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    j., 51, 103-125 (March/April 2000)

    Photodegradationf humanhair: An SEM studySIGRID B. RUETSCH, Y. KAMATH, and H.-D. WEIGMANN,TRI/Princeton, .O. Box 625, Princeton,J 08542.

    Acceptedor publicationanuary3 , 2000.SynopsisThis studyuses ield emission canning lectronmicroscopyFESEM) to monitor the effectsof UV irra-diation on the physicalnature of hair fibers. Long-term UV irradiation/humidification ycling causesthinningand fusionof the surface uticlecell, aswell as usionof the cuticularsheath nto a solid,rigid,and brittle unit. While intercellular ohesion ithin the cuticularsheath s high, possibly ue to crosslink-ing of the proteinsn the intra- and ntercellular omains,he cells hemselvesrebrittle. A newlyobservedfracture atternof ong-termUV-exposedibers uggestsusionof the regions ttackedmostseverely y UVlight into one rigid and brittle mass, ncapable f extension ue to lossof all original elasticproperties.Unlike chemical xidation,which resultsn partialdissolution1 h H202) and then complete olubilization(4 h H202) of the melaningranules, hotochemicalxidationproduces ntirelydifferent esults.Evenafterlong-termUV irradiation/humidification95% RH) cycling, he melaningranules ppearphysically ntact.Lossof colordoesnot occuras long as the melanin granulesare intact.The severityof photodegradationuring UV irradiation/humidificationycling becomes pparentuponbrief (seconds)ontactof these iberswith alkalinehydrogen eroxide.Suchcontact esultsn instantaneousdisintegration f the components ithin the cuticle cells.Formationof sac-likestructuresAllwiSrden acs)occursdue to osmoticpressurewithin seconds f exposureo alkaline hydrogenperoxidecausedby pho-tochemically egradedproteinswithin the surface uticle cells.The cells swell until they burst and theircontents rain, leavingbehind cuticularmembranes, hich may detachor fuse to the fiber surface.UVirradiationhasalsoseverely hotodegradedhe melaningranules nd preconditionedhem for acceleratedsolubilization poncontactof the fiberswith alkalinehydrogen eroxide.The effects f both relativehumidity and spectral nergydistributionon the photochemical xidationof thehair fiber arestudied.Resultsobtained t various elativehumidities n two different adingunits, namely,the QUV Accelerated eatheringTesterand the Atlas Weather-Ometer ("AW") are compared. calethinning and fusionobserved uring UV/humidificationcyclingare greatly reducedwith exposure t lowhumiditieswithout humidification ycles.Upon post-treatmentwith water, fibers rradiatedat a constant10% RH in the QUV showscale hinningand usionsimilar o that of fibersexposedo UV/humidificationcycling.This indicates hat photodegradationccurs t low humidity as well. Fibersexposed t constant20%, 50%, and 70% humidity in the "AW" showonly moderate cale hinning, evenafter post-treatmentwith water. The total solarspectrumused n the "AW" apparentlycausesesssevere hotodegradation fthe proteins han the UV light of the QUV.

    INTRODUCTIONWhen exposedo sunlight,hair is known to undergochangesn morphological, hemi-cal, and mechanical haracteristics1-4). The lower wavelength angeof the UV com-


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    104 JOURNAL OF COSMETIC SCIENCEponentof sunlight s known to be responsibleor thesechanges.n recentyears,UVradiationat lowerwavelengths assignificantly ncreased ue to the deterioration f theozone ayer. While the earth'satmosphereilters out most radiation below 295 nm,depletionof the ozone ayer, and therefore he reduction n the screening ffectof theatmosphere, ermits lower wavelengthcomponentso reach he earth'ssurface.Theselower wavelengthregionsof the UV radiation receivedby the earth are the mostenergetic nd therefore ancause evere hotodegradation.he UV rangeof the sunlightcan be divided into three wavelength egions,namely UV-A (320-400 nm), UV-B(280-320 nm), and UV-C (

