Ae-Kyoung Lee

Comparisons of computed mobile phone induced SAR in the SAM phantom to that in anatomically correct models of the human head

The specific absorption rates (SAR) determined computationally in the specific anthropomorphic mannequin (SAM) and anatomically correct models of the human head when exposed to a mobile phone model are compared as part of a study organized by IEEE Standards Coordinating Committee 34, Sub-Committee 2, and Working Group 2, and carried out by an international task force comprising 14 government, academic, and industrial research institutions. The detailed study protocol defined the computational head and mobile phone models. The participants used different finite-difference time-domain software and independently positioned the mobile phone and head models in accordance with the protocol. The results show that when the pinna SAR is calculated separately from the head SAR, SAM produced a higher SAR in the head than the anatomically correct head models. Also the larger (adult) head produced a statistically significant higher peak SAR for both the 1- and 10-g averages than did the smaller (child) head for all conditions of frequency and position.

The MOBI-Kids study protocol: challenges in assessing childhood and adolescent exposure to electromagnetic fields from wireless telecommunication technologies and possible association with brain tumor risk

The rapid increase in mobile phone use in young people has generated concern about possible health effects of exposure to radiofrequency (RF) and extremely low frequency (ELF) electromagnetic fields (EMF). MOBI-Kids, a multinational case–control study, investigates the potential effects of childhood and adolescent exposure to EMF from mobile communications technologies on brain tumor risk in 14 countries. The study, which aims to include approximately 1,000 brain tumor cases aged 10–24 years and two individually matched controls for each case, follows a common protocol and builds upon the methodological experience of the INTERPHONE study. The design and conduct of a study on EMF exposure and brain tumor risk in young people in a large number of countries is complex and poses methodological challenges. This manuscript discusses the design of MOBI-Kids and describes the challenges and approaches chosen to address them, including: (1) the choice of controls operated for suspected appendicitis, to reduce potential selection bias related to low response rates among population controls; (2) investigating a young study population spanning a relatively wide age range; (3) conducting a large, multinational epidemiological study, while adhering to increasingly stricter ethics requirements; (4) investigating a rare and potentially fatal disease; and (5) assessing exposure to EMF from communication technologies. Our experience in thus far developing and implementing the study protocol indicates that MOBI-Kids is feasible and will generate results that will contribute to the understanding of potential brain tumor risks associated with use of mobile phones and other wireless communications technologies among young people.

Study on SARs in Head Models With Different Shapes by Age Using SAM Model for Mobile Phone Exposure at 835 MHz

Four head models with the outer shapes of different ages were established using the specific anthropomorphic mannequin (SAM) model of IEEE Standard 1528. The criteria of head height, face length, head length, and head breadth by age were applied to build the models. We assumed that the shells of all the head models have the same dielectric properties with the head-equivalent tissue in order to simulate a real pressed ear. The cheek and tilt positions of three bar-type phone models were used, and the positioning processes against each head model were described in detail. Antenna input impedances of the phones under the test positions and specific absorption rate (SAR) distributions in the head models were computed using the finite-difference time-domain (FDTD) technique. Spatial peak SARs averaging over 1 and 10 g were compared for fixed input and radiated powers of all the phones. The effect of the dielectric properties in a younger head model on SAR result was analyzed. First, input resistance of the phone antennas in the cheek position gradually increased when head size grew with age, but those for the tilt position showed a slight decrease. Second, for a fixed input power, the head models by age changed peak 1- and 10-g SARs by approximately 15%. The electromagnetic absorption depths in the head models in the same test position were about the same, but the head-averaged SAR was higher in the younger model because of the smaller head volume. Third, for a fixed radiated power, the peak SARs got relatively lower in the smaller head model and higher in the larger head model, compared with those for the fixed input power since the smaller head model needs lower input power. Fourth, it was shown that simultaneous change in the conductivity and permittivity of head tissue within 20%-30% did not have a significant influence on energy absorption.

