Why Low Level Radiation Exposure Should Not Be Feared - AW, AG, LF

WhyLow-levelRadiationExposureShouldNotBeFeared AlanE.Waltar,1 AbelJ.Gonzalez,2 andLudwigE.Feinendegen3

Abstract Thepurposeofthispaperistoaddressthepublicfear thatisusuallyassociatedwithlow-levelradiationexposuresituations.Itsultimateobjectiveistoprovidepersuasiveassurances toinformedbutskepticalmembersofthepublicthatexposuresituationsinvolvinglow-levelradiationarenottobefeared.Unfortunately,justacquiescingtoanunsupportivepublicfearoflow-level radiationisnotwithoutconsequences.Itiscausingseveredisruptionstothebenefitsthatharnessedradiationcanproduceforthe well-beingofallhumanity.Inthispursuit,thepaperprovidesthe scientificandepistemologicalbasisneededforregulatoryreform byreviewingthehistoryinquantifying,understanding,modeling, andcontrollingradiationexposure,includingsomeoftheevolving contributionsoftheUnitedNationsScientificCommitteeonthe EffectsofAtomicRadiation,theInternationalCommissiononRadiologicalProtection,andthemyriadofinternationalandintergovernmentalorganizationsestablishingradiationsafetystandards.It alsoexploresthevariousinterpretationsofthelinearno-threshold modelandtheinsightsgainedfromradiationpathologists,radiation epidemiologists,radiationbiologists,andradiationprotectionists. Giventhatthelinearno-thresholdmodelissodeeplyimbeddedin currentradiationexposureguidance,despitethelackofasolidscientificbaseontheactuallyprovenradiationeffectsatlow-doses,thepapersuggestsnear-termwaystoimproveregulatoryimplementation andbetterservethepublicbyexcludingand/orexemptingtrivial low-dosesituationsfromtheregulatoryscope.Severalexamplesare givenwheretheunsubstantiatedpublicfearoflow-levelradiation hasresultedincripplingthebeneficialeffectsthatcontrolledradiationofferstoamodernsociety.

HealthPhys.125(3):207–227;2023

Keywords:InternationalCommissiononRadiationProtection (ICRP);linearhypothesis;radiation,low-level;UnitedNationsScientificCommitteeontheEffectsofAtomicRadiation(UNSCEAR); linearhypothesis;LNT

1RetiredProfessorandHead,DepartmentofNuclearEngineering, TexasA&MUniversityandPastPresidentoftheAmericanNuclearSociety;12449IngallsCreekRoad,Peshastin,WA98847; 2NuclearRegulatoryAuthority(Argentina),RepresentativeatUNSCEARandMember oftheIAEACommissionofSafetyStandards;formerlyDirectorofradiationsafetyattheIAEA;Vice-PresidentofIRPAandVice-ChairofICRP; BuenosAires,Argentina; 3NuclearMedicine,Heinrich-HeineUniversity, Dusseldorf,Germany;Lindau,Germany.

Theauthorsdeclarenoconflictsofinterest.

Forcorrespondencecontact:AlanWaltar,12449IngallsCreek Road,Peshastin,WA98847,oratemailat alan.waltar@gmail.com

(Manuscriptaccepted 9 March 2023) 0017-9078/23/0

Copyright©2023HealthPhysicsSociety

DOI:10.1097/HP.0000000000001703

www.health-physics.com

INTRODUCTION

IFPROPERLY harnessed,ionizingradiation(orradiation,inshort) hasyieldedextraordinarybenefitsintheworldofmedicine, agriculture,andgeneralindustry.Ifleftuncontrolled,however, highlevelsofradiationexposurecanleadtoseverehealtheffects.Butthisdoesnotjustifyfearingsituationsinvolvinglow levelsofradiationexposure.Thisdichotomyisusualtomany humanendeavors.Weprotectourselvesagainsthighlevelsof harmfulagents,butontheotherhand,weliveinharmonyand areunconcernedwithlowerlevelsofthesameagents.

However,experiencehasshownthatunpleasantemotionsareoftencausedwithinthepublicatlargeandintheir representativesbythemerementionoftheword “radiation.” Thereappearstobeafeelingofthreatfromradiationatany level evendowntoverylowlevelsofradiationexposure. Inspiteofearlyeffortstoputradiationinperspective(Ander andGonzalez1989),thisphysicalphenomenon,whichis strangeformanypeopleinspiteofitsnaturalism(perhaps becausetheycannottouch,see,orsmellit),hasbecomethe nemesisofmanybeneficialendeavorsforsociety.

Thepurposeofthispaperistoaddressthepublicfear thatisusuallyassociatedwithlow-levelradiationexposure situations.Itsultimateobjectiveisprovidingpersuasiveassurancestoinformedbutskepticalmembersofthepublic thatexposuresituationsinvolvinglow-levelradiationare nottobefeared.

Assuch,wefeelakeystepistoprovidescientificand epistemologicalargumentsforpersuadingourcolleaguesin theradiationprotectioncommunityandtheregulatorswho receivetheiradvicethatthecurrentsituationhascreateda stateoffearoflow-levelradiationexposureandthatthis couldbecausingmoreharmthanbenefit,violatingabasic ethicalprincipleofourprofession.Thismightinturnprovidestepsforchangingthewayregulationsareestablished andimplemented.Suchchanges,properlycommunicated, shouldeasethefearthatpermeatesalargesegmentofthe generalpublic.

Unfortunately,justacquiescingtoanunsupportivepublicfearoflow-levelradiationisnotwithoutconsequences.It iscausingseveredisruptionstothebenefitsthatharnessedradiationcanproduceforthewell-beingofallhumanity.

QUANTIFYINGRADIATION

Usually,physicalphenomenaaffectingourlivesarenot quantifiedwithsuchdetailandcontrolledsoconservatively asradiationexposure.Intheireffortstoprovidesuchcontrol, radiationhealthprofessionalshavedevelopedaratherelaboratebutsomewhatbewilderingsystembasedonaquantitythat isconfusedlytermed[radiation] dose,whichisunderstoodby mostpeopleasaquantityofamedicineordrugtakenatone time.Butforradiation,suchadoseexpressestheamountof energydeliveredbytheemittedradiationtoaunitmassoftissue.Achallengeindefinitionarisesbecausethehumanhealth effectsofradiationexposuredependuponnotonlythedoseincurredbutalsoonthetypeofradiation(i.e.,alpha,beta, gamma,etc.),theenergyoftheradiation,andthesuspected sensitivityoftheparticulartissuebeingexposed.

Toaccountforthesevariables,radiationisusuallynot controlledintermsofeitherthedoseitdeliversortheamount ofradioactivitythatgenerates suchradiation.Rather,itis “measured” intermsofanon-physicalentitythat “weights” thephysicaldose.Stateddifferently,thisphysicalradiation doseismodifiedbysubjectiveweightingfactors(i.e.,factors thatareconjecturedratherthanprovenfacts)thatestimatethe influenceofthetypeofradiationinvolvedintheexposuresituationandtheradiosensitivityofthedifferentorgansinthe bodybeingexposed.Suchweighteddoseistermed “effective dose,” butusuallyitissimplycalled “dose.”

Itistobenotedthatconventionally,theterm “dose” is generallyusedtomeantheamountofsubstanceinteracting withatargettissue.Inradiobiology,dosemeansanamount ofa “substance-equivalent” perunitmass.Thistranslatesinto aratiocausingobservedeffectsintheexposedsystem.In pharmacology,forinstance,doseisamassperbodyweight, whereasinradiationbiology,doseisaweightedratioofenergyabsorbedperexposedunitmass,whichroughlyexpressestheweightedamountoftheenergythatdifferentparts ofthehumanbodyabsorbfromradiationexposure.

