Ocular Effects of Exposure to 40, 75, and 95 GHz Millimeter Waves

Transcription

1 J Infrared Mll Terahz Waves (2018) 39: Ocular Effects of Exposure to 40, 75, and 95 GHz Mllmeter Waves Masam Kojma 1,2,3 & Yukhsa Suzuk 4 & Kensuke Sasak 5 & Masao Tak 4 & Kanako Wake 5 & Soch Watanabe 5 & Maya Mzuno 5 & Takafum Tasak 6 & Hrosh Sasak 1,2 Receved: 16 January 2018 / Accepted: 10 May 2018 / Publshed onlne: 27 May 2018 # The Author(s) 2018 Abstract The objectve of ths study was to develop a model of ocular damage nduced by 40, 75, and 95 GHz contnuous mllmeter waves (MMW), thereby allowng assessment of the clncal course of ocular damage resultng from exposure to thermal damage-nducng MMW Ths study also examned the dependence of ocular damage on ncdent power densty Pgmented rabbt eyes were exposed to 40, 75, and 95 GHz MMW from a spot-focus-type lens antenna Slght ocular damage was observed 10 mn after MMW exposure, ncludng reduced cornea thckness and reduced transparency Dffuse fluorescen stanng around the pupllary area ndcated corneal epthelal njury Slt-lamp examnaton 1 day after MMW exposure revealed a round area of opacty, accompaned by fluorescence stanng, n the central pupllary zone Corneal edema, ndcatve of corneal stromal damage, peaked 1 day after MMW exposure, wth thckness gradually subsdng to normal Three days after exposure, ocular condtons had almost normalzed, though corneal thckness was slghtly greater than that before exposure The 50% probablty of ocular damage (DD 50 ) was n the order 40 > GHz at the same ncdent power denstes * Masam Kojma m-kojma@kanazawa-medacjp Dvson of Vson Research for Envronmental Health, Medcal Research Insttute, Kanazawa Medcal Unversty, Kahoku, Japan Department of Ophthalmology, Kanazawa Medcal Unversty, Kahoku, Japan School of Nursng, Kanazawa Medcal Unversty, 1-1 Dagaku, Uchnada, Kahoku, Ishkawa , Japan Department of Electrcal Engneerng and Computer Scence, Graduate School of Systems Desgn, Tokyo Metropoltan Unversty, Tokyo, Japan Natonal Insttute of Informaton and Communcatons Technology, Tokyo, Japan Dvson of Proten Regulaton Research, Medcal Research Insttute, Kanazawa Medcal Unversty, Kahoku, Japan

2 J Infrared Mll Terahz Waves (2018) 39: Keywords Mllmeter wave Pgmented rabbt eye Corneal epthelum damage Ocular damage dose 1 Introducton Mllmeter wave (MMW) technologes have come nto wdespread use n daly lfe, ncludng n hgh-speed wreless communcatons, sensng, hgh-resoluton radar magng, spectroscopy, and automoble collson preventon systems For example, Wreless Ggabt (WGg) products, whch operate at frequences n 60 GHz band, are now commercally avalable, wth addtonal MMW frequency bands expected to be used n the ffth generaton wreless communcaton technologes [1 3] Industral development of 60 GHz technology and user expectatons have ncreased concomtantly However, ncreased publc exposure to MMW has heghtened the need to evaluate ther health effects Several n vvo studes n expermental anmals have evaluated the specfc effects of MMW [4 6] Frequency-specfc thresholds for ocular damage were observed at 35 and 107 GHz Exposure to 35 GHz MMW was assocated wth corneal damage, ncludng hgh levels of corneal epthelal njury, persstng for almost 2 days, whereas exposure to 107 GHz was assocated wth transent njury to the corneal stroma, albet more powerful n nducng mmedate corneal stromal damage [4] Under pulse wave condtons, the thresholds for corneal njury, corneal edema, and corneal epthelal defect were found to be 75 J/cm 2 for 35 GHz and 50 J/cm 2 for 94 GHz [5], suggestng that dfferent frequences have dfferent ocular effects These earler studes employed a crcular horn antenna [4, 6] or an open-ended wavegude [5] Because ther exposure methodology and expermental anmals dffered, the results of these studes cannot be drectly compared Smlarly, we reported that dfferent antennas caused dfferent ocular effects [7] To examne the clncal course of MMW-nduced ocular damage wth a hgh degree of reproducblty, we developed a model of acute ocular njury usng hgh dose 60 GHz MMW n rabbt eyes [7] The objectve of ths study was to develop models of ocular damage nduced by 40, 75, and 95 GHz MMW, allowng us to evaluate the clncal course of thermal damagenduced ocular njury We also assessed the dependence of ocular damage on ncdent power densty, as well as the frequency characterstcs of the ocular damage n rabbt eyes exposed to 40, 75, and 95 GHz MMW 2 Materals and Methods 21 Exposure System The n vvo exposure system has been prevously descrbed n detal [8] Therefore, confguraton of the exposure system s only brefly descrbed here Fgure 1 depcts a block dagram of the system whch comprses power sources, e, a sgnal generator (75 and 95 GHz) or a sgnal generator and amplfer (40 GHz), drectonal couplers, power sensors, and a power meter (E4417A, Aglent Technologes) Power sensors A and B, shown n Fg 1, are used to measure ncdent power to the antenna and reflected power from the antenna, respectvely, va drectonal couplers A spot-focus-type lens antenna, e, a concal horn