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    700 hours t alternating hree-hour ycles f humidificationat 95% RH; 40C) andUVirradiation 50C;RH decreasingrom 95% to 10% within 30 min, then remaining t10% for 2.5 h); and (b) 0, 100, 200, and 300 hours continuous UV irradiation atconstant 10% RH and 50C.At/as Weather-Ometer, ("AW"). The "AW" hasa spectral istribution anging rom UVthrough he visiblerange,simulatingsunlight.Theseexposure onditions orrespondoan "average" 5 Miami summersunlight.The energydensityat the 340-nm wave-lengthskeptconstantt 0.3 W/m . Thismeanshatat thespecific avelengthf 340nm, heenergy ensityn theQUV (0.97W/m2) s approximatelyhree imes reaterthan hatof the"AW" 0.3W/mP).However,n thewavelengthange f 300-400nm,the energy ensitiesf the QUV (300-400nm yield5.06mW/cm ) and he "AW"(300-400 nm yield4.46 mW/cm ) are similar, lthoughhe distributions quitedifferent.The specific pectraldistributions n the "AW" in the rangeof 250-300 nmyield0.012mW/cm, and 00-800nmyield36.80mW/cm .The otalenergyensityin the "AW" is 41.272mW/cm. Specific xposureonditionsf the hair fibers oUV/visible radiation in the "AW" were 0, 100, 200, and 300 hours at constant 20%,50%, and 70% RH and 50C.Post-treatmentf hair fibers n alkalinehydrogeneroxide.air fibersexposedo UV irra-diation/humidificationat 95% RH) cyclingwere subjected o post-treatmentn 6%alkalinehydrogen eroxiderom secondsp to two hours.This wasdone o illustrateandcharacterizehe severe xtent of photodegradationnflicted upon the fiber during treat-ment in the QUV.Post-treatmentf samplesrradiated t lowand intermediateumidities ith water.UV irra-diation/humidificationat 95% RH) cycling ead to extensive thinning" of the surfacecuticlecellsand "fusion"of the scale dges.Differentiationof the surface uticlecell, socharacteristic f the untreatedhair fiber, eventuallydisappearst longerexposureimes.We concluded hat the presence f moisture n the highly swollen iber during thehumidification ycle s responsibleor transportinghe degraded,ow-molecular-weightprotein ragments ut of the cuticlecell andpossiblynto the cortex, herebycausinghecollapse nd thinning of the surface uticlecell.Hair fibers rradiated t low humidities from 10% to 70% RH) in the two fadingunitsdo not show his featureof thinning and fusionof the cuticle cell. Apparently, he lackof mobile water in the hair fiber and the lack of swelling of the fiber eliminate thetransportof photodegradedragments ut of the cuticlecell. To mobilize he photode-gradedproteins,hair fibers rradiatedunder low RH conditions n both the QUV and"AW" were subjected o a water post-treatment.The fibers were immersed or 60minutes n lukewarm,deionizedwater, air-dried, and then examined ongitudinally nthe SEM. This was to establishwhetherpost-treatment n warm water would result nthinning and fusionof the cuticlecell by diffusingphotodegradedmaterialsout of thecuticlecell, assuminghat photo-oxidation f the hair proteinshad occurred t all.

    INVESTIGATIVE METHODField emissioncanninglectron icroscopy.ongitudinal and cross-sectionalegmentsofuntreatedand UV-exposed iberswere mountedon double-sidedape and coatedwithapproximately0 fkofplatinum.hehair iber opographynd nterior ere xamined

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    106 JOURNAL OF COSMETIC SCIENCEfor photo-oxidative amagen a Hitachi S-4500 digital field emission canning lectronmicroscope FESEM).


    Photochemicalegradation f hair affects oth hair proteins nd hair pigments nd hasbeenshown o occurprimarily n the wavelengthegionof 254 to 400. In recent nfraredspectroscopictudies f hair exposedo weathering,Dubief (3) shows decreasen lysineand histidine,and a drastic ncreasen cysteic cidcontent n tip endsof hair comparedto root ends.This suggestscission f the disulfidecrosslinks uring the photochemicaloxidationprocess nd formationof cysteic cid. This assumptions supported y Rob-binsandBahl 5), who haveshownby electron pectroscopyor chemical nalysisESCA)that both UV-B (280-320 nm) and UV-A (320-400 nm) oxidize sulfur in hair. Oxi-dationwasshown o occurprimarily n the fiberperiphery, amely, he cuticularsheath,producinga steepgradient o lessoxidizedhair in the fiber core.These esults re notsurprising, incedisulfide s at its highestconcentrationn the A-layer and the exocu-ticle, where the highest evel of photochemical xidation occurs.The ESCA spectrasuggesthat high levelsof cystineS-sulfonatendcysteic cidare ormed n hair exposedto photochemical xidation n the wavelength egionof 254-500 nm.Our research6) strongly upportshe results f Dubief (3) and Robbins nd Bahl (5).Using UV microspectrophotometry,e haveshown hat high levelsof photodegradationproducts re formed hroughout he hair fiber cross ection uring long-termexposurein the 290-400 nm range.UV irradiation-induced hotodegradationroductsof thehair proteins re revealed y an extension f the absorbancelateauand a shift in peaksfrom 290 nm in untreated hair to 315 nm in UV-exposedhair. There is also thedevelopment f an absorbancehouldern the 330-340 nm range,well isolated rom theabsorbancef the bulk of the hair fiber (Figure 1). Formationof the photo-oxidized airproteins anbe tracedand mapped, venquantified,by scanning cross air fiber crosssections t the wavelengths f the absorbancehoulder, }tm -- 330 nm). Thesephoto-degradation roducts reespecially ronouncedn blond unpigmented) iedmonthair,with the highest evel ofphotodegradationccurringn the cuticular egion A-layerandexocuticle),where cystine s at its highest concentrationFigure 2).It is a generallyaccepted oncept hat the mechanismor photochemical xidationofcystine ollows he C-S scission athway,wherebyoxidativescission ieldsS-sulfonicacid that is finally degraded y light to cysteic cid (7,8). In contrast o the chemicaloxidation hat follows he S-Sscission athwayand yields wo molesof cysteic cidpermole of reacteddisulfide,only one mole of cysteic cid s produced rom eachmole ofreacteddisulfide n the C-S scission athway.The progressivexidationpathwaysor thetwo typesof scission re:S-S Scissionchemical xidation):R-S-S-R --->R-SO-S-R > R-SO2-S-R R-SO2-SO-R R-SO2-SO2R 2R-SO3HC-S Scissionphotochemicalxidation):

