Noiseinducedhearingloss:AnoccupationalmedicineperspectiveEmilyZ.StuckenMDMichiganEarInstituteRobertS.HongMD,PhDMichiganEarInstituteCorrespondingauthor:RobertS.HongMD,PhDMichiganEarInstitute30055NorthwesternHighway,Suite#101FarmingtonHills,MI48334Phone(248)865-4444AbstractPurposeofreview:Upto30millionworkersintheUnitedStatesareexposedtopotentiallydetrimentallevelsofnoise.Whilereliablemedicationsforminimizingorreversingnoiseinducedhearingloss(NIHL)arenotcurrentlyavailable,NIHLisentirelypreventable.ThepurposeofthisarticleistoreviewtheepidemiologyandpathophysiologyofoccupationalNIHL.Wewillfocusonat-riskpopulationsanddiscusspreventionprograms.Currentpreventionprogramsfocusonreductionofinnereardamagebyminimizingenvironmentalnoiseproductionandthroughtheuseofpersonalhearingprotectivedevices.Recentfindings:Noiseinducedhearinglossistheresultofacomplexinteractionbetweenenvironmentalfactorsandpatientfactors,bothgeneticandacquired.Theeffectsofnoiseexposurearespecifictoanindividual.Trialsarecurrentlyunderwayevaluatingtheroleofantioxidantsinprotectionfrom,andevenreversalof,NIHL.Summary:OccupationalNIHListhemostprevalentoccupationaldiseaseintheUnitedStates.Occupationalnoiseexposuresmaycontributetotemporaryorpermanentthresholdshifts,thougheventemporarythresholdshiftsmaypredisposeanindividualtoeventualpermanenthearingloss.Noisepreventionprogramsareparamountinreducinghearinglossasaresultofoccupationalexposures.Keywords:occupationalnoiseinducedhearingloss,occupationalnoiseexposure,hearingprotectionprograms

IntroductionHearinglossisthemostwidespreaddisabilityinWesternizedsociety.Noiseexposureisthemostcommonpreventablecauseofhearingloss;someestimatethatonethirdofallcasesofhearinglosscanbeattributedtonoiseexposure.1Occupationalexposuretonoisemakesupagreatproportionofpatientsaffectedbynoiseinducedhearingloss(NIHL),makingNIHLthemostprevalentoccupationaldiseaseintheUnitedStates.2Occupationalnoiseexposurehasbeendocumentedsinceatleastthe18thcentury,whenitwasnotedthatcopperminersdevelopedhearinglossasaresultofthenoisefromhammeringonmetal.3ScopeoftheproblemWorldwide,1.3billionpeopleareaffectedbyhearingloss.4Itisestimatedthat10%oftheworld’spopulationareatriskforNIHL.2WithintheUnitedStates,16-24%ofhearinglossescanbeattributedtooccupationalnoiseexposures.5,6Infact,itisestimatedthat22millionto30millionworkersintheUnitedStatesareexposedtopotentiallydetrimentallevelsofnoise.2,7Theeffectsofoccupationalnoiseexposureplaceatremendousburdenonboththeindividualandsociety.Thefinancialburdentosocietyissignificantandcontinuestorise,withanestimated$242.4millionannualexpenditureincompensationforwork-relatedhearinglossintheUnitedStates.2Followingmilitaryservice,workersaffectedbyNIHLmakeupthemostpopulateddisabilityclassification.In2010,theU.S.DepartmentofVeteransAffairscompensatedmorethan1.4millionveteransforservice-connectedNIHLandtinnitus,resultinginmorethan$1.2billionofcompensation.3WorkersmostatriskforoccupationalNIHLincludethoseemployedinconstruction,manufacturing,mining,agriculture,utilities,transportation,andthemilitary,aswellasmusicians.Aretrospectivecohortstudyevaluatedaudiogramsof1millionemployeesexposedtoloudnoiseandfoundthejobsathighestriskforhearinglossweremining,woodproductmanufacturing,constructionofbuildings,andrealestateandrentalleasing.Withinthisgroupof1millionnoise-exposedworkers,hearinglosswasmoreprevalentamongmenthanwomen,andtheriskofhearinglossincreasedwithage.8TheconsequencesofoccupationalNIHLtotheindividualcanbedire.Hearinglosslimitsanindividual’sabilitytocommunicatewiththesurroundingworld,whichcanleadtoincreasedsocialstress,depression,embarrassment,poorself-esteem,andrelationshipdifficulties.Obstaclesincommunicationareexacerbatedbydifficultlisteningsituationssuchasenvironmentswithexcessivebackgroundnoise.Hearinglosscanaffectattentionandcognition.2Inolderadults,amildhearinglossisassociatedwithatwotimesincreasedriskofdementia,whileaseverehearinglossisassociatedwithafivetimesincreasedriskofdementia.9Associatedauralfullnessandtinnituscanalsobeextremelytroublesomeforsomepeople.