Butdifferentlythanforotherbodilyattacks,itisnotthe doseincurred(i.e.,theamountreceivedinagivenperiodof timeorduetoaparticularaction)thatisassessedbutrather theso-called “committeddose.” Thisisthelifetimedoseexpectedtobeincurredasaresultofanexposure.Moreover, controlisnotevenexercisedoversuch “committeddose” butratheroverwhatisconfusedlytermed “dosecommitment.” Thisisdefinedasthetotalcommitteddosesthatwouldeventuallyresultfromanyendeavorinvolvingradiationexposure.

IntherecentCongressoftheInternationalRadiation ProtectionAssociation(IRPA),acritiquewaspresentedincludingthepotentialdifficultieswiththesystemofradiation quantitiesandunits recognizingtheconfusionitgenerates eveninprofessionalaudiences(Gonzalez2021).Suchradiationdefinitioncomplexitiesarelikelyresponsibleformisunderstandingandevenpuzzlementamongmembersofthe publicandare,therefore,partiallyresponsibleforthefear causedbyradiation.

RADIATIONHEALTHEFFECTS

Aswithotheragentsinvolvedinourdailylife,radiationis quantifiedandradiationexposureiscontrolledbecauseofits potentialhealtheffects.Thehealtheffectsofradiationhave beenmeticulouslystudiedforwelloveracentury with farmorein-depthinvestigationsthanforanyotherknown bodilyattack.

Infact,thehealtheffectsofnootheragentarestudiedand agreeduponatthehighestinternationalandintergovernmental levelasisthecaseforradiationhealtheffects.TheinternationalintergovernmentalorganizationchargedwiththeestimationofradiationhealtheffectsistheUnitedNationsScientific CommitteeontheEffectsofAtomicRadiation(UNSCEAR), whichreportsitsestimatesannuallytotheUnitedNations GeneralAssembly(UNGA)ofthe194countriesthatare membersoftheUnitedNations.UNSCEARwasestablished bytheUnitedNationsin1955withthemandatetoassessand reportlevelsandeffectsofexposuretoionizingradiation (UN1955).Governmentsandorganizationsthroughoutthe worldrelyonUNSCEAR´sestimatesasthescientificbasis forevaluatingradiationriskandforestablishingprotective measures.Nootheragentsaffectingpeoplehavethebenefit ofasimilarlevelofscientificsupport.

Asaresult,theradiationhealtheffectsthatshouldbeof concernforsocietyarewellknown.Itisundisputablethat exposuretohighlevelsofradiationdose,deliveredatrelatively highlevelsofchangingdoserates,causeharmfulacuteeffects tothehumanbody.Thesehealtheffectsareobservableanddiagnosableinindividualsexposedtosuchhighdosesandcan beunequivocallyattestedbyaradiationpathologistasbeing attributabletoradiationexposure.Theindividualsexposedto suchhighdosesexperiencetissuereactions(oftenreferredto as “deterministic” effects,becauseabovecertainlevelsofdose, theseeffectsare “determined” tooccur)forwhichdifferential pathologicaldiagnosisandattestationareachievablethateliminatepossiblealternativecauses.Theoccurrenceofsuchadeterministiceffectisuncommoninpractice.Somedeterministic effectshavebeensufferedbyworkersasaresultofhighradiationexposuresinseriousaccidents;someothersoccurredin patientsduetotheerroneousadministrationofradiationdose inradiotherapeutictreatmentsandininterventionalradiology. Radiationsafetymeasuresareundertakentopreventsuchexposuresand,asaresult,suchadverseeffectsareexceedinglyrare, giventheusualsafetyendeavorsinvolvingradiationexposure.

Belowcertainthresholdhighdoselevels,suchacuteradiationhealtheffectsthatcanbediagnosedinindividuals andattributedtotheradiationexposuredonotoccur.However,atmodestlyhighandmediumdoselevels,somedelayedlimitedincreasesinthebackgroundincidenceofother healtheffectshavebeenobserved,suchasmalignanciesthat havebeenassociatedwithsignificantradiationexposure.These effectsoccurrandomlyandthereforearetermed “stochastic”

(fromtheGreekstokhastikós, “aimat,guess”),meaninghavingarandomprobabilitydistributionorpatternthatcanbeanalyzedstatisticallyinacohortofexposedpeople.Potentially radiation-induciblemalignanciesareindistinguishablefrom genericbackgroundmalignancies,whicharerathercommon (aroundaquarteroftheworldpopulationsufferamalignancy duringtheirlife),andtherefore:

• Theycannotbeunequivocallyattributedtoradiationexposureinanexposedindividualbecauseradiationexposureis nottheonlypossiblecauseoftheiroccurrence;and

• Theirincidenceinacohortofexposedpeopleisextremely difficulttodetectatmediumdosesandimpossibletoobserveatlowdosesbecauseofthepresenceofahighbackgroundincidence.

Itshouldbenotedthatthereareatpresentnobiomarkers ofmalignanciesthatarespecifictoradiationexposure.Evenif thesebiomarkerswerediscovered,theywouldmostprobably beunabletodistinguishpotentialeffectsfromhuman-maderadiationexposuresvis-à-visbackgroundexposure,whichis usuallyhigherthanthatcausedbyhumanactivities.

Thepresenceofsuch “stochasticeffects” hasbeenidentifiedthroughoutepidemiologicalstudiesoflargecohortsof peoplewhohaveincurredhighandmediumdoses,delivered atrelativelyhighandhighlychangingdoserates notablythe survivorsofthenuclearbombingoftheJapanesecitiesof HiroshimaandNagasaki.Theirincidenceisrelativelylow, butitisstatisticallymeasurableifthecohortofexposedpopulationislargeandtheirradiationdosesarerelativelyhigh.

Insummary,

• Attributionofstochasticeffectstoexposuresituationsis notpossibleinindividuals;

• Attributionisonlyachievablecollectivelyforalargecohortexposedtorelativelyhighdoses,anditisusually expressedasanincreaseinthebackgroundincidenceof thestochasticeffectsinthatcohort;but

• Suchcollectiveattributionis notachievableforlow-dose radiationexposuresituations;namely,forthosesituations involvingexposurelevelstypicaloftheglobalrangeofradiationbackgroundlevels.(Theselevelsarearoundthose usuallyestablishedaslimitsbyregulatoryauthoritiesand ininternationalstandards.)

UNSCEARhasissuedareportthatrecapitulatesand clarifiestheepistemologyaswellasthescientificknowledgeforattributingobservedhealtheffectsinindividuals andpopulationstoradiationexposureanddistinguishesthat frominferringconjecturalrisks(i.e.,aneducatedguess,inferredfromincompleteevidence)toindividualsandpopulationsfromanexposure(UNSCEAR2012a).Itconcluded thatincreasesintheincidenceofhealtheffectsinpopulations cannotbeattributedreliablytochronicexposuretoradiation

atsuchlowlevels.UNGAhasendorsedthisreport (UN2012).

Simplyput,inthenormalradiationexposuresituations thatarecontrolledbyregulators,theoccurrenceofradiation healtheffectscannotbeattested,eitherindividuallyorcollectively. “Risks” canbeonlysubjectivelyinferred.Effects, ifany,wouldbeunobservableduetobothepistemicand statisticallimitations.Whereasthereremainsconsiderable controversyregardingtheactualmechanisticbiologicalresponsestolow-doseexposure,theultimateeffectsonhealth (ifany)andthesubjectivelyinferredriskwouldbesosmall thattherewouldbenoreasontoharboranyfear.