3 914 J Infrared Mll Terahz Waves (2018) 39: Power source Drec onal coupler Drec onal coupler Horn antenna For reflected power Power sensor A For ncdent power Power sensor B -15cm lens Fg 1 Block dagram of the exposure system Power meter (E4417A, Aglent Technologes) antenna wth a ϕ-15-cm lens, was used to ensure localzaton to the eye By usng ths equpment, exposure of the skn, e, upper and lower eyelds surroundng the eye tssue, to MMW can be avoded preventng burns; facal burns are a lmtaton n results of MMW exposure studes due to varaton n ndvdual values of ocular damage [7] The level of exposure was calculated as the spatally averaged ncdent power densty over a crcular regon of 13 mm n dameter, the average sze of the corneal regon n Dutch-belted rabbts The specal profles of ncdent power densty radatng from the antenna aperture were measured usng open-ended wavegude probes [8] 22 Expermental Anmals All anmal experments were conducted n accordance wth the anmal study gudelnes of Kanazawa Medcal Unversty (Kahoku, Japan) and the ARVO (Assocaton for Research n Vson and Ophthalmology) statement for the use of anmals n ophthalmc and vson research [9] One hundred and thrty male Dutch-belted pgmented rabbts (12 14 weeks old, kg) were purchased from Sankyo Labo Servce Co, Inc (Toyama, Japan) and kept wth unrestrcted access to food and water At baselne, all rabbt eyes were examned usng a SL- 130 slt-lamp mcroscope (Zess, Tokyo, Japan) to ensure absence of abnormaltes n the anteror segment, and each eye was photographed Each rabbt was njected ntramuscularly (IM) wth a soluton contanng mg/kg of medetomdne hydrochlorde (Domtor, Nppon Zenyaku Kogyo Co, Ltd, Fukushma, Japan) to nduce general anesthesa, and mmoblzed n an acrylc rabbt restraner, specally constructed for studes of exposure to MMW [7] Immedately before MMW exposure, 2% ldocane hydrochlorde topcal anesthetc (Xylocane 2%; AstraZeneca, Osaka, Japan) was admnstered to each eye The upper and lower eyelds were held open wth tape Because anesthesa suppressed blnkng, salne drops, pre-warmed to C, were admnstered to the eyes as necessary to prevent damage to corneal epthelal cells resultng from corneal desccaton

