The effect of increase in dielectric values on specific absorption rate (SAR) in eye and head tissues following 900, 1800 and 2450 MHz radio frequency (RF) exposure.
ABSTRACT Numerous studies have attempted to address the question of the RF energy absorption difference between children and adults using computational methods. They have assumed the same dielectric parameters for child and adult head models in SAR calculations. This has been criticized by many researchers who have stated that child organs are not fully developed, their anatomy is different and also their tissue composition is slightly different with higher water content. Higher water content would affect dielectric values, which in turn would have an effect on RF energy absorption. The objective of this study was to investigate possible variation in specific absorption rate (SAR) in the head region of children and adults by applying the finite-difference time-domain (FDTD) method and using anatomically correct child and adult head models. In the calculations, the conductivity and permittivity of all tissues were increased from 5 to 20% but using otherwise the same exposure conditions. A half-wave dipole antenna was used as an exposure source to minimize the uncertainties of the positioning of a real mobile device and making the simulations easily replicable. Common mobile telephony frequencies of 900, 1800 and 2450 MHz were used in this study. The exposures of ear and eye regions were investigated. The SARs of models with increased dielectric values were compared to the SARs of the models where dielectric values were unchanged. The analyses suggest that increasing the value of dielectric parameters does not necessarily mean that volume-averaged SAR would increase. Under many exposure conditions, specifically at higher frequencies in eye exposure, volume-averaged SAR decreases. An increase of up to 20% in dielectric conductivity or both conductivity and permittivity always caused a SAR variation of less than 20%, usually about 5%, when it was averaged over 1, 5 or 10 g of cubic mass for all models. The thickness and composition of different tissue layers in the exposed regions within the human head play a more significant role in SAR variation compared to the variations (5-20%) of the tissue dielectric parameters.
- [Show abstract] [Hide abstract]
ABSTRACT: This paper evaluates the variability of specific absorption rate (SAR) in the human eye. This variability results from changes in ocular axial length (OAL), which is common in many ophthalmologic and vision abnormalities, including myopia. A generic eye model was reconstructed according to published data. The feasibility of using the generic model in numerical research of electromagnetic fields (EMF) was demonstrated by means of comparative simulations with eye models reconstructed from magnetic resonance (MR) scans. Free-form deformation (FFD) was used to deform the OAL of the generic eye model. Thus, 64 deformed eyes were created and were categorized according to the OAL increase. The finite-difference time-domain (FDTD) method was applied in the simulations. The results revealed that changing the OAL does not increase EMF absorption in the eyes or the eye tissues. No additional induced temperature rise was produced by the changes of OAL. The results also indicated that the non-pathological increment of the OAL, which is inevitable during the childhood, does not increase the SAR in the eyes. Bioelectromagnetics. © 2014 Wiley Periodicals, Inc.Bioelectromagnetics 02/2014; · 1.86 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: This paper deals with the numerical dosimetry for adult and children models exposed to CW signals of several wireless communication systems (UMTS, WiMax, and Bluetooth) within a partly shielded environment represented by a realistic car model. More than 20 mono- and multi-source exposure scenarios are considered. Computational results demonstrate that, for all considered exposure scenarios, the specific absorption rate (SAR) is at least 40 times (whole-body average) and 10 times (local SAR) lower than the exposure limits fixed by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The whole-body average SAR values for children are found to be typically 1.1–1.3 times higher than those of adults. Under several exposure scenarios, the local SAR in the limbs of children models is 2–3 times higher than corresponding values in adult models. The power density distributions within the car have been also analyzed for one, two, and three simultaneously emitting devices. The results show that the homogeneity of the power density distribution increases with increasing number of simultaneously operating transmitters. These data suggest that the use of several wireless communication devices within a car leads to exposure levels that are several orders of magnitude below international exposure limits, even for the multi-exposure scenarios for both adult and children models.International Journal of Microwave and Wireless Technologies 12/2011; 3(06). · 0.57 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Myelin provides the electrical insulation for the central and peripheral nervous system and develops rapidly in the first years of life, but continues into mid-life or later. Myelin integrity is vital to healthy nervous system development and functioning. This review outlines the development of myelin through life, and then considers the evidence for an association between myelin integrity and exposure to low-intensity radiofrequency electromagnetic fields (RF-EMFs) typical in the modern world. In RF-EMF peer-reviewed literature examining relevant impacts such as myelin sheath, multiple sclerosis, and other myelin-related diseases, cellular examination was included. There are surprisingly little data available in each area, but considered together a picture begins to emerge in RF-EMF-exposed cases: (1) significant morphological lesions in the myelin sheath of rats; (2) a greater risk of multiple sclerosis in a study subgroup; (3) effects in proteins related to myelin production; and (4) physical symptoms in individuals with functional impairment electrohypersensitivity, many of which are the same as if myelin were affected by RF-EMF exposure, giving rise to symptoms of demyelination. In the latter, there are exceptions; headache is common only in electrohypersensitivity, while ataxia is typical of demyelination but infrequently found in the former group. Overall, evidence from in vivo and in vitro and epidemiological studies suggests an association between RF-EMF exposure and either myelin deterioration or a direct impact on neuronal conduction, which may account for many electrohypersensitivity symptoms. The most vulnerable are likely to be those in utero through to at least mid-teen years, as well as ill and elderly individuals.Journal of Toxicology and Environmental Health Part B 09/2014; 17(5):247-58. · 5.15 Impact Factor
The effect of increase in dielectric values on specific absorption rate (SAR) in eye and head
tissues following 900, 1800 and 2450 MHz radio frequency (RF) exposure
This content has been downloaded from IOPscience. Please scroll down to see the full text.
