| Determination of exposure distribution from high frequency fields in the human body with regard to small structures and relevant thermo-physiological parameters |
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Topic Determination of exposure distribution from high frequency fields in the human body with regard to small structures and relevant thermo-physiological parameters Start 11.11.2003 End 30.11.2005 Project Management ARC Seibersdorf Research GmbH, Austria Objective Based on current scientific, documented knowledge with regard to the absorption of high frequency electromagnetic fields in the human body, extensive investigations, particularly with regard to the anatomically small and delicate organ structures of the head (e.g. eyes, inner ear, pineal gland) should be carried out. Temporal and spatial temperature distribution in the above mentioned organs as a result of radiation absorption should be the primary focus so that the biophysical validity of currently used spatial and temporal averaging can be investigated. As far as possible, the influences of thermal regulation should be taken into account. The investigation should also be performed on different head geometries (adults and children). For this task, appropriate calculation or simulation tools first have to be evaluated and then verified using simple experimental models. The development of new numerical head models with extremely high spatial resolution in the area of the mentioned organs makes up another point of emphasis. Furthermore, electrical and thermal relevant characteristics of the affected tissues are to be measured and values set forth in available literature are to be verified taking into account the representativeness of the tissue characteristics in vivo. Results The current scientific state of knowledge about the absorption of high frequency electromagnetic fields and the resulting thermal effects within the human body was summarized within a comprehensive literature review. Regarding the spatial distribution of the radiation absorption in the human body the scientific papers concentrated on the exposure of the head in the frequency range of modern mobile communications. The results show an increasingly detailed picture of absorption conditions in various head regions like skin, pinna, skull, brain, although the spatial resolution of the anatomical models remained at about 1 mm. Concerning thermal consequences of the absorption of electromagnetic fields in the tissue there is a lot of data from medical literature and a few papers particularly considering the temperature rise in the tissue due to the absorption of high frequency electromagnetic fields in the frequency range of modern mobile communications. For absorption rates which can be caused by usual devices of the mobile communication, the rises in temperature reported in the eye and in the brain are in the range of few tenths of degree Celsius. In order to perform detailed analyses of the radiation absorption in sensitive and finely structured organs and tissues, numeric, spatially highly resolved (0.001 mm3) models of the eye, the inner ear (cochlea, equilibrium organ, auditory ossicles) and the pineal gland (a gland localised within the brain and producing the hormone melatonin) were developed on the basis of human tissue samples using of a special freeze-cut technique and integrated into a commercially available head model. For the skin a three-dimensional layered model was developed based on the data from medical literature. Calculations of the radiation absorption and the resulting increase of tissue temperature for different, practically relevant exposure scenarios, were performed using generic handset models at 400 MHz, 900 MHz and 1850 MHz by the method of the finite differences in time domain (FDTD). The computations were verified by measurements using a model developed particularly for this purpose. Finally a method for determination of thermally relevant tissue parameters (specific thermal capacity and heat conductivity) in small tissue samples was developed and realised, and electrical tissue parameters of some organs were measured. The results showed that for typical exposure scenarios expected during usage of GSM 900, GSM 1800 and UMTS mobile phones (lateral exposure at the ear) maximum temperature elevations in the brain ranged between 0.013oC and 0.005 oC. Less temperature elevations were found for tissues deep inside the head, e.g. pineal gland and inner ear. In the case of more superficially located tissues (skin, fat, muscle, skull bone) RF-induced temperature elevations ranged up to 0.5oC. An exposure scenario representing the use of a walky talky at 400 MHz and 1W transmit power next to the ear yielded maximum temperature elevations in the skin, in the brain cortex and deep inside the head of approximately 1.8oC, 0.1oC and less than 0.05oC, respectively. In general, lower frequencies caused higher temperature elevations (especially in deep tissues) at comparable transmit power levels. Temperature elevations in the eye due to lateral exposure by the considered devices were negligible (<0.001oC). Considering an exposure situation where a walky talky (400 MHz, 2 W) was operated in front of the face led to maximum temperature elevations in the eye (mainly in the lens) in the range between 0.5°C and 1°C. The tissue temperature elevation caused by frontal exposure from mobile phones was much lower due to the lower transmit power and the usually larger distance between the antenna and the eye. A detailed investigation of the RF-absorption and RF-induced temperature elevations in the skin revealed that under normal use conditions of mobile phones (phone case touching the ear and the cheek) the temperature elevations in the skin resulting from the isolation from convection and from heating up of the phone case is much larger (up to 5oC) than warming caused by RF-absorption. This warming can be perceived by thermoreceptors and cause thermoregulatory reactions, which is a normal physiological process without any health consequences. A reduction of tissue perfusion mainly affected the RF-induced temperature elevations in tissues deep inside the head due to the lack of convective heat exchange with the environment. A reduction of perfusion by 50% in all tissues led to a doubling of the maximum RF-induced temperature elevations in deep tissues, whereas superficial tissues were only negligibly affected. Pulsed exposure (short SAR-peaks lasting for 10 seconds) caused an increased warming of the tissue in comparison to continuous exposure at identical 6 minute averaged SAR. In this case, deep tissues were significantly more affected than superficial tissues. With respect to spatial averaging of SAR, it has been shown that the RF-induced SAR distribution inside the head is, as expected, highly inhomogeneous due to the heterogeneity of the tissue composition in the head. In highly conductive, small tissue regions SAR-hot spots can appear. However, the corresponding temperature distribution in the tissue did not indicate significant temperature hot spots. This is due to the effective heat transfer mechanisms taking place in the tissue. The final report, which also contains all interim reports, can be downloaded as PDF-file in German (2.626 KB) with English summary. Conclusions
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