Sven Kühn

Age-dependent tissue-specific exposure of cell phone users

The peak spatial specific absorption rate (SAR) assessed with the standardized specific anthropometric mannequin head phantom has been shown to yield a conservative exposure estimate for both adults and children using mobile phones. There are, however, questions remaining concerning the impact of age-dependent dielectric tissue properties and age-dependent proportions of the skull, face and ear on the global and local absorption, in particular in the brain tissues. In this study, we compare the absorption in various parts of the cortex for different magnetic resonance imaging-based head phantoms of adults and children exposed to different models of mobile phones. The results show that the locally induced fields in children can be significantly higher (>3 dB) in subregions of the brain (cortex, hippocampus and hypothalamus) and the eye due to the closer proximity of the phone to these tissues. The increase is even larger for bone marrow (>10 dB) as a result of its significantly high conductivity. Tissues such as the pineal gland show no increase since their distances to the phone are not a function of age. This study, however, confirms previous findings saying that there are no age-dependent changes of the peak spatial SAR when averaged over the entire head.

Assessment of induced radio-frequency electromagnetic fields in various anatomical human body models.

The reference levels for testing compliance of human exposure with radio-frequency (RF) safety limits have been derived from very simplified models of the human. In order to validate these findings for anatomical models, we investigated the absorption characteristics for various anatomies ranging from 6 year old child to large adult male by numerical modeling. We 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). Our findings suggest that the reference levels of current electromagnetic (EM) safety guidelines for demonstrating compliance as well as some of the current measurement standards are not consistent with the basic restrictions and need to be revised.

Measurement, simulation and uncertainty assessment of implant heating during MRI

The heating of tissues around implants during MRI can pose severe health risks, and careful evaluation is required for leads to be labeled as MR conditionally safe. A recent interlaboratory comparison study has shown that different groups can produce widely varying results (sometimes with more than a factor of 5 difference) when performing measurements according to current guidelines. To determine the related difficulties and to derive optimized procedures, two different generic lead structures have been investigated in this study by using state-of-the-art temperature and dosimetric probes, as well as simulations for which detailed uncertainty budgets have been determined. The agreement between simulations and measurements is well within the combined uncertainty. The study revealed that the uncertainty can be kept below 17% if appropriate instrumentation and procedures are applied. Optimized experimental assessment techniques can be derived from the findings presented herein.

Dependence of the Occupational Exposure to Mobile Phone Base Stations on the Properties of the Antenna and the Human Body

This study assesses human exposure in the close vicinity of mobile phone base station antennas by finite-difference time-domain simulations. The peak spatial average specific absorption rate (SAR) and the whole-body average SAR are analyzed in three different anatomical models (55-101 kg) with respect to the basic restrictions for occupational exposure. The models are at distances between 0.5 and 4 m from various antenna types operating at frequencies ranging from 450 to 2140 MHz. The validity of the simulations is confirmed by an analysis of the impact of the mesh resolution on local and whole-body average SAR and by experimental validation of the numerical models. The results demonstrate that the whole-body absorption generally determines the maximum permissible antenna output power for collinear array antennas. Local exposure depends on various effects that fluctuate strongly among individuals. In particular for short antennas, the peak spatial average SAR can be more restrictive than the whole-body absorption because they may only expose a fraction of the body. Therefore, compliance must be demonstrated for both quantities.

Assessment Methods for Demonstrating Compliance With Safety Limits of Wireless Devices Used in Home and Office Environments

Short-range wireless radio-frequency devices are rapidly pervading home and office environments. Nevertheless, their contribution to the electromagnetic field exposure of humans has not yet been systematically assessed, nor have the procedures for testing compliance been developed. In this paper, we have assessed the range of exposure in terms of the dosimetric and incident field quantities for devices of the most common technologies, i.e., digital enhanced cordless telecommunications, wireless local area networks and Bluetooth, as well as wireless communication devices based on proprietary standards in the frequency range from 30 MHz to 6 GHz. Well-defined procedures to obtain the worst case operational modes are presented. Since operation in the near-field of these devices cannot be excluded, dosimetric evaluation is the most straightforward technique for testing compliance when low-power exclusions are not applicable. The suitability of the suggested methods is demonstrated through the examination of five classes of short-range transmitters.

Measured radiofrequency exposure during various mobile-phone use scenarios

Epidemiologic studies of mobile phone users have relied on self reporting or billing records to assess exposure. Herein, we report quantitative measurements of mobile-phone power output as a function of phone technology, environmental terrain, and handset design. Radiofrequency (RF) output data were collected using software-modified phones that recorded power control settings, coupled with a mobile system that recorded and analyzed RF fields measured in a phantom head placed in a vehicle. Data collected from three distinct routes (urban, suburban, and rural) were summarized as averages of peak levels and overall averages of RF power output, and were analyzed using analysis of variance methods. Technology was the strongest predictor of RF power output. The older analog technology produced the highest RF levels, whereas CDMA had the lowest, with GSM and TDMA showing similar intermediate levels. We observed generally higher RF power output in rural areas. There was good correlation between average power control settings in the software-modified phones and power measurements in the phantoms. Our findings suggest that phone technology, and to a lesser extent, degree of urbanization, are the two stronger influences on RF power output. Software-modified phones should be useful for improving epidemiologic exposure assessment.

