- Forums - Sign Up - Reply - Search - Statistics -
www.mast-victims.org forum / Health / BIOLOGICAL EFFECTS OF TIME VARYING ELECTROMAGNETIC FIELDS
Author Message
Catherine
# Posted: 3 Jul 2006 04:40
Reply 


Select Committee on Science and Technology Appendices to the Minutes of Evidence Z J SIENKIEWICZ
ABSTRACT
Acute exposure to extremely low frequency electric and magnetic fields of sufficient field strength or flux density will result in the perception of surface charge (electric fields only) and in the induction of electrical potentials and currents in the body which will affect electrically excitable tissue such as nerves and muscles. Acute exposure to sufficiently intense radiofrequency and microwave radiation will induce heating, resulting either in detectable rises in tissue or body temperature or in responses for minimising the total heat load. Except for the perception of surface charge, fields of the magnitude necessary to induce these responses are unlikely to be encountered except in a few occupational or medical situations.
The effects described above are well understood. However, the vast majority of people are exposed to much lower field strengths. There are several possible areas of biological interaction at low levels of exposure which may have important health implications and about which our knowledge is limited. Mechanisms of interaction have been proposed but are not established. If there are such effects, then the evidence suggests that they are subtle and may well be masked by normal biological variation.
INTRODUCTION
An extensive and diverse literature has arisen during the last two decades in which a wide variety of biological effects and responses have been attributed to exposure to electromagnetic fields. Many of these effects are known to result from the induction of surface electric charge, induced electric fields and currents within the body, or from increases in thermal loading and elevated body and tissue temperature. These particular effects and responses are well-understood and have been used by various national and international organizations as scientific bases in setting guidelines and restrictions on exposure to electromagnetic fields for human populations (eg INIRC, 1988; Allen et al, 1991; NRPB, 1993). In addition, a variety of biological responses, including a possible adverse effect on pregnancy outcome and increased risk of certain types of cancer have been reported at very low levels of exposure which appear to defy conventional explanation, as the induced fields will be so minute that they will be completely masked by thermally generated electrical noise. Possible interaction mechanisms have been suggested but none have been established experimentally in vivo.
In these lecture notes, the biological effects are discussed under three separate sections. The first section describes the well-established effects of exposure to fields with frequencies less than 100 kHz on the nervous system. The second section considers the biological effects of whole body and localised heating following exposure to RF and microwave radiation at frequencies above 100 kHz. The third section describes biological effects that are less well established, and includes effects on embryo and fetal development and cancer-related studies.
The information summarised in these notes is covered in more detail in NRPB, (1993), Sienkiewicz et al (1993), NRPB (1994), and Cridland et al (1996). Other reviews describing the general biological effects of electromagnetic fields and radiations are listed at the end of this chapter. The specific effects of electromagnetic fields on cells and cellular systems have recently been reviewed by Cridland (1993).
ELECTRIC AND MAGNETIC FIELDS OF FREQUENCIES OF LESS THAN 100 KHZ
Extremely low frequency (ELF) electric fields external to the body induce electric fields and currents within the body as a result of the time-variation of the induced surface charge density; the induced current density in general increases as the body cross section decreases and depends on the electrical properties of tissue. ELF magnetic fields induce circulating electric currents in such a way that the largest current densities tend to be induced in peripheral tissues, decreasing in magnitude towards the centre of the body.
Surface electric charge
Exposure to time-varying electric fields can result in perception effects due to the alternating electrical charge induced on the surface of the body. The perception results from the surface electrical charge causing, for example, body hairs to vibrate, and may become stressful if prolonged. About 10 per cent of adults can perceive power frequency electric fields of 10 to 15 kVm-1; the threshold for annoyance is around 15 to 20 kVm-1.
Nervous system responses
It is well established that electric currents applied directly to the body can stimulate peripheral nerve and muscle tissue, and such effects can prove fatal if breathing is inhibited or ventricular fibrillation induced. Threshold current densities for direct nerve or muscle stimulation are thought to be about 1-10 A m-2 between 10-1,000 Hz, rising progressively at frequencies above and below this range. Similarly, the exposure of volunteers to rapidly alternating (1 kHz) gradient magnetic fields in experimental magnetic resonance imaging systems has been shown to elicit nerve and muscle stimulation. In addition, short, rapidly changing magnetic field pulses have been used to stimulate nerves in a number of clinical studies.
