Issues in electromagnetic field-biointeractionsnopr.niscair.res.in/bitstream/123456789/15458/1/IJBB 36(5) 352-360.pdfIssues in electromagnetic field-biointeractions J Behari School

Indi an Journal of Biochemi stry & Biophysics Vol. 36, October 1999, pp. 352-360

Review

Issues in electromagnetic field-biointeractions

J Behari

School of Environmental Sciences,lawaharlal Nehru Uni versity, New Delhi I 10067

Low level electromagnetic ti elds have been found to produce a variety of biological e ffects, though the mechanism or such interaction is still not completely understood. Cell membrane of the brain is a critical structure perceiving the ac tion or microwaves, which has received greater attention in the recent past. The interactions o f EMF with the li ving cell s are con­sidered as stochastic resonance, cooperative effects, non-equilibrium thermodynamic process and non-linear interactions. The living cells derive the energy from noise and pumps it into the modes of excitation at the driving frequen cy of an elec­tromagneti c wave which give sufJicient amplification of the signal and increase the signal to noi se ratio. The non-linear mechanism plays their main role in the process of transmembrane coupling of the signal to the cytoplasm. The crit eria l'or safe exposure limits of electromagnetic field to humans is also discussed.

Introduction Electromagnetic fields (EMF) are invisible electri­

cal and magnetic forces radiating from objects that operate on an electric current. These fields emitted from man made devices operate at a broad range in the electromagnetic spectrum (60 Hz-15,000 Hz +) and in recent years have been shown to be detrimental to human health . The term Electropollution is often used to describe these phenomena as Non-ionizing electromagnetic radiation is emitted by: electrical power liners, power stations, transformers, e lectrical appliances, microwave towers, radio, TV and com­puter terminals, electric blankets, steel pipes, rein­forcement bars that conduct an electrical current and underground transport systems.

We are immersed in the earth's magnetic field,

which has a typical flux density of around 50 f.1T . The biological body, however, is almost completely non­magnetic .

Background of the problem Till about two decades ago it was believed that

these fields were incapable of producing any bioef­fects. In the range of extremely low frequency (ELF) the heating effects are large ly due to the production of eddy currents and to the dielectric relaxation (ex: type, 0-100 Hz). According to the poynting theorem the maximum average electromagnetic energy depo­sition in a living system is given by:

? ? Em=lf2[EE - nns +f.1H ~;ns ] ." (I)

Where Erms is the electric field and Hrms the mag­

netic field r.m.s. intensities respec tively . E and f.1 a re

the dielectric constant and magnetic permeability of the biological medium, respectively. Research with animals and in vitro investigations of tissue irradiated by electric and magnetic field i

,2 mu st involve the field to which the cells are actually exposed. The re lation­ship between the incident field and the induced field depends on the magnitude, po larization , direction of wave travel, frequency of the incident field, the length, proximity to the earth of the body and its s ize and electrical properties of the organ<; in the body. To site an exampie, with a 300A current at 110 Kv, the vertical electric field at a 10m distance is 530 vfm

and the horizontal magnetic field is 1.77 ~lT. Public concern about the possible health effects of

exposure to EMFs, particularly fields generated by 60 Hz sources, is widespread. Many people have been led to believe that we should prudently avoid EMFs, abandon electric blankets and hair dryers , move beds away from walls, use manual rather than electrical appliances, avoid cooking with a microwave oven, avoid using cordless and cellular telephones, and pos­sibly reside in a house far away from overhead. trans­mission lines. A great dea l of research has of course been done to determine the safe expos ure limits of I . 14 I h' e ectromagnetIc energy". n tis paper we present

briefly the development of the subject touching upon the landmark accomplishments in setting exposure standards.

