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ELECTROMAGNETIC RADIATION & DIGITAL COMMUNICATIONS

A Greek version of the reserch scientific activities of this topic can be found in odf.pdf  format.

 

State of the art

Humans are being constantly exposed to electromagnetic radiation (EMR), including sunlight, cosmic rays and terrestrial radiation. However, a substantial increase in exposure to non-ionizing radiation and especially to low frequency electromagnetic radiation (LF-EMR), started in the early 20th century with the generation of artificial electromagnetic fields and continued with the development of power stations, radios, radars, televisions, computers, mobile phones, microwave ovens and numerous devices used in medicine, industry and home. These technological advances have aroused concerns about the potential health risks associated with unprecedented levels of EMR exposure (Ahlbom et al 2008; HPA 2004a, 2004b; NRPB 2003; SCENIHR 2007, 2009; Valberg et al 2007).

 

The amount of energy deposited by EMR and the nature of its absorption are determined by the frequency and type of incident radiation and by the type of tissue that absorbs it. Exposure to multiple sources of non-ionizing radiation (Table 1), including residential exposure to high-voltage power lines, transformers, and domestic electrical installations, varies in duration and depends on the distance from the source. Exposure is usually due to low-frequency (LF) or extremely low- frequency (ELF) EMR radiation, it is continuous and rises among populations of the industrialized world. Exposures to ELF electric and magnetic fields emanating from generation, transmission and uses of electricity constitute a ubiquitous part of modern life (CENELEC 2008; EU 1999). Besides LF-EMR and ELF-EMR radiation, individuals are increasingly exposed to radio frequencies (RF) from television (TV) towers, radio stations, mobile phone/wi-fi systems and personal computers. In contrast to ionizing radiation, where natural sources contribute the largest proportion to population exposure, man-made non-ionizing sources tend to dominate the human exposure to electromagnetic fields. In all cases of EMR, exposure depends not only on the strength of the field but also on the distance from the source and, in the case of directional antennas, on the proximity to the main beam. The field strength often decreases rapidly with distance (IEC 2005; IEEE 2004, 2005a, 2005b; WHO 2002, 2006, 2010, 2011). There exist several possible sources of RF fields to which people may be exposed. Within the frequency band from 3 kHz to 300 GHz the sources include those used for telecommunications or security. Communications equipment cover most of the frequency range with TV and radio transmissions frequencies from about 200 kHz to 900 MHz. Personal telecommunication devices operate over the range of frequencies from 100 MHz to 3 GHz. Table 1 summarises the different types, frequency ranges and sources of non-ionizing radiation, the energy of which, even at 300 GHz, is still around three orders of magnitude smaller than the ionization threshold in matter (EPA 2013). Application and rapid development of technologies using radiofrequencies (RFs) induced a substantial increase in exposure among the general population, especially over the last 20 years. RFs are emitted by numerous sources operating in different frequency bands (Table 2). These sources can be subdivided in two broad categories: (a) ambient sources, such as broadcast transmitters (radio, TV), or mobile phone base stations and (b) personal sources, such as mobile phones, in-house bases for cordless phones (DECT – Digital enhanced cordless telephony), microwave ovens, wireless networks. Consequently, exposure to RF varies considerably across persons, space and time (Frei et al 2009a, 2009b; Viel et al 2009a). There are, therefore, significant challenges in assessing the sources of variation and related uncertainty, but also in identifying exposure relevant factors (Ahlbom et al 2004; Joseph et al 2009, 2010b, 2012; Joseph and Verloock 2010a, Mann et al 2005; Röösli et al 2008, 2010; Viel et al 2009a, 2009b; Vrijheid et al 2008).