Estimation Formulas for the Specific Absorption Rate in Humans Exposed to Base-Station Antennas

The demonstration of compliance with guidelines for human exposure to base-station antennas can be a time consuming process or often results in overly conservative estimates. To alleviate this burden and reduce the overestimation, approximation formulas for the whole-body average specific absorption rate (SAR) and the peak spatial SAR of human bodies using readily available basic antenna parameters have been developed and validated in this study. The formulas can be used for adults standing in the radiating near field of base-station antennas operating between 300 MHz and 5 GHz, at distances larger than 200 mm. It is shown that the 95th-percentile absorption for the human population can be well approximated by the absorption mechanism and statistical data of weight, height, and body-mass index of the human population. The validation was performed numerically using three anatomical human models (Duke, Ella, and Thelonious) exposed to 12 generic base-station antennas in the frequency range 300 MHz to 5 GHz at six distances between 10 mm and 3 m. From the 432 evaluated configurations, the estimation formulas for adult models are proven to be conservative in predicting the SAR exposure values of the two adults, but as expected not of the child.

Impact of pinna compression on the RF absorption in the heads of adult and juvenile cell phone users.

The electromagnetic exposure of cell phone users depends on several parameters. One of the most dominant of these is the distance between the cell phone and the head tissue. The pinna can be regarded as a spacer between the top of the phone and the head tissue. The size of this spacer has not yet been systematically studied. The objective of this article is to investigate the variations of distance as a function of age of the exposed person, and the mechanical force on the pinna and how it affects the peak spatial specific absorption rate (psSAR). The distances were measured for adults and children (6-8 years of age) while applying a well-defined force on the pinna using a custom-developed measurement device. The average distances of the pinnae to the heads and their standard deviations showed no major differences between the two age groups: 10.5 +/- 2.0 mm for children (6-8 years) and 9.5 +/- 2.0 mm for adults. The pinnae of our anatomical high-resolution head models of one adult and two children were transformed according to the measurement results. The numerical exposure analysis showed that the reduced distance due to the pinna compression can increase the maximum 10 g psSAR by approximately 2 dB for adults and children, if the exposure maximum is associated with the upper part of the phone.

Assessment of the radio-frequency electromagnetic fields induced in the human body from mobile phones used with hands-free kits.

In this study, the radiation emission from mobile phones when used with wireless and wired hands-free kits (HFK) was evaluated to determine the necessity for a dedicated compliance procedure and the extent to which the use of wired and wireless HFK can reduce human exposure. The specific absorption rates (SAR) from wireless HFK were determined experimentally. Wired HFK were evaluated dosimetrically while connected to mobile phones (GSM900/1800, UMTS1950) under maximized current coupling onto the HFK cable and various wire routing configurations. In addition, experimentally validated simulations of a wired HFK and a mobile phone operating on anatomical whole-body models were performed. The maximum spatial peak SAR in the head when using wired HFK was more than five times lower than ICNIRP limits. The SAR in the head depends on the output power of the mobile phone, the coupling between the antenna and cable, external attenuation and potential cable specific attenuation. In general, a wired HFK considerably reduces the exposure of the entire head region compared to mobile phones operated at the head, even under unlikely worst-case coupling scenarios. However, wired HFK may cause a localized increase of the exposure in the region of the ear inside the head under worst-case conditions. Wireless HFK exhibit a low but constant exposure.

Estimation of head tissue-specific exposure from mobile phones based on measurements in the homogeneous SAM head.

The maximum spatial peak exposure of each commercial mobile phone determined in compliance with the relevant safety and product standards is publicly available. However, this information is not sufficient for epidemiological studies aiming to correlate the use of mobile phones with specific cancers or to behavioral alterations, as the dominant location of the exposure may be anywhere in the head between the chin to above the ear, depending on the phone design. The objective of this study was to develop a methodology to determine tissue-specific exposure by expanding the post-processing of the measured surface or volume scans using standardized compliance testing equipment, that is, specific absorption rate (SAR) scanners. The transformation matrix was developed using the results from generic dipoles to evaluate the relation between the SAR in many brain regions of the Virtual Family anatomical phantoms and in virtual brain regions mapped onto the homogeneous SAM head. A set of transformation factors was derived to correlate the SAR induced in the SAM head to the SAR in the anatomical heads. The evaluation included the uncertainty associated with each factor, arising from the anatomical differences between the phantoms (typically less than 6 dB (4×)). The applicability of these factors was validated by performing simulations of four head models exposed to four realistic mobile phone models. The new methodology enables the reliable determination of the maximum and averaged exposure of specific tissues and functional brain regions to mobile phones when combined with mobile phone power control data, and therefore greatly strengthens epidemiological evaluations and improves information for the consumer.