    R-S-S-R > R-S-S-OH > R-S-SO2H > R-S-SO3H > R-SO3H + H2SO+ R-OH

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    Untreated Hair

    .o230. o 264. o 298. o 332. o 366. o 400.


    The conditionswe havechosenn this studymay seemsomewhat xtreme;however,wefelt they were necessaryo be able to learnabout he behaviorof the hair fiber and thechanges f its physical aturebroughtaboutby severe V irradiation.Observationsnthe FESEMhadshown hat nearlyall hair fibersexposedo long-termalternatingcyclesof UV irradiation 290-400 nm; kma = 340 nm) and humidification95% RH; 42C)displayed n unusual opography. he hair fibershavea smooth opography imilar tothat of a man-made iber, with little of the characteristic ifferentiationof the cuticle cellof undamaged air. Figure 3a shows he typical appearancend thicknessof a normalcuticle cell of unaltered,untreatedhair fibers.However,after only 100 hoursof UVirradiationand humidification n the QUV, a slight thinning of the surface uticle celland fusion t the scale dges reapparent Figure3b). After 300 hoursof UV exposure,a more pronounced ollapse f the surface uticlesand fusionof the scaleedges o theunderlyingcuticlecells s seen Figure3c). 700 hoursof UV exposure asproduced airfiberswith a smooth opography,ackingcleardifferentiation f the cuticlecells Figure3d) becausef extreme uticular hinning and fusion o the underlyingcuticlecells.Theoveralldecreasen thickness f the surface uticlecell as a functionof exposureime toUV irradiationwas obtained n the FESEM using built-in software or measuringdistances n a nanometer cale n the axial, radial, and diagonaldirection of fibers.Decreasesn scale hicknesswere measured t the samehigh magnificationand con-verted to the appropriate cale.The resultsare shown n Figure 4.Our hypothesis xplaining his photochemical amagephenomenons as follows: hisprogressivehinningand fusionof the surface uticlecell, under he conditionswe used,is most ikely due to photochemical egradation f the proteins n the surface uticlecell

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    0.00 20 40 60 80 1O0 120 140






    UV exposedhair

    0 20 40 60 80 100 120 140Distance (mm)

    Figure 2. Formation f photodegradationroductsn the hair fiber cross ection. bsorbancecans eremade t 330 nmacrossair ibercrossectionseforetop)andafter bottom) V exposure.intosoluble,ow-molecular-weighteptides, hich recapablef diffusingnto owercuticlecell layersor out of the fiber)while n a highlyswollen tateduring hehumidificationcycleat 95% RH.

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    0.0 I I I I I I I0 100 200 300 400 500 600 700

    UV Irradiation (h)Figure 4. Progressivehinningand usionof the surface uticlecellsasa functionof exposureime to UVirradiation.

    layer of the cortex nto one rigid unit causes rittle failure circumferentiallynd dis-placeshe stress f extensiono those egions f the hair fiber that are still untouchedby thisprogressiveusion.Multiple, successiveractures evelop t individual ites longthe corticalcell boundaries, hen changedirection and travel radially acrossndividualcorticalcells,and this patterncontinues ntil it tapersoff towardshe coreof the fiber,therebycreatingwhat we call the "cathedral pire" racturepattern(Figure7a). Theopposite, orrespondingite of the "cathedral pire" ractureshows hollowopening(Figure7b), surrounded y a firmly fusedwall. Higher magnificationhowshat thiswall consists f a firmly fusedcuticularsheath ndpossibly lso he outer ayerof corticalcells Figure7c,d). Somecuticular egions, referentially hoseon the sideof the fiberoriented owards he damaging ight source, avebecome ndistinguishable,igid, andvery brittle.The "cathedral pire" racturepatternclearlyshows rimary evelsof photodegradationin the fiberperipherycuticular heath) nda drop-off o lesserevelsof degradationnthe fiber interior.