Study of the tissue volume for spatial-peak mass-averaged SAR evaluation

For being adaptable to the irregular shape of a biological model and for not underestimating the spatial-peak mass-averaged specific absorption rates (SARs) of hand-held wireless telephones, a method of taking a contiguous tissue volume for the SAR-averaging-mass is proposed. The proposed method is to obtain a tissue volume for SAR averaging by summing node massed of the layers enclosing the point in which the averaged SAR is wanted in a body model until the summed mass is closest to the required mass. Therefore, at the skin layer and in the tissue region around the sinuses such as nasal cavities, the averaging volume can follow closely the shape of the region. To investigate the effect of this algorithm on SAR evaluation, SARs have been evaluated numerically for an anatomical head with a simple hand models exposed to a mobile phone, using the proposed method and the previous method of averaging over a cubic tissue volume and the results are compared. The results show that the proposed algorithm is very reasonable and leads to more stable values for spatial-peak mass-averaged SAR evaluation in an irregularly shaped model such as a realistic human model.

A Comparison of Specific Absorption Rates in SAM Phantom and Child Head Models at 835 and 1900 MHz

The specific absorption rates (SARs) in 7-year-old Korean, and 5- and 9-year-old European child head models were evaluated and compared with those of the specific anthropomorphic mannequin (SAM) phantom for mobile phone exposure at 835 and 1900 MHz. Compression of the pinnae was also considered for the 5- and 7-year-old child models during the evaluation. The cheek and tilt positions occurring when the earpiece of the phone is placed at the ear entrance canal (EEC) were analyzed against the same positions occurring when the earpiece is placed at the ear reference point (ERP). SAR variations were investigated for different skin and fat properties, as well as for different internal fat and muscle morphologies in the tissue area near the phone. A basic phone model with a monopole antenna was used for SAR calculation at each frequency. A phone model with a planar inverted F antenna was also used for verification, since the SAR results in the child models were higher than those in the SAM phantom at 1900 MHz. The spatial peak 10-g SAR values in all tissue including the pinnae and head-only were normalized to a forward power of 1 W at the feedpoint of the phone. Our results can be summarized as follows. First, a compressed pinna did not show significant changes in SAR values at 835 MHz; however, at 1900 MHz, there was an average 25% increase in spatial peak 10-g SARs for pinna-excluded tissue and a 29% increase for pinna-included tissue. Second, a phone earpiece placed at the EEC provided higher SARs in many cases compared to placement at the ERP. Third, the peak 10-g SAR was found to be very sensitive to the fat and muscle structure under the skin when touched by the mobile phone; a muscle-dominant internal head structure led to a higher peak 10-g SAR. Finally, the SAM phantom does not seem to provide a conservative estimation of child head exposure at 1900 MHz; 45% (pinna-excluded: IEEE Std C95.1) and 75% (pinna-included: ICNIRP guidelines) of the 40 total cases we reviewed showed higher SAR results than the SAM phantom.

Assessment of extremely low frequency magnetic field exposure from GSM mobile phones

Although radio frequency (RF) electromagnetic fields emitted by mobile phones have received much attention, relatively little is known about the extremely low frequency (ELF) magnetic fields emitted by phones. This paper summarises ELF magnetic flux density measurements on global system for mobile communications (GSM) mobile phones, conducted as part of the MOBI-KIDS epidemiological study. The main challenge is to identify a small number of generic phone models that can be used to classify the ELF exposure for the different phones reported in the study. Two-dimensional magnetic flux density measurements were performed on 47 GSM mobile phones at a distance of 25 mm. Maximum resultant magnetic flux density values at 217 Hz had a geometric mean of 221 (+198/-104) nT. Taking into account harmonic data, measurements suggest that mobile phones could make a substantial contribution to ELF exposure in the general population. The maximum values and easily available variables were poorly correlated. However, three groups could be defined on the basis of field pattern indicating that manufacturers and shapes of mobile phones may be the important parameters linked to the spatial characteristics of the magnetic field, and the categorization of ELF magnetic field exposure for GSM phones in the MOBI-KIDS study may be achievable on the basis of a small number of representative phones. Such categorization would result in a twofold exposure gradient between high and low exposure based on type of phone used, although there was overlap in the grouping.

Determining the influence of Korean population variation on whole-body average SAR.