IP Address: 126.96.36.199
This content was downloaded on 02/10/2013 at 14:14
Please note that terms and conditions apply.
2006 Phys. Med. Biol. 51 1463
View the table of contents for this issue, or go to the journal homepage for more
Home Search CollectionsJournals About Contact usMy IOPscience
INSTITUTE OF PHYSICS PUBLISHING
PHYSICS IN MEDICINE AND BIOLOGY
Phys. Med. Biol. 51 (2006) 1463–1477
The effect of increase in dielectric values on specific
absorption rate (SAR) in eye and head tissues
following 900, 1800 and 2450 MHz radio frequency
Jafar Keshvari1, Rahim Keshvari2and Sakari Lang1
1Technology Platforms, Nokia Corporation, PO Box 301, FIN-00045 Nokia Group,
Linnoitustie 6, 02600 Espoo, Finland
2Department of Ophthalmology, Savonlinna Central Hospital, Savonlinna, Finland
E-mail: Jafar.firstname.lastname@example.org and email@example.com
Received 16 August 2005, in final form 16 January 2006
Published 1 March 2006
Online at stacks.iop.org/PMB/51/1463
Numerous studies have attempted to address the question of the RF energy
absorption difference between children and adults using computational
who have stated that child organs are not fully developed, their anatomy is
different and also their tissue composition is slightly different with higher
water content. Higher water content would affect dielectric values, which in
turn would have an effect on RF energy absorption. The objective of this study
was to investigate possible variation in specific absorption rate (SAR) in the
the calculations, the conductivity and permittivity of all tissues were increased
from 5 to 20% but using otherwise the same exposure conditions. A half-wave
dipole antenna was used as an exposure source to minimize the uncertainties
of the positioning of a real mobile device and making the simulations
easily replicable. Common mobile telephony frequencies of 900, 1800 and
2450 MHz were used in this study. The exposures of ear and eye regions
were investigated. The SARs of models with increased dielectric values were
compared to the SARs of the models where dielectric values were unchanged.
The analyses suggest that increasing the value of dielectric parameters does
not necessarily mean that volume-averaged SAR would increase. Under many
averaged SAR decreases. An increase of up to 20% in dielectric conductivity
or both conductivity and permittivity always caused a SAR variation of less
than 20%, usually about 5%, when it was averaged over 1, 5 or 10 g of cubic
mass for all models. The thickness and composition of different tissue layers
0031-9155/06/061463+15$30.00© 2006 IOP Publishing LtdPrinted in the UK1463
1464 J Keshvari et al
in the exposed regions within the human head play a more significant role
in SAR variation compared to the variations (5–20%) of the tissue dielectric
The increasing use of wireless communication devices has led to concerns that exposure to
electromagnetic (EM)waves emittedbythesedevices maycauseadversehealtheffects. There
have also been some discussions that children’s heads would absorb more RF energy under
the same exposure conditions compared to adults’ heads. However, this is not necessarily the
case and not supported by many studies (Kuster and Balzano 1992, Schoenborn et al 1998,
Keshvari and Lang 2005, Christ and Kuster 2005).
The basic safety limits for RF exposure are defined in terms of absorbed power per unit
mass, which is expressed by SAR in W kg–1. Depending on the exposure condition, SAR is
expressed either as a localized SAR value or averaged over the whole body. It describes the
amount of energy W absorbed in a dielectric material in time (dt) and mass unit (dm).
It can be related to the electric field or temperature rise (dT) at a point by
= (σ + ωε0ε??)|E|2
where E is the root-mean-square (rms) electric field; σeffis the effective conductivity (S m–1);
ρ is the mass density (kg m–3); ω = 2πf, with f the frequency; ε0is the permittivity of free
space; ε??is the loss factor and c is the specific heat capacity (J kg–1 ◦C–1) of the material.
Relation (2) to temperature is limited to ‘ideal’ non-thermodynamic circumstances with
no heat loss by thermal diffusion, heat radiation or thermoregulation. Equation (2) shows that
the SAR varies directly with both σ and ε??, or in other words, σ and ε??indicate how much
energy will be absorbed by tissues for a given electric field E. Theoretically the greater the σ
or ε??, the greater the loss in material, and more power is absorbed for the same electric field
E. Generally, tissues with larger water content such as muscle absorb more energy than tissues
with lower water content, such as bone and fat. Both conductivity and permittivity of tissues
are frequency dependent.