    Differencesetween hemical ndphotochemicalxidation f hairproteins ndmelaninshavebeenwidely discussedn the literature.Robbins 9) reports hat both chemical ndphotochemicalxidation ttackboth the hair pigments nd proteins, nd within theproteins, rimarily he aminoacidcystine.Up to 25% of the disulfidebondsn humanhairaredegraded y "normal" leaching,nd45% of the disulfide ondsmaybe broken

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    { - .

    Figure 5. Representativeross-sectionaliewsof the cuticular heath f an untreated al,a2) and a 300-hUV-exposed b) hair fiber.

    duringsevere leaching. owever, hemical xidation f the hairpigments ccursasterthan the degradation f the proteins.UV irradiation n the wavelengthegionbetween 54 and 400 nm hasbeenshown 9)to degrade air proteinand pigment.Althoughboth hair proteins nd pigmentsabsorblight in the UV/visible region, he longerwavelengths ave been found to be lesseffective n causingphotodamage. s with chemicaloxidation,photodegradationfcystines the mostextensivehenomenonecausef the reactivity f the disulfide ond.Examinationof cross ections f hair fibers exposed p to 700 hoursUV-irradiation/humidification yclingshowed, uite surprisingly,hat the physical ppearancef themelaningranules ad not changedmuch, f at all. Figure8a,b showsmelaningranulesin cross ections f untreatedhair fibers.Thesegranules re of various izes, pherical relliptical n shape, nd appear o consist f smallergranularentities.The granules rehousedn small cavities nd appear o be connectedo the cell walls by some ntercel-lular material.Evenafter ong-termUV-irradiation/humidificationycling, he physical atureof themelaningranules ppearsntact and undegradedFigure 9a,b). In unpublishedworkcarried out at TRI, we have shown that hair fibers retained their dark brown color andthat only a few had beenfadedslightly to a lighter brown color.Since he melaningranules etained heir physicalbulk and appearance,he melaninpigment was pro-

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    30509 50 kV x4.0k"'ff25'&


    Figure 6. a-d: Typical ongitudinaliews f a long-term V-exposedair iber,displayingevereadialcracking, typical adiation-inducedamage henomenon.

    tected romdegradation, hich, n turn, prevents lossof hair color.The assumptionwasmade hat photochemicalxidationof the melaninchromophor ouldhave o occurin situ if color loss was to occur at all.This is quite in contrast o the swellingand dissolution f the melaningranules ychemicaloxidationwith hydrogen eroxide.After 1 hour of oxidationwith 6% alkalinehydrogen eroxide,moderate isintegrationf the melaningranules adoccurredFig-ure 10b,c).The subgranulartructures, hichmakeup the melaningranules, ppearmore pronounced. here are variousstages f degradation, rom mile to severe, ue topartialsolubilizationf the melanoprotein.olubilizationppearso occur referentiallyfrom the centerof the granulesFigure 10c).After hehair iberswereexposedor ourhourso oxidation ith 6% alkaline ydrogenperoxide,mostof the granular avitieswereempty(Figure 10d). The assumption asmade hat the majorityof the cavitieswereempty becausehe granulesweredissolvedawayby theperoxide. rogressiveolubilizationf themelanoproteinss heprerequisitefor solubilizationnd/ordispersionf the melaninpigments.With the melanoproteindissolvednddiffusednto the bulk of the hair fiber,the spread-out elanin igmentbecomeshe unrestrictedargetof bleaching r loss n colorof the humanhair.Wolframand Hall (10) haveobservedimilarbehaviorwith melaningranulessolatedrom hairand exposedo peroxidebleaching.

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    Figure 7. a-d: The "cathedral pire" racture atternseenuponextension f hair fibersexposedo 700 hof UV irradiation/humidificationycling.

    Using microspectrophotometry, e have shown that under the selectedconditionsofprogressivexposureo UV radiation/humidificationycling n the QUV, no or insig-nificant oss f coloroccurs ftershortexposureimesand hat onlymoderateighteningof the color s observedfter700 hoursof UV exposure.he melaningranulesemainintact as observed n the FESEM. Chemical oxidation, on the other hand, results inincreasedighteningof the hair colorwith increasedxposureime, while solubilizationof the melanin granulesoccurs.Sincemicrospectrophotometrys a non-destructiveechnique, he change n hair coloris measuredn transmitted ight at the very same previouslymarked) ocationson thevery samehair fibersunder dentical nstrumental ettingsbeforeand after the variousexposureimes to UV radiation. ncreasesn transmissionntensityare directly propor-tional to loss n hair color.The same echniques usedon hair fibersexposedo chemicaloxidation.Figure 11 compareshe decreasen hair colorcaused y chemicaloxidationwith 6% alkalinehydrogen eroxide ersus rogressive V irradiation.


