Autores
Neubauer G, Preiner P, Cecil S, Mitrevski N, Gonter J, Garn H. The relation between the specific absorption rate and electromagnetic field intensity for heterogeneous exposure conditions at mobile communications frequencies. Bioelectromagnetics. Jun 23, 2009 Ahead of print.

Background

To avoid effects of radio frequency (RF) and microwave exposure on well human being and health, the ICNIRP and IEEE put restrictions on exposure, such restrictions are defined in terms of the so-called specific absorption rate (SAR) in the frequency range from 100 kHz to 10 GHz and the power density in the frequency range from 10 to 300 GHz. These field strength values were obtained from the basic restrictions by mathematical modeling and by extrapolation from the results of laboratory investigations at specific frequencies. Plane wave exposure indicates a uniform electric field distribution in the space occupied by the human without the human body being present. However, in heterogeneous exposure, it is necessary to average the electric field strength over the space occupied by the human body to ensure that these field levels represent the whole-body SAR. The main idea of the approach developed is to model multiple incoming electromagnetic waves entering a specific volume to be investigated.

Methodology
A numerical tool solving Maxwell’s equations was used to obtain the distributions of the SAR in a phantom. Information on incoming electromagnetic waves is obtained by analyzing the wave fronts received at a specific position using the optical tool Wireless Insite. It is assumed that approximately the same wavefronts are received at all locations within the area of investigation where the phantom is located. Such an assumption leads to realistic exposure conditions as long as the phantom is located at a sufficient distance from the electromagnetic source, for example, the far field and the lengths of the different paths from the source to the phantom are similar. These conditions represented typical heterogeneous exposure scenarios for frequencies relevant for mobile communication.

Results
The results showed that plane wave exposure does not represent worst-case exposure conditions. When the electric field strength arising at plane wave exposure is compared to the electric field strength averaged over the volume of the human body occurring during multipath exposure, 12% of all heterogeneous cases examined represent worse exposure conditions than plane wave exposure for whole-body exposure at 946 MHz, 15% at 1,840 MHz, and 22% at 2,140 MHz. For whole-body exposure, 14 of 63 cases show higher SAR than homogenous exposure. Regarding partial-body exposure cases together, the relation is 33 out of 189 for scenario 1 compared to 27 out of 180 for scenario 2. Slight differences can be observed when comparing the different parts of the body: for head and limbs more cases with higher SAR have been found for scenario 1 (7/63 and 17/63 compared to 3/60 and 12/60), but the situation is different for the trunk.

Conclusions
The results of this study demonstrate that plane wave exposure is not the worst-case exposure condition. For worst-case conditions examined within the framework of this study, the whole-body SAR is up to 62% higher for heterogeneous exposure compared to frontal plane wave exposure; for partial body the SAR is 209% higher. It can also be clearly seen that the planewave exposure case is not the worst case for any of the investigated frequencies, because for several heterogeneous exposure conditions, lower averaged field levels are sufficient to reach the whole-body SAR limit. The results of this study were in accordance with previous studies.



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