Basis of the interaction Human exposure to extremely low frequency elec­

tromagnetic field can be depicted as 111 Fig. I . Be­cause of tran sformer action, changes in a magneti c

BEHARI : ISSUES IN ELECTROM AGNETIC FIELD-BIOINTERACTIONS 353

Static Voltage Source ELF Voltage Source Induced Alternating Torso Voltage and Currenl

(0) (b)

J Induced Alternating Voltage Current

(c)

Fig. I- Human body exposed to stati c and time dependent extremely low frequency electric (a, b) and magnet ic fi e ld tc ).

fi eld can induce voltages in the surrounding body tis­sues, resulting in circulating currents . For whole-body exposure, the most important of these is the induced current and voltages in the torso. The induced voltage per turn depends on the magnetic flux lines within the turn :

Y = 2 nfBA, .. . (2)

Where A is the cross-sectional area within the one­tum torso, and B is the flux density.

(i) The human can be approximated by a rectangle that has a height of 60 cm and width of 30 cm, so that the area is 1,800 cm2

, for a 200 IlT nux dens ity, at f = 60 Hz and the tota l induced voltage around the torso is 13.6 my5. The resulting current depends upon the path traversed- less current in hi gh-resi stivity re­gion (eg. bone marrow and fa t), and higher current in low-resistance path (eg. soft tissues). This can be physically described by considering a loop around the torso, whose resistance is given by

R = pI/A ... (3)

Where p is resistivity, I is the length of the path, and A is the cross section.

Since the length of the path is 180 cm., eqn . (3) re­sults in current density of 0.6 Il A cm·2 at 60 Hz, close to maximum permissible exposure leve l of I IlA cm2

.

It is apparent that , for relati vely low frequencies, the induced current density is proporti onal to frequency.

(ii) On ' hi gh-voltage power line, the load is rea­sonably constant so that the power flow is predomi­nantly a forward travelling wave, the ri sk rati o for the occurrence of leukemia near the 220 and 400 KY power lines is significantly greater than I when B > 0.1-0.2 IlT and when di stance is 50m. This study further indicates a long term exposure ri sk with a ra­tio of the order of three to four when .

E> 61 vim, B > O. 2 IlT

Wi th this va lue of inciden t electric fielJ the magni­tude on induced field is 15.5 Ilw /m('. Th is is within the range in which signifi cant cellu lar effects are al so observed ill vitro . Fa!' an incident fi eld or 3v/111 from a YLF transmitter corresponding induced field is i .66 mv/m .

(ii i) Video display term inal s (YDT) produce very low frequency electric, and magnet ic fields and ai <;o stat ic electric fie lds. Flemi ng and J ouner7 reported

354 l ND1AN 1. 1310 CHEM. B10PI1YS .• VOL. 36, OCTOBER 1999

that the spec ific absorpt ion rate induced in the em­bryo or fetu s can exceed that recommended for the general public when the mother is exposed to radio frequency radiation at occupational limits. Appl yi ng current exposure limits, the results indicate that over exposures to the embryo und fetus can occur from early pregnancy at frequencies 80-100 MHz and 300-1500 MHz. Youbicier-Simo et af. 8 have also con­cluded that VDU-generated EMFs appear as the main causative factor of the increased embryonic defect in the exposed group. Other investi gators9

.14 have re­

ported that exposure of fertile chicken eggs to a pulsed ELF magnetic field for the first 48 hr or incu­bation was accompanied by increased embryonic death .

(iv) The recent deve lopment of mobile communi­cations (900 and 1500 MHz) has drawn urgent atten­tion to the biological effects of electromagnetic fields (Fig. 2). Apart from the controversy over health due to the non-thermal effect of EMF's, the electromag­netic interaction of a portable radi o with a human head need to be quantitati ve ly evaluated in order to establish the safety of wireless communication sys­tems. Specific absorption rate (SAR) has been recog­nized as one of the most significant variable quanti­fy ing EMF interaction with hu man body, and safety guidelines recommend limits on the maximum local SAR as well as on the whole-body averaged SAR. Eva luati on of the maximum local SAR is especially importan t when part of the body is exposed to elec­tromagnetic radiati on fro m nearby sources. Thus the estimation of SAR distribution in a human head dur­ing the use of cellular telephones has become a matter

Antenna ,;

Fi~. :> - Field s induccd in human Ile~d (III(: In ccllu iar Illohiic

of great concern and has been ca lcul ated by various workers I5 ,16 .

