1 . An individual GSM-900 mobile phone will then generate an EM wave with a time-varying frequency within a 200 kHz band on a carrier frequency between 890 and 915 MHz. The intensity of the EMR will also vary in time, since the encoding of the eight separate channels occurs within a 4.615 ms period. So, to properly measure the effect of all mobile phone irradiation on biological systems, experiments should be conducted using pulsed radiation for a range of frequencies within the 850-2,100 MHz band.

The alternating electric, E. and magnetic, B, fields in the EMR interact with materials by exerting forces on charged particles, changing charge distributions in the material. In nonmagnetic materials, the E field causes polarisation (or separation) of bound charges, orientation of permanent dipoles (pairs of opposite charges) and movement of electrons and ions. The first two effects are taken into account by the permittivity, e, which is a measure of how easily the polarisation of the material changes due to an electric field. Materials primarily affected by the first two processes are called dielectrics. The third effect, the movement of both electrons and positively and negatively charged ions, is accounted for by the conductivity, s, and materials affected by the third process are known as conductors. The permittivity is typically expressed as a complex quantity

where e0 is the permittivity of free space (8.85 x 1012 F m-1), e0e' is the real part of the complex permittivity (termed the dielectric constant), j = V-1 and w = 2nf is the angular frequency in radians per second. Both e and s increase with increasing water content in the tissue being low for fat and high for blood. The variation of these parameters over the communication frequency range is not large. For testes, e is 58 and conductivity is 1.34 S m-1 at 900 MHz, whereas at 2.5 GHz, e and s are 57.5 and 2.21 S m-1, respectively [5]. Since e is high, tissue such as testicles may then be considered to be lossy dielectrics. With a lossy dielectric, the transmitted wave is attenuated as it travels into the material. Energy is transferred from the wave to the dielectric as kinetic energy of the charged particles in the dielectric. The loss is related to the average permittivity for biological tissue and depends on frequency, generally decreasing as the frequency increases since the charges cannot respond to rapid changes in high frequency fields. e represents the conduction of ions as well as friction associated with the alignment of dipoles and vibrational and rotational motion in molecules. The depth of penetration of EM radiation, defined as the distance at which power absorption is approximately 14% of the surface value, is about 4 cm at 1 GHz and 2.5 cm at 2 GHz in tissue. Real world irradiations are, however, more complicated because scattering and refraction of EM waves at interfaces means that energy is deposited in a non-uniform manner into tissue. The energy absorbed from the wave is directly related to the internal E field at the point of absorption. But the incident and internal fields can be quite different depending on the size and shape of the body, its electrical properties, its orientation with respect to the field and its frequency of the EM radiation.

The power absorbed by the sample is related to the specific absorption rate (SAR) where "specific" indicates that the parameter is normalised with respect to mass. The SAR (in W kg-1) is then the power absorbed per unit mass or

where s is the sample electrical conductivity (in S m-1), p is the sample density (kg m-3) and |E(r)|2 is the square of the magnitude of the electric field, E(r), at point r in the sample. The actual SAR delivered to a region of the body will depend heavily on the depth of the region below the skin, the electrical characteristics of the tissue between the skin and irradiated region and on the exact location and geometry of the RF source.

The US standard is that phones have a SAR level at or below 1.6 W kg-1 taken over a volume containing 1 g of tissue, whereas European standards require a SAR maximum of 2 W kg-1 averaged over 10 g of tissue. For tissue of density 103 kg m-3 and 1 Wm, a SAR of 10 W kg-1 corresponds to a field of 100 V m-1 and B field of 0.3 mT. However, the actual SAR absorbed by tissue depends on its depth below the surface, the electrical characteristics of the tissue between the source and the target and the presence of external factors which may influence the EMR delivered to the skin. For example, most men put a mobile phone in a front trouser pocket [6].A1W phone placed in the position of the front trouser pocket [7] generates SAR levels of 2 W kg-1 in the testes over the frequency range 0.9-4 GHz. This SAR rose to 4 W kg-1, if the effect of metal objects, such as keys, in the pocket was included. Given that GSM-850-900 handsets can have a peak power level of 2 W, then peak SARs in the testes could reach to more than 10 W kg-1 under worst case scenarios. However, at typical phone power levels of 0.5 W, SARs would be a more realistic 2 W kg-1.

EMR energy absorption increases the average energy level of random molecular excitation, resulting in a temperature rise. The power absorbed into a region of tissue will cause an initial increase in temperature DT in the time interval Dt given by the bioheat equation [8]

dt where T is the temperature above the mean arterial temperature, k is the thermal conductivity of the tissue (typically 0.5 W m-1 K-1), C is the heat capacity of the tissue (typically 3,700 J K" 1 kg-1), p is the density of tissue and blood (typically 1.06 kg m"3) and mb is the volumetric perfusion rate of blood (typically 0.510 x 10-6 m3kg-1 s-1). The first two terms in the above equation represent heat loss from the irradiated area by conduction through the tissue and by blood flow, respectively. The third term is the heat gain due to the irradiation and the fourth term is the temperature rise of the irradiated region with time. On commencement of an irradiation, there is minimal heat loss so the temperature increases linearly with time. For typical tissue, a SAR of 1 W kg-1 will cause a 1°C temperature increase per second. As the temperature of the irradiated region rises above the surroundings, heat energy will be transferred away from the irradiated region by thermal conduction through tissue and convective heat flow through the blood, reducing the rate of temperature increase. The sum effect of these channels for heat loss results in an effective thermal conductivity of approximately 10 W m-2 K-1. Finally, the system will come to thermal equilibrium with the EM energy delivered to the irradiated region balanced by the heat energy leaving the region in any time interval. An irradiated spherical region of tissue of mass 10 g would then typically show an equilibrium temperature rise of about 1°C at a SAR of 2 W kg-1. It is very well known that the heating of tissue will induce a stress response that is invariably damaging for the tissue involved. To study non-thermal effects of RF irradiation, the subject of this chapter, the equilibrium temperature increase should be kept below 0.1°C, requiring typical SARs of less than 0.2 W kg-1.

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