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.