4 J Infrared Mll Terahz Waves (2018) 39: Followng each ocular examnaton, all rabbts were admnstered topcal ofloxacn ontment (Tarvd; Santen Pharmaceutcal Co, Ltd, Osaka, Japan) to prevent secondary nfecton Temperature and humdty durng exposure were mantaned at 24 ± 2 C and 60 ± 10%, respectvely, usng an ar condtoner and dehumdfer Anesthesa was reversed wth mg/kg IM atpamezole hydrochlorde (Antsedan, Nppon Zenyaku Kogyo Co, Ltd, Fukushma, Japan) to help the rabbts recovery 23 MMW Exposure The center of the corneal surface of each rabbt s rght eye was postoned 135 mm from the antenna aperture and on the lne of maxmum radaton of the antenna [7, 8] The exposure pont was set usng red and green laser ponters on a target [10], and the rght eye of 105 rabbts was exposed to contnuous MMW of mw/cm 2 (40 GHz), mw/cm 2 (75 GHz), or mw/cm 2 (95 GHz) for 6 mn The left eye of each rabbt was unexposed and regarded as a control eye To assess the ocular effects of exposure to MMW for 30 mn, the rght eye of another set of 21 rabbts was exposed to 75 GHz ( mw/cm 2 ) MMW, and the left eye of each of these rabbts was treated as non-exposed control eyes All other expermental condtons were dentcal to those n rabbt eyes exposed to MMW for 6 mn In order to assess the effects of sham exposure for 30 mn or exposure to very low ntensty nfrared (IR) rradaton, another set of four rabbts was used The rght eye of each of these was sham exposure whch nvolved the power supply of all exposure systems set to the ON state, but the sgnal generator dd not feed the system wth the (MMW) sgnal The left eye of each of these was exposed wth a 60-W desk lamp, such that the corneal surface temperature reached about 38 C (IR exposure) Ocular condtons were assessed pror to exposure and 10 mn and 1 day after exposure Other expermental condtons were dentcal to those n rabbt eyes exposed to MMW for 6 mn 24 Corneal Surface Temperature Corneal surface temperatures were recorded n MMW and IR exposed eyes at 5 s pror to exposure and at 5 s before the end of MMW exposure usng a thermography camera (R300, NEC Avo, Tokyo, Japan) The same measurements were taken n control and sham exposure eyes 25 Examnaton of Ocular Injury The anteror segment was evaluated before IR and MMW exposure, and at 10 mn, and 1, 2, and 3 days after exposure Corneal epthelal damage was observed by slt-lamp mcroscopy usng a modfed method nvolvng fluorescen stanng of damaged corneal epthelal cells [4] Brefly, rabbt eyes were gently washed wth salne, and fluorescen soluton (005%, 25 μl) was nstlled nto the cul-de-sac wth a mcroppette After a sngle blnk, excess fluorescen was washed out wth salne, and mages of the anteror segment were recorded wth a slt-lamp mcroscope, followng exctaton wth blue lght and montorng wth green lght usng approprate flters (exctaton lght cuttng flter), corneal cross-sectonal thckness was measured and recorded by optcal coherence tomography (OCT, Zess model 5000, Tokyo, Japan), and corneal opacty was assessed usng a slt-lamp mcroscope

5 916 J Infrared Mll Terahz Waves (2018) 39: Categorzaton of Corneal Epthelal Injury Corneal epthelal njury was defned as a round area of epthelal njury n the central pupllary zone of exposed eyes, wth no smlar njury observed n unexposed eyes Other types of corneal epthelal njury, such as desccaton of the cornea (e, dry eye) and mechancal damage, were excluded 27 Data Analyss The probablty of corneal damage at 1 day after exposure dependng on the power denstes to dfferent frequences of MMW was evaluated by maxmum lkelhood estmaton wth probt analyss [11] Morphologcal changes n the cornea were assessed by slt-lamp mcroscopy, ncludng fluorescen stanng, and optcal coherence tomography Ocular dsorders, ncludng corneal epthelal dsorders, corneal opacty, and corneal edema, were determned by observaton at 1 day after exposure The dose-response relatonshp between corneal njury and range of power densty at each frequency was evaluated by fttng wth a cumulatve lognormal dstrbuton functon for probt analyss usng R Ver 333 software [12] The MMW power densty ndcatng the probablty of eye damage was defned as damage dose (DD) and was derved from the best-ft probt functon 3Results Representatve eye njures from exposure wth 200 mw/cm 2 at 75 GHz for 6 mn are shown n Fg 2 Slght ocular damage observed 10 mn after MMW exposure ncluded reduced corneal transparency around the pupllary area (Fg 2d), reduced corneal thckness (Fg 2e), and corneal epthelal damage as ndcated by dffuse fluorescen stanng around the pupllary area (Fg 2f) Slt-lamp examnaton at 1 day after MMW exposure revealed a round area of corneal opacty n the central area of the pupl (Fg 2g) and a round area of fluorescen stanng (corneal epthelal defect, Fg 2) at the same place Corneal edema, ndcatve of corneal stromal damage, peaked at 1 day after MMW exposure, wth thckness gradually subsdng (Fg 2h, k, and n) almost reachng that before exposure (Fg 2b) Tables 1, 2, and3 summarze the effects of exposure to 95, 75, and 40 GHz MMW, respectvely Corneal surface temperature was hghest n eyes exposed to 75 GHz MMW, followed by those exposed to 95 GHz, then those exposed to 40 GHz, n that order for the same ncdent power denstes Under the exposure condton of 95 GHz, 100 mw/cm 2,for 6 mn exposure, the corneal surface temperature reached an average of 370 ± 33 C Corneal epthelal damage was observed only n one rabbt at 1 day after exposure (Table 1) Corneal surface temperature reached 402 ± 05 C even under the same exposure condton (100 mw/ cm 2 for 6 mn exposure) at 75 GHz, corneal epthelal damage and corneal edema were observed n only one rabbt at 1 day after exposure (Table 2) The effect of 40 GHz exposure (100 mw/cm 2, 6 mn exposure) was almost the same as that of 95 GHz exposure, the corneal surface temperature was 371 ± 27 C, and corneal epthelum damage was observed n only one rabbt (Table 3) Corneal surface temperature rses due to MMW exposure vared wth frequency The extent of ocular damage and clncal course were smlar n eyes exposed to hgh doses of 40 and 75 GHz MMW for 6 mn That s, mtoss and damage to the corneal epthelum