Current densities at levels insufficient to stimulate excitable tissue directly may nevertheless affect ongoing electrical activity and influence neuronal excitability. The activity of parts of the central nervous system such as the spinal cord, the cerebellum and the cortex are known to be sensitive to the endogenous electrical fields generated by the action of adjacent nerve cells at levels below those required for direct stimulation. Neurophysiological studies of isolated nervous tissues suggest that threshold current densities may be as low as 100 mA m-2. However, even lower thresholds can be derived from studies of weak field effects on volunteers, perhaps reflecting the increased sensitivity of the intact organism compared to responses from isolated tissue. A large number of studies have reported volunteers experiencing faint flickering visual sensations—the magnetic phosphenes—during exposure to ELF magnetic fields above about 5 mT. These can also be induced by the direct application of weak electric currents to the head with threshold current densities of about 10-20 mA m-2. These results suggest a conservative estimate of 10 mA m-2 for the current density threshold for modulation of neuronal activity in the central nervous system. This value receives some support from a study of the effect of weak power frequency electric currents on human mental processes. Volunteers had electrodes attached to their head and shoulders through which weak electric currents were passed (current densities in the brain were estimated as around 10-40 mA m-2) and were required to perform a number of reasoning tasks whilst either exposed or sham exposed. The performance of most tasks was unaffected, but some changes were seen in alertness and the performance of a complex reasoning task. Unfortunately, the design of the study made it difficult to draw any definite conclusion.
Field-induced changes in neuronal excitability within the central nervous system could be responsible for producing observable changes in behaviour. However, exposure of rodents or non-human primates to 60 Hz electric fields of up to 30 kV m-1 does not appear to reduce the performance of learned tasks. Other effects on behaviour observed during exposure of animals to an electric field, such as transitory changes in arousal or activity in rodents, or in the social behaviour of baboons are more likely the result of stress caused by perception of the field by cutaneous stimulation. Similarly, exposure of rodents to magnetic fields does not cause long-term changes in behaviour, although some studies suggest transient changes in the performance of specific learning tasks with fields of about 1 mT.
Exposure has also been reported to result in mild changes in cardiac function in human volunteers; the resting heart rate was found to be slightly reduced (by about 3-5 beats per minute, well within normal values) during or immediately after exposure for 2-6 h to power frequency electric and magnetic fields of 9 kV m-1 and 20 ”T, but not after exposure to stronger or weaker fields. The small magnitude and transitory nature of this effect, however, does not suggest a health risk.
RADIOFREQUENCY AND MICROWAVE RADIATION OF FREQUENCIES ABOVE 100 KHZ
The heating effects of radiofrequency (RF) and microwave radiation are well established; the total whole body heat load experience during RF exposure is the sum of the SAR and the endogenous rate of heat production. The latter varies in normal individuals from about 1 W kg-1 at rest, to about 10 W kg-1 for short periods during hard physical exercise. Power deposition within the body is never uniform; differences in the electrical properties of tissues and the reflection and refraction of radiation at the interfaces of tissues of different electrical properties can result in localised SAR "hot spots". In addition, differences in local blood perfusion will affect heat dissipation characteristics, and some tissues are more sensitive to raised temperatures than others, and/or may be less able to effect repair.
Whole-body responses
Animals, like humans, use various physiological and behavioural mechanisms in order to regulate body temperature. The responses of the thermoregulatory system to irradiation are well established and include altered rates of metabolic heat production, food intake, activity, the vasodilation of superficial blood vessels and the behavioural selection of appropriate ambient temperatures. Thresholds for such responses have been reported in rodents and primates between about 0.3 W kg-1 and 5 W kg-1. The magnitudes of these responses appear to depend on the frequency of irradiation used, and orientation of the exposed animal with respect to the applied electric and magnetic field. These are caused by differences in power absorption by the deeper ti

