Biological effects of microwaves and radio wavesProf. E.H. Grant, Ph.D., F.lnst.P.

Indexing terms: Microwave systems, Radio waves, Radiation

Abstract: The factors determining the interaction with biological material of radiowaves and microwaves atthe macroscopic, cellular and molecular levels are briefly discussed in terms of the relevant physical par-ameters for electromagnetic radiation in the frequency range 10 MHz—300 GHz. This is followed by asummary of the results of the reported animal and in vitro experiments, and it is concluded that, with thepossible exception of a few cases awaiting further investigation, the biological effects of radiowaves andmicrowaves at these frequencies have a thermal basis for their explanation. The implication for humanexposure of what is known in respect of fundamental mechanisms of interaction and the results of animalexperiments are discussed, and it is explained why the present exposure limit of 10mW/cm2 requiresmodification at the lower end of this frequency range.

1 Introduction

It has been well known for many years that non-ionisingelectromagnetic radiation produces heat in biological tissue.This effect has been exploited in physical medicine and,more recently, in cancer therapy and other new develop-ments, and in microwave cookers. However apart from thesedeliberately produced, beneficial biological effects thereare also the possible hazards which can occur owing tounplanned exposure arising as a result of the increasing useof radio waves and microwaves in daily life. In this paper thefundamental mechanisms by which these radiations interactwith biological material will be discussed, and this will befollowed by a brief description of some of the biologicaleffects which have been observed in animals and in man. Thepaper concludes with an indication of how the possibilityof the occurrence of hazards can be reduced to negligibleproportions by the adoption of appropriately chosen maxi-mum permissible exposure levels.

2 Interaction of electromagnetic waves with a biologicalmedium

2.1 General considerationsWhen radio waves or microwaves are incident on a body themode of interaction depends upon the magnitude of thewavelength relative to the geometrical dimensions of thebody. At long wavelengths diffraction effects occur and theinteraction is small, but as the wavelength is decreased apoint will be reached where resonance is encountered and, inconsequence, considerable energy absorption takes place.At still higher frequencies the interaction can be describedapproximately in terms of reflection, transmission andabsorption, utilising mathematical expressions based onMaxwell's equations.

Considering a typical human as a prolate spheroid ofheight 1.7 m, quarter-wave resonance will occur at around35 MHz for a grounded subject, and half-wave resonance willtake place at twice this frequency under ungrounded con-ditions. Hence, for the purposes of this article, a lower fre-quency limit of 10 MHz is assumed, which will then ensurethat the resonance region is covered. Choosing this lowerlimit will also take in the point at 27 MHz which is widelyused for industrial scientific and medical uses. As the fre-quency increases, the penetration of the radiation into thetissues becomes progressively smaller, and above a few tensof gigahertz all interaction take place within the first fewmillimetres below the surface. By convention, the upperfrequency limit of the microwave region is taken as 300 GHz.

Electromagnetic waves incident on a semi-infinite planar

Paper 1679A, first received 3rd March and in revised form 15thOctober 1981Prof. Grant is with the Physics Department, Queen Elizabeth College,Campden Hill Road, London W8 7AH, England

surface undergo reflection and transmission, the relativeproportions of each depending principally on the real partof the relative permittivity (dielectric constant) of themedium. Of the fraction which is transmitted through thesurface, the amount of energy absorbed is proportional tothe electrical conductivity of the medium. The real part ofthe relative permittivity e and the electrical conductivitya of a lossy material may be written

e = e —je" = e —jo/ooe0

where e" is the dielectric loss, co is 2TT time the frequency,e0 is the permittivity of free space and e is the complexrelative permittivity. For convenience, and in accordancewith custom, e will be referred to simply as the relativepermittivity.