DEVELOPINGTHERADIATION PROTECTIONPARADIGM

Theradiationprotectionparadigm,ormodel,usedfor controllingradiationexposureisgloballyaccepted.Basedon thisparadigm,auniqueinternationalandintergovernmental systemofradiationprotectionstandardsisbeingestablished inco-sponsorshipbyallrelevantorganizationswithintheUnited Nationssystem.Asimilarapproachdoesnotexistforany otheragentaffectingpublicsafety.

Suchinternationalandintergovernmentalfocus-in-depth onradiationhealtheffects,alongwiththeassociateddevelopmentofasophisticatedsystemofsafetystandards,should havebeenareasonforreassuringthepublicthatradiationis verywellunderstoodandproperlycontrolledforpublicprotection.Unfortunately,suchanextensivefocus evenonlow radiationdoseswherenodetrimentaleffectshaveeverbeen seen maybehavingtheoppositeeffect.

Fromahistoricalperspective,assoonasxraysandradioactivitywerediscoveredbytheendofthenineteenthcentury,thenewprofessionalcommunitiesofradiologistsbecameawareofthedetrimentaleffectsofhigh-levelradiation exposure.Theyrealizedtheneedforaparadigmormodelfor theprotectionofthemselves.In1928,afewdecadesafter thesediscoveries,theInternationalX-rayandRadiumProtectionCommitteewascreatedbythe1928SecondCongress ofRadiologyinStockholm.ThisCommitteewouldeventuallyevolveintotoday’sInternationalCommissiononRadiologicalProtection(ICRP)(Lindell1996;ClarkeandValentin 2008).ICRPisanon-governmentalinternationalcharitythat providesrecommendationsonradiationprotectiontoprofessionalsusingradiationandto agenciesresponsibleforregulatingtheuseofradiation.

TheICRPactivitiesgeneratedtheprofessionofradiation-protectionists.Thisisalargecommunityofspecialists whoareinternationallygroupedundertheIRPA.Their workisgovernedbythreeprinciplesdevelopedbytheICRP sincethemid-1970s,whichhavesinceevolvedintothecurrentlyrecommendedparadigm(ICRP2007a).Theseare basedonfoundedethicaldoctrines(Gonzalez2011a;ICRP

2018):namely,thejustificationofactionsthatresultin changesoftheradiationexposuresituation;theoptimization ofradiationprotectionbyselectingthebestprotectionoptionsundertheprevailingcircumstances;andthelimitation ofthecommittedindividualdoses.Afourthprincipleonprotectionofthefutureandtheenvironmentwasimplicitin ICRPrecommendations,butitwasspecificallyaddedby theintergovernmentalinternationalorganizationsandreads asfollows: “Peopleandtheenvironment,presentandfuture, mustbeprotectedagainstradiationrisks” (IAEA2006).This universalparadigmwaselaboratedoveranumberofyears, becomingdefactouniversalaroundthe1990s(ICRP 1991).Itiscommensuratewiththe “precautionaryprinciple” recommendedbytheUnitedNationsEducational,Scientific andCulturalOrganization(WC2005).

Theinternationalandintergovernmentalradiationsafety standardsbasedontheICRPparadigmareestablishedincollaborationwiththeEuropeanCommission(EC),theFood andAgricultureOrganizationoftheUnitedNations(FAO), theInternationalAtomicEnergyAgency(IAEA),theInternationalCommissionofRadiationUnitsandMeasurements (ICRU),UnitoftheInternationalLabourOrganization(ILO), theOrganizationforEconomicCo-operationandDevelopmentNuclearEnergyAgency(OECD/NEA),thePan AmericanHealthOrganization(PAHO),theUnitedNations EnvironmentProgramme(UNEP),andtheWorldHealth Organization(WHO),allundertheaegisoftheIAEAthat issuesthestandards.Thefirststandardswereissuedin1962 (IAEA1962).Theywereverymuchimprovedattheendof lastcentury(IAEA1996a),andtheyhavebeenupdatedrecently(IAEA2014).

intherisk(i.e.,probability)ofincurringeffectsattributable toradiation.

TheICRPparadigmwasinitiallydevelopedtobesuitableforpracticaloperationalprotectionforoccupationalexposuresituationswhereinternationallegallybindingobligationsforlabormaybeinvolved(ILO1980).

InICRP’songoingeffortstoimprovetheirguidanceinresponsetoincreasingscientificinput,aformalquantitativeuncertaintyanalysishasbeenusedtocombinetheuncertaincomponentsofestimatingthechanceofincurringradiation-related malignancieswithandwithoutallowingfortheuncertain possibilityofauniversallow-dosethreshold.Whereasthe existenceofsuchalow-dosethresholdintherisk-doserelationshipisnotseenbyICRPasunlikelyforradiation-related cancersofcertaintissues(ICRP2005),itconcludedthatthe evidencedoesnotfavortheexistenceofauniversalriskthreshold.However,bothUNSCEAR(UNSCEAR2012a)and ICRPconsideredthatsucheffectsinthislow-doseregionbe referredtoasonlynotionaleffects.

Onsuchbasis,aso-called “detriment-adjustednominal probability” perunitdosecoefficientwasdevelopedby ICRP,thevaluebeingaround5%Sv 1 ofdose.Thus,dose restrictionsforradiationprotectionpurposesarecurrently implicitlyrecommendedonthebasisofariskcoefficient of0.005%mSv 1 (athousandthofaSievertistermed millisievertormSv).Itistobenotedthatwhile5%Sv 1 is mathematicallyequivalentto0.005%mSv 1,thesetwofactors areepistemologicallyverydifferent.Thefirstisbasedonmeasureddataandthesecondonconjecturalexperts’ judgments.

THECURRENTPARADIGM

TheICRPparadigmiscurrentlyusedworldwide.Itimpliesaconservativeassumption:thattheprovenriskofradiationathighdosescanbeconjecturedforlowdosesaswell, despitealackofdirectevidencesupportingsuchaconjecture andinspiteofaUNSCEARpronouncementthathealtheffects atsuchlowdosescannotbeattributed(seelastparagraph under “Radioepidemiologists” inthesectiontitled “The LinearNo-ThresholdRegulatoryApproach”).

TheICRPpurportstobeawarethattherearerecognizedexceptionstosuchanassumptionembeddedintheir paradigmbutstilljudgesthatforthepurposesofradiation protectioninthedoserangebelowabout100mSv,itisplausibletoassumethattheincidenceofdetrimentaleffectsmay riseindirectproportiontoanincreaseinthedoseintherelevantorgansandtissues.Therefore,theparadigmrecommendedbytheICRPisbasedupontheassumptionthatat anydose,includingdosesbelowabout100mSv,agivenincrementindosewillproduceadirectlyproportionateincrement

Itiscrucialtoemphasizethisimportantdifference.This paradigmisbasedonepidemiologicalfactualdatausually obtainedfromexposuresituationsinvolvinghighdoses,high dose-rate,andinmanycaseslargevariationsinthedose-rate change,andwhereanepidemicofattributableradiationeffectsisevidentandscientificallyquantifiablebyprofessional radioepidemiologists.ThiscouldbedonewellwiththecohortoftheHiroshimaandNagasakibombingsurvivorsand withthechildrenaroundChernobylcontaminatedwithhigh levelsofradioiodineviamilkingestion,justtoshowtwoexampleswhereanepidemicofradiationeffectswassuffered. Forsuchwhole-bodyradiationexposures,theprobabilityof someoneinthecohortdevelopinganattributablemalignancy isconsideredtobearound10 2 Sv 1 ofeffectivedoseincurred(basedonnothresholdbutincludingvariationswith sex,conditionsofexposure,etc.)(IAEA2014;ICRP2007a).