 

The signals generated by various sources may be different in type. The underlying waveform from a source is usually sinusoidal, the signal however may then be amplitude modulated (AM), frequency modulated (FM), pulse modulated (e.g. radar) or modulated in a more complex way (e.g. digital radio) (CENELEC 2008; ECC 2006; NRPB 2003). Exposure to EMR sources is commonly described by electric and magnetic field strength, which is however measured around the subject. Any biological effects would be the result of the exposure within the body and this is difficult to be measured directly. As it is already mentioned, the nature of the field and the characteristics of the source differ considerably from each other (Frei et al 2009a, 2009b; HPA 2004a,2004b, 2004c, 2012). At frequencies below 100 kHz, the physical quantity associated with most biological effects is the electric field strength in tissue (ICNIRP 1998, 2009). More appropriate quantity at higher frequencies is the specific absorption rate, SAR, which is related to the second power of the electric field strength in tissue (IEC 2005;NRPB 2003; SAR Database 2012). At frequencies above about 1 MHz, the orientation of the body with respect to the incident field becomes increasingly important, because the body behaves as an antenna (Fig.1), absorbing energy in a resonant manner (for standing adults the maximum absorption occurs when frequency varies between 70-80 MHz, a value that depends on the isolation status relative to the ground). As frequency increases above the resonance region, energy absorption becomes confined to the surface layers of the body, limited to the skin when frequency reaches a few tens of GHz (Ahlbom et al 2004, 2008; HPA 2012; ICNIRP 2009; NRPB 2003; SCENIHR 2007, 2009).

 

Electric field can be measured using suitable sensors such as small dipoles. Studies to evaluate internal exposure are carried out either by using computational methods or by conducting measurements in phantoms. Computational methods rely on the detailed anatomical information, in addition to information on the electrical properties of the different tissues for each frequency regime. The electric field at various points inside simple phantoms is usually measured via a robotically positioned probe, small enough to minimise the changes in the fields produced by its presence. In simple cases, estimation of the internal exposure can rely on measurement of the field outside the body accompanied by reasonable approximations (HPA 2012; NRPB 2003; WHO 2002, 2006, 2010, 2011). The strength of the electric or magnetic field can be indicated by its peak value, although it is often denoted by the rms value. For a sinusoidally varying field, the rms value equals to the peak value divided by 1.4. The power density represents the intensity of the electromagnetic field and is determined by the amount of electromagnetic energy passing through a point per unit area perpendicular to the direction of propagation (NRPB 2003). The power density of an electromagnetic wave is equal to the product of the electric and magnetic fields, although this is not true in near-field regions, i.e. when the distance from the source is comparable to the wavelength. In the near-field region the electric and magnetic fields are neither perpendicular to each other nor in phase. In general, the fields can be divided into two components: radiative and reactive (NRPB 2003). The radiative component is that part of the field which propagates energy away from the source, while the reactive component can be thought of as relating to energy stored in the region around the source. The reactive component dominates close to the source and the stored energy can be absorbed by people standing in the near-field region. However, any measurement in the near-field region is particularly difficult since, even the introduction of a small probe, can substantially alter the field. Magnetic field is measured with small loop sensors. The boundary radius depends on wavelength. Distances of about one-sixth of a wavelength from the source, define approximately the near-field boundary. The frequency range of 3 kHz to 300 GHz corresponds to the wavelength range of 100 km to 1 mm (HPA 2012; ICNIRP 2009; Lauer et al 2013; NRPB 2003;Valberg et al 2007).

 

Antennas generate electromagnetic fields across the spectrum. At very low frequencies the structures are massive with support towers 200-250 m high and the fields may be extensive over the site area. Electric field strengths of several hundred Vm-1 and magnetic field strengths in the range 2-15 Am-1 (52 Am-1 close to low frequency towers) may be encountered. The currents induced in body (Fig. 1) flow to ground through the feet and can reach a theoretical maximum of 10-12 mA per Vm-1 at a resonance frequency for an electrically grounded adult (the current is reduced to half of these values when the adult is wearing shoes) (IEEE 2005b; National Academy of Sciences 2006; Neubauer et al 2007; SCENIHR 2007, 2009). Nevertheless, the average magnetic flux density (in µT) is, generally, considered to be below maximum exposure limits established by different organizations, such as the International Council of Non-Ionizing Radiation Protection (ICNIRP, 1998) or the National Radiological Protection Board (NRPB 2003). The International Commission on Non-Ionizing Radiation Protection and the UK's National Radiological Protection Board, together with the Health Protection Agency (HPA), the Institute of Electrical and Electronics Engineers (IEEE), the International Telecommunication Union Recommendation (ITU-R 2005) and European Union committees, reviewed many relevant studies and recommended guidelines on restrictions for exposure to electromagnetic fields.