(\') Experi mental observations indicate that modu­lat ion .plays an important role in eliciti ng biologica l response, particularl y when exposed wi th weak « 5 W/kg) radio frequency and microwaves .

Research findings Until the late 1970s most scientists believed that

EMFs could have no biological impact or influence on induction of cancer. More recentl y, however, re­search findin gs suggest that these fi e lds affect bio­logical systems. Whether these effects are re lated in any way to DNA damage leading to carc inogenes is is highly speculati ve. Stud ies on bioeffects associated with EMFs may be divided into three c lasses:

In vitro experiments usi ng animal or human cells exposed to EMFs under controlled env ironment

III vivo experiments involving living animals and people exposed to fields under contro lled circum­stances; and

Epidemiological studies that retrospecti ve ly ~va lu­

ate the ex istence and extent of certai n bioeffects among specific groups of peop le ex posed to EMFs at work or at home.

The laboratory and ep idem i o l og i c~l l results de­scribed so far suggest that EMFs may interact and produce certain charges in a va ri ety of" biological systems under selected conditi ons . The mec hani sm causing these changes is poorly understood. Some of the underlying findings can be summari zed thu s: (i) EMFs are not known to cause any chromosomal dam­age, and consequently do not initiate cancer. They however, show DNA strand break l7; (ii) EMFs have been shown to increase orni thine decarboxy lase (ODC) activ ity. which is characterist ic of many can-

I!P I H b cer promoters - . owever, converse may not e true; (i ii) Changes in calcium ion efflux ~2 -27 and in­duction of cellulur transformations2x

-.12 have been re­

ported; ( iv) Alterat ions in prote in syn thes is, im mu ­nol ogical and hormone status, and metabolic compe­tence via ci rcad ian shifts may contribu k to the prog­ress of cancer in itiated by independent eve IHs. To the extent that EMFs influence such alterati ons, the) mi ght have an effect on tumour growth or inh ib it ion: (v) EMFs have been shown to de pres~ pi neal mela­tonin levels (i n anima ls), whIch in turn has been asso­CIated with cancer g r O\ [11.11

; (v i) SOI1 IC cpldc11l 1 ()ll).~i­cal stUJ ICS 01' [~,'II-;- CXplJS urC and C<ln,_'l, sho w ,il l UI1-

BEHARI: ISSUES IN ELECTROMAGNETIC FIELD-BIOINTERACTIONS 355

confirmed link between exposure and nervous system cancer and leukemia34

.

Although the above arguments are consistent with the hypothesis that EMFs may playa role in cancer promotion, none of the findings constitutes a proof as yet.

Mechanism of interaction It is now widely accepted that the plasma mem­

brane is the target for the interac tion of electromag­netic fields with ce ll (Fig. 3). An ELF-EMF, both through the EF and through the MF by means of the Faraday induction effect, will produce electric cur­rents in the ionic aqueous solutions surrounding the cell. However, the cell membrane is a strong dielec­tric ( £-6 ) barrier for the passage of these currents to the intracellular medium, except for a very small re­duced fraction. Therefore if the ELF-EMF have to produce any effects inside the living cells perhaps they ought to be through suitable alterations at the membrane level. Thi s in turn conveys signals across the membrane body to produce biochemical and physiological response. Since the di electric constant of the membrane is relatively large and therefore in­duced intensity currents at the ELF regime are small, it is speculated that ELF-induced alterations must reside at the cell membrane outer surface, invol ving onl y the counter ion layer, ion channel permeability, glycoproteins and ligand receptors.

This leads to enzyme activation , gene induction, protein synthes is, and ultimately mitogenes is and cell

75 m V resting potential

+~

Nucleus

7.5 nm cell membrane

Fig. 3-An idealized exci tab le cell at rest in the human body. 175mV across a thickness of 7.5 nm correspond s to an elect ric field often milli on Vm· I

] .

proliferation35. Two signaling systems in volve re­

ceptor tyrosyl kinases and receptor phospholipid breakdown in the cell membrane. In the second sys­tem, which is widely distributed and found in all that are calcium dependent, a li gand binds to its receptor and triggers phospholipid breakdown in the cell membrane, leading to the generati on of the "second messengers". Any such actions would modul ate ex­isting cellular signaling pathways which transmit ex­tracellular signals from the cell membrane to the nu­cleus. These pathways are known to in vo lve a cas­cade of phosphorylation reacti ons catalyzed by spe­cific protein kinases including protein kinase C (PKC) .

