Energy absorption by the human body from RF and microwave emissions in Sri Lanka

Transcription

1 N I Sri Lankan Journal of Phsics, Vol. 7 (26), T I T S U T SRI OF P YS.. LAN KA ICS INSTITUT OF PYSICS - SRI LANKA nerg absorpion b he human bod from RF and microwave emissions in Sri Lanka M. A. A. Karunarahna and I. J. Daawansa* Deparmen of lecronic and Telecommunicaion ngineering, Universi of Morauwa, Morauwa Absrac This paper presens he compued Specific Absorpion Rae (SAR) in he human ee using measured field srenghs from RF signals radiaed b FM radio, TV and mobile cellular base saions operaing in he 9 M band, in differen pars of Sri Lanka. The SAR values in he ee are well below he inernaionall recognied FCC sandards on human eposure o RF radiaion. Calculaions were carried ou using he Finie Difference Time Domain (FDTD) mehod. The esimaed SAR disribuion in some organs of he human bod due o eposure from signals of frequencies ranging from M o 25 M, based on an inciden field of srengh V/m, are also presened. Some of he mos vial organs of he human bod, he ee, he esis, he brain and he kidne, have been eamined. Resuls indicae ha in all he organs invesigaed ecep in he esis, he maimum SAR increases wih increasing frequenc. Among he four organs he highes SAR of value.38 W/kg is in he ee and i occur a he highes frequenc. The SAR disribuion in he ee shows ha beond 9 M is maimum value shifs owards he inerior of he ee and a 25 M i is almos a he cenre of he ee. In he kidne, a low frequencies, he highes SAR is a he fron laers and i ges shifed o he cenre of he kidne a 9 M and beond his frequenc he peak SAR shifs owards he inerior. SAR values inside he four vial human bod organs analed in his paper are below FCC sandards and herefore a presen we are safe.. INTRODUCTION The rapid deplomen of wireless communicaion ssems such as cellular phones has caused an increased concern for he poenial dangers o public healh as a resul of eposure o elecromagneic waves. Signals are ransmied in he form of elecromagneic waves in all direcions b FM radio ransmiers broadcasing in he VF region and TV ransmiers broadcasing in he VF and UF * Corresponding Auhor, -mail: indra@en.mr.ac.lk

2 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), regions. In cellular mobile ssems and fied wireless local loop ssems, base saions ransmi in he UF and microwave regions. Compuer neworks and he Inerne are accessed using wireless echniques a frequencies in he microwave range from 2 o 4 G, using blueooh echnolog for shor disances, WiFi echnolog for longer disances and WiMa echnolog for ver long disances such as o 2 km. As he number of users grows, more base saions are insalled o increase he ssem capaci. In urban areas, base saions are closer ogeher bu operaed a lower power levels han he rural areas where cells end o be larger in sie. Thus we are consanl being eposed o RF and microwave radiaion from a varie of wireless communicaion ssems. The elecromagneic waves of differen power levels and differen frequencies penerae ino he human bod causing healh risks; his is of grea public concern. The rae a which elecromagneic energ is absorbed b he issues of he human bod is usuall quanified b he specific absorpion rae or he SAR value. I is a quani ha depends on he issue mass and he srengh of he elecromagneic waves inside he bod as given in equaion (), SAR 2 where is he elecric field inensi of he elecromagneic wave in he issue, s and r are he conducivi and issue mass respecivel, of he human bod organ under eaminaion. To avoid adverse healh effecs, several repued organiaions such as he Insiue of lecrical and lecronic ngineers [] (I), Federal Communicaions Commission [2] (FCC), he Naional Council on Radiaion Proecion and Measuremens [3] (NCRP) and he Inernaional Commiee on Non- Ioniing Radiaion Proecion [4] (ICNIRP) and he Naional Radiaion Proecion Board [5] (NRPB) have adoped eposure guidelines for he general public as well as for RF workers in he course of heir regular duies. According o FCC sandards, he safe level of SAR for he general public is.6 W/Kg in g of issue. Sandards also specif he eposure level in erms of he inciden elecric field srengh, (V/m), and he power densi, S (W/m 2 ). posure Quoien is a useful dimensionless quani ha epresses he eposure due o elecric fields of muliple radio signals a an locaion as given in equaion (2). n S i posure.. Quoien (2) MP where S i is he power densi and MP is he maimum permissible eposure of power densi a he i h frequenc. For safe, he eposure quoien mus be less han or equal o one. 2 i i () 2. BIOLOGICAL FFCTS DU TO XPOSUR I has been known for man ears ha eposure o RF radiaion can be harmful due o he abili of RF energ o hea biological issue rapidl. posure o ver high RF power densiies of he order of 36