For a biological material the variation of e and o withfrequency is complicated and typically exhibits the kind ofbehaviour shown in Fig. 1. Three clearly defined dispersions,a, j3, 7, are present, and there is in addition a much smaller 5dispersion occurring at frequencies of tens and hundreds ofmegahertz. The mechanisms underlying these dispersionshave been described in full elsewhere [1], but will be brieflysummarised here. The origin of the a dispersion is still notcompletely understood, but the movement of counterionsaround the outer surfaces of the cell membranes is oneunderlying mechanism in many cases. Other properties of theouter cell membrane which contribute towards the mem-brane capacitance are also involved. The j3 dispersion isprincipally due to the charging of the cell membranes, andwas accounted for mathematically by Wagner in an appli-

10

103

10

10*

10

1 102 )* 106 108 1010 1012

frequency,GHz

Fig. 1 Relative permittivity e and conductivity a of typicalbiological tissue

The four principal dispersions (or, (3, 8, y) are indicated in order ofascending frequency. permittivity conductivity

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cation of Maxwell's equations for the case of an inhomo-genous dielectric material. This is therefore now known asthe Maxwell-Wagner effect, and is particularly important forbiological materials, where considerable heterogeneityexists. Another effect occurring in the same frequencyregion is the relaxation of polar macromolecules, such asproteins, in their aqueous environment. For example thehaemoglobin molecule has a dipole moment of around400 D and a relaxation frequency about 2 MHz; for a smallermolecule, such as ribonuclease, the corresponding values are340 D and 5 MHz. The 5 dispersion is difficult to defineaccurately, owing to its small amplitude, but its size andposition in the spectrum are consistent with the hypothesisthat its origin is due, at least in part, to the rotation of watermolecules tightly bound to neighbouring macromolecules.This 'bound water' or 'water of hydration' plays an essentialrole in maintaining the structural integrity of the macromol-ecule, and therefore the hydra ted molecule, considered asone unit, is an entity of biological importance. The 7 disper-sion is due to the dispersion of all the rest of the water in thebiological system, and behaves in a similar manner to ordi-nary pure water, with the value of e falling from around80 to near 5 as the frequency changes over the approximaterange 1-100GHz. It is, therefore, important to realisethat absorption of microwave energy (i.e. at frequenciesabove 300 MHz) by a biological tissue is largely due to themotion of water dipoles and dissolved ions. This may beseen from Fig. 1, where it is noticed that the total conduc-tivity (the electrical parameter determining energy absorp-tion) increases markedly in the microwave region. This hasanother important practical implication. The penetration ofan electromagnetic wave in to a medium is inversely depend-ent on the attenuation coefficeint a of the wave, which itselfincreases with frequency in a manner similar to that of theconductivity. The attenuation coefficient appropriate tothe electric field, for an electromagnetic wave passingthrough water, is shown in Fig. 2. At 9 GHz a takes a valuenear 3 cm"1, which (assuming exponential attenuation)means that the electric field falls to about one third of itsinitial value in approximately 3 mm. Therefore, at thehigh-frequency end of the microwave region all interactionsare essentially confined to the surface, or just below it.

The dielectric properties of the tissues comprising a

20

15

10

0-1 1 10frequency , GHz

Fig. 2 Variation of attenuation coefficient a of pure water withfrequency in microwave region

living organism are one set of parameters determining howan incident electromagnetic wave interacts with it. Anotherconsideration concerns geometrical factors and, in particular,the magnitude of the long axis of an animal in relation tothe wavelength X of the incident radiation. When this lengthis near to 3 A for a grounded animal, or U for anungrounded one, the animal acts a dipole receiver andconsiderable energy absorption results. For example for aman of 1.7 m height, regarded as a prolate spheroid of axialratio about 4.5 the frequency for quarter-wavelength reson-ance is 32 MHz. Far-field exposure at this frequency forradiation of a given power density will give rise to an energyabsorption about 15 times greater than that correspondingto far-field exposure of the same power density at 1 GHz.This is one reason why any protection guide [2] which doesnot take into account frequency variation is inadequate.Another reason concerns the production of standing waveswithin the body. For example, the skull, acting as a cavity,will produce nodes and antinodes in the electric field inrespect of radio waves or microwaves which have penetratedin to it. The antinodes, frequently referred to as 'hot spots',will produce enhanced local energy deposition at thesepoints. Standing waves may also be produced within differ-ent layers below the skin, and this phenomenon! is observedwith 2450 MHz microwaves, where the wavelength in aque-ous tissue is little more than 1 cm.

This Section has referred to the principal macroscopicparameters which determine how radio waves and micro-waves interact with biological material. It will now be necess-ary to consider the modes of interaction at a deeper level.