Butacritiquetothiscoefficientisthatexpressingit withthequantityeffectivedoseandtheunitSievertisequivocalbecausethisquantityandthisunitweredefinedforlow doses.Anevenmoreseriousproblemarisesduetothescientificdatabeingaccumulatedfromsituationswheretherewas anepidemicofradiationeffectsbeingextrapolatedtoradiationexposuresituationswherethereisnoobviousepidemic,

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thedosesarelow,thedoserateislow,andthechangeinthe doserateisverylow.

Theparadigmthenconjecturesthatfactualepidemiologicaldatasupportingtheriskof10 2 Sv 1 areapplicable tolow-radiationdose,lowdose-rate,lowchangesindoserate,andsituationsimposinga detriment-adjustednominal riskcoefficient of10 5mSv 1.Theequation10 2 Sv 1 =10 5mSv 1 isatthecoreofaconceptualproblemthatthispaperaimsto underline:whereasthisequationismathematicallycoherent,itcanbestronglyarguedthatitisepistemologicallyincoherent.Theleftrepresentsaprovenfact;namely,atruth demonstratedbyevidence,whiletherightrepresentsaninferredconjecture;namely,anopinionorconclusionbased onincompleteinformation.

Nevertheless,ontheICRPbasisoutlinedabove,atheoreticaldose-responserelationshiphasbeensynthesizedby theUnitedNationsEnvironmentalProgramme(UNEP 2016)asinthegraphshowninFig.1.

Thedosesareexpressedas:

•“Highdoses” (afewthousandsofmSv);

•“Moderatedoses” (afewhundredsofmSv);

•“Lowdoses” (afewtensofmSv);and

•“Verylowdoses” (afewmSv).

• Itshouldbenotedthattheglobalaveragenaturalbackgrounddoseis2.4mSvy 1 (UNSCEAR2008c).Typical highbackgroundvaluescouldbearound10mSvy 1,and veryhighnaturalbackgroundvalueshavebeenfoundina fewareasoftheworldwheretheyareevenwellabove 100mSvy 1 (UNSCEAR2008c).

• Theprobabilitiesareexpressedinpercentagesbetween 0%and100%,where:

• 100%correspondstothecertaintythattheeffectwilloccur;and

• 0%correspondstothecertaintythattheeffectwillnotoccur.

Ithastobenotedthattheprobabilitiesareoftwo distinguishabletypes:

• Frequentistprobabilities,whichareinthehigh-dosearea, arebasedonevidence;namely,onthetruthfulandverifiableexistenceofanincreaseinthefrequencyofradiation healtheffectsinacohortofexposedpeopleandaredefinedasthelimitoftherelativefrequencyofincidence oftheeffectinaseriesofcertifiableepidemiologicalstudiesonsuchcohorts;and

• Subjectiveprobabilities(sometimesalsoconfusedlytermed “Bayesian”),whichareconjecturedforthelow-dosearea, expressedasapossibleexpectationthatradiationhealtheffectsmightoccur,andarequantifiedbyapersonalbeliefor expert’sjudgement;thatis,notnecessarilysubstantiatedby thefrequencyorpropensitythattheeffectsactuallyoccurat suchlevelsofdose.

Bothfrequentistandsubjectiveprobabilitiesaremathematicallycompatiblebutepistemologicallyverydifferent: thefirstisbasedonfactualevidence,andthesecondis basedonsubjectiveconjectures(i.e.,extrapolationsthat lackexperimentalevidence).

Wefurthernotetheimportanceofdistinguishingbetweenthefollowingfactors:

• Verifiedobservationsofhealtheffectsinexposedindividualsandpopulations,whichallowtheiroccurrence tobeattestedbyqualifiedprofessionalsand,therefore, sucheffectstobeunambiguouslyattributedtotheexposuresituationsthatgeneratedthem;and

• Theoreticalprojectionsofhealtheffects(markedasbiologicallyplausibleinFig.1),forwhichoccurrenceisfeasible butnotverifiable;namely,thoseprojectionsallowingonly someconjecturalinferenceofrisks.

Asindicatedbefore,giventhecurrentstateofknowledge, radiationhealtheffectsinindividualsexposedtoradiationcan onlybeattributedwithconfidenceiftheywerediagnosedand theiroccurrenceattestedbyaradiopathologicalspecialist.These deterministiceffectsareusuallyacuteandoccurearlyinindividualsexposedtohighdosesofradiation.Theydonotoccur unlessthedoseexceedsacertainhighthresholdvalue.

Inthelow-andmedium-doseregions,thereiscurrently nofactualwaytoattributeradiationhealtheffectstoaspecificindividualwhoispartofanexposedcohortbecause, forthetimebeing,therearenobiomarkersthatallowthat determination.Notwithstanding,collectiveincreasesinthe backgroundincidenceofhealtheffectsassociatedwithradiationexposurecanbedeterminedasaresultofepidemiologicalstudies.But,asemphasizedbefore,statisticaland epistemicuncertaintiesmakeanyfactualdeterminationof suchdamageunfeasiblewhenthedosesarelow.Itisonly throughradiationbiologythatsomemechanisticinsightis

Fig.1. Atheoreticaldose-responserelationship.

possible.Researchoverthepastcoupledecadeswithinthe radiobiologycommunityhasprovidednewinsightsregardingtheexistenceofhealtheffectsatlowdoses,including claimsondetrimental,beneficial,ornoeffects.Additionalresearchislikelynecessarybeforesuchfindingscanachievethe reproducibilityandreliabilityrequiredtoincludethecomplexitiesnecessaryindevelopingauniversallyacceptedradiobiologicalmodelforevaluatingtheultimatehealtheffectsoflow-doseradiationexposure.

Thissituationhasresultedinadilemmaforregulators duringtheimplementationstage.Therandomnatureofstochasticeffects,alongwiththeparadigmrecommendedby ICRPandestablishedininternationalandintergovernmentalsafetystandards,makeitimpossibletoderiveacleardistinctionbetween “safe” and “dangerous.” Thishascreated difficultiesinexplainingthecontrolofradiationexposures, howeversmalltheymaybe,andhaslikelybeenasignificant causeofthepublicfearofradiation.Themajorimplication oftheICRPparadigmandoftheconsequentinternational andintergovernmentalsafetystandardsisthatsomeriskis assumed notwithstandingtheclearevidencethatfactual healtheffectsfromlowradiationdoselevelshavebeen conjecturedbutnotproveninpeople.

DOSERATEANDDOSE-RATECHANGE

Theradiationprotectionparadigmanditsderivedsafety standardsarebasedonthequantitydose,incurredorcommittedoveraperiodoftime,usually1y.However,oneareathat hasreceivedlittlefocustodateistheinfluenceofboththe doserateandtherateofchangeofthedoserate(namely,of thefirstandsecondtime-derivativesofdose)ontheultimate healtheffectsofradiationexposure,and,therefore,onthe protectionparadigmanditsderivedstandards.Thedoserate

seemstobecertainlyrelevant,andtherateofchangeofdose ratemayberelevantaswell,buttheinformationavailableon thelatterisminimal,althoughsomeexperimentalevidence onitsinfluenceexists(Brehwensetal.2010).