 

Recommended restrictions are based on biological data relating to thresholds for adverse direct and indirect effects of acute exposure. Direct effects are those resulting from the interactions of electromagnetic fields with the human body (basic restrictions). Indirect effects are those resulting from an interaction between electromagnetic fields, an external object and the human body (e.g. to avoid burns). As compliance with the basic restrictions cannot be easily determined, ICNIRP recommends reference levels as values of measurable field quantities for assessing whether compliance with the basic restrictions is achieved (ICNIRP 1998; NRPB 2003). Table 3 summarises the reference levels for electric field intensity (in V/m), magnetic flux density (in µT) and power density (in W/m2). Corresponding values for occupational exposure are about five times higher (HPA 2012; ICNIRP 1998; NRPB 2003). Radiocommunications Agency (now Ofcom: http://www.ofcom.org.uk/) supported in 2003 measurements in the UK that gave range and geometric mean (in parenthesis) of power density values in μW m-2 from all signals: (a) indoor 2-1000 (75), (b) outdoor 50-1700 (240) and (c) all locations 3.5-1100 (110) (HPA 2004c; NRPB 2003).

 

Since the introduction of mobile phones in the early 90s, there has been a constant and rapid increase in the number of base stations. Joseph et al (2010b) compared the total radio frequency electromagnetic field (RF-EMF) exposure in five European countries and found that in outdoor urban environments mobile phone base stations are a major, if not the largest, source of environmental RF-EMF. There has been concern about potential health effects of the electromagnetic waves emitted by these base stations (Neubauer et al 2007; Valberg et al 2007), which have led to studies assessing the relationship between RF-EMF and the health impact on the general population. To date, no consistent health effect has been found (HPA 2012; NRPB 2003; Röösli et al 2010). However, if there are health effects, they are likely to be small and subtle and, as such, large population samples and a reliable exposure assessment are needed to confirm or reject the hypothesis of a certain health effect, minimizing statistical uncertainties (Briggs et al 2012; NRPB 2003; SCENIHR 2007, 2009). In general, in the last few years, several countries have performed measurement studies using exposimeters and some of the results have already been published (Bolte et al 2008; EU 1999; Frei et al 2009a; Joseph et al 2009, 2010b, 2012; Joseph and Verloock 2010a, Röösli et al 2008; Thomas et al 2008a, 2008b; Thuróczy et al 2008; Trcek et al 2007; Viel et al 2009a, 2009b). In some of these studies, measurements were performed in different microenvironments such as offices or outdoor urban areas, to characterize typical exposure levels in these places (microenvironmental studies). Other studies, were population surveys where the personal exposure distribution in the population of interest was determined. The strategies for the recruitment of the study participants as well as the data analysis methods differed between these studies and therefore, a direct comparison of their results is difficult.

 

In Table 4, Reference Levels for exposure to Electric Field, Magnetic Field and Wave Power Density are shown for mobile phones, as well as Wi-Fi frequencies for general population and workers (in parenthesis), according to ICNIRP and NRPB guidance. The Greek Atomic Energy Agency, according to EU recommendations, made a series of electromagnetic field measurements in selected Greek regions. Table 5 gives average and maximum values of Electric Field, Magnetic Field and Wave Power Density measured, together with the Reference Levels estimated for Greek environment, for mobile phones frequencies. Depending on the particular environmental situation, two groups of Reference Levels are established in Greece: (a) 70 per cent of the proposed values for general purpose and (b) 60 per cent of the proposed values for regions with more sensitive population (GAEA 2010). In Table 5 the 70 per cent Reference Levels are given.