2.2 Cellular and molecular interactionsIt is well known that energy deposition, and consequenttemperature rise, in a medium exposed to radio waves andmicrowaves occurs principally owing to dielectric relaxationof polar molecules, or motion of ions throughout a lattice.This is equally true for biological and nonbiologicalmaterials, and both phenomena are fairly well understoodand therefore need not be pursued here. Apart from thevery-high-frequency end of the microwave region (consideredlater), all molecular and ionic responses are classical innature and involve photon energies which are very much lessthan kT at body temperatures. The question arises, however,as to whether there are modes of interaction which areunique to biological material, and which derive from thepresence of cellular and other structures.

A biological membrane typically has a width in theregion of 10~8 m and an electric field across it of around10 6 -10 7 V/m, i.e. the membrane would be sustaining apotential difference AFmof 10-100 mV in the normal state.Any field set up across the membrane arising from an exter-nal field could, in theory, stimulate the membrane andproduce biological activity. Therefore, if a field Eo externalto the biological organism produces a field E in the tissue,such that the product of E and the membrane width iscomparable with AF m , a biological effect may occur. Inpractice, however, E is considerably less than Eo, owing tothe high relative permittivity of tissue, and at microwave andhigh radio wave frequencies the membranes impedance islow, as can be seen from the tailing off of the |3 dispersion(Fig. 1). For these two reasons any voltage developed acrossthe cell membrane will be very small unless the externalfield strength is so high that strong thermal effects will alsobe observed. For example for an incident power densityof 100W/m2 (10mW/cm2), which corresponds to a fieldstrength in air of around 200 V/m, the potential producedacross a membrane would typically be of the order of 10" 7 Vat microwave frequencies. This is negligible compared with

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the value of AVm. The figure of 10mW/cm2 is the maximumrecommended exposure level which has been adopted byWestern countries on the basis of the heat stress produced.This will be referred to later in the paper.-

Another possibility to be considered is whether cells ormolecules can be directly orientated by the mechanical forceacting on them, and arising as a direct result of an externalelectric field. These so called 'field-force' effects will beobserved when the changes in potential energy of the par-ticle, due to its interaction with the field, become greaterthan kT. The threshold field is inversely related to the sizeof the particle and, to take an important example, is around1 kV/m for an erthrocyte [3]. This is the field strength inthe tissue, and is therefore orders of magnitude greaterthan the field which would be produced by radiation ofincident power density of 10mW/cm2. For individualmacromolecules the threshold field would be higher still,the value depending on the molecular dipole moment andon the length of the molecule. For respective values of theseparameters of 100D and lOnm the field strength requiredfor direct orientation of the molecule turns out to be ofthe order of 1000 kV/m. Finally the phenomenom of inter-molecular resonance should be considered. For pure waterthe lowest frequency at which such an absorption bandhas been observed is 5 THz, which is well outside the micro-wave region. This kind of absorption is nonclassical, andmust be explained in terms of quantum theory. It is possiblethat such resonant absorption could exist at lower fre-quencies in other molecules, but it would be highly unlikelyto occur at a frequency less than that corresponding toaround lcm"1 (30 GHz), and it is probable that thethreshold frequency would be higher. Thus, resonanceabsorption is not likely to be of great significance in relationto the biological effects of radio waves and microwaves as awhole, although it would be of interest to accurately measureat millimetre and sub-millimetre wavelengths the permit-tivity and conductivity of material of biological interest inorder to check this possibility, particularly if there is to beincreasing use of microwaves of frequencies of tens ofgigahertz.

Apart from the possibility of resonance absorption at veryhigh frequencies, it appears conclusive that if the well estab-lished methods of interaction are considered, any effects ofa nonthermal nature will occur only at field strengths con-siderably higher than those at which observable heating willalready have taken place. Recently, however, theories havebeen proposed, one, particularly, due to Frohlich [4], postu-late the existence in biological materials of coherent electricvibrations and highly polar metastable states which can beexcited through metabolic energy. Such an effect, referred toas biological pumping could, according to this theory, betriggered by electromagnetic radiation of considerably lowerpower density than that required to produce thermal heating.This, therefore, would be a co-operative phenomenonwhereby microwave energy is stored for a time by thesystem. Therefore, frequency-dependent processes areinvolved and, according to Frohlich's theory, these low-leveleffects would be particularly likely to occur at millimetrewavelengths.