Muchofourdatabaseforhighradiationdosecomes fromtheatomicexplosionsinJapan,anditisabundantly clearthatnotonlythedosebutalsothedoserateandtherate ofchangeofthedoserateforthosesituationswasenormous. However,inmostradiationexposuresituations,thedose,the doserate,andthedose-ratevariationarelow,andthustherate ofchangeofdoserateisbasically zero.Fig.2illustratesthese importantdifferences.

Oneofthefewexposuresituationsthatfeatureschanges inthedoserateistheexposureofaircrewandpassengersto cosmicraysduringsomeminutesfollowingdepartureand beforelanding,whereboththedoserateandtherateof changeofdoseratevary.Butthechangeisprotractedover timeand,furthermore,theavailableepidemiologicaldata forthesecohortsisminimalforestimatinghealtheffects. Hence,weremainopentolearningmoreabouthowthis mayhavefactoredintoourunderstandingoftheactualhealth effectsofionizingradiationatlowdoses.

ATTRIBUTIONVS.INFERENCE

Fromthediscussionabove,itcanbeconcludedthatthere arecurrentlytwodomainsinthedose-effectsrelationship,namely:

• Adomainaboveacertaindoserange,inthemoderateand highdosearea,whereeitherpathologicaldiagnosisorepidemiologicalevidencecanbegatheredinordertoattributetoradiationeitherdeterministiceffectsinindividuals orstochasticeffectsincohortsofexposedpeople;and

• Adomainbelowthisrangewherethereisbiologicalinformationsuggestingthatradiation healtheffectsmightbeplausibleandthatariskcouldbesubjectivelyinferred,e.g.,for regulatorypurposes.Inthislowandverylowdomain,the risksinthecurrentregulatoryframeworkaregenerally inferredbytheexpertjudgmentofradio-protectionists.

Unfortunately,theattributionoffactualeffectsoflow-level radiationexposure(usuallytermedepistemology)isfartoo oftenbasedonconjecturedestimates;namely,opinionsor conclusionsinferredfromincompleteinformation.Thisisa seriousproblem.Understanding thatsuchriskisconjectural iscrucialforthedebateontheperceptionoflowradiation

4GonzalezAJ.Attributionsofhealtheffectstoradiationvis-à-visinference ofradiationrisk:ICRPrecommendationsandUNSCEARreporting.In: InternationalJointConferenceRadio2017.VCongressoBrasileirode ProteçãoRadiológica,VICongressodeProteçãoContraRadiaçõesde PaísesdeLínguaPortuguesaeVIICongressoInternacionaldeRadioproteção Industrial.OrganizadopelaSociedadeBrasileiradeProteçãoRadiológicaem cooperaçãocomaAgênciaInternacionaldeEnergiaAtômica,Sociedade PortuguesadeProteçãocontraRadiaçãoeABENDI,CidadedeGoiânia, 25

29September2017.

Fig.2. Ontheleftthefigurepresentstheidealizedtimevariationofdose (D),doserate(dD/dt)andchangeofdoserate(d2D/dt2)causedbytheexplosionsinHiroshimaandNagasaki;ontherightthesamevariablesare presentedforatypicalradiationexposuresituation.

dosesbecausethishasnotbeenexplainedtomembersofthe publicandtheirrepresentativesinpositionsofauthority.The epistemologicallimitsaroundtheradiationsciences,which areessentialforclarifyingtheradiationprotectionparadigm, havebeenamplydebatedintheliterature(Gonzalez2011b, 2014aandbWielandandGonzalez2018)4 andrecently havebeenaddressedbytheIAEA(IAEA2022a).

STUDYINGRADIATIONEFFECTS

Therearefourprimaryprofessionalspecialtiesfocused onstudyingradiationhealtheffects:

• Radiation-pathologists.Expertswhoareabletodiagnose,attestandattributetoparticularexposuresituations theendresultofradiation-induceddiseases inexposed individuals.Theseexpertsdiagnoseonthebasisoftheir professionalexperienceandalsousinglaboratorysamplesofbodytissuefordiagnosticorforensicpurposes. Theirfieldofcompetenceandpossibilityofdiagnosis, attestation,andattributionarelimitedtodeterministiceffects,andthereforeonlytohighdoses;

• Radiation-epidemiologists. Expertswhousemedical statistics(specificallythestatisticsofepidemics)toestimatetheprevalenceofhealtheffects,suchasstochastic malignancies,thatcouldbeassociatedtoradiationin exposedcohortsofpeople(notinindividuals).Sincethese effectsusuallyhavearelativelyhighbackgroundincidence,epidemiologistsaimatquantifyingincreasesin suchincidenceinacohortfollowingtheirradiationexposure.Bothepistemicandstatisticallimitationsrestrictthe possibilitiesofepidemiologicalestimatesofstochasticradiationeffectstomediumandhighdosesincurredbycohortsof people.However,epidemiologists’ competenceforestimationandattestationshouldinprincipleberestrictedtosituationswherethereisanepidemicinthecohort,namelywhere changesintheincidenceofeffectscanbeobservedandmeasured(asitwasthecaseintheHiroshimaandNagasakicohort).Alas,manyradiationepidemiologistshaveextended theirestimatestothelow-doseregionbyconjecturingthat thesameepidemicsthattheycanobserveandquantify athighdosesalsooccuratlowdoses–despitethatsuch anepidemiccannotbeseenbutonlyconjectured;

• Radiation-biologists.Expertswhofocusonthemechanisticbiologicalchangesattributedtoradiationexposure (e.g.,evaluatingtheprogressionofmolecularchanges causedbyradiationupthroughcells,tissue,andorgans). Theycanprovidescientificinsighttothemechanismsof inductionofradiationhealtheffects,andtheyarealsoable toattesttheoccurrenceofradiationexposureinindividualsbyusingbiologicalindicators.However,theycannot yetreliablyattesttotheoccurrenceofultimatehealtheffectsineitherexposedindividualsorincohorts;and

• Radiation-protectionists.Expertsassociatedwithconjecturingandinferringradiationrisksonthebasisoftheinformationprovidedbyradiation-pathologists,epidemiologists,and biologists,inordertoprovideguidancefortheprotectionof peopleandtheenvironmentagainstexposuretoradiation.

Afterincludingasummaryofwhatismeantbythe term “risk,” thisarticlewillthenfocusonsomeofthekey issuesrelatedtotheabovedisciplines.Itisnotedthatitis uncommonthatspecialistsinthesefourverydifferentdisciplinesdirectlyinteractwithanopeneartounderstandthe fullcomplexityoftheeffectsoflow-levelradiation.Asaresult, thelackofconvergenceintheirworkhasledtosomeconfusionand,understandably,haslikelybecomeanotherelement leadingtothecurrentunnecessarypublicfearoflow-levelradiation.Amongseveralattemptstobringthesedisciplinestogether,theAmericanNuclearSocietyjoinedwiththeHealth PhysicsSocietyinsponsoringaconferencein2018(Health Physics2020)tobringtogetherkeyinternationalspecialistsinthesefour(plusrelated)disciplines.Asummaryof theconferenceoutcomeisreferenced(Feinendegen2020).

RADIATIONRISK

Theword “risk” canmeanmanydifferentthingstomany differentpeople,includingprofessionals.Followingisashort overviewofthisconceptinrelationtoradiation,whichhasbeen amplydiscussedattheinternationallevel(Gonzalez2019).

Fortheprofessionalcommunity, riskisformallydefined ininternationalsafetystandards(IAEA2007)as,alternately:

• Theprobabilityofaspecifiedhealtheffectoccurringina personorgroupasaresultofexposuretoradiation;

• Themathematicalmean(expectationvalue)ofanappropriatemeasureofaspecified(usuallyunwelcome)consequence;and

• Amulti-attributequantityexpressinghazard,danger,or chanceofharmfulorinjuriousconsequencesassociated withactualorpotentialexposures.