OccupationalNIHLhasbeenassociatedwithanincreasedriskforwork-relatedinjuries.Excessivenoisehamperstheabilitytohearwarningsignals,monitorequipment,respondtoenvironmentalsounds,andcommunicatewithotherworkers.Onestudyof46,550workersexposedtonoisefoundastatisticallysignificantincreaseinriskforinjuryrequiringhospitalizationwitheachdecibelofhearingloss.Thehazardratioforinjurywas1.01foreachdecibelofhearingloss.10Anotherstudyevaluated1,062workersinatractormanufacturingplant.Inthisstudy,workersexposedtosoundintensities>85dBAaveragedover8hourshada1.52foldincreasedriskofinjurycomparedtoworkersexposedto<85dBA.11Undiagnosedhearinglosshasbeenassociatedwithanexcessmortalityof10-20%overthecourseof20years.12Noise-inducedhearinglosshasalsobeenassociatedwithanumberofnon-auditorysequelae.Themostprevalentoftheseareannoyance,cognitiveimpairment,sleepdisturbance,andadversecardiovascularhealth.2Atheoryhasbeenproposedthatnoiseexposureleadstostimulationoftheautonomicnervoussystemandendocrinesystem,leadingtoincreasedstresshormonereleaseandelevationsinbloodpressureandheartrate.This,inturn,mayleadtoanincreasedriskofhypertension,ischemicheartdisease,andstroke.13,14Epidemiologicalstudieshavedemonstratedahigherprevalenceofcardiovasculardiseaseandmortalityinpopulationsexposedtohighnoiselevels.15,16,17,18,19,20,21PathophysiologyNoiseinducedhearinglossmaybeinflictedbyshortburstsofloudsoundorcontinuouslyelevatednoiselevels.2Suchexposuresleadtocochlearhaircelldamage,damagetosurroundingsupportingcells,andultimatelydegenerationofassociatedauditorynervefibers.Thelevelofinnereardamageandassociatedhearinglossarecorrelatedtotheintensityanddurationofnoiseexposure.3Sublethallevelsofnoisedamagemayleadclinicallytotemporarythresholdshifts,inwhichanindividualexperiencesdepressedsensorineuralhearingthatusuallyrecoverswithin24to48hours.Moreseveredamageleadstoapermanentthresholdshift,clinicallymanifestedasanirreversibledecreaseinhearingthresholds.Recentevidencesuggeststhateventemporarythresholdshiftsfromwhichanindividualrecoversarenotinnocuous,assuchinsultsmayacceleratetheprocessofage-relatedhearingloss.22,23,24Theamountofhearinglossinflictedbyaspecifiednoiseexposureisvariableandspecifictoanindividual.Likemanydiseaseprocesses,thepathophysiologyofnoise-inducedhearinglossrepresentsacomplexinteractionbetweengeneticandenvironmentalfactors.Someauthorshaveestimatedthatupto50%ofindividualvariationsinhearinglossduetonoiseexposuremaybeattributedtohereditaryfactors.25Inaddition,otherpatient-relatedfactorssuchasage,pre-existingsensorineuralhearingloss,hypertension,diabetesmellitus,smokinghistory,and

