AGNIR Working Group
Publication date: 2001
This report reflects understanding and evaluation of the current scientific evidence as presented and referenced in the document. The report was first published on 31 July 2001, and has minor amendments as of 12 November 2001.
The version of the report published here includes a supplementary NRPB Technical Note on the characteristics of radio signals from base stations and mobile terminals using TETRA at 400 MHz.
This report by the National Radiological Protection Board's Advisory Group on Non-ionising Radiation gives advice on possible health effects of Terrestrial Trunked Radio (TETRA). It has been prepared, at the request of Government, as a consequence of a recommendation by the Independent Expert Group on Mobile Phones (IEGMP) in May 2000 that:
as a precautionary measure, amplitude modulation around 16 Hz should be avoided, if possible, in future developments in signal coding.
The IEGMP recommendation was made because of the results of a number of studies on the effects of radiofrequency (RF) fields on the rate of loss of radiolabelled calcium from brain and other tissues. These studies, most of which were carried out in the late 1970s and early 1980s on isolated tissues, had suggested that when the RF signal was modulated at around 16 Hz the rate of calcium efflux was increased. IEGMP concluded that although no obvious health risk was suggested, as a precautionary measure, amplitude modulation around 16 Hz should be avoided, if possible.
The system being used for commercial applications and by emergency services in the UK and in a number of other countries uses a network of base stations to serve terminals that are either vehicle mounted or in the form of separate handsets. Its operation results in power modulation of some of the RF signal at a pulse frequency of 17.6 Hz. As a consequence of the recommendation by IEGMP, concerns have been raised about the health implications of the use of this system.
In the UK a TETRA system is presently operated by Dolphin (for commercial use) and trials are under way by BT Airwave for the police and possibly for other emergency services. The system uses two carrier bands, between 380 and 395 MHz and between 410 and 425 MHz, although it is possible that other bands could be assigned if the demand grows. Fixed base stations serve mobile terminals that are in the form of hand portables (similar to mobile phone handsets) or built into vehicles (these are called mobiles by the police). The base stations provide the service either directly or indirectly via repeaters that are generally built into vehicles.The information from mobile terminals and repeaters is carried by the radio signal using phase modulation. The voice information is concentrated into bursts (or time-slots) 14.2 ms long, which occur every 56.7 ms (a frame) and this allows up to four users to communicate on the same frequency channel. This corresponds to a duty factor of a quarter and a pulse frequency of 17.6 Hz. It is notable that the signals from base stations are continuous, not pulsed, in contrast to those from mobile terminals and repeaters, which are pulsed.
In the most common mode of use, the network continually adjusts the power radiated by a TETRA mobile terminal to the minimum necessary to maintain radio-contact during a call. This technique of adaptive power control (APC), which conserves the battery of a hand portable, is also a feature of GSM (Global System for Mobile Communication) mobile phones used commonly in the UK and around the world.
Exposure to RF fields from the TETRA system can arise in a number of different ways. As hand portables are not intended for use by the general public, the predominant exposure from these devices will arise in the course of work. Similarly, occupational users will normally receive greater exposure from vehicle-mounted terminals than will the general public. Exposure of workers in other occupations may also occur in the vicinity of base station antennas, particularly when they are located at rooftop sites. The exposures of the general public, at normally accessible positions in the vicinity of TETRA base stations, will be small fractions of the exposure guidelines and will be comparable with exposures due to the ambient field strengths arising from the operation of other telecommunication systems.
Guidelines advising limits on exposure to RF radiation are expressed in terms of specific energy absorption rate (SAR). [SAR is the rate at which energy is absorbed by unit mass of tissue in an electromagnetic field and is measured in watts per kilogram (W kg -1). Relevant guidelines average exposures of the head over 10 g and 6 minutes.] Little work has, however, been published on the assessment of SAR in the head from TETRA hand portables under simulated conditions of use. The results of very limited experimental work known to the Advisory Group suggest that when the present generation of hand portables, with 1 W and 3 W power outputs, are operating at full power, the exposure of the head will be below the guidelines on exposure limits recommended for occupational exposure by NRPB and by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). Actual SAR values would commonly be less than those determined experimentally at maximum operating power, both because of APC and because most calls will be short. The average output powers of these hand portables are 0.25 W and 0.75 W; less than the 1.5 W average power output of the analogue hand portables presently being used by the police. This suggests that exposure from the TETRA hand portables will generally be less than that from the present system.