6 J Infrared Mll Terahz Waves (2018) 39: Anteror segment Photo Cornea tomographc mage Fluorescen stanng Before exposure A B C 10 mn after exposure Moss+ 1 day after exposure 2 days after exposure 3 days after exposure D G J (nflammaton) damage stromal Corneal E H K M N O Fg 2 Representatve examples of eyes wth corneal damage and ther clncal courses followng exposure of eyes to 75 GHz MMW at 200 mw/cm 2 for 6 mn The numbers ndcate corneal thckness n mllmeters a Anteror segment photo, b Corneal tomographc mage, and c fluorescen stanng at before exposure condton d, e,andf At 10 mn after exposure g, h,and At 1 day after exposure j, k,andl At 2 days after exposure m, n, and o At 3 days after exposure were observed mmedately after rradaton wth ether mw/cm 2 at 40 GHz (data not shown) or 300 mw/cm 2 at 75 GHz The level of ocular damage, however, was mlder n the eyes exposed to 300 mw/cm 2 at 95 GHz exposure, wth mtoss and promnent corneal epthelal damage rarely observed mmedately after exposure n these eyes Damage to both the corneal stroma (corneal opacty) and corneal epthelum was observed at 1 day after MMW exposure At 3 days, corneal opacty was observed n all exposed eyes, Table 1 Summary of the results of exposure of rabbt eyes to 95 GHz MMW Damage + (n/n)ndcatesdamage was observed n n of N exposed eyes l l l l l F I L Corneal epthelal cell damage (surface)

7 918 J Infrared Mll Terahz Waves (2018) 39: Table 2 Summary of the results of exposure of rabbt eyes to 75 GHz MMW Damage + (n/n)ndcatesdamage was observed n n of N exposed eyes l l l l l wth clary njecton peakng n some, although these symptoms gradually mproved Damage to corneal epthelal cells peaked at 1 day after exposure, gradually mprovng over the next 1 2 days The most severe damage to the corneal stroma was observed at 3 days after exposure These levels of ocular damage and clncal course observed n eyes exposed to 40 and 75 GHz MMW were smlar to those reported for 60 GHz MMW [7] Table 3 Summary of the results of exposure of rabbt eyes to 40 GHz MMW Damage + (n/n)ndcatesdamage was observed n n of N exposed eyes l l l l l

8 J Infrared Mll Terahz Waves (2018) 39: Although exposure to 40, 75, and 95 GHz at 50 mw/cm 2 for 6 mn dd not nduce ocular dsorder n any eye, corneal epthelal damage was observed n all fve eyes exposed to 75 GHz MMW at 50 mw/cm 2 for 30 mn In addton, one of these fve eyes also showed corneal edema and opacty ndcatng damage to the corneal stroma (Table 2) Exposure to 75 GHz MMW at 50 mw/cm 2 for 30 mn resulted n a corneal surface temperature of 371 ± 08 C, smlar to rabbt body temperature In contrast, none of the unexposed left eyes (control) showed any ndcaton of eye damage Fgure 3 shows representatve expermental results of MMW sham exposure to rght eyes and smultaneous IR exposure wth a 60-W desk lamp to left eyes for 30 mn Exposure to sham exposure ncreased corneal surface temperature by 14 C, from 325 to 339 C, whereas IR exposure ncreased corneal surface temperature by 51 C, from 327 to 378 C, whch s almost the same as the body temperature of these rabbts Corneal thckness at 10 mn after sham and IR exposure was reduced to 021 and 024 mm, respectvely (Fg 3g, l) Shamexposed eyes were negatve for fluorescen stanng, but damage to corneal epthelal cells was observed n IR-exposed eyes One day later, the sham-exposed eyes appeared normal, although ther corneas were slghtly thnner than normal (Fg 3m, n, and o) In contrast, IR-exposed eyes at 1 day after exposure showed corneal epthelal damage accompaned by corneal edema (Fg 3q, r) The assocaton between ocular damage and the power densty at dfferent MMW frequences was assessed by determnng the probablty of corneal damage at 1 day after exposure by maxmum lkelhood estmaton (MLE) wth probt analyss (Fg 4) The markers plotted n each graph denote the dependence of probablty of eye damage relatve to the number of eyes exposed to the power densty (dose) followng actual MMW exposure Table 4 and Fg 5 show the 10, 50, and 90% probablty of ocular damage DD 10% (DD 10 ), DD 50% (DD 50 ), and DD 90% (DD 90 ), as determned by MLE, followng exposure to 40, 75, and 95 GHz Fg 3 Representatve corneal damage to eyes exposed to MMW sham and IR rradaton wth a 60-W desk lamp for 30 mn