useofototoxicmedicationsmayaltertheamountofdamageincurredtotheinnerearasaresultofnoise.1Noiseexposuretypicallyaffectsthehaircellsnearthebasalturnofthecochlea,leadingtoacharacteristicdecreaseinhearingthresholdsbetween3kHzand6kHz.Thispatternofhearinglosswillproduceadistinctivedipat4kHzonpure-toneaudiometry,oftentermedthe“noisenotch”(Figure1).Thenaturalresonancefrequenciesoftheouterear,inparticulartheearcanal,appeartoemphasizethedamagetothisfrequencyregion.Thelowerfrequencies,includingthemainspeechfrequencies,areoftensparedinitially.Withprogressivedamage,however,thenoisenotchwillflattenasthresholdsdecreaseatthesurroundingfrequencies.3Individualswithseverehearinglossasaresultofnoiseexposuremayhaveaudiogramsthatareindistinguishablefromothercausesofhearingloss.OccupationalNIHListypicallybilateralandsymmetric.Theremaybe,however,someasymmetryinhearingloss,particularlyifthereisdifferentialexposuretosoundbetweenthetwoears.Whenadiscrepancyispresent,occupationalNIHLiscommonlymoresevereintheleftear,thoughthereasonsforthishavenotbeenscientificallybornout.3Somehavehypothesizedthataright-handedmachineoperatormaybemorelikelytolookovertheirrightshoulder,leavingtheleftearturnedtowardamachineengine.Inhuntersorotherswhoshootfirearms,hearinglosstendstobemoresevereintheearclosesttothebarrelofthegun;theoppositeearisrelativelyprotectedbytheheadshadoweffect.Musiciansmayhavediscrepanciesbasedontheirlocationwithinthebandororchestra.Brassplayersgenerallyproducethegreatestamountofnoise,26andmusiciansseatedwithonesideclosertoanoise-producinginstrumentmayhavegreaterexposureinthatear.PreventionProgramsAtthepresenttime,occupationalNIHLisnotatreatabledisease.Itis,however,entirelypreventable.Afocusisplaced,therefore,onpreventionofNIHLbeforepermanentdamagetotheinnerearcanoccur.Preventionstrategiesmaybefocusedonreducingnoiselevelsinanoccupationalenvironmentoronreducingtheamountofenvironmentalnoisethatistransmittedtoanindividual’sinnerearsystem.Manycountrieshaveimplementedlegalstandardsregardingoccupationalnoiseexposureandtheneedforprotectionaboveacertainexposurelevel.IntheUnitedStates,occupationalnoiseexposureisregulatedbytheOccupationalSafetyandHealthAdministration(OSHA,adivisionoftheU.S.DepartmentofLabor)andtheNationalInstituteforOccupationalSafetyandHealth(NIOSH,adivisionoftheU.S.CentersforDiseaseControlandPreventionwithintheU.S.DepartmentofHealthandHumanServices).ThesetwogoverningbodiesworktogethertodevelopandenforceworkplacesafetyregulationsinthepreventionofoccupationalNIHL.Inordertomonitoroccupationalnoiseexposures,noiselevelsmustbequantifiedandexpressedasanumericalvaluethatcanbesubjectedtostandardizedregulations.Forregulatorypurposes,noiselevelsaregenerallymeasuredinthe