3 Observed biological effects of radio waves andmicrowaves

There has been a recent explosion of interest in the biologicaleffects of radio waves and microwaves, and this is witnessedby the fact that, of the 5000 or so publications which existin this area, more than 4000 have appeared during the pastdecade. The quality of the published experimental work is

extremely variable, but there are, unfortunately, more thanthe normal share of papers describing work carried out usingsmall samples, with experimental errors not rigorouslyconsidered, and frequently with inadequate statistics andinconclusive or nonexistent tests of significance. All thismakes evaluation of the work rather difficult in some cases.

A considerable amount of work has now been carried outon experimental animals in order to determine the effects ofradio waves and microwaves, and numerous phenomena havebeen observed which can be attributed to heat dissipation.These will now be considered in turn, but no attempt willbe made to give a detailed description, or a comprehensivelist of References. For this information the literature shouldbe consulted (Reference 5 and numerous papers in /. Micro-wave Power and Bio electromagnetics / . ) . One group ofphenomena of great potential importance are ocular effects,some of which are irreversible. For example, if the tempera-ture of the lens of the eye, initially at 37°C, is elevated toaround 42°C irreversible denaturation of the lens proteins,with the consequent production of cataract, may result.Such an effect has been well known for many years inrespect of glassblowers and furnace workers, due in thesecases to excessive absorption of infra-red radiation by theeye. In view of the proximity of the infra-red and microwaveregions in the electromagnetic spectrum, cataracts inducedby microwaves are therefore to be expected provided thatthe incident power density is high enough. From experimentscarried out on the New Zealand white rabbit, the lens ofwhich bears anatomical similarities to that of the human, itis clear that the threshold power density for cataract induc-tion is at least 120mW/cm2 for an exposure time of greaterthan 20 min. The lens is particularly vulnerable to tempera-ture elevation arising from microwave absorption, in that thewater content (and hence attenuation coefficient) is high [6]and there is no blood supply to provide cooling. At highmicrowave frequencies, i.e. tens of gigahertz, the penetrationin to the eye is poor, and effects have been observed in theconjunctiva and cornea, as well as in the lens. After the lensthe other organ of the body particularly susceptible tobiological damage, again on account of poor heat conductionpathways, is the testis. It is well known that immersion inwarm water will produce histological changes and temporarysterility if a temperature rise of more than a few degreesCelsius is produced. In experiments carried out on dogsit has been observed that exposure at power levels of20mW/cm2 for several hours a day continuously for morethan one year did not have any effect on reproductive per-formance, whereas at exposures of more than 50mW/cm2

various forms of testicular damage have been observed.Results of other animal experiments have been well

summarised elsewhere [7,8] , where it is shown that the vastmajority of biological effects, whether cardiovascular, neuro-endocrinological, cellular and genetic, central nervous systemor behavioural, can be accounted for, by general agreement,in terms of heat dissipation. In other words the observedbiological effect would be precisely the same if the equiv-alent temperature rise had been produced in the same part ofthe animal by agencies other than nonionising radi-ation. There are, however, some reported effects in animalsoccurring at incident power densities of only a few milliwattsper square centimetre, and which may be difficult to explainin terms of a thermal mechanism. Recent experiments whichhave been reported include the alteration of the permeabilityof the blood-brain barrier in the rat [9] and the synergisticrelationship between microwave exposure and drug action[10].

Apart from the animal experiments there has been asteady growth of data accumulating as a result of exper-

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iments carried out to study the interaction between radiowaves or microwaves and biological material in an in vitrosituation. In one such experiment, carried out by Bawin,Kaczmerek and Adey [11], an efflux of calcium ions wasobserved from isolated chick brain tissue exposed to radiowaves of frequency 147MHz, modulated at 0.5, 3, 6, 9,11, 16, 25, and 35 GHz. The incident power density was0.8 mW/cm2. Independent studies by Blackman, Elder, Weil,Benane and Eichenger [12] confirmed that low-intensityamplitude-modulated radiation of this frequency can affectcalcium binding in brain tissue. A modulating-frequencywindow and a power density window between 0.1 —1 mW/cm2 were specified to account for the phenomenonand the possibility of direct action between the radio wavesand the nervous system was proposed. An alternative expla-nation in terms of a surface transduction mechanism waspropounded by Schwan [13].