    Hair fibersexposedo long-term 700 h) UV irradiation/humidificationycling,and

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    Figure 8. a,b: Melanin granulesn cross ections f untreatedhair fibers.

    subsequently ubjected o treatmentwith alkaline hydrogenperoxide, evealsignifi-cantlymoredamage o the hair fiber than had originallybeensuspected.Short-termalkalineperoxidebleaching f long-termUV-irradiated hair fibers esultsninstantaneousisintegration nddissolution f elements f the cuticularcells, using hehair fibers irmly together.After drying, these iberscan no longerbe freelyseparated,and forcefully ulling them apartresultsn their tearingand ibrillation Figure12a,b).

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    "6"K ;i :" '. 2.' ,B.;..

    Figure9. a,b:After ong-term V-irradiation/humidificationycling,hephysicalature f themelaningranulesappearsntact.

    The cross-sectionaliewof these ibersconfirmshe observationsadeduring helongitudinaltudy. hecrossectionshow artially isintegrated,issolved,nd usedsurfaceuticle ellsFigure 2c,d).We also bservedhatsome f themelanin ranulesappearo havebeensolubilizedftermerely15 minutesof peroxidereatment.Suchsolubilizationf themelanin ranulesn untreatedairoccurssually fterseveralours(4 h) of V exposureas,without oubt, everelyegradedhe

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    Figure 10. Intact (a) andpartially b,c)and completelyd) solubilizedmelaningranules efore ndafter 1h and 4 h of oxidationwith 6% alkalinehydrogen eroxide, espectively.

    hair fibers (cuticle cells and melanin granules)and preconditioned hem further toaccelerated isintegration.After oneand two hoursof alkalineperoxidebleaching f long-termUV-irradiatedhairfibers, the cuticula has completelydisintegratedand dissolvednto an unrecognizable,thin, film-like layer of cuticularmembranes nveloping he outer cells of the cortex(Figure 13a,b). The disintegration, issolution nd fusionof the cuticle cellsare soadvanced hat the individual cuticle cells are no longer identifiable. The original to-pographyof the hair fiber no longer exists. nsteadof surface uticle cellswith theircharacteristiccale-like tructure, bumpy surface overed y a thin film is displayed.This thin film may consist f cuticular emnants, he cell membranes, nd the CMC,which existsbetweencuticula and cortex. The correspondingross-sectionaliews ofthese ibersdemonstratehe progressiveamage nflictedupon the hair fiber by long-term UV exposure,Figure 13c,d). The low magnification rosssection Figure 13c)shows he fused cuticula and merely a cortex containingempty cavities Figure 13d)where once he melanin granuleswere housed.Besides evereoxidative damage o thecuticula, long-term U irradiation has also photochemically egraded he melaningranules, nd has preconditionedhem for acceleratedissolution uring subsequentshort-term reatmentwith alkaline hydrogenperoxide.These observations learly show that long-term UV exposurecauses everechemical

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    Transmission Intensity(%FS)605040302010

    0o I 4

    Peroxide Treatment (h)Transmission Intensity(%FS)




    0 100 300 500 700QUV Exposure ( hours )

    Figure 11. Comparisonf ossn haircolordue o chemicalxidation ith 6% alkaline ydrogeneroxide(top) and UV irradiation bottom).

    degradationot onlyof the cuticula ndmelanin ranules, ut alsoof the hairfiber ngeneral.Opticalmicroscopy.he unusual esults emonstratedn the SEM studyproduced everalquestionsbout he drasticchangesn the physical atureof the fiber's opography.Therefore,ong-termUV-irradiatedhair fiberswereobservedn the opticalmicroscopeduringalkaline eroxidereatments, hichprovided ome nswerso questionsbout

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    006020 5. 0 RV xG00 50. O,mtl. doGo 1 kV x 5 B.Figure 12. Longitudinal a,b) and cross-sectionalc,d) viewsof long-termUV-irradiated hair fibersaftersubsequent5-rnin reatmentwith 6% alkalinehydrogen eroxide.

