Kühn S, Jennings W, Christ A, Kuster N. (2009). Assessment of induced radio-frequency electromagnetic fields in various anatomical human body models. Phys. Med.Biol. 54 875-890.
Testing for compliance with safety limits of human exposure to electromagnetic fields (EMFs) from fixed base station transmitters is based on reference levels proposed by the International Commission on Non-Ionizing Radiation Protection (ICNIRP, 1998) and ANSI/IEEE C.95 (2001). Due to the lack of computational power at the time of standard development, these reference levels were derived from very simplified models of the human anatomy. In most currently available publications, conclusions are based on large adult models with only one published paper dealing with anatomically correct child models. Also, the results in the literature are mainly based on frontal exposure with co-polarized electric field vector incident to the human body. Therefore, an evaluation for different incidence directions and polarizations is required. Finally, the spatial averaging of incident fields as proposed by ICNIRP 1998 has never been validated with respect to its consistency with the basic restrictions for localized exposure.
The objectives of the study were: 1) to evaluate the consistency of reference levels of international standards for limiting human exposure to EMF with the basic restrictions and 2) to address the scientific basis for methods to demonstrate compliance of fixed transmitters with electromagnetic safety standards.
The study is based on numerical evaluations on the induced SAR of various human anatomies from homogeneous and inhomogeneous field distributions. The human models, ranging from 6 year old child to large adult male, were exposed to plane-wave and scattered field distributions with the frequency from 50 MHz to 2.5 GHz. The authors address the exposure to plane-waves incident from all major six sides of the humans with two orthogonal polarizations each. Worst-case scattered field exposure scenarios have been constructed in order to test the implemented procedures of current in situ compliance measurement standards (spatial averaging versus peak search). For all numerical evaluations, the simulation platform SEMCAD X (SPEAG, Switzerland) was used. The software includes finite difference time domain (FDTD) and alternating direction implicit (ADI-FDTD) solvers, unconditionally stable implementations of conformal C-FDTD and C-ADI-FDTD solvers as well as hardware accelerated implementations of those.
Results and Interpretation
The findings of this study suggest that the reference levels of current RF electromagnetic safety guidelines for demonstrating compliance as well as some of the current measurement standards are not consistent with the basic restrictions for all human anatomies. In particular, children might be exposed above the basic restrictions for incident fields at the reference levels. It was also shown that spatial averaging of incident fields is only correlated with whole-body averaged SAR. Using a spatial averaging method of incident fields to demonstrate compliance with the basic restrictions for localized SAR might not be conservative. The reference values need to be revised to make them consistent with the basic restrictions. Since only a limited number of different anatomies have been evaluated in this study, it can be assumed that the discrepancies are larger if all anatomies are covered. An extended evaluation is necessary to determine the exposure of a certain proportion of the population, using a sufficient set of anatomies and generic models to cover whole-body and peak 10 g spatial averaged SAR. Also, the frequency range should be extended to include frequencies above 2.45 GHz and below 50 MHz. Currently, compliance should only be shown using the incident peak field, which will be conservative for both whole-body averaged and localized SAR.
The current reference values as proposed by ICNIRP and IEEE are not consistent with the basic restrictions and should be revised. Spatial averaging of the incident field is only suited for demonstrating compliance with whole-body averaged SAR and may result in underestimation of the peak spatial SAR limits. Peak field measurements considering the directivity of the locally incident field are more sound and rigorous.