Biomedical effects of low frequency electromag­netic fields on signal transduction have ga ined con­siderable interest as results obtained from various species point to a possible interacti on between ELFs with ion transport mechanism in the cell membrane'6. Ion channels of different spec ies have been ex ten­sively stl,.ldied and a direct correlati on has been shown between particul ar ion currents , the membrane poten­tial , the control of ciliary activity and the sw imm ing behaviour of cell s37

.38 . Hemmersbach et (/ 1 .. 1<1 observed a dose dependent increase in the mean swimming velocity and a decrease in the linearity of cell tracks in all wild-type cell s. These authors ha ve concluded that there is a direct effect of low frequency electro­magnetic fi eld (50Hz, flux densiti es 0.5-20 mT) on the transport mechani sms of the cell membrane for ions controlling the motile acti vity of cilia .

The mechanism leading to changes in biosynthes is has been elusive, in large measure because it is as­sumed that magnetic fields do not act directly on the cell components, only indirect ly through their in ­duced electric fi elds. Low-frequency electri c fi e Ids do not penetrate cells very effecti ve ly because of the low dielectric constant of the cell membrane, but low­frequency magnetic fi elds do penetrate. The electri c fi elds they induce in ce ll s are very small because of cell dimensions, so an indirect route via the mem­brane has been assumed. However, it is also poss ibl e for the magnetic fields to interact directl y.

Interaction of electric and magneti c fields with b . II d d~o ~ l b ' mem rane components IS we ocumente " ut It

is currently believed that interaction with membrane is the only pathway. Acco(ding to thi s view, EM fie lds must be transduced by the membrane via an enzym'e cascade into a biochemical (second) messen-

356 INDIAN 1. BIOCHEM. BIOPHYS., VOL. 36, OCTOFlER 1999

ger that can reach the nucleus and cause changes in DNA.

EMF - Biointeraction Macromolecular resonance

When collisional perturbations are very brief in comparison with the fie ld period , every collision is quite effective in intermpting the molecule's radiation absorption-emission process . In a fluid with a large rate of collision a collisional broadened relaxation­type spectrum results. On the other hand, for colli­sions for long duration compared to the EMF period , the spectrum becomes resonant, even for a high rate of co li isions, because each coli ision causes, statisti­cally, little disruption. The collision time in a macro­molecular fluid is a fixed function of temperature, and therefore the EMF frequency determines the kind of spectrum (rotational or resonant). Attenuation of MW field s is dominated by the presence of water, which also shields other possible bimolecular absorp­tion processes. Experiments suggest resonant absorp­tion in growing cells at 41 GHz fi elds and in DNA at II GHz42.4.,.

Mechanism of signal amplification Amplification process at the membrane site is by

far the least understood problem as of date. Amplifi­cation of extremely weak fields can be provided by highly cooperative non-equilibrium biomolecular processes at membrane level, viz. cell membrane be­haves as extremely efficient energy amplifier under weak stimuli. Cooperativity can be defined within biological systems, as any dynamical or coherent os­cillation by which an ensemble of macromolecules acts together to change the ensemble from one stable state to another. In the process, the trigger or input signal to the system may be weak and the response will be an amplified signal, probably of many orders of magnitude. In this regard many processes can be chosen in organized biomolecular systems44

.

Circumscribing ELF-EMF or ELF-modulated HF rield interaction will play their main role in the proc­ess of transmembrane coupling of the signal to the cytoplasm. It is now well accepted that soliton waves are responsible for such coupling. Such sol itary waves, involve a high degree of non-linearity in the mo lecular binding. Soliton is a highly non-linear vi­bration propagated through the protein.