3 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), mw/cm 2, can resul in heaing biological issues and hereb increasing he bod emperaure [6]. Tissue damage could also occur during eposure o high RF levels because he bod is unable o cope wih or dissipae he ecessive hea ha could be generaed. Under cerain condiions, eposure o RF energ a power densi levels of o mw/cm 2 can resul in measurable heaing of biological issue [6] bu no necessaril cause issue damage. Two areas of he bod, he ees and esis, are known o be paricularl suscepible o heaing b RF energ because of he relaive lack of available blood flow o dissipae he ecessive hea. I is repored ha shor erm eposure o ver high levels of RF radiaion have caused caaracs in rabbis [6]. Temporar serili, caused b such effecs as changes in sperm coun and sperm mobili, is repored o be possible afer eposure of he esis o high-level RF radiaion. Mice and rabbis have been emploed for mos of he eperimenal invesigaions on biological effecs of RF eposure. The RF eposure levels in differen pars of Sri Lanka are known [7]. The measured values consised of fields radiaed b a large number of FM radio, TV and cellular mobile base saions. From several spo measuremens aken from over 2 major owns in he counr i has been shown ha he eposure levels a hese locaions are well below FCC sandards [2,7]. This paper repors he SAR values calculaed for he human ee, using he measured values of he elecric field srengh in differen ciies in Sri Lanka and he are compared wih he FCC sandards. The ee is aken for he calculaions because i is one of he mos sensiive organs of he human bod. This paper also repors he resuls of esimaed SAR disribuion in differen organs of he human bod such as he ee, brain, kidne and esis, due o differen inciden RF signals a differen frequencies. 3. FILD DISTRIBUTION INSID T BODY AND SAR CALCULATIONS Several mehods have been described in he lieraure [8] for numerical calculaions of raes of elecromagneic energ absorpion. One common echnique is he Mehod Of Momens or MOM which is a frequenc domain echnique. The oher common mehod is he Finie Difference Time Domain or FDTD which is a ime domain mehod. The FDTD mehod firs proposed b Yee [9] and laer developed b man researchers [-2] is he mos ofen used numerical echnique o solve he elecromagneic dosimer problems [3-5]. This paper repors resuls of SAR calculaions using FDTD mehod o eamine he SAR disribuion in differen organs of he human bod. 3. Field Disribuion inside he bod The elecric field inensi a differen poins ha make up he organ under invesigaion is obained using he FDTD mehod. In his mehod, Mawell s equaions in he differenial form are solved when an inciden uniform plane elecromagneic wave propagaes hrough he bod as shown in 37

4 38 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), The componens of and fields are posiioned a half sep inervals around a uni cell. The parameers σ, µ and ε are he conducivi, permeabili and permiivi respecivel of he bod. The vecor equaions (3) & (4) represen a ssem of si scalar equaions, which can be epressed in he recangular coordinae ssem (,, ) as; Fig.. This is carried ou in a ime-sepped manner unil convergence is reached. The Mawell s ime-dependen curl equaions are, The bod under invesigaion is divided ino a large number of small cubic cells of sie δ, δ, δ and he invesigaion is carried ou a δ ime inervals unil sead sae is reached. Fig. Uniform plane wave inciden on he bod. The propagaion of an inciden RF wave, i, hrough he bod having elecrical properies, σ, µ and ε is governed b equaions (5) o (). The equaions are solved using FDTD mehod b convering he si parial differenial equaions ino si difference equaions from which all si field componens, (5) (6) (7) (8) (9) () (3) (4)