Catherine
# Posted: 3 Jul 2006 04:44
Reply 


Whole-body responses
Animals, like humans, use various physiological and behavioural mechanisms in order to regulate body temperature. The responses of the thermoregulatory system to irradiation are well established and include altered rates of metabolic heat production, food intake, activity, the vasodilation of superficial blood vessels and the behavioural selection of appropriate ambient temperatures. Thresholds for such responses have been reported in rodents and primates between about 0.3 W kg-1 and 5 W kg-1. The magnitudes of these responses appear to depend on the frequency of irradiation used, and orientation of the exposed animal with respect to the applied electric and magnetic field. These are caused by differences in power absorption by the deeper tissues of the body; in particular, the less effective stimulation of the temperature receptors in the skin results in less effective thermoregulation.
The performance of learned tasks seems particularly sensitive to RF radiation; thresholds for decreased performance in both rats and primates have been reported as lying between 2.5 and 8 W kg-1; concomitant rises in rectal temperature were around 1șC. Exposure to RF radiation can also modify the action of drugs whose effectiveness can be altered by heat-induced changes in body physiology; changes have been recorded in the duration of barbiturate-induced anaesthesia and in the permeability of the blood-brain barrier, but only at levels of irradiation sufficient to raise body temperature.
Other easily demonstrable effects are generally consistent with responses to non-specific stressors such as heat. The acute exposure of primates to microwaves or RF at SARs of 3 to 4 W kg-1, sufficient to raise rectal temperature by 1 to 2șC, resulted in increased stress hormone (plasma cortisol) levels; similar effects have been reported in rats. In addition, microwave-induced changes have been reported in the levels of circulating white blood cells (increased levels of neutrophils and decreased lymphocyte levels) in rats and mice following thermal exposures which are similar to the changes induced by the injection of stress hormones, suggesting a common aetiology. Reported changes in white blood cells (natural killer cell and macrophage) activity have also been linked to heat induced stress.
Similar sorts of changes could be expected in humans following RF-induced increases in body temperature. It is known that most healthy people can tolerate short-term rises in body temperature by up to about 1șC, although individuals vary widely in their ability to tolerate increased body temperatures; some individuals cannot tolerate rectal temperatures of 38șC, others continue to perform well even at higher temperatures. However, prolonged exposure at body temperatures in excess of 38șC is known to increase the risk of heat exhaustion and reduce mental performance. Experiments have been carried out with volunteers which investigated the relationship between whole body SAR, body temperature rise and the ensuing physiological responses. One study reported that following exposure to RF magnetic fields at a whole body SAR of 3W kg-1 for 20 minutes, body temperature was increased by up to 0.7șC without having stabilised and the resting heart rate was elevated by up to 45 per cent. The total heat load experienced by the volunteers, resulting from the sum of the SAR and metabolic heat production, can be estimated as about 5 W kg-1, which represents a typical heavy workload for many industrial jobs. Adverse environmental conditions and moderate physical exercise will reduce the tolerable level of RF or microwave energy absorption, whilst people under medication or with clinical conditions which compromise thermoregulation may be more sensitive to RF or microwave induced heating.
Localised responses
The lens of the eye is regarded as potentially sensitive to microwave irradiation because of its lack of a blood supply and consequent limited cooling ability, and its tendency to accumulate damage and cellular debris. In anaesthetized rabbits, high local temperatures induced by acute exposure of the head to microwave radiation between about 1 and 10 GHz have been shown to induce lens opacities (cataracts); the threshold SAR in the lens was between about 100 to 140 W kg-1. Primate eyes were found to be less susceptible to cataract induction, possibly because they are more recessed in the skull, and so better shielded, and have thinner lenses which can dissipate heat more effectively. However, thresholds for chronic exposure have not been defined. In humans, cataracts have been historically associated with chronic exposure to infrared radiation, indicating that some degree of caution should be exercised.