    this extremecuticulardisintegrationduring chemicaloxidationof UV-exposed ibers.Long-term UV irradiation causes xtensiveoxidativedegradationof the proteinsof thecuticula as manifested n an Allw6rden-like reactionduring subsequent hemicaloxi-dationof UV-exposed air fibers Figure14a-f). After only secondsf alkalineperoxidetreatment(Figure 14b,c), small bubblesdevelopalong the hair surface.These smallbubblesor half domes esemble he famousAllw6rden sacs ormedby the Allw6rdenreaction.Allw6rden first detected hesesacs t the surface f wool fibersduring treat-ment with chlorinewater. He thus alsodiscoveredhe epicuticlesurrounding he cuticlecell. The epicuticle s a semipermeable, artly proteinaceousmembrane, 25 A inthickness, hichallowssmallmoleculeswater) o diffuse nto the cell, rapidlyswellingdegraded, ydrophilic, ow-molecular-weighteptide ractions. he highly hydrophilicmolecules re not capableof diffusing out and remain trapped nside the cell. InAllw6rden'sreaction, he hydrophilicmolecules ontinue o attract water from theoutsidesolution,and thus thesesacs re formedby the resultingosmoticpressure.In our studyconcerningong-term UV exposure f humanhair, the bubbles esemblingAllw6rdensacs re formedby diffusionof peroxide, mmoniumhydroxide, nd waterthrough the epicuticle nto the cuticle cell, where the alreadyphotolyticallydegradedproteins renow urtherdegraded y the peroxide ndswollenby water.The hydrophilicprotein ractions restill too bulky to diffuseout through he semipermeablepicuticle.They remaintrappedwithin the cuticlecell, attractingwater and forming the sac-like

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    B0011 5.0 kV

    Figure 13. Longitudinal a,b) and cross-sectionalc,d) viewsof long-termUV-irradiated hair fibersandsubsequent-h treatmentwith 6% alkalinehydrogen eroxide.

    structures Figure 14b,c). Eventually, hesesac-likestructures swollencuticle cells)burst and their contents rain (Figure 14d), leavingbehind membranes r shellsofcuticlecells Figure 14e),whichmay detach rom the hair fiber and drift away Figure14f), or may remain attachedand form a thin film-like layer enveloping he outercortical cells, as has been shown n the SEM study.EFFECTS OF RELATIVE HUMIDITY AND SPECTRAL ENERGY DISTRIBUTION ONPHOTOCHEMICAL OXIDATION

    Moistureplaysan important role in the photobleaching f human hair (3,11,12). Pho-tochemical xidation also ermed "weathering" r "photo-bleaching")amages air atany given RH and increasesith durationof exposure. he wet mechanical ropertiesof photo-bleachedair fibersdecrease ith increasing xtentof damage 11). Reductionin wet mechanical ropertieswas ound o be the mostseverewhen hair is weathered thigh or low RH. These iber properties re leastaffectedwhen hair is exposed t a RHof 30%.We have also investigatedhe effectsof both the relative humidity and the spectralenergydistributionon photochemicalxidationof hair fibers.We comparedhe resultsobtainedat various elativehumidities n two different ading units, namely, he QUVacceleratedeatheringesterand the AtlasWeather-Ometer.

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    Figure 14. Longitudinal iewsof long-termUV-irradiated air fibers ndsubsequentxposureor varioustimes o 6% alkalinehydrogen eroxide:a) no peroxide ost-treatment,b) 5 s, (c) 20 s, (d) 15 rain, (e) 15rain (focused n domeof fiber), and (f) 60 rain of peroxidepost-treatment.UV exposuren the QUV. Fibersexposed o continuousUV light at constant10% hu-midity in the QUV do not show hinning and fusionof the cuticlecells Figure 15a),aswasobservedn our earlierstudies n which hair had beenexposedo UV irradiation/humidification at 95% RH) cycling n the QUV. However, upon subsequentmmer-sion n water, collapse nd fusionof the surface uticlecell occursFigure 15b), with aseveritysimilar to that observedn hair fibers exposed o irradiation/humidificationcycling n the QUV. This indicates hat photodegradationad occurred t a low con-centrationof water in the fiber structure,but that the degraded rotein fractions unlikein the swollen iber), werenot ableto diffuse rom the cuticlecell due to lack of adequateamountsof water to promoteswelling.This thinning and fusionphenomenon an be easily explained.At very low relative

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    Figure 15. Longitudinaliews f hair ibers xposedo (a) 300 h of continuousV irradiation t constant10% RH, 50C,and (b) subsequent-h immersionn lukewarmdeionizedwater.humidities,below25%, water molecules re principallybound o hydrophilic itesofthe keratin iberby hydrogen onds, nd he water s not mobile 13). As the humidityincreases,dditionalwater s sorbed, ccupyinghe remainingadsorption itesassociatedwith the protein.At high RH, whenall the adsorptionitesof the keratinareoccupied,water molecules re no longer bound and are more mobile within the fiber. Thisfacilitates iffusion f degraded, ighlysoluble,ow-molecular-weighteptide ractions

