| Exposure from transmitters worn near the trunk of the body |
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Topic Exposure from transmitters worn near the trunk of the body Start 01.08.2004 End 31.07.2005 Project Management Integrated Systems Laboratory, Swiss Federal Institute of Technology, Zürich Objective With respect to the application of new technologies (UMTS, WLAN), it is expected that an increasing number of mobile transmitters will be carried in the area of the underbelly (fanny pack) or chest area (shirt pocket). Reliable data of the expected specific absorption rate in the affected tissues as a basis for risk estimations are not available. This research project will answer the questions of a) whether the torso has distinct, sensitive areas which, in light of increased exposure under the conditions discussed, need to be more extensively examined and b) whether an effective radiation protection concept needs to be addressed. Exposures of organs close to the surface (in particular, liver, kidneys, spleen, gonads, breast) resulting from transmitters carried on the body are to be determined and evaluated. Respective SAR values and temperature distributions are to be calculated for different exposure geometries, various standard transmitters and standard body dimensions (child, male, female). The frequency range to be covered is in the area of 0,4 GHz to 10 GHz in the frequency bands TETRA, GSM, UMTS, WLAN. The validity of the used calculation method and the used frequency dependent tissue conductivities has to be proven. Results As a first result the researchers presented a review of literature about existing computational and measurement techniques for determination of electromagnetic fields and the resulting temperature distribution within the human body. The recent scientific literature on the exposure of humans by body-worn transmitters was evaluated as well. The literature study can be downloaded as a PDF-file in German (2.585 kB). Computations in a layered model have shown that if a tissue layer with low permittivity (e.g. fat, bone) is surrounded by two layers with high permittivity (skin, muscle, internal organs), than under far-field-like conditions at certain layer thicknesses standing waves can develop in this tissue layer, which cause an increase of the SAR values in the neighbouring layers, particularly in the skin. Compared with homogeneous phantoms under otherwise identical conditions a rise of the SAR value by up to 5 dB was determined. Since this happens only at a distance of the field source of several cm from the body, the limit values are not exceeded. In near-field-like conditions standing waves do not arise. The results of the generic model were verified in anatomical models under the exposure by the following devices: walkie talkie at 450 MHz, mobile phones at 900 (GSM) and 1950 MHz (UMTS) and computer with WLAN at 2450 MHz. A flat phantom supplied predominantly conservative values in comparison to the exposure by the mentioned devices. Only in some exceptional cases the flat phantom underestimated the exposure by max. 5 %. The maximum exposure was always located at the body surface, the exposure of internal organs was considerably smaller. During the use of a laptop with WLAN the maximum exposure was located in the thighs, the exposure of the sex organs was considerably smaller. The standing wave effects observed in the generic model could be confirmed in the anatomical model, however they were less pronounced. The temperature rise due to HF exposure was calculated in the layered model in the frequency range from 30 MHz to 5.8 GHz. The maximum temperature rise occurred close to the surface of the body, predominantly in the skin. After 30 min it amounted at free convection (heat emission from the body surface to the environment) up to 0,8 oC, under adiabatic conditions (without heat emission from the skin) after 6 min up to 2,5 oC and after 30 min up to 3.5 oC. The results obtained in the generic model were verified in anatomical models. The frequency 450 MHz was chosen, since due to the higher penetration depth the strongest heating of the internal organs is expected. The maximum temperature rise occurred in the uppermost tissue layers, about 7 – 22 mm under the body surface, and amounted to 0.16 – 0.23 oC at free convection and to 0.23 – 0.31 oC at adiabatic conditions. The heating up of the internal organs was considerably smaller and reached 0.11 oC, the warming of the gonads was maximally 0.04 oC. The final report can be downloaded as PDF-file in German (1.287 KB) with English summary. References
Conclusions Due to not yet observed and in this extent unexpected effects, which result from the layered structure of the body tissue, an increased absorption of electromagnetic fields in comparison with homogeneous phantoms or body-simulating liquids arises. These result particularly from standing wave effects at far field exposure and cannot be reproduced in homogeneous phantoms for SAR estimation. Since they arise only within the far-field range, at a distance of several cm from body, the limit values are nor exceeded. The exposure of different internal organs was determined. No elevated absorption in the single organs could be observed. In all observed cases the SAR value at the body surface was larger than in the internal organs. The rise of temperature in layered tissues, induced by exposure, can amount at extreme conditions (maximum absorption, no thermal regulation, no heat emission by the skin) over 3.5 oC and be thus clearly higher than expected so far. However, this scenario is very unrealistic. With free convection a maximum of 0.8 oC was determined. Using anatomical models the values determined are up to 0.31 oC at the body surface. The temperature rise determined under these conditions for the internal organs is negligible as long as the SAR limit values are kept. |