CSIRO - June 1994 BIOLOGICAL EFFECTS AND SAFETY OF EMR |
9.0 MECHANISMS OF INTERACTIONSUMMARYHeating effects of microwave radiation are reasonably well understood and lead to significant physiological effects. Difficulties exist in determining the in situ heating in relation to applied dose due to the heterogeneous nature of body tissues. However, non-thermal subtle effects are considerably more difficult to recognise and understand. It has been suggested that non-equilibrium processes are significant in the bioenergetics of living systems (Adey 1993), challenging the traditional approach of the chemistry of equilibrium thermodynamics. Rather than observing traditional dose-response effects, there have been a number of reports claiming both amplitude and frequency response "windows". The concept of an all-or-none effect at specific exposure conditions challenges conventional assumptions that the magnitude of a response increases with increasing "dose". If this can be reliably substantiated, it adds weight to the argument that there are significant as yet, unexplained, non-thermal mechanisms involved in biological effects of EMR. In recent years, a number of reports of effects of EMR have appeared which are incompatible with the concept of bulk heating and heat exchange. The altered flux of calcium ions across cell membranes has been commonly reported. The issue has not been resolved but the phenomenon may be due to molecular vibration of receptors rather than due to EM induced voltage activated effects (Moolenar et al 1986; Hoth & Penner 1992) on channels in cell membranes. Evidence exists that microwave radiation interacts directly with cell membranes to induce functional alterations in membrane components, including ATP-ase and ion channels. The role of free radicals is becoming appreciated from evidence of free-radical reactions in melanin-containing membranes leading to changes in membrane state. The reported effects are unexpected from the existing knowledge on physical interactions since they do not appear to be described by classical intensity- or dose- response relationships. It seems to be unlikely that a single biophysical interaction mechanism will be adequate to explain all of the reported non-thermal effects of RF and microwave radiation.
9.1 MECHANISMSAn important consideration in estimating the effects of dosimetry is the coupling of RF and microwave radiation to biological systems. This depends on the orientation of the subject (animal, human or culture vessel containing cells) relative to the field, and on its dimensions relative to the wavelength. Coupling to a body is maximal when its long axis is oriented parallel to the electric field and when its length is similar to the wavelength. Therefore, maximum (resonant) absorption for an exposed subject is frequency dependent and occurs at approximately 40 MHz for an average (electrically grounded) man, 600-700 MHz for a rat and 2500 MHz for a mouse whole body (Durney et al 1978). It is well known that the distribution of RFR in an exposed object depends on many factors including frequency, orientation of exposure, dielectric constant of the constituent tissue. The design of experimental protocols is critical if the results are to provide meaningful extrapolation to a particular RF source. Cellular telephones are used in a specific manner. Most people would hold a phone to the same ear in the same orientation and proximity to the skull. Usually one would expect the antenna to be close to the parietal bone (although many airport officials have a peculiar habit of holding the large portable phones in front of their mouth so that they look across the top of the antenna). However, assuming normal usage patterns it would make sense to design experiments so that the RF source was located towards the lateral aspect of the skull. Chou et al (1985a) found significant differences in local SARs in eight different regions of the brain of rats and these all changed in each of seven different exposure arrangements. Lai et al (1984a) reported a difference in microwave response with pentobarbital depending on whether the rat was facing toward or away from the source of irradiation in a waveguide when the average whole body SAR remained constant; patterns of energy absorption in the brain differed substantially. If the wavelength is smaller than the overall dimensions of the body, reflection and refraction of radiation at the interfaces of different tissues with different electrical properties, (air/skin boundary), can result in localised “energy hot spots”. These hot spots can occur within the whole body at frequencies near body resonance, or within parts of the body such as the head at higher frequencies up to about 2-3 GHz. As frequencies increase and wavelengths decrease, power absorption per unit mass of tissue increases and penetration decreases. Above 10 GHz, absorption would be expected to be largely confined to the skin. When the wavelength is larger than the exposed body, contact with other conducting bodies (including the earth) will cause induced electric currents to flow within the body and between the ground. Hot spots will be felt in regions of the body where the current flow is constricted by small cross-sectional areas, particularly in occupational exposures. Operators of RF sealer-welder equipment (13.5 or 27 MHz) have experienced SARs in the wrists and ankles above 20 W/kg while the SAR