Simulation of the temperature elevation in children exposed to plane wave electromagnetic fields (10 MHz–1 GHz) at the ICNIRP reference level

Abstract

Because of a lack of thermal models, to date, limitation of exposure to an electromagnetic field (EMF) has been based on restricting intracorporal specific absorption rates. To allow convenient compliance checks, reference field values have been defined. If they are met, compliance with basic restrictions is assumed. This article demonstrates that this assumption is not valid in every case. It has therefore been investigated as to whether the biological goal of limiting tissue heating is still met, in particular with regard to children. The thermal solver applied is based on the bioheat equation, with implemented additional improvements that allow consideration of blood flow and metabolic rate as a function of local tissue temperature rise and, in addition, adapt the blood temperature relative to the absorbed power. As a further improvement, heat exchange at the tissue/air boundary has been modeled, with radiation, convection, and sweating considered as well. The mathematical equations describing these additional thermoregulatory mechanisms were taken from the literature and unified in the thermoregulatory model used for this study. For the investigated case of plane wave exposure, the results confirm the violation of the basic restrictions in five of the six models when exposed to reference EMF levels. However, using thermal modeling, it was possible to demonstrate that heating remained within the biological tolerances. In particular, temperature elevation of the body core temperature remained <0.014°C and the local peak temperature did not exceed 1°C.

The Impact of Thermal Modeling on Limiting RF-EMF

“…In this particular case the excess was 23%. This finding is in agreement with the literature reporting overexposures up to 45% [7][8][9][10][11][12][13][14][15].…”

“…In a second step the calculated SAR distributions were used for thermal simulation which was based on the bioheat equation [19] which had been further improved by accounting for additional and time-dependent thermoregulatory processes. These were heat-dependent changing metabolic rate A, blood perfusion and vasodilatation B, heat exchange S via tissue/air interfaces at the skin and within the lung, and by heat transfer due to insensible water loss and sweating [7]. This resulted in the following improved bioheat equation After initially calculating the steady-state starting condition, induced tissue temperature changes were determined as a function of time over 60 min to assure T reached the new steady-state.…”

“…In the meanwhile, progress in numerical simulation and digital anatomical modeling allowed demonstrating that RF EMF reference values are not conservative enough to justify presumption of conformity with basic restrictions in any case. Studies reported up to 45% excess of basic restrictions at exposure scenarios of persons with small body height such as children or undersized adults [7][8][9][10][11][12][13][14][15].…”