Testicular temperatures are normally several degrees centigrade below body temperature, and it has been known for some time that male germ cells are sensitive to elevated testicular temperatures. Two animal studies have reported that chronic exposure at about 6 W kg-1 may have resulted in transient infertility in male rats. In one study, body temperature rose by about 1.5șC during exposure. In the other study, testicular temperature rose to about 37.5șC, a rise of about 3.5șC which was considered to be the minimum exposure required to cause a slight loss of male fertility in rats. In humans, it has been reported that repeated heating of the testis by 3-5șC will result in a decreased sperm count persisting for several weeks.
Heat has been shown to be teratogenic in various animal species including primates and has been associated with central nervous system and facial defects in children whose mothers developed moderate to severe hyperthermia, especially during the first trimester of pregnancy. The embryo and fetus may be particularly sensitive to RF-induced heating, since heat loss across the placenta will be less effective than heat exchange in other, well vascularised tissues. In rats, acute exposure at 11 W kg-1, raising maternal temperatures to 43șC, was sufficient to induce embryo and fetal death and developmental abnormalities; chronic exposure at 6 to 7 W kg-1, usually raising maternal temperatures to between 39 and 41șC, was reported to induce growth retardation and subtle behavioural changes. In general, even prolonged exposure at less than 4 W kg-1 had no effect.
Surface heating
The absorption of RF and microwave radiation can be detected by temperature sensitive receptors in the skin. Power flux densities of around 300 W m-2 at 3 GHz have been detected experimentally during exposure for 10 seconds; radiation of higher frequencies applied for similar lengths of time have been detected at lower power flux densities because of their greater absorption by the skin. It is considered that the avoidance of the perception of skin warming for frequencies where the penetration depth is greater than the thickness of the skin (ie frequencies below about 10 GHz) does not provide a reliable mechanism of protection against potentially harmful exposure. The avoidance of the perception of skin warming may give adequate protection at frequencies above 10 GHz, although its general effectiveness may be reduced if other factors in the environment compete for attention.
Pulsed radiation
People with normal hearing have perceived pulse-modulated RF radiation of frequencies between about 200 MHz and 6.5 GHz; the sound has been variously described as a buzzing, clicking, hissing or popping noise, depending on modulation characteristics. Prolonged or repeated exposure may be stressful. It seems most likely that the sound results from the thermoelastic expansion of brain tissue following a small but rapid increase in temperature on the absorption of the incident energy. The perception threshold for pulses shorter than 30 ”s depends on the energy density per pulse and has been estimated as about 400 mJ m-2 at 2.45 GHz, corresponding to an estimated specific energy absorption in the head of about 16 mJ kg-1. However, a reduction in ambient noise has been reported to reduce this to about 280 mJ m-2, an SA of 10 mJ kg-1 at 2.45 GHz.
Exposure to very intense pulsed radiation has been reported to suppress the startle response and evoke body movements in conscious mice. Specific absorptions were 200 mJ kg-1 [for 1 ”s pulses] and 200 J kg-1 [for 10 ”s pulses] for suppression of the startle response and evoked body movement respectively. The mechanism for these effects is not well established although they may well be related to microwave hearing; auditory thresholds for rats are several orders of magnitude lower, about 1-2 mJ kg-1 per pulse (<30 ”s) for rats.
POSSIBLE EFFECTS OF TIME VARYING ELECTROMAGNETIC FIELD EXPOSURE
The effects of exposure to time-varying electromagnetic fields (EMFs) that have been described so far are well established and fairly well understood. There are in addition a large number of biological effects that have been reported in cell cultures and in animals, often in response to relatively low field levels, which are not well established but which may have health implications and are subject of much ongoing research. These include research on the effects of ELF fields on the body's daily (circadian) rhythms and on growth and development of the embryo and fetus, on the effects of ELF fields and RF radiation on carcinogenic processes, on the existence of specific frequency and amplitude "window" effects, and on the effects of low level pulsed RF radiation. Studies on the possible effects on embryo and fetal growth and development, and on carcinogenic processes can be linked to epidemiological studies of pregnancy outcome and the risk of cancer.
Circadian rhythms and melatonin secretion
Some evidence has been published to suggest that chronic exposure to ELF electric fields can modulate specific circadian rhythms, natural daily cycles of body function such as the secretion of various hormones, levels of alertness, activity and body temperature which continue even when normal "time signals", such as light and dark periods, are removed. In animals, such effects have been found after exposure to power frequency elect

