Electromagnetic radiation and health risks: cell phones and microwave radiation in New Zealand.
Electric fields arise from electric charges at rest, while magnetic fields arise from charges in motion. Acceleration and deceleration of electric charges gives rise to electromagnetic radiation.
This radiation takes the form of waves of electric and magnetic energy that move out in space, traveling at the speed of light. Although their speed through space is fixed, these waves vary in frequency.
Radiation is classified according to its frequency, and the different classes may be arranged along a continuum, called the electromagnetic spectrum. At one end of this spectrum is high frequency radiation such as x-rays, cosmic rays and nuclear radiation, and at the other end is low frequency radiation such as AC power. At the lower end are frequencies less than 1 wave/second, Hz. Towards the centre of the spectrum is visible light and infrared, while microwave and radiofrequency are just below this.
An important distinction is made between ionizing radiation (frequencies above about [10.sup.16] Hz) and non-ionizing radiation (frequencies below this figure). This is because the energy in ionizing radiation is sufficient to knock electrons off molecules, creating free radicals which in living organisms may be directly harmful. The harmful effects of non-ionizing radiation are caused by different means, including the heating effect of molecular excitation.
The cause of biological effects by exposure to microwave and radiofrequency radiation at frequencies greater than 30 Mhz, too weak to cause heating, are poorly understood at present.
Measuring Electromagnetic Radiation
The electric field is measured in Volts/metre and the magnetic field in Amps/metre, however it is more convenient simply to measure the amount of radiated power. This is known as power density, and is usually measured in units of Watts/square metre, but in this document it is more convenient to use the smaller unit of microwatts/square centimetre ([micro]w/[cm.sup.2]), (1 W/[m.sup.2] = 100 [micro]W/[cm.sup.2]).
Sources of Radiofrequency Radiation
There is some background radiofrequency/microwave (RF/MW) radiation from the earth, from living organisms, and from extraterrestrial sources. Levels vary according to atmospheric factors and variation in the magnetic properties of the ionosphere, but typically they are extremely small relative to average field levels from devices used for telecommunication or navigational purposes. Sources include medical applications such as diathermy and magnetic resonance imaging (MRI), industrial applications such as heat sealing, detection applications such as security alarms and radar, and an enormous range of telecommunications devices.
Many of these applications create fields that are confined to the immediate vicinity and are of little consequence to the wider environment. Significant exposure of the general public arises mainly from telecommunications, especially radio and TV broadcasting. In fact, the purpose of broadcasting is to reach as many radio or TV receivers as possible. The field strengths required to operate receivers are quite small, but there will necessarily be much larger fields near the transmitter. Cellular phone networks use relatively low-powered transmitters to restrict coverage to a circumscribed locality and thereby enable particular carrier frequencies to be used simultaneously at different cell sites in the same general area. Compared with TV and radio broadcasting, the radiation power levels near cell sites are therefore relatively small. Broadcast transmission antennae are designed to confine the radiation so that it doesn't go in directions where it is not required or not wanted.
Biological Effects of Radiofrequency Radiation
Studies of biological effects of RF/MW exposure began in the 1950's and by 1990 there were over 10,000 published studies worldwide (1). There are numerous international scientific journals that publish reports or studies in this area, and several that are confined only to this area. Dozens of books have been written on the topic and several reviews have been published which have attempted to analyse and summarise the findings of this enormous body of research. In 1993, over 30 conferences were planned for the dissemination and discussion of the results of newly completed research on biological effects of electromagnetic fields. This area of research is developing rapidly, and it is generally accepted that there is much more to learn than already is known.
Numerous effects have been described, but most are consistent with the view that the heating (thermal) effect of the field is responsible. However, a number of recent studies have demonstrated effects under conditions that seem to rule out thermal mechanisms. shown include the following:
* Changes in cell membrane permeability to potassium, sodium, and calcium
* Changes in composition or behaviour of blood-forming and immunological cells
* Alteration of calcium ion exchange in nerve tissue
* Changes in the firing pattern of neurons
* Changes in levels of cancer-related enzymes
Some of these results are disputed, and have not been reproducible in other laboratories, or no one else has tried to replicate the work.
Studies of Living Animals
Studies in which living animals are exposed to RF/MW radiation do not permit the same rigorous control of experimental variables that is possible in in-vitro studies, but they have the advantage that a whole living organism is being studied, making clearer the possible health significance of any effects that might be found. The majority of studies found effects only under exposure conditions that involved significant heating of the whole animal.