9 920 J Infrared Mll Terahz Waves (2018) 39: Fg 4 Results of maxmum lkelhood estmaton wth probt analyss for exposure to 40, 75, and 95 GHz MMW The markers n each graph denote the dependence of the probablty of damaged eyes relatve to the number of eyes exposed to the ndcated power densty (dose) The number of rabbt eyes at each power densty corresponds to the sze of the damond-shaped markers The blue and red lnes ndcate the lower and upper lmts on 95% confdence ntervals, respectvely MMW The 50% probablty of ocular damage (DD 50 ) was n the order 40 (206 mw/cm 2 )>95 (146 mw/cm 2 ) 75 GHz (143 mw/cm 2 ) at the same ncdent power denstes 4 Dscusson 41 Comparson Between our Methodology and that of Prevous Studes The man purpose of ths study was to develop an approprate rabbt model of thermally nduced ocular damage usng dfferent frequences of MMW (40, 75, and 95 GHz) Ocular Table 4 Power denstes for DD 10% (DD 10 ), dose level to cause ocular damage wth 10% probablty, DD 50% (DD 50 ), dose level to cause ocular damage wth 50% probablty and DD 90% (DD 90 >) dose level to cause ocular damage wth 90% probablty estmated from MLE results for exposure to 40, 75, and 95 GHz MMW Frequency Power densty for DD 10 [mw/ cm 2 ] Power densty for DD 50 [mw/ cm 2 ] Power densty for DD 90 [mw/ cm 2 ] 40 GHz GHz GHz

10 J Infrared Mll Terahz Waves (2018) 39: Fg 5 DD 10% (DD 10 ), dose level to cause ocular damage wth 10% probablty, DD 50% (DD 50 ), dose level to cause ocular damage wth 50% probablty and DD 90% (DD 90 ) dose level to cause ocular damage wth 90% probablty estmated from results of MLE for exposure to 40, 75, and 95 GHz MMW Error bars ndcate 95% confdence ntervals for each marker damage, ncludng damage to the corneal epthelum, corneal edema, and corneal opacty, resultng from exposure to hgh ntensty MMW dffered somewhat by frequency, but was generally smlar Rosenthal et al [4] classfed corneal dsorders caused by MMW exposure nto Bsuperfcal and deep keratts^ They also pont out that surface keratts wll heal wthn 24 h and deep keratts wll be Bpersstent keratts and leucoma (permanent scar n the stroma)^ [4] Heat generated by MMW energy s absorbed n the cornea, resultng n thermal damage of the cornea The ntal step of corneal dsorder, occurrng 10 mn after exposure to MMW, ncluded thnnng of the cornea (corneal desccaton) due to thermal dsturbance and partal dropout of corneal epthelal cells, as shown by dffuse stanng wth fluorescent dye These fndngs seem to be surface keratts descrbed by Rosenthal et al [4] Corneal njury at 1 day after exposure was characterzed by crcular fluorescence stanng, ndcatng a defect n the corneal epthelum, and confrmed by slt-lamp mcroscopy Damage to the corneal epthelum can be repared by the prolferaton [13] and mgraton [14] of cells n the basal layer A study of a mechancally nduced corneal epthelal dsorder n rabbt eyes of dameter 6 mm, reported that ths dsorder was repared wthn 48 h [15] Smlarly, we found that fluorescen stanng dsappeared about 2 days after MMW exposure Corneal edema seen after MMW exposure convexfed to the corneal epthelum sde, whereas corneal edema due to corneal endothelal damage protruded on the corneal endothelal sde Corneal edema or opacty s thought to be due to the entry of water nto the corneal stroma from the ste of the corneal epthelum defect caused by thermally nduced cell death Taken together, these fndngs ndcate that ths corneal dsorder resulted prmarly from corneal epthelal cell death nduced by MMW exposure, whereas corneal edema and corneal opacty were secondary fndngs to corneal epthelal cell death These are the reasons we used