to the whole body may be approximately 0.4 W/kg (NRPB 1991). Many of the biological effects of RF and microwave radiation which have significant implications for human health can be related to the induced heating. Heating from microwave and RF radiation best relates to SAR rather than to incident power density to account for differences in coupling. Temperature rise and specific energy absorption are related as shown: where T = temperature rise (°C), J = specific energy absorption (J/kg) and htc = relative heat capacity (= 0.85). This simple worst case situation neglects the effects of cooling. However, as a “rule of thumb” a SAR of 5 W/kg applied for one hour would increase temperature by 5°C. Localised heat may be dissipated by the blood through thermoregulatory processes, although the rate of cooling varies considerably for different organs and tissues. Temperature is profoundly affected by many factors which may confound interpretation of results from different experiments. The degree of thermal stress imposed on an animal (or human) by a given SAR is strongly affected by ambient temperature, relative humidity and air flow. The induced thermal load would be expected to increase with increasing body mass, at least in small animals. Some argument for a conservative extrapolation of effects from laboratory animals to humans comes from the observed differences in responses of mice and rats in haematology, immunology, reproduction and development (Gordon 1987; Gordon & Ferguson 1984). The difference in the ability of different species to regulate body temperature is a further confounding factor. It would be unnecessarily simplistic to assume that bulk heating occurred evenly over body tissues and fluids and that it is the only mechanism that can result in a significant effect. Exposure conditions in an RF microwave field are altered considerably by the presence of an object which can profoundly perturb the field, depending on its size, orientation and electrical properties. Refractions and differential absorption within a biological system can result in very complex non-uniform field distributions, and energy deposition. Transmitted energy can be focussed to very localised sites within a body organ. Absorbed RF energy can be converted to other forms of energy and interfere with the function of biological systems. While most of the energy is converted into heat, this does not provide an adequate explanation for a number of biological effects associated with exposures to low level EM radiations. Evidence of a detectable response to minute temperature increase comes from the observations of human perception of pulsed RF fields. The rapid rate of temperature increase has been attributed to the phenomenon of thermoelastic expansion of brain tissue. This effect creates a response through an auditory pathway. At the cellular level, there is a large body on data on cell membrane responses which has developed the concept of a signal transduction pathway modifying cell behaviour following stimulation by low level microwave fields that do not produce a measurable temperature rise. It has been suggested that the resonant excitation of particular molecules may elicit specific biological effects that are independent of heating. At frequencies between 1 and 10 GHz, there have been reports for and against resonant absorption of microwave energy by DNA molecules producing mechanical vibration in the DNA (Edwards et al 1984; Gabriel et al 1987, 1989). Based on evidence of the effects of amplitude-modulated radiofrequency fields in different biological endpoints, it has been suggested (Adey 1983, 1989), that co-operative interactions on the surface of cell membranes may allow weak fields to influence cellular processes through signal amplification and transduction processes. Possible Non-thermal MechanismsThere are difficulties encountered in attempting to ascribe acceptable mechanisms for the observed non-thermal effects of RF (and ELF) radiation. The process has been impeded for the following reasons:
A theory to adequately explain EMR bioeffects must incorporate a biophysical-interaction mechanism consistent with modulation- and intensity-windowed responses, occurring under conditions of low-field-energy coupling to living systems. It is claimed (Cleary 1990) that there is unambiguous evidence of direct effects of RF and microwave radiation from the results of in vitro studies. Effects include, altered cell proliferation, cell membrane receptor and mediated events, and alterations in membrane channels. Although detailed biophysical interaction mechanisms for these effects are not currently available, it is considered that interactions at the microscopic level are related to the dielectric properties of biomacromolecules and molecular assemblages in the form of membrane receptor units, ion channels and enzyme complexes. Membrane OrderingThere is evidence from various studies of microwave frequencies to demonstrate a direct effect on the cell membrane that may be due to alteration of the membrane molecular composition. Evidence of direct interactions at the molecular level comes from results of studies on transmembrane ionic fluxes and Na+/K+ ATP -ase catalytic activity. The direct effect of microwave interaction is restricted to a limited temperature range, implicating the involvement of membrane lipid phase transition (Liburdy 1992, 1994). A theorectical model was