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    from the cuticlecell. This acceleratesegradation ecause ow the radiationcanattacknew proteins atherthan the alreadydegraded roteins.Similar phenomena avebeenobserved y Ratnapandian t al, (11).Exposureo theso/atspectrumn the "AW." Fibersexposed o continuousrradiationatconstant 20%, 50%, and 70% relative humidity in the "AW" show the extent ofphotodegradationo be less han n hair fibersexposedn the QUV (to eithercontinuousor cycling conditions), venafter subsequentmmersion n water. This was somewhatunexpected,specially incehair fiberswereexposedo UV irradiation n the 250-400nm rangeaswell as to visible ight at 400-800 nm wavelengths.While fibersexposedto continuousUV light at constant10% RH in the QUV show extremecollapse ndfusionof the cuticulaafter subsequentmmersion n water, fibersexposedn the "AW"displaymore moderate uticularcollapse nd fusion.Micrographs epict argepores nthe scale acesFigure16b) and smallporesor openings t the fusedscale dgesFigure16b,c),most ikely areaswheremoistureand solubilizedmaterialsescaped uring im-mersionn water.We concludedhat the increasedamage o fibersexposedn the QUVmustbe dueto radiation mittedat }tmof 340 nm in the UV-A range,since t is thatwavelengthat which the radiation energy n the QUV is approximately hree timesgreater han that of the "AW."There is no SEM evidenceof melanin granule degradation,even in the presence fmoisture or either wavelength ange,(QUV or "AW"). After long-term exposureoirradiation n both fading units, the melanin granulesappearphysically ntact, eventhough n some nstanceshe melaningranulesmay havebecome lightly "grainier" tmoderate evelsof degradation. s long as the melaningranules re intact, lossof haircolor does not occur.Lossof hair co/or. sing microspectrophotometryn transmittedwhite light, we havemeasuredoss n hair colorasa functionof exposureime to radiation n both the QUVand "AW." Since he transmissionntensity was measured t the same ocationon thesame hair fibers before and after the variousexposure imes, the true color loss wasmonitored,or example,or heavilyor weaklypigmentedhair fibers.Figure17 showsheincreasesn transmissionntensity TIt-TIo), indicativeof loss n coloras a functionofexposure ime to radiation n both the QUV and "AW."The followingsummarizeshe effects f both the RH and the spectral atureof radiationon the photolysisof hair pigment:(a) At low RH conditions 10%, 20%, and 50% RH), independentf thespectralange,there is a lack of loss n hair color in both the UV range of the QUV and theUV/visible range of the "AW." In other words, at low RH, neither UV norUV/visible radiation appear o causesignificantphotodegradation f melanin inbrown hair.(b) Moderate ossof hair color s observed hen ong-termexposureo radiation n theUV/visible range s combinedwith a higher relativehumidity (70% RH).(c) Moderate ossof hair color s alsoobserved pon long-term exposureo alternating

    three-hourcyclesof humidificationat 95% RH and UV radiation,during whichthe relative humidity is decreased ithin 30 minutes from 95% to a low of 10%RH, then remainingat the 10% RH for the duration of the radiationcycle.From this microspectrophotometrictudy it can be concluded hat the high relativehmidity s theprimarycontributingactor o acceleratedoss n hair color, ince ow RH

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    Figure 16. Longitudinal iewsof hair fibersexposedo (a) 300 h of continuous V radiationat constant20% RH and 50C in the "AW," and (b,c) subsequent -h immersion n lukewarm deionizedwater.

    conditions o not result n significant oss n hair color,evenafter long-term exposure.Increased xposureime and spectral angeappear o be secondaryactors.Also, thereappearso be an initial periodof greatresistanceo fading,since olor ossdoesnot occurduring shortexposureimes under any of the conditions nvestigated.However, t isclearlyshown hat the combination f both the high RH conditions nd the light in thevisible rangehas the most damagingeffectson hair color.According o Wolfram and Albrecht 2), partial solubilization f the melanoproteinssa prerequisite or the decolorization y disintegration nd dispersion f the melaninpigmentswithin the hair fiber.

