Untersuchungen zu altersabhängigen Wirkungen hochfrequenter elektromagnetischer Felder auf der Basis relevanter biophysikalischer und biologischer Parameter

Objective

Within the framework of the study children’s heads and the source of RF-radiation will be modelled as realistic as possible. Amount and the distribution of energy absorption and the temperature increase will be examined in anatomically correct head phantoms of children. Age dependent differences of physiological, biophysical or anatomic parameters will be taken into account as far as possible. Target structures such as the pineal organ, hippocampus, hypothalamus and bone marrow will be included in the model. The results of the project will help to verify standards and exposure limits and to determine energy absorption in relevant target structures.

Methods

This study investigates the absorption of electromagnetic fields in the heads of mo-bile phone users with particular emphasis on differences between adults and children due to age dependent changes of anatomy and biophysical quantities. The SAR and the temperature rise induced by RF absorption are analyzed numerically and experimentally using highly accurate anatomical head models, particularly developed measurement and exposure setups and phantoms of heads of children. In detail, the study encompasses the following work packages:

  • In order to determine the absorption in different regions of the brain and tissues of the head, three anatomically correct MRI-based head models (three children aged 3, 6 and 11) and one adult (SAM Phantom) are improved and complemented by the following structures: hypothalamus, hippocampus, pineal gland, eye and bone mar-row.

  • The variability of the absorption of electromagnetic fields of mobile phones is ana-lyzed depending on age dependent changes of dielectric tissue properties using data of a most recent experimental evaluation

  • The changes in distance between the telephone and the skull due to mechanical stress caused by pressing the phone against the ear are evaluated by measuring the pinna thicknesses of 40 children and 28 adults. A particular gauge is developed which can register distances as a function of applied force.

  • The segmentation of the anatomical head models is adapted according to these measurements.

  • The exposure of children and adults to electromagnetic fields is evaluated system-atically for different types of mobile phones in both GSM bands. The following as-pects are discussed in detail:

    • Peak Spatial Average SAR as a function of age dependent tissue parameters with respect to the standardized SAM phantom

    • absorption in the subregions listed above as a function of age dependent tissue parameters (hypothalamus, hippocampus, pineal gland, eye and bone marrow)

    • differences in absorption in brain regions at the surface due to anatomical differ-ences

    • impact of mechanical stress of the ear

  • The temperature rise due to the absorbed RF power is evaluated numerically in the models of the three children and of the adult using worst case assumptions for the thermal parameters.

  • A measurement and exposure setup is developed for the experimental validation of the numerically assessed temperature increase. The temperature in the auditory canal and on the cheek are evaluated in 8 children and 16 adults.

  • The measurements of the volunteers are approved by an ethics committee. After the measurements, experimental and numerical results are compared.

  • In order to measure the Peak Spatial Average SAR in the heads of children, two head phantoms are developed based on MR images of children. The SAR of two telephones is measured using these phantoms and compared to the results of the SAM phantom.

At the beginning of the study the researchers presented a review of the scientific knowledge about the determination of RF-energy absorption in the heads of children. Different computational and measurement techniques, existing head models and relevant studies are discussed.

The review is available as PDF-file in German (2191 KB).

Result

The main results regarding the peak spatial average SAR, which is relevant for the compliance testing of mobile phones, confirm the conclusions of previous studies.

  • Age dependencies of dielectric tissue properties do not lead to typical changes of the peak spatial average SAR. This is valid for all configurations analyzed here (phone models, positions, etc.).

  • The geometrical properties of the head do not have a systematic impact on the peak spatial average SAR, i. e., a correlation between the size of the head and the peak spatial average SAR could neither be established numerically nor experimentally. Differences are merely due to individual anatomical properties. This is valid for all configurations analyzed here (phone models, positions, etc.).

  • The thickness of the pinna has a large impact on the peak spatial average SAR (up to 2 dB) if the antenna or the maximum of the current distribution is located above the user’s ear. Characteristic differences between adults and children with respect to the pinna thickness, which defines the distance between the cell phone and the user’s head, could not be observed.

  • In all investigated cases, the current methods for compliance testing prove to be conservative.

Major age dependent changes were observed for the exposure of particular tissues and brain regions. These concern bone marrow as well as regions both inside the brain and at its surface.

The findings can be summarized as follows:

  • The exposure of regions inside the brains of young children (e. g. hippocampus, hypothalamus, etc.) can be higher by more than a factor of two in comparison to adults. If the maximum of the Peak Spatial Average SAR of the brain tissue is close to the value averaged over all head tissues, it can reach the order of magnitude of the Peak Spatial Average SAR of the phone. A higher Peak Spatial Average SAR in brain tissue is highly unlikely.

  • The exposure of the bone marrow of children can exceed that of adults by about a factor of ten. This is due to the strong decrease of the electric conductivity of this tis-sue with age. The direct comparison of the SAR in bone marrow with the Peak Spa-tial Average SAR appears difficult because if the cubical averaging volume must contain marrow tissue only.

  • The exposure of the eyes of children is higher than the exposure of the eyes of adults. Regarding thermal effects, however, this does not represent a problem be-cause the exposure of the eyes by mobile phones is very low in general.

  • Because of differences in their positions with respect to the ear, brain regions at the surface can exhibit large differences in exposure between adults and children. The cerebellum of children can show a peak spatial average SAR more than twice as high as for adults. It should be noted that these differences strongly depend on the current distribution on the phone.

Children often show a higher exposure of regions inside the brain and on its surface than adults. With the exception of bone marrow, the conductivity of which strongly decreases with age, the higher exposure of children is due to differences in anatomical proportions. For exposure of the surface of the brain, the current density distribution or the near field must be regarded.

The following observations could be made for thermal exposure:

  • The evaluation of the temperature increase induced by absorbed mobile phone radiation does not suggest any systematic differences between adults and children. The choice of the thermal tissue parameters can lead to large uncertainties.

  • Measurements of the temperature increase in adult volunteers lead to an average temperature increase of 0.05 °C in the auditory canal or on the cheek after 7.5 min-utes of exposure. Whereas this increase can be attributed to the absorbed RF power, the temperature increase due to electric losses in the mobile phone is significantly higher.

  • • The results of the temperature measurements do not indicate a higher increase of the tissue temperature of children than of adults due to mobile phone use.

The final report is available as PDF-filein German (2667 KB).

References

  • Andreas Christ, Marie-Christine Gosselin, Maria Christopoulou, Sven Kühn and Niels Kuster (2010) Age dependent tissue-specific exposure of cell phone users, Physics in Medicine and Biology 55, 1767-1783
  • Andreas Christ, Marie-Christine Gosselin, Sven Kühn and Niels Kuster(2010) Impact of pinna compression on the RF Absorption in the heads of adult and juvenile cell phone users, Bioelectromagnetis 31: 406-412

Conclusion

On the one hand do the main results of the study, regarding the peak spatial average SAR, which is relevant for the compliance testing of mobile phones, confirm the conclusions of previous studies showing that in all investigated cases, the current methods for compliance testing prove to be conservative.

On the other hand especially younger children often show a higher RF-exposure of regions inside the brain and on its surface than adults. With the exception of bone marrow, the conductivity of which strongly decreases with age, the higher exposure of certain structures in the heads of children is due to differences in anatomical pro-portions. The dosimetric character of the study does not allow to draw conclusions about biological effects or health relevance. Previous results of experimental and epidemiologic studies do not provide indications of age dependent negative health effects. Nevertheless more research is needed, particularly about possible long term effects of near field RF-exposure on a developing brain. The results of the study support the precautionary advice given by the BfS particularly for children.