originally appearing in The Lancet 2000; 356: 1833-36
by G. J. Hyland
Department of Physics International Institute of Biophysics
University of Warwick Neuss-Holzheim,
Coventry, UK Germany
Although safety guidelines--to which mobile telephones and their base-stations conform--do protect against excessive microwave heating, there is evidence that the low intensity, pulsed radiation currently used can exert subtle non-thermal influences. If these influences entail adverse health consequences, current guidelines would be inadequate. This review will focus on this possibility. The radiation used is indeed of very low intensity, but an oscillatory similitude between this pulsed microwave radiation and certain electrochemical activities of the living human being should prompt concern. However, being so inherently dependent on aliveness, non-thermal effects cannot be expected to be as robust as thermal ones, as is indeed found; nor can everyone be expected to be affected in the same way by exposure to the same radiation. Notwithstanding uncertainty about whether the non-thermal influences reported do adversely affect health, there are consistencies between some of these effects and the neurological problems reported by some mobile-telephone users and people exposed longterm to base-station radiation. These should be pointers for future research.
Public concern over possible adverse health impacts from exposure to the radiation used in GSM (Global System for Mobile communication) mobile telephony shows little sign of abating, despite assurances from the industry and official bodies such as the UK National Radiological Protection Board (NRPB) that all is well. In March, 1999, the UK Government set up the Independent Expert Group on Mobile Phones, under the chairmanship of Sir William Stewart. The Stewart Report,1 published in May, 2000, makes some sensible recommendations, but unfortunately some of its greyer areas are now being exploited by the industry to obfuscate the issue. As yet unresolved is the question of adverse health impacts provoked by the contentious non-thermal effects of the low intensity, pulsed microwave radiation (MWR) used. For these effects are not taken into account in current safety guidelines,2 which simply restrict the intensity of the radiation to prevent tissue heating in excess of what the body's thermoregulatory mechanism can cope with. Whilst these guidelines, which are the result of careful investigation over many years, are clearly necessary, the question remains as to whether they are comprehensive enough. For in the case of living systems (and only living ones) there are many reports over the past 30 years that MWR can exert non-thermal influences, at intensities well below those necessary to cause any detectable heating.3
The purpose of this review is to introduce clinicians to the physics of mobile telephony and to explain how low-intensity, pulsed microwaves can affect living organisms, both thermally and non-thermally; and then to identify some of the reported biological impacts of exposure to this radiation, particularly those provoked by the contentious non-thermal effects. It is thereby hoped to alert clinicians to the possibility that certain presenting symptoms might well be a consequence of non-thermal exposure to this kind of radiation. A companion Lancet review4 covers the epidemiological evidence for effects of mobile telephony on human health.
Physics of mobile telephony
Mobile (cellular) telephony is based on two-way radio communication between a portable handset and the nearest base-station. Every base-station serves a cell, varying from hundreds of metres in extent in densely populated areas to kilometres in rural areas, and is connected both to the conventional land-line telephone network and, by tightly focused line-of-sight microwave links, to neighbouring stations. As the user of a mobile phone moves from cell to cell, the call is transferred between base-stations without interruption.
The radio communication utilises microwaves at 900 or 1800 MHz to carry voice information via small modulations of the wave's frequency. A base-station antenna typically radiates 60 W and a handset between 1 and 2 W (peak). The antenna of a handset radiates equally in all directions but a base-station produces a beam that is much more directional. In addition, the stations have subsidiary beams called side-lobes, into which a small fraction of the emitted power is channelled. Unlike the mean beam, these side-lobes are localised in the immediate vicinity of the mast, and, despite their low power, the power density can be comparable with that of the main beam much further away from the mast. At 150-200 m, for example, the power density in the main beam near ground level is typically tenths of a µW/cm2.
A handset that is in operation also has a low-frequency magnetic field associated, not with the emitted microwaves, but with surges of electric current from the battery that are necessary to implement "time division multiple access" (TDMA), the system currently used to increase the number of people who can simultaneously communicate with a base-station. Every communication channel has eight time slots (thus the average power of a handset is 1/8 of the peak values cited above--ie, is between 0·125 W and 0·25 W), which are transmitted as 576 µs bursts. Together, the eight slots define a frame, the repetition rate of which is 217 Hz. The frames transmitted by both handsets and base-stations are grouped into "multi-frames" of 25 by the absence of every 26th frame. This results in an additional low-frequency pulsing of the signal at 8·34 Hz, which, unlike that at 217 Hz, is unaffected by call density, and is thus a permanent feature of the emission. With handsets that have an energy-saving discontinuous transmission mode (DTX), there is an even lower frequency pulsing at 2 Hz, which occurs when the user is listening but not speaking.
Biological impacts: thermal
Heating of biological tissue is a consequence of microwave energy absorption by the tissue's water content. The amount of heating produced in a living organism depends primarily on the intensity (or power density) of the radiation once it has penetrated the system, on certain electrical properties of the biomatter, and on the efficiency of the body's thermoregulation mechanism. Above a certain intensity of the microwaves, temperature homoeostasis is not maintained, and effects on health ensue once the temperature rise exceeds about 1°C. Safety guidelines impose upper limits on the radiation intensity to ensure that this does not happen. Heating occurs whether the organism is alive or dead. The frequency of the radiation, as opposed to the intensity, is taken into account only in so far as it affects (via size resonance) the ability of the organism to absorb energy from the irradiating field.
Amongst the most thermally vulnerable areas of the body,2 because of their low blood supply, are the eyes and the testes, and cataract formation and reduced sperm counts are well-documented acute exposure hazards. Animal studies indicate that a variety of behavioural and physiological disorders can be provoked by temperature rises below 1°C--ie, under much less acute exposure conditions.
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