5 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), 35-47,,,,, can be obained. Following Yee s noaion, we assume ha he grid poins of he bod are defined using (i, j, k) wih coordinaes given as, ( iδ, jδ, kδ ), where δ = δ = δ = δ is he sie of he uni cubic cell. For good accurac in FDTD simulaions, he cell dimensions should be less han one enh of he wavelengh in he medium when i has he highes permiivi. i.e: m f () For sabili, he ime sep is given b,..c r 2c (2) In he FDTD mehod, he radiaion condiion is no implici and herefore as he RF wave propagaes inside he bod, he scaered waves mus be runcaed a he edge of he compuing field. The runcaion used is in accordance wih Baliss and Turkel [6], and i resuls in an error as given b equaion (3). c rror 2 f r where f is he frequenc of he inciden signal, r is he disance from he cener of he scaering sie o he edge of he compuing field and c is he veloci of ligh in free space. quaion (3) implies ha problems using lower inciden frequencies ma require a larger volume surrounding he bod o avoid errors due o imperfec runcaion condiions. Thus a compuing field of approimael 5δ o δ is used for minimum runcaion error [7]. 2.5 (3) 3.2 SAR disribuion When he -field disribuion inside he bod is esimaed, he SAR disribuion is also easil obained from equaion (). The region wih he maimum field corresponds o he region wih maimum SAR value. ence he maimum SAR and he SAR disribuion for differen bod organs of differen shapes and sies having differen elecrical properies, σ, µ and ε can be esimaed from he -field disribuion. 4. ORGAN MODLS FOR SAR CALCULATIONS For SAR calculaions, he bod organs under invesigaion are modeled in such a wa ha calculaions can be carried ou wihou much difficul. Spherical models are used for he human ee [8] and esis [9] wih average radii being 8.75 mm and 25.5 mm respecivel. A cubical bo model of 39

6 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), side 2 cm is aken for he brain. A recangular bo model of 6 4 cm is aken for he kidne [9]. Tissues of brain, ee sclera, kidne and esis are aken as homogeneous. For elecrical characeriaion of he issues, he permiivi and conducivi daa repored in Gabriel s paper [2] and heir highes values in he range of frequencies of ineres are aken for SAR calculaions. According o equaion (3), he lower inciden frequenc dicaes a larger surrounding volume o minimie errors. Raher han using an enormous amoun of compuer resources, he cell sie was increased a lower frequencies, wihou sacrificing accurac and sabili of he soluions. Using he smmerical properies of he organs, onl one fourh of he organs are used for SAR evaluaion, in order o reduce he compuaional ime. For evaluaion of he SAR in he human ee a he frequenc of 9 M, he ee was divided ino a number of cubic cells of sie δ =.25 mm and enclosed in o a recangular bo of sie 39δ 39δ 39δ, and laice runcaion condiion was applied for he FDTD mehod. Fig. 2 shows one forh of he ee model a his frequenc. The inciden plane wave is assumed o propagae in he + direcion. This wave is generaed a he laice plane =3δ. A planes =, =39δ, =δ/2 and = he field componens and are deermined using laice runcaion condiions. Fig. 2 The ee model and he aes All models seleced for SAR evaluaion in he ee, brain, esis and kidne are shown in Fig.3. (a) cubical bo model; brain (a) Spherical model; ee (a) recangular bo model; kidne (a) Spherical model; esis Fig. 3 uman organ models and heir dimensions (a) Brain (b) e (c) Kidne (d) Tesis 4