11 922 J Infrared Mll Terahz Waves (2018) 39: corneal epthelal dsorder n the present study to determne the end pont of ocular dsorder by MMW exposure 42 Effects of Penetraton Depth of MMW We found that the degree of ocular dsorder dffered dependng on the MMW frequency Ths dfference was lkely due, at least n part, to dfferences n penetraton depth The corneal penetraton depth of MMW by frequency dfference was estmated by calculaton [8] usng measurements of delectrc propertes [16] These estmates found that the depths of corneal penetraton n eyes exposed to 40, 75, and 95 GHz, were 057, 036, and 032 mm, respectvely Normal rabbt corneal thckness s around mm (OCT-measured data of Dutch rabbt) The corneal penetraton depth of 75 and 95 GHz MMW was roughly comparable to the corneal thckness of these rabbts, but penetraton of MMW at 40 GHz reached the anteror chamber whereas that of 75 and 95 GHz remaned wthn the cornea of these rabbts Sasak et al reported that the temperature of the rabbt cornea rses wth ncrease n frequency n a smulaton experment usng the same exposure equpment under the same exposure condtons [8] as the present experment One of the causes of the dfference n penetraton depth between 75 and 95 GHz n the smulaton result of Sasak et al [8] and n the results of our anmal experments s consdered to be the tear flm Sasak s numercal model of the rabbt cornea was represented as homogeneous tssue snce 90% of cornea s occuped by the stromal layer, and no tear flm was consdered The tear flm layer (thckness range, μm) coverng the surface of the cornea has roles such as mantanng eyeball mosture, lubrcaton n blnkng, smoothng the corneal surface to mantan a clear vew, and protecton of the ocular surface [17] The lpd layer consttutng the tear flm layer s very thn about 100 nm [18], but t acts as a Bseal^ to prevent evaporaton of water from the ocular surface [17] Although the tear flm layer ncludng the lpd layer s very thn, ts mportance s evdent snce ts absence accompaned damage to the corneal surface (Fg 3g) n the 30-mn sham exposure experment suppressng blnkng of rabbts Smulaton experments ncludng a tear flm layer n a cornea model are urgently needed Under condtons of much hgher frequency exposure such as 95 GHz, heat absorbed by the cornea s not only transported nto the nner part of the eye by aqueous humor convecton, but effectvely dsspated to the outsde of the eye by transpraton As descrbed above, snce the state of the tear flm nvolves transpraton from the cornea, t s necessary to consder the state of the tear flm and the transpraton of heat energy from the cornea In addton, analyss of the transpraton of the heat to outsde of the cornea s necessary to understand the heat transportaton dynamcs drven by hgher frequency exposure, especally when penetraton depth s less than corneal thckness 43 Effects of Blnkng In ths study, rabbts were prevented from blnkng by fxng the eyelds wth tape Irradaton of the eyelds wth MMW resulted n eyeld closure due to nflammaton of eyeld skn, completely suppressng the effect of MMW exposure on the eye tself [7] Moreover, the temperature on the corneal surface was ncreased for about 1 s by blnkng, but gradually decreased thereafter, n normal physologcal condtons [19] In contrast, hgh frequency energy of GHz was absorbed by the skn surface and the surface of the eyeballs