 

Regarding non-ionizing electromagnetic radiation, the energy carried (and potentially transferred) is measured in electrovolts (eV). Biological tissues exposed to radiofrequencies absorb energy and develop an induced current density from the external field. Specific absorption rate (SAR) is the quantity showing the rate at which energy is absorbed by a particular mass of tissue and depends on the density and the electrical conductivity of the tissue, as well as on the electric field strength (second power) (IEC 2005; NRPB 2003; SAR Database 2012). SAR is measured in watts per kilogram. As SAR varies from point to point, may be ascertained by averaging over a small mass or over the whole body mass. The most commonly used methods for experimental measurement of SAR involve measurement of the internal electric field strength or the rate of temperature rise, both methods however, being very difficult in practice (NRPB 2003).Thermal effects from RF electric fields occur because most biological tissues are electrically conducting. Electric fields inside human tissue generate currents and their dissipation leads to energy absorption and, hence, to increase in temperature. The latter is considered as a cause leading to biological effects. Except thermal effects, non-thermal effects are associated with changes in protein conformation (different dipole moment and energy, transitions that would result in changes in protein folding), conformational changes in the ATPases associated with cell membrane ion channels (ion pumping across membranes produced by RF fields), heat shock proteins (an increase in unfolded protein produces an increase in aggregation), changes in binding ability of Ca ions to cell receptor proteins. In general, the interaction of RF magnetic fields with tissue would be expected to be much weaker than that of RF electric fields. Possible exceptions might be expected to include interaction with tissues like human brain, containing particles of magnetite. RF magnetic fields could interact either by ferromagnetic resonance or by mechanical activation of cellular ion channels. Positive findings are not yet confirmed. The literature on non-thermal effects is inconsistent (Ahlbom et al 2004, 2008; HPA 2004a, 2004b, 2004c, 2012; ICNIRP 2009; NRPB 2003; WHO 2002, 2006, 2011). With regard to the effects of RF radiation on the nervous system, IEGMP (Independent Expert Group on Mobile Phones) concluded that changes in neuronal excitability will occur when exposure induces significant heating by about 1 grad or more (NRPB 2003; SCENIHR 2007, 2009).

 