dBAscaleandexpressedasan8-hourtime-weightedaverage(TWA).ThedBAscaleisadecibelscaleinwhichthelowfrequenciesarerelativelyde-emphasized.Thisscaleisappliedwhenmeasuringoccupationalnoiseexposurebecausehigh-frequencynoiseexposurescausegreaterthresholdshiftsthanlow-frequencyexposuresofequivalentintensity.27OSHAandNIOSHhavebothsetforthregulatoryguidelinesforacceptableoccupationalnoiseexposuresandrecommendationsforhearingconservationprograms.OSHAregulationscarrytheweightoflaw;however,NIOSHrecommendationsaremorestringentandarethoughttobebackedbystrongerscientificevidence.27UnderOSHAcriteria,themaximum8-hournoiseexposureis90dBA.Fornoiseabove90dBA,exposuretimemustbereducedby50%forevery5dBincrease(e.g.maximumallowableexposuretimefor95dBAnoiseis4hours,etc.).Thisisreferredtoasthe5-dBtradingruleor5-dBexchangerate.Incontrast,NIOSHrecommendationsarecalculatedusinga3-dBexchangerate,inwhichexposuretimemustbehalvedforevery3dBelevationinnoiselevels.Inaddition,NIOSHsetstherecommended8-hourTWAexposureat85dBAratherthan90dBA.27Thesechangesleadtoconsiderablymoreprotectiveregulationsforworkers,asisdemonstratedinTable1.OSHAmandatesthatahearingconservationprogrambeprovidedforworkerswhoareexposedtonoise>85dBoveran8-hourTWA.Thishearingconservationprogrammustincludethefollowingelements:(1)employersmustmeasurenoiselevelsonthepremises,(2)freehearingprotectivedevicesmustbeprovidedifnoiseexceedsexposurelimits,(3)employeesmustbetrainedregardingnoiseexposureandtheuseofpersonalhearingprotectivedevices,(4)employeesmustbeprovidedwithbaselineandannualaudiogramstoassesstheirhearing,and(5)recordsofalltestresultsmustbekeptandmadeavailabletoemployees.27Withregardtonoiseregulation,noisesourcesortheenvironmentmustbealteredasmuchaspossiblethroughengineeringorschedulingchangestodecreasenoiselevelsintheworkplace.28Mechanismstoreducenoiseproductionandexposurethroughengineeringandadministrativecontrolsmayprovidethemosteffectivemeansforreducingworkers’exposuretonoise,butoftenthesemechanismsareunrealisticorimpossibletoimpose.Whentheon-the-jobenvironmentalnoiselevelscannotbebroughtdowntoacceptablestandards,everyemployeemustbeprovidedwithappropriatepersonalhearingprotectivedevicesandinstructedontheiruse.Itmaybethatcurrentregulations,whicharebaseduponrecoveryofpure-tonethresholdsfollowingnoiseexposure,will,inthefuture,needtotakeintoaccounttherecentlyreportedfindingsthatevenrecoveryofthresholdsfollowingtemporarythresholdshiftsdoesnottrulyindicateanundamagedear.22,23,24Personalhearingprotectivedevicesincludebothearmuffsandearplugs.Expandablefoamplugsprovidesimilarnoiseattenuationtoearmuffs,thoughothertypesofearplugsprovidelessprotection.Advertisedattenuationlevelsofhearingprotectivedevicesrangefrom15-28dBwhentestedinalaboratorysetting.Inrealworkplaceconditions,mostearmuffsandexpandablefoamplugswillprovide10to15dBattenuation.Theuseofearmuffsandplugstogetherimprovesattenuationby5to10dBovereitherdevicealone.Aprotectiveeffectof10to15dBisrelevanttoa

noise-exposedworker,as10dBofattenuationwillbringnoiselevelsintotheacceptablerangeinmorethan90%ofexposedworkers.27Datahasshownthatearplugsmaynotprovidetheadvertisedlevelofprotectionifemployeesarenotinstructedontheirproperuse.29ThemostimportantvariableinpreventingoccupationalNIHLwithpersonalhearingprotectivedevicesisconsistentuseofthedevice.DatasuggestthatpersonalhearingprotectivedevicesareeffectiveinpreventingNIHL;29,30,31,32however,studieshavedemonstratedthatmanyworkersdonotregularlyusethesedevices.33,34,35,36,37Non-useofhearingprotectivedevicestendstobemoreprevalentincertainoccupations,particularlyamongagriculturalworkers.38Musiciansrepresentanotherclassofworkersmanyofwhomdonotregularlyusehearingprotectivedevices.26Studiesevaluatingthesuccessofhearingprotectionprogramshavedemonstratedconflictingresults.ACochraneReviewof15studiesincluding79,986participantsfoundthattheregularuseofpersonalhearingprotectivedevicesaspartofastronghearinglosspreventionprogramwasassociatedwithlesshearingloss.29Otherstudieshavedemonstratedsignificantcontinuedhearinglosseveninthepresenceofahearinglosspreventionprogram.Groenewoldandcolleaguesstudiedaudiometricdatafrom19,911workerswhowereexposedtosignificantnoise,stratifiedintogroupsbasedonsurveyresponsesintothoseworkerswhoneverusedhearingprotectionandthosewhoalwaysusedhearingprotection.DatadidnotshowanysignificantdifferenceinOSHAthresholdshiftsbetweenthetwogroups,thoughtherewasasignificanttrendtowardincreasedriskforhighfrequencythresholdshiftwithoutuseofprotectivedevices.39FutureDirectionsResearchintotheunderlyingdiseaseprocesseshasproducedsomeheadwayinachievingpreventativeandtherapeutictreatmentsforoccupationalNIHL.SeveralauthorshavetheorizedthatoxidativestressplaysaroleinthepathogenesisofNIHLonamicrocellularlevel.40,41Accordingly,severalantioxidantcompoundshavebeenexploredfortheirprotectiveproperties.Inastudylookingatthetreatmenteffectoftheantioxidantcompoundmagnesiumafterimpulsenoise,magnesium-treatedguineapigshadimprovedhaircellpreservationcomparedtogroupstreatedwithmethylprednisoloneandwithplacebo.42Theantioxidantprecursorofglutathione,N-acetyl-L-cysteine,hasbeenshowntohaveprotectiveeffectsagainstnoiseexposureinchinchillastudies.43Humanstudiesonsuchcompoundsarepreliminaryandhavehadconflictingresults.Onetrialinwhich31normal-hearingparticipantsweretreatedwithN-acetylcysteineasaprotectivemechanismagainstnightclubmusicexposuredidnotdemonstrateaprotectiveeffect.44Adouble-blind,placebo-controlledcrossoverstudy,however,performedon53noise-exposedworkersdiddemonstratethatN-acetylcysteinesignificantlyreducedtheincidenceoftemporarythresholdshift.45Interestingly,variabilityinresponsetothiscompoundwasassociatedwithgenetic