Recently Frohlich [4] and Berteaud and Servantie [14]have collected together the experimental evidence for theexistence of low-level microwave biological effects, much ofwhich refers to work carried out at millimetre wavelengths.The significance of a great deal of this work is difficult toevaluate, but one notable investigation is that carried out byGriindler and Keilmann [15] on the influence of millimetremicrowaves on the rate of growth of yeast cells. In theseexperiments yeast cells were exposed to microwaves in thenarrow frequency range of 41.6—42.1 GHz, and of incidentpower density of around 2mW/cm2. The growth rate ofthe yeast cells was plotted as a function of frequency,within the above range, and was found to rise and fall regu-larly around the value of growth rate observed in the absenceof the field. Such a phenomenon could be accounted for byFrohlich's theory of coherent processes [4], but is quiteinexplicable in terms of a thermal effect as ordinarily under-stood. The latter could produce an increase in growth rate,but not a reduction. These experiments are presently beingrepeated by Griindler, and the results will be awaited withinterest.

The experimental evidence summarised in this Section isintended to give an overall view of how effects of biologicalimportance are produced by radio waves and microwaves.Certain experiments which require novel mechanisms fortheir interpretation have been highlighted in order to empha-sise the fact that not everything is universally understood inthis field. It must be stressed, however, that the vast majorityof observed phenomena in this area can be explained bytraditional electromagnetic field theory together with theapplication of well established physiological principlesgoverning heat dissipation. A considerable amount of soundexperimental work has been carried out in order to establishthis strong position.

4 Implications of current knowledge in respect of healthhazards in man arising from exposure to radio wavesand microwaves

In the UK, official interest in the possible health hazards ofnonionising electromagnetic radiation was first shown in1960 when the British Post Office issued a code of safetyprecautions for their workers. In the same year an editorialon the subject appeared in The Lancet [16]. The consensusof opinion at that time was that working personnel shouldnot be exposed to a power density of greater than10mW/cm2, this being the figure which had been proposedby Schwan [17] in the USA during the 1950s. The basis forSchwan's proposal was as follows. It is well known that theBasal metabolic rate (BMR) for an average man under typicalenvironmental conditions corresponds to around 100W.

Given a total skin surface area of 2 m2, and assuming radi-ation to be incident from one direction only (i.e. interactingwith half the surface of the body), microwaves of incidentpower density 100W/m2 (lOmW/cm2) would give rise toan additional thermal burden equal to the correspondingto the BMR. This, it was proposed, would be an acceptablethermal load for the body to tolerate.

It will be appropriate at this stage to consider the reportedcases of biological effects in humans arising from exposure toradio waves and microwaves. That radio waves produce heatin human tissue was observed by d'Arsonval as long ago as1892, and this fact has formed the basis of RF therapeuticheating in hospital departments of physical medicine. Morerecently microwaves have been used for cancer treatment[18], and tumour regression has been observed as a resultof heat dissipation arising from the absorption of micro-waves. There is consequently no difficulty in acceptingthat nonionising electromagnetic radiation produces clearlydefined biological effects in humans. The exposure powerdensities during therapeutic use are typically of the orderof a few hundred mW/cm2. Turning now to inadvertantexposure of persons to radio waves or microwaves, thenumber of reported cases of deleterious effects is very small,and even then there is considerable disagreement as towhether the effects are clearly due to the radiation at all.Cases of microwave cataract have been claimed by Zaret onseveral occasions, one of which was reported in the NewYork State Journal of Medicine [19]. The article was fol-lowed by a discussion, which was reported immediately afterthe article in the same journal issue, and it is clear from thisand subsequent pronouncements, at conferences and inpublications, that the substantial majority of medical andscientific opinion rejects Zaret's interpretation. Notwith-standing this Zaret has more recently alleged (Reference 14,pp. 197—204) cataract production arising from low levelradiation emanating from visual display units. In this case itis not specifically stated that the cataract is due to radiowaves or microwaves, but it is implied by the author'sstatement that the cataracts exhibit characteristics resem-bling nonionising radiation injury. In both of these reports[19] (Reference 14, pp. 197—204), the power densities towhich the subjects were exposed would appear to be (atmaximum) between one and two orders of magnitude lowerthan that required to produce a temperature in the lenssufficiently high to produce protein denaturation, and hencecataract. Thus, some alternative explanation would have tobe sought for these observations. Other reported injuries tohumans arising from inadvertent exposure are internalburning (referred to in Reference 16, but without any speci-fied value for power density), and temporary sterility (Refer-ence 8, p. 61) in a technician whose job involved servicingradar antennas. In the latter example the documentation isnot complete, and in any case the exposure level was esti-mated as being around 30 000 mW/cm2. Next, in this groupof effects, it is convenient to refer to the phenomenonof microwave hearing, it having been known for some timethat hissing, clicking or buzzing noises can sometimes beheard by being in the presence of pulsed microwave fields.This is now recognised as being due to small expansionswithin the skull as a result of temperature increases as lowas 10~5°C. However for microwaves consisting of pulses oflength 10 jus the rate of rise of temperature during eachpulse would be ^ C s " 1 , which would be sufficient to launcha pressure wave and stimulate the cochlear. Before con-cluding the Section it would be appropriate to make briefreference to pacemakers. Whereas some of the older devicescould be affected by power densities corresponding to10mW/cm2 in air, modern shielded devices are unaffected