Theseareverydifferentconcepts,andtheusebyprofessionalsofallofthemhascertainlynothelpedtofacilitate theunderstandingofriskatlowdoses.

Butforthenon-professionals,themeaningofriskcan beverydifferentthanthedifferentconceptsusedbyprofessionals.Thegeneralpublicoftenassociateriskwithavariety ofconnotationsincludingchance,plausibility,likelihood, prospect,hazard,imperiling,jeopardizing,gambling,betting, wagering,venturing,danger,peril,threat,menace,fear,and endangering.Threat,menace,andfearhavebeenthemain publicconnotationsofradiationriskandhavefueledthepublicfearofradiation.

Historically,thequantificationof “risk” wasformedon aretrospectiveanalysisofactualexperienceonfactual

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frequenciesofpastoccurrencesandwasthusquantified witha “frequentist” probability;namely,aprobabilitycalculatedfromthefrequenciesoffactualoccurrenceoftherisky event.However,overtime,theconceptof “risk” evolved,particularlyforradiationprotection,andisnowoftenalsoreferredtoasasubjectiveprobabilityofoccurrenceresulting fromexperts’ judgments,whicharesometimesbasedonestimatesoffrequentistprobabilitiesbutfromdifferentsituations.Thus,dueinlargeparttothelackoffactualdataof healtheffectsatlowdoses,conjecturesaremadeextrapolatingriskestimatesfromhigh-doseradiationexposuresituationstolow-dosesituations.

Theconceptoffrequentistprobabilitywasandcontinues tobethebasisforepidemiologicalestimatesandshouldprovidethecorescientificbasisforradiationprotectionatdoses abovethelevelsatwhichstochasticeffectsareseen.Originally, radioepidemiologicalestimatesweregenerallyperformed usingdataonfrequenciesofsuchactualhealtheffectsfactually incurredintheaftermathofpastradiationexposuresituations. Sucheventsusuallyinvolvedrelativelyhighdosesandhigh andhighlychangingdoserates,suchasthoseaffectingthe cohortofHiroshimaandNagasakisurvivors.Alas,this frequentistradioepidemiologicalpracticewasextendedto low-doseradiationexposuresituationsforwhichaprevalenceofradiation-induceddiseaseisnotidentifiable;namely, wheretherearenotfrequenciesprovidingabasisfortheassignmentoffrequentistprobabilities.Thesesituationswould epistemologicallybeconsideredtobeoutsidethedomain ofradioepidemiology.However,extrapolationsfromthe frequentistexperiencehavebeenusedforprovidingestimates and,withthehelpofexperts’ subjectivejudgments,notional radiationriskshavebeeninferredasasubjectiveprobability quantifyingapotentialincrementoverthebackgroundprevalenceofeffects.

Onthesebases,bothfrequentistandsubjectiveradiation risksareexpressedinradioepidemiologyindistinctlyasexcess absoluterisk,meaningtherateofdiseaseinanexposedpopulationminustherateofdiseaseinanunexposedpopulation.They arealsoexpressedasexcessrelativerisk,i.e.,therateofdisease inanexposedpopulationdividedbytherateofdiseaseinanunexposedpopulationminusone.Inadditiontothesetwomain denotations,alargenumberofrisk-relatedterminologiesare somehowconfusedlyusedinradioepidemiology,anotation thatmaybecontributingtopublicfear.

Thepuzzlementcausedbythedifferentdenotations andconnotationsoftheword “risk” havepermeatedthe fieldofradiationprotection.Inthedescriptionsoftheradiationprotectionparadigmandintheradiationsafetystandards,thewordriskisusedprolificallyandindissimilar ways,withoutspecificallydescribingthemeaningofthe termeachtimeitisused.Notably,adetriment-adjustedrisk isdefinedastheprobabilityoftheoccurrenceofastochastic healtheffect,modifiedtoallowforthedifferentcomponents

ofsuchdetrimentinordertoexpresstheseverityoftheconsequence(s).Thedetrimentisamultidimensionalconcept includingstochasticandconjecturalcomponents,suchas probabilityofattributablefatalcancer,weightedprobabilityofattributablenon-fatalcancer,weightedprobabilityofsevereheritableeffects,andlengthoflifelostifthe harmoccurs.

Thus,theradiationprotectionparadigmisbasedona radiationweightedrisk-dosemodelestimatedbyextrapolationofhigh-dosedata,withoutadosethreshold.

Initspublication60,ICRPhadengagedinasophisticatedanalysisoftheconceptofriskbutavoidingthecrucial distinctionbetweenfrequentistandsubjectiveprobabilities. Thatrecommendationincludedtheformulationofprecise, butagainconjectural,quantitiesdefiningrisk,including thetotalprobabilityofdeath(usedasareference)andthe conditionalandunconditionaldeathprobabilityrates.These quantitieswouldeventuallyevolveintoseveraltypesofestimatesthatcanbeusedtocalculateconjecturallifetime risk;namely,thesubjectiveprobabilitythatindividualswill develop,ordiefrom,aspecificdiseasecausedbyradiation exposure.Theseestimatesinclude:

• Theexcesslifetimerisk,whichisthedifferencebetween theproportionofpeoplewhodevelopordiefromthediseaseinanexposedpopulationandthecorresponding proportioninasimilarpopulationwithouttheexposure;

• Theriskofexposure-induceddeath,whichisdefinedas theincrementinacause-specificdeathrateaccumulated overalifetime,asanadditionalcauseofdeathintroduced byradiationexposure;

• Theriskoflosinglifeexpectancy,whichdescribesthe decreaseinlifeexpectancyduetotheexposureofinterest;and

• Thelifetimeattributablerisk,whichisanapproximation oftheriskofexposure-induceddeathanddescribesexcess deaths(ordiseasecases)overafollow-upperiodwith populationbackgroundratesdeterminedbytheexperienceofunexposedindividuals.Thelifetimeattributable riskisusedbyICRPtoestimatelifetimerisksandforestablishingdoserestrictions,butunfortunatelyitwasnot clearlyindicatedthatlifetimeattributableriskisaconjecture(i.e.,basedonopinionsorconclusionsinferredfrom incompleteinformation).

Itistobeemphasizedthatthedistinctivenessofthe frequentistvis-à-vissubjective interpretationofriskisnotspecificallyrecognizedintheICRPparadigm.Thefrequentist probabilitiesarisingfromtheepidemiologicalstudiesofpopulationsexposedtorelativelyhighdosesandhighdoseratesare usedtofiguresubjectiveradiationriskestimatesforlow-dose low-dose-rateexposuresituations,andnospecificdistinctions aremadebetweenthesetwoprobabilities.

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Itiscleartherearemanynuancesthatcomeintoplay whendiscussingrisks,andradiationrisksinparticular.A comprehensivediscussionofsuchnuancesisaddressedin theliterature(Gonzalez2015).Forthepurposeofthispaper, itshouldbesimplyrecognizedthatallfourfieldsoftherelevantsciences,i.e.,radiationepidemiology,radiationbiology,radiationpathology,andradiationprotection,playan importantroleinunderstandingtheactualhealtheffectsof radiation.Whereasdisagreementoftenexistswithinthese fieldsregardingtheinterpretationofthehealtheffectsresulting fromradiationexposure,thereisampleandconvincingevidencethatthereisnolegitimatereasonforthepublictoremain fearfulofexposuretolowradiationlevels.

Insummary,

• Aprovableradiationriskforindividualsoccursonlyfor exposuretohigh-doseradiation(thedeterministicdomain andthemediumandhighdose-epidemiologicaldomain).