    CONCLUSIONSLong-termexposureo UV irradiation n the QUV results n photo-oxidative egrada-tion of the hair proteins, specially f cystine,which occurs t its highestconcentrationin the cuticulardomainswhere the degradations most severe.UV irradiation (QUV)results n amino acid degradation s ndicatedby shifts n the UV absorbancepectrumto higherwavelengths nd development f an absorbancehoulder.Cycling UV irradiationat 10% RH with humidificationat 95% RH (QUV) leads oextensive thinning"of the surface uticle cell and "fusion"of its scaleedge to the

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    20 20t6 16 12

    Figure 17. Loss n hair colordue to progressiveight exposuren the QUV and "AW," shownas ncreasein transmissionntensity %) of hair fibersdue to light exposure.

    underlyingcuticle ayer.The characteristicifferentiationof the surface uticle cell ofuntreated hair gradually disappearswith increasingexposure.Besides hinning andfusionof the surface uticlecell, therealsooccursusionof the complete uticularsheathinto a rigid, brittle unit. While intercellularcohesionwithin the cuticular sheath shigh, possiblydue to free radical-initiated rosslinking f the CMC, the cuticle cellsthemselvesrebrittle. A newlyobservedracturepatternof long-termUV-exposediberssuggestsusionof the regionsattackedby UV light into one rigid and brittle mass,incapable f extension ue to lossof all original elasticproperties.While chemical xidation esultsn partial (1 h H202) and then complete olubilization(4 h H102) of the melanin ranules, hotochemicalxidation roducesntirelydifferentresults. Even after long-term exposureto severe conditions of UV irradiation/humidification cycling, the melanin granulesappearphysically ntact with little changein their physical ature.Lossof colordoesnot occuras ong as he melaningranules reintact.Long-termUV irradiation/humidificationyclinghasseverely amaged he hair proteinsand preconditionedhem for accelerated isintegration uring subsequentreatmentswith alkaline hydrogenperoxide.Such contactrestfits n rapid disintegrationand dis-solutionof elementsof the cuticle cells, restilting n formationof Allw/Srden acs iaosmosis.UV irradiation has also severelydamaged he melanoproteins nd precondi-tioned them for acceleratedisintegration nd solubilization y the alkalinehydrogenperoxide, s ndicated y SEM micrographsf the emptycavitieswhereonce he melaningranuleswere housed.While photochemical xidationof hair occurs t all humidity conditions,t appearsobe morepronounced nd rapid at high relativehumidity and/orhumidificationcycling.

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    ContinuousUV irradiation at constant10% RH (QUV), without alternating cyclesofhumidification, does not show the thinning and fusion of the surfacecuticle cells.However,photochemical egradation as occurred, s subsequentmmersion n watercauses uticle thinning and fusionof a magnitudesimilar to that experienced y fibersexposedo UV irradiation/humidificationycling QUV).Low and moderateRH conditions uring exposureo unfilteredsolar ight in the "AW"also ail to producecuticle thinning and fusion,and hair fibersstill display he char-acteristic uticle cell differentiation.Subsequentmmersion n water showsonly mod-eratecuticle thinning and fusion.This suggestshat the solarspectrumof the "AW"causesesssevere evelsof photo-oxidative egradationof the proteins han the UVirradiation in the QUV.High relative humidity is the primary contributing actor to acceleratedoss n haircolor.The combinationof both the high RH conditions nd the unfiltered solar ighthave he most damagingeffectson hair color.REFERENCES(1) E. Hoting, M. Zimmermann, and S. Hilterhaus-Bong,Photochemical lterations n human hair. I.Artificial irradiation nd investigationsf hair proteins,J.Soc.Cosmet.hem., 6, 85-99 (1995).(2) L.J. Wolfram and L. Albrecht,Chemical- nd photobleachingf brownand red hair,J. Soc.Cosmet.Chem.,82, 179-191 (1987).(3) C. Dubief, Experimentswith hair photodegradation,osmet.oilerr.,107, 95 (1992).(4) J. B. Speakmannd P. R. McMahon,The actionof light on woolandrelated ibers,N. Z.J. Sci.Tech.,20 (1939).(5) C. Robbins nd M. Bahl,J. Soc.Cosmet.hem., 5, 379-390 (1984).(6) S. B. Ruetsch,Y. Kamath, and H.-D. Weigmann, unpublished bservations.(7) C. R. Robbins,J. Soc.Cosmet. hem., 2, 339 (1971).(8) W. E. Savigeand J. A. Maclaren, n The Chemistryf Organic alfar Compoands,. Kharash,C.Y.Meyers,Eds. Pergamon ress, ew York, 1966), Vol. 2, pp. 367-402.(9) C. R. Robbins,ChemicalndPhysical ehavior f HamanHair, 2nd ed. (Springer-Verlag, ew York,1994), pp. 103-104, 108.(10) L.J. Wolfram and K. Hall,J. Soc.Costa. hem., 6, 247 (1975).(11) S. Ratnapandian, . B. Warner,andK. Kamath,Photodegradationf humanhair,J. Soc. osmet.hem.,49, 309-320 (1998).(12) F. Leroy, A. Deftandre,and J. C. Garson,Photoagingof human hair, 7th International air Science

    SymposiamBad-Neuenahr,1990).(13) C. R. Robbins,Chemicalnd Physical ehavior f HamanHair, 3rd ed. (Springer-Verlag, ew York,1994), p. 78.