7 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), OBSRVATIONS & RSULTS 5. SAR Levels in he e of he Public The SAR values in he ee are evaluaed using FDTD mehod for he public using spo field measuremens [7] aken in differen urban areas. As he human ee is one of he mos sensiive organs of he bod, he SAR values for g issue of he ee are evaluaed for he wors-case siuaion for each ci for compliance esing. The maimum SAR values obained are given in Table. The oal SAR g for a paricular frequenc range is calculaed b mulipling he maimum SAR value b he number of received signals of ha frequenc range. This procedure is applied o he oher hree frequenc ranges as well and finall he grand oal SAR g is calculaed for each sie. As seen in he resuls, he oal SAR g values in each ci are well below he safe limis. The highes SAR value of.922 W/kg was observed from Badulla; measuremens were made a he Uva provincial cricke grounds. The measuremens indicae ha he highes conribuor o he SAR a Badulla is from he mobile base saions; perhaps he measuremen sie which is he Uva provincial Table. Maimum SAR in he ee of he general public Ci No. of received signals SAR g (W/kg) for he human ee FM TV (VF) TV (UF) Base Saions Toal FM TV (VF) TV (UF) Cellular mobile base saions Toal Colombo For Nuwara-lia Colombo Uni Kegalle Dambulla Maale Kuliapiia Kand Gampaha Maara Galle Kurunegala Morau. Uni Ambalangoda Anuradhapura Mahiangana Badulla Ranapura Chilaw Pulam Negombo

8 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), cricke grounds is surrounded b a significan number of mobile base saions. The esimaed SAR value a he sie is 5.76% of he FCC sandard of.6 W/kg for he general public. Resuls in Table also show ha he main eposure from all sies is due o he cellular mobile base saions. As he cellular mobile base saions are rapidl on he increase, he SAR would increase beond he values given in he able and some public concern has o be epeced as high SAR could cause healh risks.there are no such field measuremens or SAR calculaions repored from neighbouring counries for comparison. Up o now here are ver few resuls repored from oher pars of he world [2] and he include onl emissions from one or wo ransmiers and as such he esimaed SAR values are ver low. 5.2 Maimum SAR in he human bod organs: ee, brain, kidne and esis The maimum SAR in four vial organs of he human bod due o RF & microwave eposure from a uniform field of srengh V/m is eamined. This field srengh is a reasonable value for wors case esimaes. SAR calculaions are carried ou for inciden signals of frequencies from M o 9 M in he range where field srengh measuremens are carried ou. Calculaions are eended o frequencies from 9 M o 25 M which are used a presen in cellular mobile ssems and fied wireless communicaion ssems. The human bod organs are modeled as menioned earlier in his secion. The SAR a a single frequenc is esimaed as a uniform plane sinusoidal wave of ha paricular frequenc is made o propagae ino he organ. The maimum SAR values for g of he issue are shown in Table 2. Organ Model Table 2. Maimum SAR g values in he brain, ee, kidne and he esis Maimum SAR g (W/kg) M 2 M 5 M 9 M 8 M 25 M Brain Cubical bo e Spherical Kidne Recangular bo Tesis Spherical Table 2 shows ha in all organs ecep he esis, he maimum SAR g values increase wih increasing frequenc. Our calculaions show ha he highes SAR is in he ee and i is approimael.3 W/ kg and occurs a he highes frequenc of 25 M. The FCC sandard is.6 W/kg. Such high values of SAR are considered o be a possible cause of ee caarac [22-23]. The SAR in he brain is higher han ha of all oher organs a low frequencies. In he case of he esis, he maimum SAR increases wih increasing frequenc; reaches a peak a 8 M and falls off as he frequenc coninues o increase. This is clearl seen when he SAR disribuion is eamined as discussed in he ne secion. 42

9 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), A M Fig. 4 SAR in he ee J Cell number A 9M A M 5.3 SAR disribuion in human bod organs Fig. 5 SAR in he kidne A 9M The disribuions of SAR in he four organs, ee, brain, kidne and he esis due o RF and microwave signals of differen frequencies is eamined a a field srengh of V/m. Calculaions are carried ou a from M o 25 M. From M o 9 M, here are some differences in he shape of he SAR disribuion curves in he cases of kidne and esis. For clari, Figs. 4, 5, 6 and 7 show he SAR disribuions in he ee, brain, kidne and he esis a M and 9 M. The horional aes show he deph of each organ in he direcion of propagaion of he elecromagneic wave or he signal. The origin corresponds o he cenre of he fron laer of he organ. Fig. 6 indicaes ha he highes SAR values are observed a he fron laers of he brain a all frequencies and an increase in he frequenc does no affec he shape of he curve. In he case of he kidne, a low frequencies, he highes SAR is observed a he fron laers and his ges shifed owards he cener of he kidne a 9 M. 43