Despite the rapid growth of new technologies using RFs, information on the exposure of individual persons for these and older RF sources is scarce and even less is known about the relative importance of different sources. Existing RF sources are operated in different frequency bands and can be subdivided in two broad categories: (a) external sources, such as broadcast transmitters (radio,TV) or mobile phone base stations, and (b) internal sources, such as mobile phones, in-house bases for cordless phones (DECT), or microwave ovens. The relative contribution of these sources to exposure depends on individual home and workplace circumstances. For a given source, the actual exposure to RF depends on a number of factors. Regarding mobile phones, the characteristics of a certain phone (particularly type and location of the antenna), the way the phone is handled, the distance from the base station, the frequency of handovers and RF traffic conditions are of prime importance (Ahlbom et al 2004, 2008; Briggs et al 2012; Inyang et al 2008). Similarly, RF fields from mobile phone base stations also exhibit a complex pattern, influenced by numerous factors, such as, the output power of the antenna, the direction of transmission, the attenuation due to obstacles or walls, and any existing scattering from buildings and trees (Joseph et al 2009, 2010b, 2012; Joseph and Verloock 2010a, Mann et al 2005; Neubauer et al 2007). There are, therefore, significant challenges in assessing the exposure of individuals in the general population to RF signals, including the number and range of sources involved and the effect of the environment on signal's strength, as people move around. In principle, two different types of RF-EMF exposure sources can be distinguished: (a) sources which are applied close to the human body usually causing high and periodic short-term exposure mainly to the head (e.g. mobile phones) and (b) environmental sources which, in general, cause lower but relatively continuous whole-body exposure (e.g. mobile phone base stations). While exposure from mobile phones can be assessed using self-reported mobile phone use or operator data (Vrijheid et al 2008), valid assessment of exposure to environmental fields is more challenging. Frei et al (2009a) studied temporal and spatial variabilities of personal exposure to radio frequency electromagnetic fields. They concluded that exposure to RF-EMF varied considerably between persons and locations but was fairly consistent within persons. Mobile phone handsets, mobile phone base stations and cordless phones were important sources of exposure in urban Switzerland. Their results revealed mean weekly exposure values to all RF-EMF sources equal to 0.13 mW/m2 (0.22 V/m) with the range of individual means between 0.014–0.881 mW/m2). Exposure was mainly due to mobile phone base stations (32.0%), mobile phone handsets (29.1%) and digital enhanced cordless telecommunications (DECT) phones (22.7%). Persons owning DECT phones (total mean 0.15 mW/m2) or mobile phones (0.14 mW/m2) were exposed more than those not owning DECT or mobile phones (0.10 mW/m2). Mean values were highest in trains (1.16 mW/m2), airports (0.74 mW/m2) and tramways or buses (0.36 mW/m2) and higher during daytime (0.16 mW/m2) than night-time (0.08 mW/m2). However Frei et al (2010) claim that “exposure to RF-EMF in everyday life is highly temporally and spatially variable due to various emitting sources like broadcast transmitters or wireless local area networks (W-LAN). The use of personal exposure meters (exposimeters) has been recommended in order to characterize personal exposure to RF-EMFs (Neubauer et al 2007). Several exposure assessment studies have been conducted so far using exposimeters (Joseph et al 2008; Kühnlein et al 2009; Thomas et al 2008a; Thuróczy et al 2008; Viel et al 2009b), which allow capture of exposure from all relevant RF-EMF sources in the different environments where a study participant spends time (Neubauer et al 2007; Radon et al 2006). They are suitable for measuring RF-EMF from environmental far-field sources like mobile phone base stations, but are less able to accurately measure exposure to personal mobile or cordless phones (Inyang et al 2008) because measurements during personal phone calls are dependent on the distance between the emitting device and the exposimeter”.Joseph et al reported their research (2012) about in situ electromagnetic radio frequency exposure to existing and emerging wireless technologies by using spectrum analyzer measurements at 311 locations (68 indoor, 243 outdoor), subdivided into six different categories (rural, residential, urban, suburban, office and industrial), geographically spread across Belgium, The Netherlands and Sweden. The maximal total value was measured in a residential environment and found to be equal to 3.9 Vm-1, mainly due to the GSM900 signal (11 times below the ICNIRP reference levels). Exposure ratios for maximal electric field values ranged from 0.5% (WiMAX – Worldwide Interoperability for Microwave Access) to 9.3% (GSM900) for the 311 measurement locations. The exposure ratios for total exposures varied from 3.1% for the rural environment to 9.4% for the residential environment. Exposures were log-normally distributed and were in general the lowest in rural environments and the highest in urban environments. The dominating outdoor source was GSM900 (95th percentile of 1.9 Vm-1) while indoor DECT dominated (95th percentile 1.5 V m-1 ) if present. The average contribution to the total electric field was more than 60% for GSM. Except for the rural environment, average contributions of UMTS-HSPA (High Speed Packet Access) were more than 3%. The contributions of LTE (Long Term Evolution) and WiMAX were on average less than 1%.

As far as exposure to non-ionizing radiation is concerned, absorbed energy from human body is very low, but becomes significant if it is continuous for very long periods of time accounting the fact that the related effects are not yet well known.