Peopleoughttofearriskduetotheeffectsresultingfrom highandmediumdoses,butsucheffectsarenotattributablefollowingexposuretolowradiationdoses.

LEGALCONSIDERATIONS

Theabilitytoattributehealtheffectstospecificradiation exposuresituationscaninfluencethelegalcapabilitytoimputedamagesfromthosesufferingdetrimentaleffectsto thoseresponsiblefortheexposure.Theimputationmayincludeassigningliabilitiesforphysicalinjuriesorharmful effectsinflictedonthosewhoreceivedtheexposure.

Fordeterministiceffects,thelegalprocessisstraightforward,andtheradiopathologistdiagnosingtheeffectisaproper andsufficientexpertwitnesswhocanlegallyattestincourtto thefactualoccurrenceoftheeffects.Butforstochasticeffects, individualattributionisnotfeasible.Insomelegaljurisdictions (Gonzalez2022),collectiveimputation(sometimestermedas “classaction”)isfeasible,andradioepidemiologistswouldbe appropriateexpertwitnesses.However,suchclassactionsare notobtainableinalljurisdictions(Gonzalez2022).

Forsituationswheretheradiationexposureisinthe loworverylowregion,thereisnofactualbasistoimpute damagesincetheinferencesofriskarebasedonlyonsubjective(conjectural)expertjudgment.Fartoooften,thiscaveatismissedinlegalproceedings,andawardsforworkers ormembersofthepublicsubjectedtotheselow-doseexposuresarebasedonemotionsorpoliticalconveniencerather thanscience.Thesesituationsareoftenpublicizedandhave alsolikelycontributedtothepublicfearofradiation.

Insomejurisdictionswherelegislationisnotcodifiedand foroccupationalcases,ithasbeenagreedtousefactualexposuredatatodetermineaconjectural “assignedshare” oftheradiationdamages.This “assignedshare” isdeterminedbyfirst evaluatingthehypotheticalrelativeriskofthesituation.The

excessriskisthentheriskincurredbeyondtherelativerisk, andtheassignedshareissimplydeterminedastheexcess relativeriskdividedbytherelativerisk.

Theoverallissueofattributingworkers’ harmtooccupationalexposurewasaddressedbytheInternationalLabor Organizationwithoutaclearoutput(ILO2010).Thewider issueofimputingharmtoradiationexposuresituationshad beendiscussedintheinternationallegalliterature(Gonzalez 2002,2014c)andhasbeenoneofthetopicsofarecent IAEAbookonnuclearlaw(IAEA2022b).

Adefinitiveclarificationoftheimputabilityofharmto radiationexposuremightbecrucialforlimitingthepublic fearoflowradiationdoses.

THELINEARNO-THRESHOLD REGULATORYAPPROACH

Duringtheearlyyearsfollowingthediscoveryofradiation,thisnewphenomenonwasgenerallyreveredasanexcitingnewmethodologyfordiagnosingbrokenlimbsduringWorldWarIandevenasa “tonic” forimprovinghealth. Butlikemanyothernewdiscoveries,itwassometimesused inexcess causingskinburnsandotherhealthimpairments andthispromptednewexperimentstodeterminesafe doselevels.

PerhapsthemostinfluentialworkwasdonebyHerman Muller(Calabrese2019)whenheirradiatedfruitfliesand concludedthattherelationshipbetweendoseanddamage waslinear,withnothreshold.Eventhoughhisdatawereobtainedatveryhighdoserates(thousandsoftimeshigher thanlow-levelradiation),hewaslaterawardedaNobel Prizeforrelatedmutationalstudies,providingahighmeasureofnotoriety,withafollow-upendorsementofhislinear modelbytheNationalAcademyofSciences(NAS)inthe mid-1950s(CalabreseandGiordano2022).

Althoughtheremayhavebeenotherfactorsinaddition toMuller’searlyworkthatcameintoplay,hisworkleddirectlytotheacceptanceofthelinearno-threshold(LNT) model.Thus,inspiteofthemanycaveatsintheICRPrecommendations,LNThasbeenadoptedininternationalsafety standardsandalmostuniversallyusedwithintheinternational radiationprotectioncommunityasapracticaltoolforregulatingradiationexposure.

Butamajorcontroversyisbuildingwithintheradiation protectioncommunity.Someresearch(Calabreseetal.2022), alongwithanextensivevideoseriespioneeredbytheHeath PhysicsSociety(Cardarellietal.2023),provideconsiderablesupportforhowprofessionaldeceptionmayhavecrept intotheadoptionoftheMuller’soriginalLNTpremise.

GiventhequestionableethicsrevealedintheCalabrese investigations(Cardarelli2023),itisnotsurprisingthatthe meaningoftheLNTmodelismultipleandimprecise.Itis intendedtomean “alineardose-responserelationshipwith

nothresholdofdose,” butthereisambiguityinitsprecise understanding,withinterpretationsincludingthefollowing:

• Forsomeitisa premise;namely,anunderlyingassumptionthatradiationcarriesariskatanylevel;

• Forothersitisa hypothesis;namely,asuppositionor proposedexplanationoftherelationbetweenhealtheffectsandincurredradiationdose,whichismadeonthe basisoflimitedevidenceasastartingpointforfurtherinvestigation;and

• Forothersitisa model;namely,asimplifieddescription ofacomplicatedphenomenonthatisusefulonlyfor practicaloperationalpurposes.

ThesevariousviewsoftheLNThavepermeatedthe differentprofessionalcommunitiesinvolvedintheissue. Giventhiscontroversy,LNThasbeenusedwithdifferent denotationsbytherelevantprofessionalcommunitiesand withmanyconnotationsabsentspecializedaudiences,thus likelybecomingasignificantcauseofconfusioncontributingtothepublicfearofradiation.

LNThasbeenviewedbythevariousradiationprofessionalcommunitieswiththefollowingsimplifieddescriptions.

Radiopathologists

Forradiopathologists,LNTisneitherapparentnorneeded, neitherasa premisenorasa hypothesisnorasa model. Radiopathologistslookforfactuallydiagnosedeffectson individualswhohavebeenexposedtohighradiationdoses. Thedose-responserelationshipisasigmoidcurvepresentingadefactodosethreshold.

Belowthedosethreshold,radiationeffectsare,fora radiopathologist,neitherdiagnosablenorattributableinindividuals.Itshouldbenoted,however,thatradiopathologists mayusespecializedbioassayspecimens(suchassomehematologicalandcytogeneticsamples)asbiologicalindicatorsof radiationexposure,evenatdosesbelowthethreshold.Nonetheless,UNSCEARhasindicatedthatthepresenceofsuchbiologicalindicatorsinsamplestakenfromanindividualdoes notnecessarilymeanthattheindividualwouldexperience healtheffectsduetotheexposure(UNSCEAR2012b).

Radioepidemiologists

Forradioepidemiologists,LNTisa hypothesis,anepidemiologicalconjecturebywhichchangesinthebackgroundincidenceofdeleteriouseffectsassociatedwithradiation,suchasmalignancies,perunitdose,canbemeasured. Indeed,radiationexposuresituationsinvolvingrelatively highdoses,deliveredatrelativelyhighdoseratesandinvolvingsubstantivetimechangesofthedoserate,havebeen made.Butalltoooften,radioepidemiologistshavepresumedsuchchangesoccurequallyatradiationexposuresituationsinvolvinglowdosesandlowdoserateswithno changeinthedoserate inspiteofthefactthatepidemio-

logicalevidenceisnotachievableinsuchsituationsdueto epistemicandstatisticallimitations.

