Summary of Oral Evidence presented to IEGMP by Dr P J Dimbylow, National Radiological Protection Board (NRPB), on Friday 10 September 1999

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Dr Dimbylow described the method of calculating exposure to radiofrequency (RF) fields from handheld mobile phones in the near field. Recent work has used a voxel phantom, based on medical imaging data. Thirty-eight different organs and tissues are identified. Calculations of exposure use information on tissue conductivity. In general, modelling studies with mobile phones indicate that they comply with exposure guidelines although the calculations that have been carried out are generic. In general, high specific absorption rates (SARs) are associated with high conductivity tissues. Hence high SARs are confined to the eye and the surface tissues on the side of the head; there is little penetration beyond the skull. SAR from a phone on one side of the head falls rapidly through the brain.

Although the source characterisation obtained with both Finite Difference Time Domain (FDTD) and Numerical Electromagnetics Code (NEC) methods are similar, they do depend on the model used. This is particularly true with respect to the transceiver, for which a generalised model is used because data on specific phones is extremely limited. It was accepted that in theory some phones could exceed guidance from the International Commission on Non-Ionizing Radiation Protection assuming worst case conditions (radiated power of 2 W (watts)), but this was thought to be very unrealistic as radiated power is typically much lower (around 0.125 W). At present there are no standards relating to measurement of SAR, and it is difficult for members of the public to tell whether phones comply with guidance. They are, however, covered by consumer protection legislation, which requires them to be safe, and the courts would interpret this as compliance with NRPB guidance. In addition, there is no suggestion of non-compliance from available data. There was some discussion on the effect of antenna position, but moving the position of the antenna could result in deposition into other critical organs. This discussion is also relevant to satellite phones; Niels Kuster (Swiss Federal Institute of Technology) has performed some measurements and could be asked for evidence. In relation to base stations, exposures of the public are only small fractions of exposure guidelines.

It was noted that whilst calculation of SAR was important for thermal effects, it might be less relevant to other putative mechanisms of pathogenesis; the model could also be used to calculate currents. In addition, it was recognised that as the model is limited to a resolution of 2 mm, it could not be used to predict currents on a microscopic scale. This could be particularly important as brain tissue is inhomogeneous in conductivity. It was also suggested that RF currents could be rectified into DC currents, which could have biological activity. There was interest in how induced currents compared with endogenous brain currents and the extent to which the effects of RF radiation on neurons had been examined.

Calculations of SAR, as a measure of energy deposition, depend upon the size of the individual and the dielectric properties of tissues. Whilst size has been taken into account in the calculations no account has been taken of possible changes in tissue conductivity with age. This is a potentially important issue. It was suggested that Camelia Gabriel (Microwave Consultants) might have some information as to possible changes with age.

First issued 5 April 2000 | Last updated 9 May 2000