In other European countries, security authorities have already been using TETRA radio networks for some time. Similar to Germany, other European countries are currently setting up their nationwide networks. In some of these countries, the introduction of the TETRA standard was accompanied by scientific research. Within the framework of the volunteer study to investigate the effects of the TETRA radio signal on cognitive functions, scientists of the Berlin Charité's "Competence Center Sleep Medicine" performed a literature review on health effects of digital radio, bringing together data from publications and reports on the various national scientific results. Overall, there is much less data for TETRA than for public mobile communications.

The few results published previously do not show any health-relevant effects of TETRA. In one study, a slight influence on memory was found. However, because of the high number of tests performed in the study, this observation might as well be purely coincidental. Therefore, this result has to be verified.

Volunteer with EEG-electrodes glued on the head and the flat exposure antenna mounted on the left ear. Source: Charité Berlin

In this study, volunteers were exposed in the head region to a TETRA signal at SAR values of 1.5 W/kg and 6 W/kg.

A corresponding exposure setup was constructed and characterised by IMST GmbH (Kamp Lintfort).

It consisted of a flat antenna worn on the left side of the head, which was supposed to simulate the exposure to a TETRA mobile device.

It can be worn comfortably for up to eight hours, even during sleep.

The actual exposure of individual brain regions was calculated in detail by Seibersdorf Laboratories GmbH.

At the highest level of radiated power, which leads to an exposure of 6 watt per kilogramme, the exposure setup causes the temperature on the head’s surface to increase by almost 1 °C. Since the study had to be performed in a blinded way (neither the volunteer nor the directly involved scientist knew the actual exposure situation), it was important to prove whether the volunteers were able to perceive this warming.

Therefore, a pilot study was performed. It could be shown that the volunteers were not able to judge correctly the actual temperature increase. Therefore, there was no risk of unblinding (the volunteers were not able to discover the specific exposure situation).

The exposure had no substantial effects on the sleep macrostructure (sleep latency, duration and structure of particular sleep stages). The same holds true for sleep spindles (characteristic signs of sleep in the EEG).

The power spectra of the EEG changed under exposure, the power in the beta frequency range (13 – 22 Hz) was reduced. The observed effects occurred primarily after a longer exposure (at the end of the night) and did not show any dependence on the exposure intensity.

Well-being and subjectively perceived sleep quality were not influenced by the exposure.

The power spectra of the waking EEG modulated under exposure as well. The power in the beta frequency range increased (not dose-dependent) during exposure at one location. Additionally, the theta frequency range (4.0 – 7.75 Hz) of the waking EEG modulated at three locations. The power of these slow waves was higher under exposure. These results did not reflect in behavior or well-being.

Slow brain potentials evoked by acoustic and visual stimuli modulated only in one of several tests during exposure. Daytime tiredness or alertness, as measured by EEG as well as by the diameter of the pupil, were not influenced by the exposure.

The results of visual and acoustic tests on attention (precision, reaction times) did not show any influence of the exposure. In memory tests with various degrees of difficulty, the tests with intermediate difficulty showed deviations in both directions under exposure - during an exposure of 1.5 Watt per kilogramme, the number of correct responses was higher and during an exposure of 6 Watt per kilogramme it was lower than during sham exposure. At the highest and the lowest degree of difficulty, there were no exposure-dependent differences. These results are probably coincidental.

Well-being, anxiety, depression and the occurrence of symptoms were not influenced by exposure during the day.

The study duration was 20 weeks per person. Among women, sleep as well as cognitive performance are strongly influenced by the menstrual cycle. During data collection, it would have been necessary to account for the phase of the menstrual cycle. This would have had to result in an even more comprehensive planning of the study design.

In sleep research, the group of young men at the age of 18 – 30 years is an established homogeneous reference group which has already been investigated intensively.

Sleep structure as well as cognitive performance depend strongly on age. Therefore, different age groups should not be mixed up within one investigation.

Separate studies on older individuals of both sexes are desirable and planned by the BfS.

Sleep is a very complex biological process controlled by the central nervous system. It is a very well defined biological state, which reacts sensitively to external influences. Therefore, sleep is a suitable model for the investigation of possible effects on the brain.

Sleep EEG can be described very precisely and is much more suitable for the investigation of minor effects than the waking EEG, which is very variable and depends strongly on sensory perception and other factors like, for example, the degree of alertness (sleepiness).

Furthermore, it is not clear whether an exposure immediately before falling asleep might influence the sleep. For this reason, exposure started before sleep began.

Limit values generally provide sufficient protection against known health impact due to electromagnetic fields. There is always a safety factor between thresholds for proven health consequences and limit values.

For the general public this safety factor is higher in order to protect especially sensitive individuals (children, adolescents, old and sick persons) as well.

Occupationally exposed persons normally are healthy adults, especially sensitive population groups do not need to be considered. Occupational exposure is not permanent, but restricted to working time. For these two reasons, a lower safety factor is used for occupationally exposed persons.