Age-dependent tissue-specific exposure of cell phone users

Bron: iopscience.iop.org/article/10.1088/0031-9155/55/7/001/meta;jsessionid=364FB06A81D1BEEFC3BA62CB56583AB9.c1.iopscience.cld.iop.org

Andreas Christ1, Marie-Christine Gosselin1, Maria Christopoulou2, Sven Kühn1,3 and Niels Kuster1,3
Published 5 March 2010 • 2010 Institute of Physics and Engineering in Medicine
Physics in Medicine and Biology, Volume 55, Number 7

Abstract
The peak spatial specific absorption rate (SAR) assessed with the standardized specific anthropometric mannequin head phantom has been shown to yield a conservative exposure estimate for both adults and children using mobile phones. There are, however, questions remaining concerning the impact of age-dependent dielectric tissue properties and age-dependent proportions of the skull, face and ear on the global and local absorption, in particular in the brain tissues. In this study, we compare the absorption in various parts of the cortex for different magnetic resonance imaging-based head phantoms of adults and children exposed to different models of mobile phones. The results show that the locally induced fields in children can be significantly higher (>3 dB) in subregions of the brain (cortex, hippocampus and hypothalamus) and the eye due to the closer proximity of the phone to these tissues. The increase is even larger for bone marrow (>10 dB) as a result of its significantly high conductivity. Tissues such as the pineal gland show no increase since their distances to the phone are not a function of age. This study, however, confirms previous findings saying that there are no age-dependent changes of the peak spatial SAR when averaged over the entire head.

1. Introduction

Potential differences between adults and children in exposure to cell phone radiation have been the subject of ongoing debate regarding both dosimetric aspects and the possibly greater sensitivity of children, in particular their developing brain. Whereas recent interpretations of epidemiological studies indicating an increased risk for glioma and acoustic neuroma (Khurana et al 2009) are being controversially discussed within the scientific community (Khurana et al 2008, Kundi 2009), the correlation of different ailments of the central nervous system (CNS), such as Alzheimer's and migraine or vertigo, with electromagnetic field exposure has gained additional research interest among epidemiologists (Huss et al 2009, Schüz et al 2009).

During the last decade, the dosimetric analysis of cell phone exposure focused on testing the compliance of mobile phones with basic restrictions, i.e. the peak spatial average specific absorption rate (psSAR) as defined by ICNIRP (1998), IEEE (2005)1. Anatomical head models of adults and children were used to assess the psSAR in the human head for different phones, the results of which were compared to those assessed for the specific anthropometric mannequin (SAM). The SAM was proposed by the standards (CENELEC 2001, IEEE 2003, IEC 2005) for demonstrating cell phone compliance with safety limits. A large number of studies evaluated the psSAR in the SAM phantom with respect to anatomical head models, and all reviews of these studies came to the conclusion that the psSAR assessed with the SAM is a conservative measure for the exposure of both adults and children (Martens 2005, Wiart et al 2005, Christ and Kuster 2005) and that variations in psSAR among different models can be attributed to individual anatomical differences but not to age-dependent changes in head size (Kainz et al 2005). Remaining open issues, however, concern the effect of age-dependent changes of the dielectric tissue parameters on the psSAR as well as systematic differences in the local exposure, in particular of the cortex. Regarding the incomplete knowledge of these aspects, the higher lifetime exposure and the developing bodies of children, the World Health Organization recommends the development of a precautionary approach (Kheifets et al 2005).

Age-dependent changes of the loss of tissue conductivity have been repeatedly reported in the past (Thurai et al 1984, 1985, Peyman et al 2001, Gabriel 2005, Schmid and Überbacher 2005), but the available data did not suffice for a comprehensive analysis. A systematic evaluation of the age-dependent changes of the dielectric properties of a large number of different tissues has only been published very recently (Peyman et al 2009). Moreover, differences in the exposure of particular brain regions due to growth-dependent changes of the proportions of the head and the face have not been evaluated. A large number of numerical studies on absorption in the heads of children use scaled models of adults. Since this approach can lead to large uncertainties with respect to the local exposure of tissues and therefore to the interpretation of the findings, it has repeatedly been discouraged (Bit-Babik et al 2005, Wiart et al 2008). Several head models of children were recently developed based on magnetic resonance imaging (MRI), and systematic differences in the exposure of the peripheral brain regions of adults and children were observed (Wiart et al 2008).

This study aims at clarification of the remaining open questions discussed above using anatomically correct models of adults and children. In detail, its objectives are assessment of differences of the psSAR considering age-dependent changes of the anatomy of the head and dielectric tissue properties, and assessment of the differences of the local exposure of the cortex and other distinguished brain regions.

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