Catherine
# Posted: 3 Jul 2006 04:48
Reply 


Circadian rhythms and melatonin secretion
Some evidence has been published to suggest that chronic exposure to ELF electric fields can modulate specific circadian rhythms, natural daily cycles of body function such as the secretion of various hormones, levels of alertness, activity and body temperature which continue even when normal "time signals", such as light and dark periods, are removed. In animals, such effects have been found after exposure to power frequency electric fields, but at levels well above threshold for the perception of the surface electric charge. In humans, one study has suggested that very low level (2.5 V m-1) 10 Hz, square wave electric fields could alter the sleep and activity patterns of volunteers living in environments artifically isolated from normal temporal cues. However, the study was carried out in the presence of unquantified power frequency electric and magnetic fields and it is difficult to draw any firm conclusions from the results.
Exposure to electromagnetic fields has been reported to inhibit the night-time peak in the synthesis of melatonin, believed to be a natural inhibitor of certain tumours. Hence it has been suggested that this may be a route by which ELF fields could influence tumour progression. However, although several groups have reported the field-dependent reduction in melatonin, this effect has not always been successfully replicated, and the link between exposure to electromagnetic and melatonin inhibition remains tentative.
Reproduction and development
A large number of animal studies have been carried out which have investigated the potential for ELF and very low frequency (VLF) electric and magnetic fields to affect embryo and fetal development. Most well-conducted studies of the effects of exposure to power frequency electric fields on mammalian development have reported the absence of consistent and reproducible teratogenic responses. A number of studies have reported abnormal development of chick embryos exposed to pulsed or sinusodial magnetic fields greater than about 1 ”T at frequencies between 10 and 1,000 Hz. In contrast, other studies, including one which examined the effect of exposure to VLF sawtooth magnetic fields characteristic of visual display unit emissions, failed to find such effects. A large scale study in which replicate experiments were carried out in six different laboratories recently failed to establish that such effects occur reproducibly. Studies of possible magnetic field exposure on mammalian development are more relevant to humans. In general, most studies report a lack of effect on any developmental end-point; the few positive effects reported are not consistently demonstrated in different studies and may perhaps reflect the spurious significance seen when large numbers of parameters are analysed separately. Most studies of mice and rats exposed throughout gestation to power frequency fields up to 20 mT or VLF sawtooth magnetic fields of up to 200 ”T have not found decreased levels of post-implantation survival. Similarly, most studies of rodents exposed during gestation to power frequency magnetic fields of up to 20 mT, or VLF fields of up to 200 ”T have found no significant effects on the incidence of gross external abnormalities, nor on the incidence of visceral or skeletal abnormalities, although several studies have observed an increased number of skeletal variants which also occur spontaneously. In addition, studies report a lack of consistent effect on postnatal developmental and juvenile and adult behaviours.
Carcinogenesis
There is much experimental evidence that electromagnetic fields cannot cause genetic damage and it is therefore extremely unlikely that they could have any effect on the initiation of cancer. It is generally accepted that if ELF fields do affect carcinogenesis it is likely to be at the level of promotion, possibly stimulating the proliferation of potentially malignant cells. Studies at the cellular and subcellular levels have investigated the possibility that exposure to ELF fields may result in interactions at the cell membrane which trigger cell signalling pathways leading to increased cell division. Other studies have looked for the effects of ELF fields on the expression of proto-oncogenes known to be involved in regulating cell growth. Positive effects have been reported in some laboratories, although some of these results have been small and variable. In general, however, the more carefully conducted studies have failed to find any clear effect of exposure to ELF magnetic fields. In the RF region, there are some data from cellular studies using an atypical mouse fibroblast cell line that exposure may induce dose-dependent DNA changes leading to uncontrolled cell proliferation which can only be revealed by the concurrent action of a chemical promoter. The relevance of these results to normal human cells is, however, questionable.