 

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Table1

Frequencies and sources of non-ionizing radiation

Frequency

Type of radiation

Sources

0 Hz–300 kHz

Low frequency to extremely low frequency (LF–ELF) electromagnetic radiation

Electrical fields of devices, conventional electrical network, video monitors, sections of AM radio

3 kHz–300 MHz

Radio frequencies (RF)

Sections of AM radio, FM radio, medical short-wave, nuclear magnetic resonance (NMR)

300 MHz–300 GHz

Microwave (MW)

Domestic microwave devices, mobile telephones, microwave for medical physical therapy, radar and other microwave communications

3 *1011 – 3*1014 Hz

Infrared (IR)

Solar light, heat and laser therapy devices

1014 –1015 Hz

Visible light

Solar light, phototherapy, laser

1015 –1017 Hz

Ultraviolet (UV)

Solar light, fluorescent tubes, food/air sterilization, radiotherapy, etc.

High frequency ultraviolet (UV) is considered as ionizing radiation

Table 2

Personal exposure meter frequency bands (EME SPY 120, Satimo, France)

Band name

Active sources

Range (MHz)

FM

VHF broadcast radio

88–108

TV 3

Digital audio broadcasting

174–223

Tetrapol

Terrestrial trunked radio

380–400

TV 4&5

UHF broadcast television

470–830

GSMa Txb

GSM mobile phones (900 MHz)

880–915

GSM Rxc

GSM base stations (900 MHz)

925–960

DCSd Tx

DCS mobile phones (1800 MHz)

1710–1785

DCS Rx

DCS base stations (1800 MHz)

1805–1880

DECTe

Digital enhanced cordless telephony

1880–1900

UMTSf Tx

3 G mobile phones

1920–1980

UMTS Rx

3 G base stations

2110–2170

WiFi

Wireless networks and microwave ovens

2400–2500

a Global System for Mobile Communications

b Transmitted radio signal from the point of view of a mobile phone

c Received radio signal from the point of view of a mobile phone

d Digital Communication System

e Digital Enhanced Cordless Telephone

f Universal Mobile Telecommunication System

Table 3

ICNIRP reference levels for general public exposure to time-varying electric and magnetic fields (rms values)

Frequency range

E-field intensity (V/m)

B-field intensity

(µT)

Wave Power Density (W/m2)

0–1 Hz

4×104

1–8 Hz

10,000

4 × 104 / f2

8–25 Hz

10,000

5000 / f

0.025–0.8 kHz

250 / f

5/ f

0.8–3 kHz

250 / f

6.25

3–150 kHz

87

6.25

0.15–1 MHz

87

0.92 / f

1–10 MHz

87 / f1/2

0.92 / f

10–400 MHz

28

0.09

2

400–2000 MHz

1.375 × f1/2

0.0046 × f1/2

f / 200

2–300 GHz

61

0.2

10

f: frequencies as indicated in the column of frequency range

Table 4

Reference Levels for exposure to Electric Field, Magnetic Field and Wave Power Density for mobile phones, as well as Wi-Fi frequencies for general population and workers (in parenthesis), according to ICNIRP and NRPB guidance.

 

ΜΗz

Electric

Field

(V/m)

Magnetic

Field

(A/m)

Wave Power

Density

(W/m²)

900

(GSM)

41.25 (90)

0.11 (0.24)

4.5 (22.5)

1800

(DCS)

58.34 (127.3)

0.16 (0.34)

9 (45)

2100

(UMTS)

63.01 (137.5)

0.17 (0.37)

10.5 (52.5)

2400

(Wi-Fi)

67.36 (147)

0.18 (0.39)

12 (60)

Table 5

Average and maximum values of Electric Field, Magnetic Field and Wave Power Density measured, together with the Reference Levels estimated for Greek environment, for mobile phones frequencies, according to the Greek Atomic Energy Agency.

 

Average values

(all related frequencies)

Maximum

values

Ref LevelsGSM 900

Ref LevelsDCS 1800

Ref LevelsUMTS 2100

Electric

Field (V/m)

0.25 – 5.0

20

34.5

48.8

51

Magnetic

Field

(A/m)

0.005 - 0.01

0.05

0.093

0.131

0.134

Wave Power

Density

(W/m²)

0.0001 - 0.05

1

3.1

6.3

7

 

tb

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