polymorphisms.ThoseworkerswiththeglutathioneStransferasesnullgenotypedemonstratedincreasedresponsestothetreatmentcompound.Anothercompound,D-methionine,maysoonundergoclinicaltrialswiththeUSArmyforitsuseasanotoprotectiveagent.26D-methionineisaningredientofyogurtandcheesethathasshownaprotectiveeffectagainstNIHLinanimalstudies.46,47Thisagentwaseffectiveevenwhenadministeredafterthenoiseexposure.EachofthesecompoundshasshownpromiseinpreventingorreversingtheeffectsofNIHL,andhopefullyinthefuturewilladdtothearmamentariumofprotectivetoolsagainstoccupationalNIHL.ConclusionOccupationalNIHListhemostprevalentoccupationaldiseaseintheUnitedStates.Theburdenofdiseaseiffeltbyboththeaffectedindividualandbysociety.Treatmentstrategiesareinthedevelopmentalstages;beforetheybecomeuniversallyavailable,themainstayofcareisinprevention.PreventionprogramstoreduceoccupationalNIHLcanbeeffective,buttheyrequirecompliancewithOSHAandNIOSHstandardsandconsistentuseofpersonalhearingprotectivedevices.

KeyPoints- OccupationalNIHListhemostprevalentoccupationaldiseaseintheUnited

States.- OccupationalNIHLcanresultinpsychosocialisolationandanincreasedrisk

ofdementiaintheelderlypopulation,aswellasanincreasedriskofinjury,heartdisease,andstroke.

- Theeffectsofnoiseonanindividual’shearingthresholdaredependentonacomplexinteractionbetweengeneticandenvironmentalfactorsthatisuniquetoanindividual.

- ThemostimportantwaytomitigateoccupationalNIHListhroughpreventionprogramsthatemphasizeexposurereductionsandtheuseofpersonalhearingprotectivedevices.

- AntioxidantcompoundshaveshownpromiseaspreventativeandpossibletherapeuticagentsagainstNIHL.

MaximumExposureDurationMaximumExposureLevelindBA

OSHAHCA(1983)Regulations

NIOSH(1998)Recommendations

85 8hours88 4hours90 8hours 2hours,31minutes95 4hours 47minutes,37seconds97 3hours 30minutes100 2hours 15minutes105 1hour 4minutes,43seconds110 30minutes 1minute,29seconds115 15minutesorless 28secondsTable1.ComparisonofOSHAandNIOSHregulationsfornoiseexposure.

Figure1.Pure-toneaudiogramdemonstratingtheclassic“noisenotch”at4kHz.

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