A number of large scale animal carcinogenesis studies have been completed in which animals have been exposed to power frequency magnetic fields, sometimes in conjunction with chemical carcinogens and promoters in order to test for any promoting or co-promoting activity. Studies of tumour promotion have generally yielded equivocal results, although an increased incidence of mammary tumours has been reported in two studies of rats treated with a chemical carcinogen and exposed to magnetic fields. A co-promoting activity, in which exposure to magnetic fields enhanced the effect of a carcinogen in the presence of sub-optimal levels of a chemical promoter, has been reported but the authors have experienced difficulty in repeating their own studies. Overall, the available experimental evidence remains contradictory and does not provide a clear indication that ELF electromagnetic fields affect tumour promotion.
Few large scale animal carcinogenesis studies have been carried out with RF and microwave radiation. One study, in which rats are exposed for most of their lifetime to low level pulsed microwave radiation, reported that the exposed group had a significantly higher incidence of primary malignant tumours compared to the control group. However, the tumour incidence in the exposed group did not appear enhanced compared to values reported elsewhere in stock rats of the same strain; rather, values were low in the control group. Thus, these data do not provide clear evidence of an increase in tumour incidence as a result of exposure to microwaves. Other studies have reported that the chronic microwave exposure of mice at high SARs (2 to 8 W kg-1) resulted in an increase in the progression or development of spontaneous [mammary] or chemically-induced [skin] tumours; heating effects, however, could not be ruled out.
Frequency/amplitude specific effects
The possibility that only certain combinations (windows) of EMF frequency and amplitude could elicit biological effects has been reported in studies in which exposure to very low levels of amplitude-modulated RF radiation, too low to involve heating, altered the brain activity in cats and rabbits, the activity of an enzyme involved in tumour promotion, and was shown to affect calcium ion mobility in brain tissue in vivo and in vitro. Effective SARs in vitro were less than 0.01 W kg-1 occurring within modulation frequency windows usually between 1-100 Hz and sometimes within power flux density windows. The changes in calcium ion mobility have not been easy to corroborate, two groups have failed to observe these effects in similar studies. It has been suggested that the observed responses may result from the amplitude modulation signal, rather than the RF carrier; similar effects have been reported at ELF frequencies. The biological responses to low level ELF electric fields (less than .100 V m-1) include the altered mobility of calcium ions in chick and cat brain tissue, changes in neuronal firing patterns in rodents and in electricity activity of the brain, and changes in the operant behaviour of non-human primates. Extension of the work which identified changes in the exchange of 45Ca2+ in isolated chick brain in response to very weak electromagnetic fields led to the proposal of resonant interactions in which it has been suggested that the local geomagnetic field plays a role in transduction. It has been reported that combined static and time varying magnifying fields can affect calcium-dependent movement of marine micro-organisms or the uptake of radio-labelled calcium ions when the ELF frequence is at an appropriate resonance frequency for calcium ions. In addition, other recent experiments report that the performance of an operant task and a spatial memory task are impaired in rats only during exposure to combined static and resonant ELF magnetic field.
Low level pulsed RF effects
Recent well-conducted studies by one group of research workers suggest that the retina, iris and corneal endothelium of primate eyes are susceptible to low-level microwave irradiation, particularly to pulsed radiation. Various degenerative changes, particularly of the light-sensitive cells in the retina, have been reported; specific energies per pulse [10 ”s pulses at 100 pulses per second] were 26 mJ kg-1 and even as low as 2.6 mJ kg-1 after the application of a drug used in the treatment of glaucoma. Exposure to low levels of pulsed or continuous wave RF or microwave radiation have been reported to affect neurotransmitter metabolism and the concentration of receptors involved in stress and anxiety responses in different parts of the rat brain. For pulsed radiation, the threshold specific energy per pulse was approximately equal to the microwave auditory threshold.