12 J Infrared Mll Terahz Waves (2018) 39: [20] MMW-nduced ncrease n corneal surface temperature to above body temperature may result n a blnkng response to reduce corneal surface temperature Based on Japanese gudelnes for tme of exposure to rado waves [21], we set the average MMW exposure tme at 6 mn Blnkng ntervals of rabbts have been reported to range wdely, ncludng 522 mn [22], 63 mn [23], 7 mn [24], and 20 mn [25] Thus, eyeld retenton for 6 mn was unlkely to nduce severe stress n rabbts In contrast, blnkng ntervals of human eyes are much shorter, rangng from 3 to 10 s and ncludng 3 4 s[26], 701 s [22], and 10 s [25] Therefore, even f humans are exposed to the same MMW dose (such as 75 GHz 30 mn exposure or sham+ir for 30 mn exposure), humans are consdered to be at less rsk than rabbts 44 Rsks of Long-term Exposure Ocular damage was more serous n rabbt eyes exposed to MMW for 30 mn than for 6 mn Corneal surface temperatures durng exposure to 75 GHz at 50 mw/cm 2 for 6 and 30 mn were 376 ± 15 C and 371 ± 08 C, respectvely The mean corneal surface temperatures of normal and anesthetzed Dutch rabbts have been found to be 3467 ± 077 C, (range, 3207 to 3700 C) and 3453 ± 108 C (range, 3019 to 3671 C) [27] Because normal rabbt body temperature s 383 C [28], the corneal surface temperature durng MMW exposure to 75 GHz at 50 mw/cm 2 s comparable to body temperature We sought to determne whether the corneal epthelal damage observed n rabbts exposed to 50 mw/cm 2 MMW for 30 mn was due to MMW or to hgher temperatures nduced by exposure to a thermal source such as unntentonal IR exposure Because we dd not observe any ocular dsorder n unexposed control eyes durng exposure to MMW for 30 mn, we assessed the effects of sham exposure or exposure to very low ntensty IR rradaton We found no evdence of corneal epthelal damage after sham exposure for 30 mn, although some eyes showed corneal desccaton In contrast, corneal epthelal damage was nduced by thermal exposure (IR exposure) around body temperature under condtons of corneal dryness, ndcatng that corneal dsorders resulted from the ncrease n corneal surface temperature to about body temperature and were not unque to MMW (such as general hypertherma) It s an mportant fndng that corneal epthelal damage s nduced by MMW exposure, accompanyng very strong pan, although t s cured wthn 1 2 days after njury Snce humans blnk more frequently than rabbts, exposure to these levels of MMW s unlkely to nduce corneal epthelal damage n humans 5 Concluson We created rabbt models of ocular dsorder by exposure to 40, 75, and 95 GHz MMW In assessng the relatonshp between the degree of ocular dsorder and ncdent power densty of these three frequences, we found that ocular damage nduced by exposure to 40, 75, and 95 GHz was essentally the same, wth no characterstc specfc to each frequency We also found that corneal surface temperature durng exposure was ncreased n the order 40 < 95 < 75 GHz, but there was no relatonshp between degree of ocular damage and eye surface temperature In addton, we performed probt analyses by usng expermental data and estmated probabltes for ncdence of ocular damage dependng on the power densty DD 50, defned as dose level to cause ocular damage wth 50% probablty, was n the order 40 GHz (206 mw/cm 2 ) > 95 GHz (146 mw/cm 2 ) 75 GHz (143 mw/cm 2 ) At the same

13 924 J Infrared Mll Terahz Waves (2018) 39: ncdent power densty, the degree of ocular damage was greater n the eyes exposed for 30 mn than for 6 mn These fndngs suggested that damage to the corneal epthelum was not nduced by corneal dryness alone, but by exposure of dred corneas to MMW-nduced heat to above body temperature Acknowledgments The authors are ndebted to Dr Taj Saka, Ms Yoko Yamashro, Dr Nala Hasanova, Mr Cheng-Yu Tsa, and Ms Mar Seto for ther techncal assstance, and to Mr Davd Prce for Englsh proofreadng Fundng Ths work was supported by the Mnstry of Internal Affars and Communcatons, Japan (Grant numbers n 2016 and n 2017), and partally supported by JSPS KAKENHI Grant Number 15K12209 Complance wth Ethcal Standards Conflct of Interest The authors declare that they have no conflct of nterest Open Access Ths artcle s dstrbuted under the terms of the Creatve Commons Attrbuton 40 Internatonal Lcense ( whch permts unrestrcted use, dstrbuton, and reproducton n any medum, provded you gve approprate credt to the orgnal author(s) and the source, provde a lnk to the Creatve Commons lcense, and ndcate f changes were made References 1 D Colomb, B Thors, and C Törnevk, BImplcatons of EMF exposure lmts on output power levels for 5G devces above 6 GHz^, IEEE Antennas and Wreless Propagaton Letters, vol 14, pp , B Thors, D Colomb, Z Yng, T Boln, and C Törnevk, BExposure to RF EMF from array antennas n 5G moble communcaton equpment^, IEEE Access, vol 4, pp , T Obara, T Okuyama, Y Inoue, Y Aok, S Suyama, J Lee, and Y Okumura, BExpermental tral of 5G super wdeband wreless systems usng massve MIMO beamformng and beam trackng control n 28 GHz band^, IEICE Trans Commun, vol E100-B, No 8, pp , S W Rosenthal, L Brenbaum, I T Kaplan, W Metlay, W Z Snyder, and M M Zaret, BEffects of 35 and 107 GHz CW mcrowaves on the rabbt eye^, Bologcal effects of electromagnetc waves Selected Papers of the USNC/URSI Annual Meetng, Boulder, Colorado, October 1975 Rockvlle, Maryland, US Department of Health, Educaton, and Welfare, HEW Publcaton (FDA) , vol 1, pp , S Chalfn, J A D Andrea, P D Comeau, M E Belt, and D J Hatcher, BMllmeter wave absorpton n the nonhuman prmate eye at 35 GHz and 94 GHz^, Health Phys, vol 83, pp 83 90, H A Kues, S A D Anna, R Osander, W R Green, and J C Monahan, BAbsence of ocular effects after ether sngle or repeated exposure to 10 mw/cm 2 from a 60 GHz CW source^, Boelectromagnetcs,vol20, pp , M Kojma, M Hanazawa, Y Yamashro, H Sasak, S Watanabe, M Tak, Y Suzuk, A Hrata, Y Kammura, and K Sasak, BAcute ocular njures caused by 60-Ghz mllmeter-wave exposure^, Health Phys, vol 97, pp , K Sasak, T Saka, T Nagaoka, K Wake, S Watanabe, M Kojma, N Hasanova, H Sasak, K Sasak, Y Suzuk, M Tak, Y Kammura, A Hrata, and H Shra, BDosmetry usng a localzed exposure system n the mllmeter-wave band for n vvo studes on ocular effects^, IEEE Trans Mcrowave Theory Tech, vol 62, pp , Publshed by the ARVO anmals n research commttee, BToolkt for bomedcal researchers usng laboratory anmals^, Researchers_Usng_Laboratory_Anmals/ Accessed 26 december, 2017

