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 young 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. Older women were examined in a follow-up study. They showed more effects on the sleep EEG than men, but no health effects were observed.

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. Therefore, this group was examined at first.

In two follow-up studies, older people of both sexes were examined separately. The results were comparable to the results obtained in young men and no health relevant effects of the TETRA terminals were observed.

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

For the general public this reduction 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. Occupationally exposed persons are aware of the exposure and are informed about possible effects and protective measures. For these reasons, a lower reduction factor is used for occupationally exposed persons.

The radio terminals can be operated in different modes. The Trunked Mode Operation (TMO), i.e. using the base stations of the radio network, can be considered as typical. For this operation, there is currently no multi-timeslot allocation used and therefore the time averaged transmitting power equals 0,25 W. Under these conditions, the German occupational exposure limit (10 W/kg), as well as the limit recommended by ICNIRP (International Commission on Non-Ionizing Radiation Protection) for general public exposure (2 W/kg) is met for all considered scenarios. In most cases, the SAR is even well below this value. Only when using a handheld radio terminal inside a car in an untypical position close to the metallic coachwork, it is almost reached.

In Direct Mode Operation (DMO) without using the radio network, multi-timeslot allocation is possible, although according to the Federal Agency for Digital Radio of Security Authorities and Organisations (Bundesanstalt für den Digitalfunk der Behörden und Organisationen mit Sicherheitsaufgaben, BDBOS) rather untypical in daily use. In this case, the average transmitting power increases to 1 W. For all considered scenarios in this operation mode, the German occupational exposure limit (10 W/kg) is still met. The recommendation for the general public is however partly exceeded. This is especially the case with the handheld radio terminal in touch with the pinna and the helix antenna very close to the head (tilt position). If the handheld radio terminal is additionally operated inside a car, in an untypical position, where the handheld radio terminal is in contact with the head touching the metallic coachwork for more than 4,5 minutes, and all four time slots allocated, exposure can reach up to 80% of the occupational limit.

The mobile radio terminals in cars can be operated in direct mode with a comparatively high transmitting power of up to 10 W. Exposure of a person directly beside a car close to an external vehicle antenna, is within the given exposure limits. A direct contact with the antenna would, however, lead to significantly higher SAR values.

The maximum temperature rise in tissue due to the absorbed radiation energy in TMO-mode (0,25 W average transmitting power) is 0,25 K--Kelvin, which is below the 1 K--Kelvin temperature rise the ICNIRP guidelines are based on. This temperature rise is limited to the surface of the skin, namely on the pinna (typical phone use) and the tip of the nose (walkie-talkie scenario). Highest temperature increase in the eyes was found using the handheld radio terminals in front of the face, with a value of 0,075 K--Kelvin. Using the handheld radio terminal in cheek position results in a maximum temperature increase of 0,015 K--Kelvin in the nearer eye and only 0,001 K--Kelvin in the other.

Energy from high-frequency fields gives rise to tissue heating which might have detrimental effects on health. In order to exclude such detrimental effects, energy absorption is confined based on limits.

For the frequency ranges applied in mobile communications, the distribution of energy absorbed in biological tissue and the resulting rise in temperature have been thoroughly investigated within the scope of different international studies. However, in the frequency range used for TETRA, the data base is clearly smaller. The present project is intended to close the knowledge gaps, thus delivering a broader data base for risk estimates in this particular frequency range.

The radio sets actually applied are modelled using computer codes which allow for a realistic simulation of the time and space distribution of the resulting electromagnetic fields.

Subsequently, the distribution of these fields within the human body is calculated using high-resolution "body models". These body models include

• the anatomical structure of the human body based on magnetic resonance tomography images, and
• the dielectric properties of the different types of biological tissues.

As a result, such simulations reveal the distribution of the electric and magnetic field within the body.

The absorption rates (SAR values) are calculated using this distribution together with the material properties. Based on the result, a similar numerical procedure is applied to predict the rise in temperature due to the radiation energy absorbed.

Measurements of radiation exposure are carried out on measurement phantoms within which values of field strength are measurable. These are then converted into SAR values by making reference to the electrical and physical material properties of the tissue simulating liquid within the phantom. However, such phantoms are filled with a homogeneous fluid which corresponds to the actual properties of human tissue only on average. The real anatomical structure of the human body with its various organs and tissues and different blood perfusion is not taken into consideration.

Computer simulations provide very detailed information on the interior of the body, which is not directly accessible by means of measurement.

• The exposure of, and temperature rise in individual organs are calculable allowing for different perfusion rates with high local resolution.
• Specific usage scenarios can be emulated by simulations.

The computer simulation is verified by reference to measurements and is adapted to reality.

The first step is to check compliance with occupational protection limits for the frequency range used by TETRA when applying the radio sets under realistic working conditions assuming a realistic body model, viewing both typical applications and situations expected to entail particularly high radiation exposures (so-called "worst-case-scenarios").

The second step involves checking whether compliance with the limits actually ensures prevention of a potentially detrimental rise in temperature in the tissue. It is of particular importance that computer simulation permits to assess the rise in temperature even for very small tissue volumes. This is most notably relevant to the eye which is poorly supplied with blood on the one hand and may be exposed to higher field intensities during a phone call on the other hand.

The producers are obliged to assess the SAR values of the devices according to the European Standard "EN 62209-1" and to make sure that the radio sets used comply with the limits applicable for partial body exposure in the corresponding frequency range. (See also Specific Absorption Rates (SAR) for mobile phones.)

There are only a few studies dealing with the TETRA technology in the international literature. Comparable projects involving detailed studies of specific, realistic use cases based on computer simulation are not known to the BfS.

Terminals cause significantly higher exposures of users than base stations. Health-relevant effects, if any, are therefore expected from terminals. As a consequence, the present study will focus on the influence of terminals.

The Federal Network Agency (Bundesnetzagentur, BnetzA) is responsible for base station checks. Like for all stationary base stations transmitting at equivalent isotropic radiated power (EIRP) levels exceeding ten watts, a site approval by the Federal Network Agency is required for transmitters of BOS digital radio, too. BNetzA performs regular checks as to whether the requirements for site approval are observed.