It is considered possible by the Advisory Group, that if future developments in the TETRA system required the use of more than one of the four time-slots in the frame (as for data or video transmission), then under these circumstances SARs could be up to four times larger and, for the 3 W hand portables, could exceed exposure guidelines for workers. The use of more than one time-slot could, in addition, change the frequency of pulse modulation. The guidelines could also be exceeded for a mobile terminal of 10 W and above if the head were placed near to the antenna. On the basis of existing data, careful thought needs to be given to the design and conditions of use of any future systems with respect to ensuring compliance with exposure guidelines.
The initial report describing the phenomenon of calcium efflux was published in 1975. It suggested that exposure of isolated chick brain hemispheres to RF fields modulated at around 16 Hz, at levels too low to cause bulk heating, nevertheless could cause a small increase in the movement of calcium ions. Subsequent studies, some by other research groups but with essentially the same methodology, reported similar findings using this and a variety of other preparations and exposure conditions. Generally, however, no obvious effects were found using the carrier frequency alone, and the phenomenon occurred only with particular combinations of modulating frequency, carrier frequency and power density, giving rise to particular SARs.
The existence of changes in calcium efflux, and their significance if they occur in living tissue, are much disputed. The design and interpretation of the early studies were not ideal and they were predominantly carried out using non-living tissue. Since the early 1980s a number of generally better designed studies have failed to detect an increase of calcium efflux from tissues as a result of RF exposure under a variety of conditions and modulations. If the phenomenon is biologically significant, concomitant changes would be expected in the functions of nervous tissues that depend on the movement of calcium ions, but none has been unambiguously shown to occur. For example, changes in neuronal excitability have been reported but mostly under conditions of exposure to RF fields sufficient to cause biologically significant heating. Similarly, changes in brain wave activity in animals may be largely attributable to subtle heating effects. Suggestions that pulsed RF fields could trigger epileptic fits or otherwise affect epilepsy sufferers appear to be unjustified.
There do not appear to be any studies on people that provide direct information on health effects of exposures to RF fields at about 16 Hz modulation. Overall, research on exposure to low level RF radiation in general has not provided any persuasive evidence that it causes disease in people. Although, when viewed as a whole, the epidemiological research that has been carried out does not give cause for concern, it has too many limitations to provide assurance that there is no hazard.
It is recognised that calcium plays an important role in many biological processes, especially in the function of nerve cells. Moreover, as the Independent Expert Group on Mobile Phones pointed out, there is evidence that RF fields, amplitude-modulated at about 16 Hz, may influence the leakage of calcium ions from tissues. However, findings have been contradictory: they are more uncertain for living than for non-living tissue, and no associated health risk has been identified. It is notable that the signals from TETRA base stations are not pulsed, whereas those from mobile terminals and repeaters are. Although areas of uncertainty remain about the biological effects of low level RF radiation in general, including modulated signals, current evidence suggests that it is unlikely that the special features of the signals from TETRA mobile terminals and repeaters pose a hazard to health.
The Home Office studies about TETRA Health and Safety include a response to this document.
Sir Richard Doll, Imperial Cancer Research Fund Cancer Studies Unit, Oxford
Professor A J Swerdlow, Institute of Cancer Research, London
Professor C Blakemore, University of Oxford
Professor L J Challis, University of Nottingham
Professor D N M Coggon, Medical Research Council, University of Southampton
Dr L A Coulton, University of Sheffield
Professor M D Rugg, Institute of Cognitive Neuroscience, London
Dr R D Saunders, National Radiological Protection Board, Chilton
Dr H Walker, Department of Health, London
Dr A F McKinlay, National Radiological Protection Board, Chilton
Dr C R Muirhead, National Radiological Protection Board, Chilton
Dr J W Stather, National Radiological Protection Board, Chilton
Mr S G Allen, National Radiological Protection Board, Chilton
Dr S M Mann, National Radiological Protection Board, Chilton
Dr Z J Sienkiewicz, National Radiological Protection Board, Chilton
Last reviewed: 20 October 2009