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by field strengths of 200 V/m (in tissue) which, at all fre-quencies of interest, is at least an order of magnitude greaterthan the field arising from an incident wave of this powerdensity.

The other group of effects which appear in the literatureare the behavioural effects which have been reported, chieflyfrom observations carried out in Eastern European countries[5]. Symptoms such as listlessness, headache, fatigue, irrita-bility and sleep disturbances have been described amongworkers exposed to power levels as low as 1 mW/cm2. It isextremely hard to evaluate these reports, and many scientistsfind it difficult to conclude that the effects are clearly andunambiguously linked with the exposure to nonionisingelectromagnetic radiation. Nevertheless, it must be immedi-ately added that the connection is accepted in EasternEuropean countries, and accounts for why their protectionguides [2] adopt much lower maximum permitted powerlevels than those in the West.

Turning now to the standards recommended in the USAand the UK, it is necessary to consider how safe is the10mW/cm2 limit (now 25 years old), and whether it is inneed of modification. Several points require to be borne inmind, one of which is the importance of distinguishingbetween a biological effect and a biological hazard. Anabsorption of microwaves which causes a change in bodytemperature of 1°C or less would not, in the judgment ofmany people, be considered a hazard. Temperature risestwice as high as this can be produced by walking or byemotional stimuli. On the other hand a rise in temperaturein the lens which produces irreversible denaturation of thelens proteins, and consequently cataract, is clearly universallyunacceptable, and must be termed a hazard. The parameterof immediate and indirect importance is, therefore, the risein temperature of the tissues, and not the incident powerdensity. The temperature rise produced by a given powerdensity will depend on the dielectric properties of thetissues, the efficiency of the thermogulatory system of thebody and on the environmental conditions. The latter con-sideration frequently gets overlooked when figures are pro-duced in relation to microwave hazards. A power density of100mW/cm2 could be a life saver at the North Pole whereasan exposure of 10mW/cm2 at the Equator might cause thesame subject to die of heat stroke. It therefore follows thatprescribing a 'safe' exposure level with any great precision isdifficult owing to the large number of parameters involved.Careful attempts to calculate temperature rise in terms ofthe incident power density have, however, been made forvarious situations by Tell and Harlen [20]. They concludethat an exposure of 10mW/cm2 would typically cause a riseof not more than 1°C in the superficial tissues for a manexposed for 3h to radiation of frequency 2.45 GHz. Atlower frequencies, however, the presence of body resonance,and of hot spots, produces a very different picture andlocalised temperature increases as high as 7°C may be pro-duced [20] in parts of the body for an exposure of10mW/cm2. This would be the figure in the event of reson-ance occurring, i.e. if the absorbing man were behaving as ahalf-wave dipole standing in the far-field. If there were anyreflecting obstacles in the vicinity the rise in temperaturewould be even higher. Therefore the 10mW/cm2 standard,although adequate at frequencies in excess- of 1 GHz, is toohigh in repect of lower frequencies, and needs revision.Various protection guides [2] are being drafted in the USAand elsewhere which will comply with these requirements,

and indications are that the values will need to be lowered to1 mW/cm2 (10W/m2) in the frequency region betweenaround 30—300 MHz. These figures refer, of course, towhole-body exposure under far-field conditions. For localexposures, such as would arise for example with a handheld transmitter, electric fields higher than those appropriateto whole body irradiation can be tolerated provided that theenergy absorbed by the whole body is acceptably low.Hence a CB radio emitting up to several watts of powerwould be acceptable according to this criterion.