Compliance of the ICNIRP reference and IEEE action levels with the basic restrictions on whole-body average (WBA) SAR was investigated based on age, physique, and posture under isolated and grounded conditions. First, Korean male models 1, 3, 5, 7, and 20 years of age with body sizes in the 50th percentile were developed and used as the test subjects: 1y(50th), 3y(50th), 5y(50th), 7y(50th), and 20y(50th). The effects of age-dependent dielectric properties due to the water content of the tissue on WBA SAR were analysed, and showed that the changes in WBA SAR are marginal. At the ages of 1, 5, and 20, thin models 1y(10th), 5y(10th), and 20y(10th) with body sizes in the 10th percentile for the horizontal plane were added in order to determine the influence of physical variations of the population. We considered standing postures with arms up and arms down. The WBA SAR for each human model was calculated when exposed to a vertically polarized plane wave in the frequency range of 10 MHz-3 GHz using the finite-difference time-domain method. The evaluated WBA SAR-based safety factor of each model is discussed for exposure to the ICNIRP reference and IEEE action levels. Finally, the lowest external electric field strength required to produce the basic restrictions on the WBA SAR, 0.08 W kg(-1), was obtained. The results showed that the ICNIRP public reference level is not conservative in the frequency range of 20-200 MHz for an arms-up posture, in the range of 40-200 MHz for an arms-down posture, and above 1 GHz for both postures. The IEEE action level is different from the ICNIRP reference level below 30 MHz, where most cases showed a safety factor of less than 50, which is the minimum value compliant with the basic restrictions for exposure to the general public.

SAR Comparison of SAM Phantom and Anatomical Head Models for a Typical Bar-Type Phone Model

The current IEEE 1528 and IEC 62209-1 standards specify a simplified physical model of the human head to provide conservative measurement procedures of the peak spatial-average specific absorption rate (SAR) of a mobile phone. This means that the evaluated SAR in a specific anthropomorphic mannequin (SAM) head model should be higher than in the heads of a significant majority of users under normal operational conditions. In this paper, the conservativeness of the SAM is investigated by numerically comparing the SAR in the SAM with those in four anatomical head models at different ages for exposure from a typical bar-type mobile phone. A numerical bar-type phone model with an internal antenna at the bottom of the phone body was implemented at 835 and 1850 MHz. This model provides an SAR pattern and levels similar with the commercial bar phones released in Korea. For two standard test positions, spatial peak 1- and 10-g SARs were calculated for both the SAM phantom and anatomical head models. The SARs were also calculated when the antenna is located on top of the phone. The results show that the SAM phantom provides a conservative evaluation for a phone model with the antenna on the top. However, when the antenna is located at the bottom, the hotspot in the SAM occurred farther from the antenna feed point, and, thus, produced lower 1- and 10-g SAR results compared with the anatomical models.

Numerical Implementation of Representative Mobile Phone Models for Epidemiological Studies

This paper describes an implementation method and the results of numerical mobile phone models representing real phone models that have been released on the Korean market since 2002. The aim is to estimate the electromagnetic absorption in the human brain for casecontrol studies to investigate health risks related to mobile phone use. Specific absorption rate (SAR) compliance test reports about commercial phone models were collected and classified in terms of elements such as the external body shape, the antenna, and the frequency band. The design criteria of a numerical phone model representing each type of phone group are as follows. The outer dimensions of the phone body are equal to the average dimensions of all commercial models with the same shape. The distance and direction of the maximum SAR from the earpiece and the area above –3 dB of the maximum SAR are fitted to achieve the average obtained by measuring the SAR distributions of the corresponding commercial models in a flat phantom. Spatial peak 1-g SAR values in the cheek and tilt positions against the specific anthropomorphic mannequin phantom agree with average data on all of the same type of commercial models. Second criterion was applied to only a few types of models because not many commercial models were available. The results show that, with the exception of one model, the implemented numerical phone models meet criteria within 30%.

Mobile phone types and SAR characteristics of the human brain

Mobile phones differ in terms of their operating frequency, outer shape, and form and location of the antennae, all of which affect the spatial distributions of their electromagnetic field and the level of electromagnetic absorption in the human head or brain. For this paper, the specific absorption rate (SAR) was calculated for four anatomical head models at different ages using 11 numerical phone models of different shapes and antenna configurations. The 11 models represent phone types accounting for around 86% of the approximately 1400 commercial phone models released into the Korean market since 2002. Seven of the phone models selected have an internal dual-band antenna, and the remaining four possess an external antenna. Each model was intended to generate an average absorption level equivalent to that of the same type of commercial phone model operating at the maximum available output power. The 1 g peak spatial SAR and ipsilateral and contralateral brain-averaged SARs were reported for all 11 phone models. The effects of the phone type, phone position, operating frequency, and age of head models on the brain SAR were comprehensively determined.