Laboratory Studies of Humans
There are a few studies of controlled exposure of human volunteers for brief periods. These studies have established thresholds for feeling warmth and pain due to RF/MW radiation, and for the peculiar phenomenon of microwave hearing, in which individual pulses of RF/MW are experienced as clicks, buzzes or chirps.
Accidental Overexposures of Humans
There have been several surveys and reports of brief exposures to radiation levels above the recommended safety limits. The reported health effects include severe anxiety, hypertension, headache, nausea and fatigue.
Epidemiological Studies of Human Populations
There are numerous epidemiological studies of groups of humans who are known to have been exposed, usually in their work, to higher than background levels of RF/MW radiation.
In these studies, health-related data usually comes from medical records, questionnaires completed by the workers, and physical examinations. Estimation of actual exposure levels is usually a problem in these retrospective studies, because it has to be estimated from records or calculated from transmitter data. There is also a problem locating control groups who are equivalent to the exposed workers with respect to other risk factors. These problems create doubt about the actual levels of exposure that might be responsible for any health effects that may be found, and limit the certainty that health effects are due to radiation exposure rather than to some confounding variable such as work stress or chemical exposure. There is nevertheless some indication that chronic exposure may increase the incidence of physical symptoms such as heart disease, cancer, birth abnormalities, pregnancy miscarriage, memory problems and lens opacities. Subjective symptoms include neurasthenia, headache, irritability, sleep loss, and concentration problems.
Conclusions Regarding Biological Effects
There is clear evidence of a range of biological effects, including effects adverse to the health of exposed animals and humans, resulting from radiation doses at levels high enough to cause tissue heating (the so-called thermal threshold). However, there is disagreement among scientists about whether there is conclusive evidence of adverse effects of doses below this threshold.
Exposure Standards (2)
There is a need to set limits on the amount of exposure to radiant energies individuals can accept with safety. The objective of protection is to prevent injury to people. Protection standards should be based on scientific evidence, but are quite often the result of empirical approaches to various problems reflecting current qualitative and quantitative knowledge.
In most biological processes, there is a certain range between those levels that produce no effects and those that produce detectable effects. A detectable effect is not necessarily one that is irreparable or even a sign that the threshold for damage has been reached. Ultimately, a clear differentiation has to be made between biological effects per se that do not result in short-term or latent functional impairment of normal body activity either temporarily or permanently.
"Effect" is a neutral word, implying neither-benefit nor harm. Confusion has arisen because of the frequent implication that an "effect" is per se deleterious.
There are profound differences of opinion as to what constitutes a no-effect level of an environmental agent for a man. The preponderant opinion in the United States holds that slight deviations within homeostatic limits of biological change are not deleterious (Hatch, 1972). All necessary life processes required by living organisms are associated with perturbations of a steady state. For all bodily functions, there are constant deviations from a steady state. Such deviations represent necessary accommodative change to environmental alterations in its broadest sense (Dinman, 1973). There has been a rather uncritical tendency by some to interpret demonstrated biological response, of whatever kind and intensity, as evidence of impending loss of health and, in the extreme, to regard the response itself as a direct expression of injury (Hatch, 1973).
Microwave exposure standards are generally based, with some variation, on those developed in the United States, Russia, Poland, and Czechoslovakia. The U.S. Protection Guide of 10 mW/[cm.sup.2] was suggested about 20 years ago by Schwan and his associates, which was based on the "thermal load" that a standard (healthy) adult could tolerate and dissipate under usual environmental conditions without a rise in body temperature. Intensive investigation was subsequently carried out by the U.S. Department of Defense into the biological effects of microwave radiation (Michaelson, 1971). None of these investigations produced any evidence of a hazard at the proposed limit of 10 mW/[cm.sup.2]. Indeed, no conclusive evidence was established for any effect below the level of 100 mW/[cm.sup.2] that could be considered hazardous for man.
In the former USSR, the major scientific basis utilised for setting standards derived from reactions of the higher nervous system and physiological alteration. Feasibility did not seem to be considered in the standard-setting process, although there is some question as to whether such standards represent goals or working realities (Dinman, 1973). Because such minimal functional changes are considered as designating the borderline between harm and safety, and since a safety margin is then applied, former Soviet standards in general tended to be lower than those found in the United States.
The no-effect level is a puristic concept, because there is some biological response when the organism encounters exogenous material (Dinman, 1972; Hatch, 1972). Whereas in the United States it is clearly understood that such responses are not deleterious per se, in the USSR this was not explicitly recognised.