A point to mention is the failure of equilibrium thermodynamic processes in cooperati vity effects to

explain the effects of ELF-EM"). It seems that this can give rise to intensity, frequency and exposure time "window responses" in the interact ion of ELF­EMF- and ELE modulated HF fields with membranes. The huge electrical field gradient ("" I 05 vim) through the membrane, precludes any ionic equ ilibriull1 proc­esses for cell excitation produced by the tin y induced voltage by ELF-EMF - 10.6 times weaker than the transmembrane gradient46

. It has also been shown that such weak field intensity can influence the action of messengers, antibody and cancer promoter molecul es at their membrane receptor sites. Therefore amplifi­cation of such extremely weak field s is needed to ttrigger the above processes. The amp I i fication ca n be provided by highly cooperative non-equ ilibrium bio­molecular processes at membrane level (Fig. 4).

Stochastic resonance The basic issue relating to low level EMF biointer­

action is to know as to how the cells recognize the existence of the primary weak signa l and the source

EXTERNAL SIGNAL

(First Messenger) j

T

RECEPTOR

,~ TRANSDUCER

~

T AMPLI1'1 ER

T Phosphorylated PREfRSOR

SECOND

MESfNGER

INTERNAL

HFlCTOR

CELLULAR RESPONSES

Fi g. 4-Diagrammatic representati on (, I" s ignal tr:l11sductioll path­ways by membrane receptors.

BEHARI: ISS UES IN ELECTROMAGNETIC F1ELD-BIOINTERACTIONS 357

of energy required for their amplification by several orders of magnitude. The cell derives energy from the noi se and pumps it into the modes of excitation at the driving frequency of an electromagnetic wave, which give sufficient amplification of the signal and in­crease the signal to noi se rati o. Such a mechani sm manifests itself by the appearance of sharp peaks in the power spectrum of the system at the driving fre­quency and some of its hi gher harmonics and is named stochastic resonance47 .

The membrane is clearl y a site of interaction, but there can be other alternations. Furthermore, the role of the membrane in interac tions that stimulate tran­scription , a process that occurs at a considerable dis­tance from the membrane, inside the cell nucleus still remains an open question .

Because magnetic fi elds are essentially unattenu­ated by cell membranes, membrane processes may not be necessary for the signal to reach the DNA in the nucleus and stimulate biosyntl1etic stress response. Some experiments usi ng a cell-free bacterial extract­with (no membrane) showed EM-stimulated protein synthes is48

• Magnetic field also reduces the integrity of intercellular contacts without interfering with other cell functions49

.

In search for a general mechani sm of interaction of EM fi eld with cell s, Na+-K+ ATPase, the "ion pump" enzyme in cell membrane has been studied50-52

.

Changes in enzyme activity in either electri c or mag­netic fi elds show that the most important fac tor af­fecting signal transduction is the level of enzyme ac­tivation, the maximum biological (ATP-splitting) ac­ti vity of the enzyme under a given set of conditions (i.e., ion concentration, temperature) . Enzyme func­tion involves communication between ATP-splitting catalytic sites and ion-binding sites in different parts of the molecule in a coordinated sequence, and ion­binding sites in enzyme is essential and is probably brought abou t by charge movements during function . Such movements have been observed in the micro­second range.

The characteristic dependencies of electric and magnetic fi eld effects on enzyme activation can be explained by a signal transduction model in which enzyme ac ti vation increases with the concentration of mobile charges within the protein5

' .

It appears that magnetic fields always stimulate, but electric fields can inhibit as well as stimulate, de­pending on the level of ac tivati on.

Recent measurements on DNA have shown that the double helix is also capable of very hi gh electroni c

d . h h h k db' S~ -S6 con uctlon t roug t e stac ease pairs' . . A direct EM fi eld-DNA interacti on should be con­

sidered as a possible mechani sm. Interaction with electrons in the stacked base with DNA cou ld cause changes in charge fl ow, and non-uniform charging can lead to bending of a heli x and can initi ate tran­scription . In loops of DNA, currents moving in oppo­site directions in adjacent strands would experience repulsive forces in EM fi elds, and thi s could lead to structural changes .

