Topic

Possible influence of high frequency electromagnetic fields of mobile communication systems on the induction and course of phantom auditory experience (tinnitus).

Start

01.09.2005

End

31.08.2007

Project Management

Tübingen Hearing Research Center

Objective

The microwave hearing is a known phenomenon in the field of radar radiation. However, only few studies are available on the effects of high frequency electromagnetic field of mobile phones on the hearing system. These indicate possible physiological effects, but no health impairment. On the contrary, there are hints from the public and from physicians discussing hearing disorders and tinnitus. This discrepancy should be clarified. The aim of the Project is to clarify, if and at which intensity HF-EMFs of mobile phones can induce tinnitus. The influence of pulsed fields of mobile phones according to the GSM standard on the initiation of tinnitus will be investigated by means of behavioural physiology and molecular biology. Such an experimental approach with the rat as an animal model is established at the Tübingen Hearing Research Center. In this project, rats will be exposed below and above the allowed partial body SAR-limits and then tested for tinnitus. The threshold values of a possible effect and the time course of any influence will be determined. The mechanism of action will be elucidates as far as possible.

Results

As a first result the researchers provided a review of literature on possible adverse health effects of high frequency electromagnetic fields on the hearing system. The few scientific papers available on this topic at that time (end of 2005) were critically evaluated. The scientific literature does not provide any indication that electromagnetic fields of mobile phones may affect the hearing ability in a negative fashion. Regarding a possible influence of mobile phone fields on the induction of phantom sound perception (tinnitus) there are so far no data available in the scientific literature.

The text of the review (Appendix II of the printed final report) is available as PDF-file in German (110 kB).

The exposure setup for the animals (rats) used in this study was developed and dosimetrically characterized by IMST (Institut für Mobil- und Satellitenfunktechnik GmbH). Local exposure of the ear region of freely moving rats was achieved by a portable circular loop antenna fastened to the body of the animals around the neck. The antenna was fed by a flexible cable with bite protection and was well accepted by the animals. The exposure intensity used, expressed as SAR-values, was 0 (sham) and 0.02, 0.2, 2 and 20 W/kg. The exposure took place for four weeks, five days a week for two hours daily and was accomplished in a blinded fashion. Preliminary tests showed, that even at an exposure with 20 W/kg the temperature directly beneath the antenna and in the ear canal increased only slightly (max. 1,5°C). The core temperature was not affected.

The technical documentation of the exposure setup (Appendix I of the printed final report) is available as PDF-file in German (1.201 kB).

Rats were trained before the beginning of the exposure to respond to the change between silence and sound perception with a defined behavioural change. The animals respond to the perception of sound by running back and forth between two reward areas, in which a drop of sugar water is offered. During silence they sit on a resting platform and wait for the next sound perception. This performance can be achieved by rewarding with sugar water during a sound is played and by punishments with weak electrical shocks if the rats try to access the feeder during silence. An increased activity of the animals during silence points to the perception of phantom sound (tinnitus). The behaviour of the animals showed a significant dependence on the time course of the experiment, since the animals forgot their task during the six weeks of investigation a little. This effect was identical in the sham exposed and all exposed groups. During the four weeks of exposure and two recovery weeks there was no significant influence of the exposure intensity at any time.

By means of molecular biology the expression of activity-dependent genes was examined immediately after the four weeks of exposure and after a recovery phase of two weeks. Using RT-PCR (reverse transcriptase polymerase chain reaction) the activity of the genes Arg3.1/Arc (an activity-dependent gene), BDNF (brain derived nerve growth factor), c-FOS (immediately early gene) in the brain (auditory cortex, AC; inferior Colliculus, IC), and the spiral ganglia of the cochlea was measured. From scientific literature it is known, that during a tinnitus the expression of Arc3.1/Arc is decreased in the AC and the expression of BDNF and c-FOS is increased in the IC and in the cochlea. Additionally, an increase of c-FOS expression points to stress.

The comparison of exposed and sham exposed animals did not show any quantitative changes in the expression of the genes mentioned above. This means that there is no indication of an induction of tinnitus due to the exposure. In the AC the variability, but not the average value of the expression of the genes BNDF and Arg3.1/Arc, was significantly increased in animals exposed to 2 W/kg and 20 W/kg. This could be interpreted as a variable reaction of the animals to the slight warming underneath the antenna at higher SAR values.

The comparison of cage controls with exposed and sham exposed animals showed four significant changes in the expression of individual genes as well as a nearly continuous and significant change of the variance. Due to a total twelve comparisons this result can be partially explained but statistical chance, but it points also to an influence of the treatment of the animals accompanying the exposure. The observed effects - a reduced expression of BNDF and c-FOS in exposed as well as sham exposed animals in comparison to cage controls - disappeared partially during the recovery phase, in which no further handling of the animals took place, and point to an increased activity and memory performance. On the contrary, an increased expression of these genes would be expected as a consequence of tinnitus or stress.

The final report (without appendices) can be downloaded as a PDF-file in German (1.068 kB) with English abstract.

Conclusions

By means of behavioural tests, as well as using molecular biology, it could not be shown that electromagnetic fields of mobile phones induce tinnitus after four weeks of regular exposure with levels up to 20 W/kg. Other negative influences on the health and the hearing system were not found either. The used methods were sufficiently sensitive and showed also small consequences of changed environment and treatment of the animals. The effects caused by these influences were rather in opposite direction as expected due to an induction of tinnitus, and showed also clearly that the animals did not suffer stress. The results shown here are in accordance with other national and international research projects, which likewise show no negative influence of electromagnetic fields of mobile phones on the hearing system.