10 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), A M SAR(W/kg) A 9M Fig. 6 SAR in he brain A M A 9M Fig. 7 SAR in he esis When he frequenc of he inciden signal is increased beond 9 M, in he ee and he esis, an increase in he SAR value is observed owards he inerior of he organ. The ee and esis are said o be more sensiive o higher frequencies [2]. In he case of he ee, he posiion of he highes SAR ges shifed owards he cener of he ee as he frequenc increases. This implies ha when he ee is eposed o higher frequencies, ho spos are creaed owards is cener as seen in Fig. 8. A similar behaviour is observed in he esis, as shown in Fig. 9. owever he peak value decreases beond a cerain frequenc, in his case a around 8 M. In he case of he kidne, he SAR disribuion is such ha, alhough a low frequencies he SAR is high a he surface, his value drops o less han half a high frequencies. owever, here is a broad peak owards he middle of he esis a high frequencies and he peak becomes narrower and sharper wih increasing frequenc; is magniude graduall drops as he frequenc increases. This is shown in Fig.. Is biological implicaions are no e known. 44

11 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), M 3 M 8 M 25 M M 3 M 8 M 25 M Fig. 8 SAR disribuion in he ee Fig. 9. SAR disribuion in he esis M 3 M 8 M 25 M Fig. SAR disribuion in he kidne 6. CONCLUSIONS All field measuremens aken indicae ha he esimaed SAR g value in he ee issue of he public, in major ciies of Sri Lanka, is reasonabl below he FCC sandard. owever he esimaed SAR values are no oo far awa from safe sandards. Our measuremens were confined o FM radio, TV and Cellular Mobile operaing in he 9 M band. We could no ake field measuremens from CDMA fied wireless neworks operaing in he 25 M band, as he wen ino operaion recenl. The major conribuion o he SAR is from he mobile cellular base saion emissions. For heoreical calculaions of SAR in he several organs of he bod, we considered signals of frequencies ranging up o 25 M. owever calculaions are based on an inciden field of srengh of V/m which is close o he wors case considering he measured field srenghs. I is also he field srengh considered b man oher researchers. Of he four vial bod organs eamined, he highes SAR levels are seen o be a he fron laers of he organs up o a frequenc of 9 M ecep in he case of he kidne. A peak occurs in he SAR in he middle of he ee and in he esis a high 45

12 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), frequencies. The similar peak in he SAR curve is seen in he kidne. Is magniude ges smaller when he frequenc goes beond a cerain high frequenc and he peak ges shifed o he inerior wih increasing frequenc. owever, all esimaed values are reasonabl less han he public safe limis specified b FCC. When he 3G mobile communicaion ssems become operaional in he ver near fuure, public would be eposed o more microwaves and he eposure levels would go up. The ee and esis would be eposed o higher frequencies such as 35 M and beond. As such ho spos would be creaed close o he cener of he ee and he esis; his cerainl should be of public concern. As he demand for elephone and broadband inerne access increases, he wireless nework providers would insall more base saions wih he resul ha he srengh of he field eposed o, b he public, would go up and wih ha he SAR would go up. ence i is advisable o carr ou SAR evaluaions periodicall o check and mainain SAR a accepable levels for public safe. Our resuls indicae ha he maimum SAR values inside he four vial human bod organs analed in his paper are below FCC sandards and herefore a presen we are safe. ACKNOLDGMNTS The auhors are graeful o he Telecommunicaion Regulaor Commission for he assisance eended in order o carr ou calculaions. RFRNCS. I sandard for safe levels wih respec o human eposure o radio frequenc elecromagneic fields 3 k o 3 G, I Sd. C95., (Insiue of lecrical and lecronics ngineers, New York, NY, 999). 2. valuaing Compliance wih FCC Guidelines for human eposure o Radio Frequenc lecromagneic Fields, FCC OT Bullein.65, Washingon D.C., Augus Biological ffecs and posure Crieria for Radio Frequenc lecromagneic Fields, NCRP Repor 86, (Naional Council on Radiaion Proecion and Measuremens, Behesda, MD,986), ICNIRP, Guidelines for Limiing posure o Time-Varing lecric, Magneic and lecromagneic Fields (up o 3 G), ealh Phsics, 74, (998), NRPB, ealh ffecs from RF lecromagneic fields, Repor of an independen Advisor Group on Nonioniing Radiaion, 4, (23) James C. Lin, Biological Bases of Curren Guidelines for uman posure o Radio-Frequenc Radiaion, I Anennas and Propagaion Magaine, 45, (23) M. A. A. Karunarahna and I. J. Daawansa, uman posure o RF radiaion in Sri Lanka, Sri Lankan Journal of Phsics, 6, (25) R. J. Spiegel, A review of numerical models for predicing he energ deposiion and resulan hermal response of humans eposed elecromagneic fields, I Trans. Microwave Theor Tech., 23, (975)