There are a number of studies (Gandhi et al 1996, Gandhi and Gang 2002, Burkhardt
and Kuster 1999, Dimbylow 1993, 1994, Keshvari and Lang 2005, Martinez-Burdalo et al
2004, Schoenborn et al 1998, Martens et al 1995) which have investigated the SAR difference
between children and adults. Most of them have used computational techniques. In previous
numerical SAR studies, the researchers have assumed the same dielectric parameters for both
child and adult head models.
Peyman et al (2001) reported measured changes in the dielectric properties of various
tissues in the rat. This triggered the discussion about the validity of the studies, which have
used the same dielectric parameters both in child and adult head models. They reported that
conductivities of both cerebral tissue and the cranium decrease in rats from birth up to an age
of 70 days (figure 1(a)). This raised questions that how much would the SAR increase if larger
dielectric values were applied for child models.
Dielectric variation effect on SAR1465
Figure 1. (a) Left: conductivity of rat tissues in the head at 900 MHz, adapted from Peyman et al
(2001). Right: the variation in total body water (TBW) content of rats with age expressed in ml of
water per kg of body weight, adapted from Altman (1974). (b) The variation in total body water
(TBW) content of humans with age expressed as ml of water per kg of body weight, adapted from
the published data compiled by Altman (1974). The horizontal error bars indicate the age range
for the data collected at each point and the vertical error bars indicate the range of TBW data for
each data point.
The main objective of this study was to respond to this question by investigating the effect
of increased dielectric values on mass-averaged SAR. The distributions in human ear and eye
regions were studied using anatomically correct MRI-based head models as well as a layered
flat phantom model. The ear and eye regions were exposed to a half-wave dipole antenna at
900, 1800 and 2450 MHz frequencies.
2. Dielectric characteristics of biological tissues and their age dependence
Information about the dielectric properties of biological tissues is essential to both RF and
thermal dosimetry. Parameters such as the conductivity, σ, and the dielectric permittivity,
1466 J Keshvari et al
ε, of human tissues are fundamental parameters in RF dosimetry because of their role in the
field distribution inside the body. In general, the parameters of interest are the real part of the
dielectric constant and the conductivity.
The dielectric properties of tissues above 100 MHz are determined by the intra-cellular
electrolytes, principally water (0.9% saline solution). These properties are consistent with
the suspensions of low conductivity and low permittivity particles such as cells in an aqueous
electrolyte. At the microwave frequencies, tissue properties can be attributed to their ‘free’
water content and the dispersion of normal bulk water.
Brain growth is the result of an increase in both the number of brain cells and their weight
during the first year of life (Fein 1978). After the first year, only the weight of the cells
increases. Myelination of the brain mainly occurs in the first 2 years of life (Holland et al
1986, van der Knaap and Valk 1990). Unlike intercellular fluid, myelin does not contain free
ions. Thismeansthatiftheamountofmyelininthebrainincreases, itwillresultinareduction
of the ion concentration and consequently to a reduction of the overall electrical conductivity
of brain tissue. This occurs during early development, but there is very little change thereafter.
It is known that total body water content (TBW) between different individuals may differ
and tissues with larger TBW are more conductive. Newborn infants have larger water content
than older infants and children. This is partially because the newborn has less fat and a
greater proportion of body mass composed of visceral organs. The extra cellular fluid (ECF)
content of extra cellular sodium and chloride. Most of this neo-natal ‘excess’ ECF is lost
during the first 10 days of life through insensible perspiration, which can amount from 5% to
10% of the infant’s birth weight. Until about 2 years of age, the infant maintains a larger ECF
than the adult in terms of overall percentage of body water.
There is a very good correlation between the age changes in TBW and the conductivity
in physiological organs. Like tissue conductivity, the TBW is highest at birth and decreases
steadily to about the age of 20–25 days, and then flattens off (figure 1(a)). Thus, it appears
reasonable to make the general assumption that at the age at which the TBW of an animal
on animal data. By using human TBW data, tissue conductivity changes can be calculated
for children as well. In figure 1(b), a compilation of published TBW data for children of
varying ages is shown (Altman 1974). The data show an initial decline after birth and then a
relatively constant TBW after the age of 1 year. Consequently, it may be concluded that tissue
conductivity in children also ceases to decline after infancy. Hence, using increased dielectric
values in the 3 year old and 7 year old child head models does not reflect the realistic exposure
conditions but they are rather used in this study to investigate SAR variations by changing the
One would always anticipate higher RF energy absorption in the tissues with larger TBW
such as infants and very young children compared to adults; however, the results of this study
will show that it is not always the case.
3. Materials and methods
Material and methods used in this study are explained in detail in our previous paper (Keshvari
and Lang 2005). Only a short description of the head models, exposure conditions and
computational models is given here.
Finite-difference time domain was applied as a numerical method to carry out the SAR
calculations. The calculations were carried out using a commercially available EM solver,
SEMCAD 1.8.9b. The grading mesh technique was used to reduce the number of voxels