14 J Infrared Mll Terahz Waves (2018) 39: MKojma,YSuzuk,C-YTsa,KSasak,KWake,SWatanabe,MTak,YKammura,AHrata,K Sasak, and H Sasak, BCharacterstcs of ocular temperature elevatons after exposure to quas- and mllmeter waves (18 40 GHz)^, J Infrared Mll Thrahz Waves, vol 36, pp , C I Blss, BThe Method of Probts, Scence, vol 79, ssue 2037, pp 38 39, R Core Team (2017) BR: A language and envronment for statstcal computng^ R Foundaton for Statstcal Computng, Venna, Austra Accessed 24 September J S Fredenwald and W Buscke, BMtotc and wound healng actvtes of the corneal epthelum^, Arch Ophthalmol, vol 32, pp , C Hanna and J E O Bren, BCell producton and mgraton n the epthelum layer of the cornea^, Arch Ophthalmol, vol 64, pp , C E Crosson, S D Klyce, and R W Beuerman, BEpthelal wound closure n the rabbt cornea^, Invest Ophthalmol Vs Sc, vol 27, pp , K Sasak, Y Ismura, K Fuj, K Wake, S Watanabe, M Kojma, R Suga, and O Hashmoto, BDelectrc property measurement of ocular tssues up to 110 GHz usng 1 mm coaxal sensor^, Phys Med Bol,vol60, pp , T J Dursch, W L, B Taraz, M-C Ln, C J Radke, BTear-flm evaporaton rate from smultaneous ocularsurface temperature and tear-breakup area^, Optom Vs Sc, vol 95, pp 5 12, JPCrag,ATomlnson,BImportence of the lpd layer n human tear flm stablty and evaporaton^, Optom Vs Sc, vol 74, pp 8 13, C Purslow and J S Wolffsohn, BOcular surface temperature, A revew^, Eye & Contact Lens, vol 3, pp , Internatonal Commsson on Non-Ionzng Radaton Protecton, BGudelnes for lmtng exposure to tme varyng electrc, magnetc and electromagnetc felds (up to 300 GHz)^, Health Phys, vol 74, pp , Mnstry of Internal Affars and Communcatons, Japan, BRado-radaton protecton gudelne^ n Japanese Accessed20July D R Korb, J V Grener, T Glonek, A Whalen, S L Hearn, J E Esway, and C D Leahy, BHuman and rabbt lpd layer and nterference pattern observatons^ Adv Exp Med Bol, vol 438, pp , B Schwartz, BThe effect of ld closure upon the ocular temperature garademt^, Investgatve Ophthalmology, vol 3, pp , B Schendowch, BThe scence and art of blnkng^, Natonal Keratoconus Foundaton Webste, nkcforg/scence-and-art-of-blnkng/ Accessed 20 July A Ludwg and H Remann, BEye^, In: Practcal pharmaceutcs: An nternatonal gudelne for the preparaton, care and use of medcnal products ed by Y Bouwman-Boer, V Fenton-May, and P Le Brun (Sprnger Internatonal Publshng, Cham, 2015), p P Bernard, BOphthalmc drug delvery^, In: Modfed-release drug delvery technology (Drugs and the pharmaceutcal scences) ed by M J Rathbone, J Hadgraft, and M S Roberts (CRC Press, Florda, 2002), p G W Mkesell, Jr, BCorneal temperatures A study of normal and lasernjured corneas n the Dutch belted rabbt^, Am J Optom & Physol Optcs, vol 55, pp , V Aspnall and M Cappello, BSmall exotc mammals^, In: Introducton to veternary anatomy and physology textbook, 3rd edton (Elsever Health Scences, London, 2015), p166