13 M. A. A. Karunarahna and I. J. Daawansa, Sri Lankan Journal of Phsics, Vol. 7, (26), K. S. Yee, Numerical soluion of iniial boundar value problems involving Mawell s equaions in isoropic media, I Trans. Anenna Propaga., 7, (966) A. Taflove and M.. Morris, Compuaion of he elecromagneic fields and induced emperaures wihin a model of he microwave irradiaed human ee, I Trans. on Microwave Theor Tech., 23, (975) 66.. K.Umashankar and A. Taflove, A novel mehod o anale elecromagneic scaering of comple objecs, I Trans. lecromagn. Compa., 24, (982) A. Taflove and K. Umashankar, Radar cross secion of general hree-dimensional scaerers, I Trans. lecromagn. Compa., 25, (983) Dennis M. Sullivan, O. M P. Gandhi and Allen Taflove, Use of he Finie-Difference Time-Domain Mehod for Calculaing M Absorpion in Man Models, I Trans. on Biomedical ngineering, 35, (988) Dennis M. Sullivan, David T. Borup and O. M. P. Gandhi, Use of Finie-Difference Time-Domain Mehod in Calculaing M Absorpion in uman Tissues, I Trans. on Biomedical ngineering, 34, (987)2. 5. sing-yi Chen, ou-wa Wang, Curren and SAR Induced in a uman ead Model b he lecromagneic Fields Irradiaed from a Cellular Phone, I Trans. on Microwave Theor Tech., 42, (994) A. Baliss and. Turkel, Radiaion boundar condiions for wave like equaions, Commun. Pure Appl. Mah., 33, (98) Mahew N.O. Sadiku, Numerical Techniques in lecromagneics. (Florida: CRC Press 992) Allen Taflove, Morris. Brodwin, Compuaion of he lecromagneic Fields and Induced Temperaures Wihin a Model of he Microwave-Irradiaed uman e, I Trans. on Microwave Theor Tech., 23, (975) arold llis, Clinical Anaom A revision and applied anaom for clinical sudens, (Blackwell scienific publicaions, London, 98). 2. C. Gabriel, Compilaion of he dielecric properies of bod issues a RF and microwave frequencies, Tech. Rep. AL/O-TR-(996) (Brooks Air Force, Brooks AFB, TX, 996). 2. P. Bernardi, M. Cavagnaro, S. Pisa and. Piui, uman eposure o radio base saion anennas in urban environmen, I Trans.Microwave Theor Tech., 48, (2) J. C. Lin, Caaracs and Cell-Phone Radiaion, I Anennas and Propagaion Magaine, 45, (23) I Recommended Pracice for Measuremens and Compuaions of Radio Frequenc lecromagneic Fields Wih Respec o uman posure o Such Fields, k-3 G, I Sd. C95.3, (I, New York, 22). 47