proposed (Robertson & Astumian 1992) of the effect of alternating electric fields on reaction rates of membrane-associated enzyme molecules, induced by conformational change. The process has been termed electroconformational coupling. The model describes field-induced alteration in enzyme (ATP-ase) activity due to the resonant coupling of the electric field to an oscillatory activation - energy barrier of the enzymatic reaction. The model predicts effects on Na+/K+ ATP -ase inhibition by RF radiation in frequency windows, but not amplitude windows. Investigation of the possible role of melanin and free radicals in cell membranes (Phelan et al 1992) provided some interesting data. Radiation with 2.45 GHz pulsed wave, SAR 0.2 W/kg was applied to melanin-containing cells and liposomes. Exposure of B16 melanoma cells for 1 h changed membrane ordering, as measured by electron-paramagnetic-resonance (EPR) spectroscopy. Microwave exposure caused a shift from a fluid-like phase to a more solid or ordered membrane state. Similar results were obtained with liposomes that contained melanin, a redox polymer. Neither amelanotic B16 melanoma cells or liposomes exhibited the microwave-facilitated increase in membrane ordering. The microwave effect was inhibited by the free-radical scavenging agent superoxide dismutase (SOD), leading the authors to conclude that the microwave effect was mediated by the generation of oxygen radicals. The results provide evidence of a direct specific microwave effect on a cell membrane that is dependent upon membrane composition. Changes in membrane order are known to alter the function of integral membrane proteins, such as ATP -ase, as well as membrane permeability. Consequently, microwave-induced membrane order could have physiological implications. The effect of microwave radiation on membrane order required the presence of melanin in this study. Other studies of microwave effects on Na+/K+ ATP -ase in the absence of melanin, as reviewed above, suggest the possibility of similar changes in membrane order that were related to temperature. The possibility that microwave radiation may exert a general effect on membrane order as mediated by temperature-dependent generation of oxygen radicals is supported by the results of Liburdy & Vanek (1985), who reported that temperature-dependent effects of microwave radiation on red-cell membrane permeability are dependent upon oxygen tension and the presence of antioxidants. Thus the importance of free radicals in membrane-mediated effects of microwave radiation are becoming more widely accepted as a significant factor.
Other Evidence Of Direct Interaction
Additional evidence of a direct effect of microwave radiation on biomembranes was reported by Bolshakov and Alekseev (1992). Exposure of molluscan neurons to pulse-modulated (PM) 900 MHz microwave radiation caused increased rates of rapid, burst-like changes in firing rates. The threshold for the effect was 0.5 W/kg. Continuous wave (cw) radiation did not affect the firing rate at equivalent SARs. Mediator-induced activation of acetyl-choline, dopamine, serotonin, or gamma-aminobutyric acid (GABA) receptors was unaffected by either PW or CW microwave radiation, suggesting a direct effect of the microwave exposure on neuronal membranes.
Resonant Frequency EffectsAn early publication that created a great deal of interest reported cell killing at specific frequencies in the GHz range (Grundler et al 1978). Strangely, although this work has frequently been used as an example of the peculiar phenomenon of frequency windows, there has been little work to either verify the effect or to investigate the mechanism. The feeling amongst the scientific community is divided, although there is some scepticism of the effect, nevertheless, the researchers are highly regarded. A similar status applies to the report of enhanced, resonant absorption of microwaves by DNA molecules (Edwards et al 1984) in the 1 to 10 GHz frequency range. The reported effect could have implications to genetic effects from microwave exposure. Again, although this was a significant finding, it was some time before an attempt was made to duplicate the study. However, when it was tested (Gabriel et al 1987) the authors carried out duplicate tests on the dielectric properties of the same type of plasmid DNA in two separate laboratories. A reference sample was used to normalise the data against water and avoid recording artefacts from impedance mismatch within the system. The attempted verification failed to achieve the reported result. It is suggested that the effect may have been due to a measurement artefact. However, a recent report of possible rearrangement of brain tissue DNA (measured electrophonetically) following low level microwave exposure (Sarkar et al 1994) may keep the debate alive for some time. The enhancement of the effect involving corneal lesions when irradiation follows the application of the drug timolol maleate is an important finding. This may be particularly relevant to the upper GHz frequency range. It is possible that the presence of a film enhances energy coupling to the cornea and through differences in molecular composition an impedance mismatch occurs at the aqueous humour, resulting in concentrated deposition of energy in the _ 2mm thick cornea. The authors suggest the involvement of free radicals. Continued...
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