The position regarding low-level thermal effects andnonthermal effects should continue to be monitored closely.Fortunately, most of such reported effects [4] occur atfrequencies of tens of gigahertz, where the penetration into human tissue is not more than 1-2 mm, thus minimisingthe possibility of constituting a health hazard.

5 References

1 GRANT, E.H., SHEPPARD, R.J., and GRANT, E,H.: 'Dielectricbehaviour of biological molecules in solution' (Oxford UniversityPress, 1978)

2 HARLEN, F.: 'Microwave and radio frequency exposure limits',IEEProc. A, 1981, 128, (9), pp. 589-592

3 SCHWAN, H.P.: 'Field interaction with biological matter', Ann.NYAcad. Sci., 1977, 303, pp. 198-213

4.FROHLICH, H.: 'The biological effects of microwaves and re-lated questions', Adv. Electron. & Electron Phys., 1980, 53, pp.85-152

5 CZERSKI, P. (Ed.): 'Biologic effects and health hazards of micro-wave radiation' (Polish Medical Publishers, 1974)

6 DAWKINS, A.W.J., GABRIEL, C, SHEPPARD, R.J., andGRANT, E.H. 'Electrical properties of lens material at microwavefrequencies',^^. Med. Biol, 1981, 26, pp. 1-9

7 MICHAELSON, S.M.: in STUCHLY, M.A. (Ed.): 'Microwavebioeffects and radiation safety' (International Microwave PowerInstitute, 1978), pp. 55-94

8 STUCHLY, M.A.: 'Health aspects of radio frequency and micro-wave radiation exposure (Part 2)' (Department of National Healthand Welfare, Ottawa, 1978)

9 OSCAR, K.J., and HAWKINS, T.D.: 'Microwave alteration of theblood-brain barrier system in rats', Brain Research, 1977, 126,pp. 281-293

10 THOMAS, J.R., BURCH, L.S., and YEANDLE, S.S.: 'Microwaveradiation and chlordiazepoxide; synergistic effects on fixed-interval behaviour', Science, 1979, 203, pp. 1357-1358

ll.BAWIN, S.M., KACZMAREK, L.K., and ADEY, W.R.: 'Effects ofmodulated VHF fields on the central nervous system', Ann. NYAcad. Sci, 1975, 247, pp. 74-80

12 BLACKMAN, C.F., BENANE, S.G., ELDER, J.A., HOUSE, D.E.,LAMPE, J.A., and FAULK, J.M.: 'Induction of calcium-ionefflux from brain tissue by radio frequency radiation', Bioelectro-magneticsJ., 1980, 1 pp. 35-43

13 SCHWAN, H.P. 'Tissue determinants of interactions with electricfields', in ADEY, W.R., and BAW1N, S.M. (Eds.): 'Brain inter-actions with weak electric and magnetic fields. Neurosciences Res.Prog. Bull. 15', (MIT Press, 1977)

14 BERTEAUD, A.J., and SERVANTIE, B. (Eds.): Electromagneticwaves and biology'. URSI/CNFRS International Symposium,Paris, July 1980

15 GRUNDLER, W., and KEILMANN, F.: Z. Naturforsch; Teil C,1979, 33, pp. 15-22

16 Radio-frequency radiation, The Lancet, Aug 27 1960, pp. 484-486

17 SCHWAN, H.P., and LI, K.: 'Hazards due to total body irradiationby radar',Proc. IRE, 1956, pp. 1572-1581

18 HAND, J.: 'Electromagnetic techniques in cancer therapy byhyperthermia',/££"Proc. A, 1981, 128, (9), pp. 593-601

19 ZARET, M.M.: 'Cataracts following the use of microwave oven',NY J. Medicine, Oct. 1974, pp. 2032-2034, discussion pp.2034-2048

20 TELL, R.A., and HARLEN, F.: 'A review of selected biologicaleffects and dosimetric data useful for development of radiofre-quency safety standards for human exposure', J. MicrowavePower, 1979, 14, pp. 405-424

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