Electromagnetic field and bone healing While there are questi ons of possible hazards of

electromagnetic fields, its role in acce lerated healing of bone fracture is one outstanding example of thera­peutic applications.

Promotion of the process of bone repair was achieved in the past by autogenous bone grafti ng)7 application of various growt h factors (mainl y bone morphogenetic protein57-60. Use of electromagnetic fi elds61-64

, and more recentl y non-quantitati ve stud ies . . 6S 66 h I b d 1I1 vlfro .. ave a so een attempte .

Electrical or electromagneti c stimulati on is gener­ally believed to enhance bone repair. However the effects of these fields are in part determ i ned by thei r physical signal characteristics. Low frequ ency field s have been shown to decrease bone loss and maintain bone mass in models of bone resorption by decreasing osteoclastic bone remova l. Studies on the effects of these field s upon osteoclasts ill vit ro show a decrease in both osteoclastic differentiation and lysosomal en­zyme content67.

The observation that bone ti ssue is maximall y sen­sitive to induced electric fi elds in the 15 to 30 Hz range has interesting applicati ons. Results indicate that the extremely low frequency field components of the clinical PEMF signals are far more capable of stimulating bone-remodeling act ivity than hi gher fre­quency components . Th is observat ion seems to lend strong support to the contenti on that the endogenous electric fi elds arising through electrok ineti c or pie­zoelectric processes may be an integral part of bone homeostas is.

The ability of bone ti ssue to respond to an induced sinusoidal electric field at intensities below I mY /m presents an intriguing issue in signal detec ti on. One approach to thi s problem is to assume th rou gh the same process as is known to control e lectri call y ex-

358 INDIAN J. BIOCHEM. BIOPHYS., VOL. 36, OCTOBER 1999

citable cells of the body that is, through perturbation of the cell membrane potential. Extremely low fre­quency fields have been shown to interfere with parathyroid hormone receptor signaling process with a decrease in osteoclastic differentiation .

Although it has been confirmed that low fre­quency, low energy pulsed electromagnetic field (PEMF) stimulate healing of ununited fractures in h 64 68 h' h ' f .. f' II umans . , t elr mec anl sm 0 action IS not u y understood ,

Laser radiation in bone healing Use of the effect of low energy laser irradiation in

regeneration processes other than in bone following trauma has been investigated.>so far in the skin69, in h I d . h I 70-72 d ' t e centra an penp era nervous systems an m

skeletal I11USC I es7J-76. Under the weight of ex perimental evidence it is

suggested that laser energy is photochemical in nature and the energy is probably absorbed in intercellular chromophors and converted to metabolic energy , probably by invol vi ng the respiratory (cytochrome) chain77.78. Barushaka et al. 79 have shown that He-Ne irradiation can promote bone repair in the rat tibia using biochemical and hi stomorphometric methods. Yaakobi et al. 80 studied the enhancement phenomenon of He-Ne laser irradiation by direct measurement of rate and extent of calcification .

Safety standards Particular attention has been focu sed recently on

the potential health effects of radi o frequency (RF) and mi crowave fields, which are used extensively in te lecommunications. The transmission of information via RF or microwave signals is accompli shed by ap­pl ying some form of modul at ion to a carrier wave, which changes some aspect of this wave as a function of the transmitted information. Basic modulation schemes modify the carrier wave's amplitude, fre-

h 81 82 quency or p ase . . Results indicate that amplitude-modulated micro­

waves at a SAR of 2.5 W/kg, corresponding to a plane wave equivalent power density of approxi­mately I mW/cm2 are capable of altering biological act ivity in cell cultures ill vitro. The rad iation from digi tal cellular phones can cause sign ificant changes in ODC ac tiv ity, but not from analog phones (evi­dently because they are FM). The data suggest that the same coherence requirements necessary for ELF­induced bioeffects apply to the modulation of ELF

amplitude-modulated microwaves. 1t is now clear that the use of SAR alone is inadequate for setting safety standards. The type of modulation must also be con­sidered.