Catherine
# Posted: 3 Jul 2006 04:50
Reply 


SUMMARY AND CONCLUSIONS
Many acute effects of exposure to electromagnetic radiation are relatively well established and are used as a basis in setting guidance and restrictions on human exposure (see table 1). The potentially adverse effects on vision and cognitive function during exposure to electric and magnetic fields at frequencies below about 100 kHz can be avoided by restricting the current density induced in the head and trunk. Above about 100 kHz many of the effects of acute exposure to sufficiently intense radiofrequency and microwave radiation result from induced heating. Adverse effects such as heat exhaustion and reduced mental performance resulting from whole body RF heating can be avoided by restricting the whole body SAR; in addition, adverse effects on heat sensitive tissues such as the brain, the lens of the eye and the developing embryo or fetus, and on less sensitive tissues of the trunk and limbs can be avoided by restrictions on the localised SAR in the tissue. Above about 10 GHz, recommendations pertaining to long wavelength infrared radiation are applicable since absorption is largely confined to the skin and cornea. In addition, it is recommended that conditions under which the auditory effect can be invoked by exposure to pulsed RF and microwave fields should be avoided, as should the annoying effects of power frequency electric fields caused by the direct perception of surface charge. In some cases, these effects may be avoided by engineering or administrative controls.
Most people are exposed at levels much too low for the effects described above to be significant. There is little biological evidence to suggest that exposure to low-level fields causes adverse health effects. In particular, pregnancy outcome in mammals does not appear to be affected by exposure to levels of low frequency electric or magnetic fields encountered in domestic and office environments. Recent evidence suggests that high peak power pulsed RF and microwave radiation may engender specific behavioural effects, possibly as a consequence of audition of the field; other effects of pulsed radiation at very low levels of exposure, such as degenerative changes in the tissues of the eye and changes in neurotransmitter metabolism, have yet to be confirmed. Much recent biological research has focused on carcinogenic processes, following a number of studies linking the incidence of some adult and childhood tumours to mostly surrogate estimates of electromagnetic field exposure such as job description. Electromagnetic fields are not mutagenic and so are unlikely to initiate tumours. Recent studies have mostly concentrated on aspects of cellular metabolism relevant to tumour promotion or on an enhancement of the incidence of spontaneous or chemically-induced tumours in animals. However, there is little persuasive evidence to suggest that electromagnetic fields are able to influence any of the accepted stages in carcinogenesis; few clearly reproducible effects are apparent. The results of the large scale animal studies are mostly equivocal, although tumour progression may be enhanced at thermally significant levels. Some data challenge the conventional assumption that the magnitude of an effect increases with increasing exposure. In general, however, the effects following exposure to low level fields have not been well established and they do not provide a basis for restrictions on human exposure.
Table 1
BIOLOGICAL BASIS FOR RESTRICTIONS ON HUMAN EXPOSURE—NRPB 19931

Dosimetric quantity Adverse effect Restriction

Frequencies of less than 100 kHz

Surface electric charge (50/60 Hz) Stress of direct perception Avoid2
Induced current density
(10 Hz-1 kHz) Visual and mental disturbance 10 mAm-2 (head and trunk)
Frequencies between 100 kHz and 10 GHz

Specific Absorption Rate (SAR) Body temperature > 38șC 0.4 W kg-1 (average over body)
Head and fetus > 38șC 10 W kg-1 (average over 10 g)
Trunk > 39șC 10 W kg-1 (average over 100 g)
Limbs > 40șC 20 W kg-1 (average over 100 g)
Frequencies between 10 GHz and 300 GHz

Power density incident on body Excess surface heating of the skin and cornea 100 W m-2
Pulsed radiation of frequencies between 100 MHZ and 10 GHz

Specific energy per pulse (SA) Stress of perception Avoid2


Notes:
(1) The full basic restrictions currently advised by NRPB are given in the accompanying notes[8].
(2) These effects can also be avoided by administration or engineering controls.