The New Zealand Standard
The residential safety limit for non-occupational exposure is defined by the interim New Zealand Standard NZS 6609.1 for any period up to and including 24 hrs per day. This standard set a level at one fifth (20%) of the occupational exposure limit for mean power flux density (1 mW/[cm.sup.2] for frequencies in the range 30 MHz - 300 Hz); this gives a maximum field strength of 0.2 mW/[cm.sup.2] (200 microwatts/[cm.sup.2] or 2 W/[m.sup.2]). This interim standard (AS 2772: 1990, NZS 6609:1990) set a higher level than the Eastern European and Soviet standards, however, it takes a position that is below most other Western standards, such as the 1982 & 1992 ANSI and the 1986 NCRP U.S. Standards that have set exposure limits based on established scientific evidence of adverse thermal driven effects on body tissues. This interim standard goes further than many of the Western standards, to recommend that wherever possible... "as the effects of such exposures to electromagnetic fields are only imperfectly understood, it is recommended that the levels of all electromagnetic fields to which people are non-occupationally exposed, should be kept as low as reasonably achievable."
All these issues came before the Auckland City Council Planning Commissioners when hearing applications for the establishment of cellphone repeaters. They found that as the New Zealand Standard is currently under review and that the Auckland City Council felt that the likely result would be a downward revision of maximum exposure levels for the general public, they would impose their own standard. "It has been suggested that industry (and consequently the people that it serves) would experience serious problems if a downward revision exceeded a factor of 4 and this may be a suitable pragmatic compromise between extreme caution and the practicalities of maintaining a functioning radio communication system." (3)
It was decided that in Auckland City the standard should be 50 microwatts/[cm.sup.2] rather than the 200 microwatts/[cm.sup.2] recommended by the interim New Zealand Standard NZS 6609.1.
The level of electromagnetic radiation at any point near a radio transmitter can be measured (detailed procedures for measurement are set out in the second part of the interim Standard, NZS 6609.2). However, the strength of the field can also be calculated if the performance characteristics of the transmitter are known.
Data in the report showed the measured and calculated field strengths in microwatts/[cm.sup.2] from a study of a Telecom Cellular repeater site at the Three Kings Shopping Centre. The levels of radiation below this typical cellphone repeater antenna, mounted 15-20 metres above the ground and connected to a 3x5 watt per channel repeater, were calculated. The highest anticipated power level on the ground was less than 5 microwatts/[cm.sup.2], even when hot-spots caused by reflections were considered. This is a further order of magnitude below the safety levels discussed above.
When actual measurements were taken at a point 2 metres above the ground, the highest measured level observed was 1.5 microwatts/[cm.sup.2] at a point 25 metres from the base of the tower. If this level is compared to the 200 microwatts/[cm.sup.2] recommended by the New Zealand Standard or the conservative level of 50 microwatts/[cm.sup.2] adopted by the Auckland City Council, it can be seen that the public can be reassured that there is little or no risk from these sites.
Professor Beale concluded his report to the Auckland City Council, "With regard to cellphone repeater sites, the likely maximum exposure level near a typical site of around 4 microwatts/square centimetre affords a reasonable margin of safety relative to known biological effects of continuous RF/MW fields, including athermal effects of uncertain biological significance. It conforms with adopted standards in all Western and most Eastern European countries. It is considerably lower than existing exposure levels in some residential areas in New Zealand."
From the information available, it is clear that radiofrequency radiation is a risk to health at high levels. It is also apparent that low levels are "safe." The precautionary approach has already been taken by incorporating a five times lower safety factor over the occupational exposure standard (which is set at a level ten times lower than that at which adverse effects would occur) into the residential standard with the possibility that this standard may be lowered further up to four times. Even if this were the case, the radiation levels emitted by cellphone repeaters would still comply, hence there is little to be achieved by individual local authorities adopting their own "standards."
The author would like to acknowledge the advice and comments received from Mr. Martin Gledhill at the National Radiation Laboratory in the preparation and reviewing of this paper.
Editor's note: This article was reprinted with permission from the New Zealand Journal of Environmental Health, April 1995, 18(2):7-9. It originally appeared as part one of a two-part series. Because we are only reprinting the first of this two-part series, portions of the article have been slightly modified to accomodate this format. Additionally, please note that there are a few sources referred to in the text of this article that are not listed in the references section. The Journal of Environmental Health was unable to obtain additional information regarding these sources.
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|Publication:||Journal of Environmental Health|
|Date:||Jul 1, 1996|
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