Exposure standards for RF energy spec i fy the maximum levels of power density for human ex po­sure. These exposure limits depend on radi ati on fre­quency, exposure duration and the population ' ex­posed (occupational or general public). Compliance with set limits is usuall y determined by averaging measurements over a given period of lime (e .g. I or 6 min), Exposure limits (mW/cm2, Vim , or Aim) for radiation protection purposes are deri ved from fun­damental SAR values .

The thresholds of revers ible behav ioural di srupti on had been found for whole-body-averaged rates of EM energy absorption (S ARs) of the order of 4 to 8 W Ikg, in spite of the considerable difference in carrier fre­quency (600 MHz to 2450 MHz) spec ies (rodents versus primates) and modes of irrad iation (cav ity, waveguide and plane wave). The standa rd s are de­signed to ensure that whole-body-ave raged SAR would not exceed 0.4 W/kg fo r any human we ight or age group. Dosimetric informati on on EM energy ab­sorption in human was used to obtain power density as a function of frequen cy so that under worst cir­cumstances (E-vector paralle l to the length of the body ; grounded and ungrounded conditions), the whole-body-averaged SAR distributi on, less than 0.4 W/kg, factor of five lower than those fo r occupati onal exposure, since the general pub li c was perceived as a potent ia lly more sensitive popu lat ion and can be ex­posed fo r longer periods than workers.

Recognizing the highl y non-uniform nature of SAR di stributi on, there may be some regions where there is high local SARs. The standard prescribes th at loca l SAR "in any I g of ti ssue" should not exceed 8 W Ikg.

Exposure limits below 10 MHz we re not based on the SAR limit , but on the levels of e lectri c fi eld strength that minimize possib le RF shocks when making contact with conduct ing objects. The va lues of magnetic field strength are deri ved rrom the equivalen t plane wave condition.

Below resonance, where the human body is much less than 0.4 wavelength long, one must c nsider separately the electric and magneti c fie ids because their effects are diffe rent. For thi s, the magneti c and electric field curves are separate below lOa MHz (Fig. 5). For a plane electric fi eld in vacuum (or air), the strength E is gi ven by

..

BEHARI : ISSUES IN ELECTROMAGNETIC FIELD-BIOINTERACTIONS 359

200 ......................... 100 50 Earth 's magneti c

Field 20

1--0 :i

"U 10 -.; t::: .~ ;:; 5 c OIl 1 000 '" ::;:

2 600 VIm 500

.§ 200 >

O. "U -.; 100 t::: u

0.2 '5 50 u

" ill 0.1 20

Magnetic field

Electric fi eld

27 VIm

cit'· ens band

cellul ar phones r cordless phones r .r f microwave OVC'f

O.I f1T

Wavelength. cm

2 Wlm'

Power density

police , adars

100

50

20

10

2

Fig. 5-Maximum permi ssible ex poSure (MPE) limits.

E = -.J 377P,

where P is the power density. For the magnetic fi e ld

H = -.Jp /377

For P = 2 W m-2, we get H = 0.0728 Am-I, (0 .09 15

IlT). An important fea ture to RF standards is that the

most restri ctive li mits are imposed in the human reso­nance frequency range (30-300 MHz). In addit ion, the general shape of the curve of exposure limit wi th fre­quency is based on RF absorpt ion patterns in humans; the hi gher the absorpti on at a given frequency, the more restrictive is the exposure limit. There is a need to imp lement safety standards in our country.

Elect romaglletic fi eld st ress Electromagnetic fi e ld exposure to hu mans forms a

stress syndrome. T his is, as of today is c lassified as non-speci fic stress. It renders the body defence mechani sm weak and prudent avo idance is recom­

mended .

Geopathic stress Geopathic stress is a result of subtl e e lectromag­

netic freq uencies e mi tted by the earth th at are harmful to biological syste ms. Sources of Geopathi c stress are water veins, underground st reams crossi ng, ground water currents , Fault lines and cavities, mineral de­posi ts, eiec tI ically chat ged ground , global grids­Hartmann n t or Curry grid, alld Curry grid I un di -

agonally to the Hartmann net. These can be parti cu­larl y harmful when 2 grids cross each othe r.

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