Catherine
# Posted: 3 Jul 2006 04:51
Reply 


FURTHER READING
Allen, S G, Bernhardt, J H, Driscoll, C M H, Grandolfo, M, Mariutti, G F, Matthes, R, McKinlay, A F, Steinmetz, M, Vecchia, P and Whillock, M. Proposals for basic restrictions for protection against occupational exposure to electromagnetic non-ionising radiations. Recommendations of an international Working Group set up under the auspices of the Commission of the European Communities. Physica Medica, VII, 2, 77-89 (1991).
Anderson, L E. Biological effects of extremely low-frequency electromagnetic fields: in vivo studies. Radiat Protect. Australia, 9, 98-108 (1991).
Bernhardt, J H. Non-ionizing radiation safety: Radiofrequency radiation, electric and magnetic fields. Phys Med Biol, 37, 807-844 (1992).
Cridland, N A, Electromagnetic fields and Cancer: A review of relevant cellular studies. Chilton, NRPB-R256 (London, HMSO) (1993).
Cridland, N A, Sienkiewicz, Z J, Kowalczuk, C I, and Saunders R D. Recent biological studies relevant to carcinogenesis. IN Biological Effects of Magnetic and Electromagnetic Fields (Ueno, S, ed). New York, Plenum Press (1996).
Elder, J A, and Cahill, D F, eds Biological Effects of Radiofrequency Radiation. North Carolina, Health Effect Research Laboratory, United States Environmental Protection Agency, EPA-600/8-83-026F (1984).
Grandolfo, M, Michaelson, S M, and Rindi, A, eds. Biological Effects and Dosimetry of Static and ELF Electromagnetic Fields. New York, Plenum Press (1985).
INIRC. Guidelines on limits of exposure to radiofrequency electromagnetic fields in the frequency range from 100 kHz to 300 GHz. Health Phys, 54, 115-123 (1988).
NCRP. Biological Effects and Exposure Criteria for Radiofrequency Electromagnetic Fields. Bethesda, Maryland, National Council on Radiation Protection and Measurements, NCRP Report No 86 (1986).
NRPB. Electromagnetic fields and the risk of cancer. Report of an Advisory Group on Non-ionising Radiation. Doc NRPB, 3, 1, (1992).
NRPB. Board Statement on Restrictions on Human Exposure to Static and Time Varying Electromagnetic fields and Radiation. Doc NRPB, 4, 5 (1993).
NRPB. Health effects related to the use of visual display units. Report of an Advisory Group on Non-ionising Radiation. Doc NRPB, 5, 2 (1994).
Sienkiewicz, Z J, Cridland, N A, Kowalczuk, C I, and Saunders, R D, 1993. Biological effects of electromagnetic fields and radiation. IN The Review of Radio Science 1990-1992 (Stone, W R, ed). New York, Oxford University Press, pp 737-770.
UNEP/WHO. Extremely Low Frequency (ELF) Fields. Geneva, World Health Organization Environmental Health Criteria 35 (1984).
UNEP/WHO. Magnetic Fields. Geneva, World Health Organization Environmental Health Criteria 69 (1987).
UNEP/WHO. Electromagnetic Fields (300 Hz-300 GHz). Geneva, World Health Organization Environmental Health Criteria 137 (1993).
http://www.parliament.the-stationery-office.co.uk/pa/cm199899/cmselect /cmsctech/489/489a12.htm

Your reply
Bold Style  Italic Style  Underlined Style  Image Link  URL Link 

» Username  » Password 
You can post anonymously by entering a nickname with no password (if that nickname has not been taken by another member) or by leaving both fields empty. If you have an account you can also log in from this page without posting a message.
 

These forums are running on forum script miniBB™ © 2001-2020