Stateddifferently,forradiation-epidemiology,LNT meansthattheincidenceofeffectsperunitdoseathighdoses, highdoserates,andhighdose-ratechanges,whichisbacked upwithepidemiologicalevidence,remainsthesameatlow dosesandlowdoserateswithnosignificantchangesinthe doserate,inspiteoftheabsenceofepidemiologicalevidence. Epidemiologyisaveryserviceablesciencewherean epidemicexists,suchastheepidemicofradiationeffects thatfollowedthebombingofHiroshimaandNagasakior theepidemicofpediatricthyroidcancerthatfollowedthe highthyroiddosesincurredbychildrenthatingestedhighly contaminatedmilkfollowingtheChernobylaccident (UNSCEAR2008c).Butsuchanepidemichasnotbeen visibleforlow-levelradiationexposuresituations.Itisonly presumedtoexist.Theexistenceofeffectsisjustconjectured because shouldtheyexist theycannotbeseenduetothe epistemicandstatisticallimitationsofepidemiologicalsciences.Itissimplyunfeasible(namely,impossibleandimpracticaltoachieveeasilyorconveniently)toaccruesufficientstatisticaldataforlow-doseexposuresituationsinorder toobserveactualhealtheffectsduetothoseepistemological constraints.

UNSCEARhasbeenevaluatingepidemiologicalstudies ofcancerandcardiovasculardiseasesforyears(UNSCEAR 2006a).Inrecentyears,UNSCEARconductedareevaluation forinferringcancerrisktoexposureatlowdoseratesfromenvironmentalsources(UNSCEAR2017a).Theoverallresults ofthosestudiesimplythattheriskofcancerperunitdoseis smallerforlow-dosethanforhigh-dosesituations although uncertaintystillexistsduetothelowstatisticalpowerassociated withlow-doseradiation.Environmentalradiationexposuretypicallyresultsinlowandmoderatedoses,andtherefore,potential excesscancerrisksmaybeexpectedtobesmallornonexistent. Theestimationofsuchsmallandinferredincremental risksofcancerfromprotractedradiationexposurescould easilybeaffectedbyconfoundingduetoothercancerrisk factors.Itisimportanttoemphasizethatradiationisjust oneoftheriskfactorsforcancer.Itisunlikelythatradiation actsindependentlyfromotherriskfactorsincancerdevelopment(suchassmoking,diet,etc.).Thismaycontribute tothedifferencesbetweenstudyresultsbecausetheexistenceofconfoundersandtheirassociationwithradiationexposurecanvary.Ananalysisaccountingfortheeffectsof confoundersalsosetsrequirementsforsamplesizeina study.Butevenwithinthiscontext,meta-analysisorpooled analysisofpiecemealdatahaslimitationsnomatterhow largethesamplesize.Demonstratingtheinsignificanceof low-doseexposurecomparedtootherlifestylefactorsin well-organizedepidemiologicalstudies(e.g.,Kudoetal. 2022)willbemoremeaningfulasreal-lifeevidence.Precise estimatesofhealtheffectsandtheirfrequenciesneedsufficient

follow-up,caseascertainmentthroughhigh-qualitycancer registrysystems,andaccurateinformationonriskfactorsother thanradiationexposure.Thisemphasizestheneedforprospectivelong-termstudieswithhigh-qualitydosimetry,aswell ascomprehensiveandaccurateoutcomedataandinformation oncancerriskfactorsotherthanradiationexposure.

Thequalityofradioepidemiologicalstudiesisakeyissue. Thecurrentpreferredmethodsofevidencesynthesisaresystematicreviews,meta-analyses,andpooledanalyses,which areregardedasthestate-of-the-artscientificstandardsfor poolingresearchdataandaredeemedsuperiortotraditionalnarrativereviews.Thereareprinciplesandcriteriaforensuringthe qualityofthereviewsofepidemiologicalstudiesthattakeinto accountthesescientificdevelopments.Itshouldbenotedthat thespecificnatureandscientif iccontentsofsuchstudiesdo notallowforamechanisticapplicationofgenericqualityassurancecriteriaand,therefore,UNSCEARhasdevelopedanapproachtoassessthequalityofsuchstudiesandtosynthesize thefindingsfrommanystudiesintoitsinferenceofradiation risks.Thisispublishedasareportonprinciplesandcriteria forensuringthequalityoftheUNSCEAR’sreviewsofepidemiologicalstudiesofradiationexposure(UNSCEAR2017b).

UNSCEARhasdiscussedtherelevanceofthedoseand thedose-rateeffectivenessfactor(DDRF),aradiationprotectionconcept,inthecontextofscientificevaluationsofepidemiologicalstudiesofcancerriskfromlowdose-rateexposure. Itconcludedthatthedose-responserelationshipsdependona largenumberoffactorssuchthatthescientificevidenceregardingapossiblereductionintheradiation-inducedeffects perunitdoseatlowdosesandlowdoseratesrelativetoacute exposureswithmoderateorhighdosescannotbeexpressedby asinglevalue.Duetothisplethoraoffactors,theappropriatenessandneedoftheDDRFconcepthavebeendeeply questioned(Gonzalez2017).

UNSCEARcontinuestoreviewthedevelopmentsin epidemiological,biologicalandstatisticalanalysesthatcontributetoimprovedinferenceofrisk,ifany,following low-doseandlowdose-rateenvironmentalexposures.Meanwhile,UNSCEARencouragesapplyingradiobiologicaldata derivedoverthepastcoupledecadestohelpunderstandthe lackofobservablecancerincidentsfollowingtheexposure tolowdoses,evenwithoutknowingwithcertaintytheprecisemechanismsinvolved.

Insummation,theradioepidemiologicalmeaningof LNTshouldreflectthefactthattheepidemiologicaloutcomes fromlow-doseradiationexposuresituationsareonlyconjecturalinferences;namely,theeffectsarenotattestableandcannotbeattributed.Lookingtothefuture,UNSCEARrecommendscombiningamechanisticunderstandingoflow-dose radiationcarcinogenesiswithepidemiologicalstudiesusing mathematicalmodelingintegratingdatafromexperimental systems(e.g.,dose-responsedataforinductionofkeymutationsorepimutations).

Radiation-protectionists

Forradiation-protectionists,LNTrepresentsa model,i. e.,asimplifieddescriptionofreality,whichispracticaland workableformanagingoperationalradiationprotection, particularlyinoccupationalradiationprotection,byexercisingprotectionagainstadditionaldosesregardlessofthe levelofaccumulateddose.

Ifthismodelisnotusedfortheprotectionofworkers,it maynecessitateassigningdifferentprotectionforthesame increaseindoses,dependingontheaccumulateddose.This couldunavoidablycreatediscriminationamongworkers, suchasage-relatedconsiderations,whichispreventedby thecurrentinternationallaborlegislation.

Radiobiologists

Forradiobiologists,LNTwasoriginallytakenasa premisepostulatingthatatlowradiationdoseexposures,a givenincrementindosewouldproduceadirectlyproportionateincrementintheprobabilityofincurringcellulareffectsthatwouldevolveintomalignanciesorheritableeffectsattributabletotheradiationexposure.

Theearlyradiobiologicalassumptionwasthatthemain interactionsofradiationwithlivingmatterwere “targeted” directandindirectinteractingeffectswiththecellularDNA causingmutationsthatcouldevolveintoassociateddetrimentaleffectssuchasmalignancies.However,biologicalresearch overthepastfewdecadeshasincreasedtheunderstandingof howradiationinteractswiththenon-linearcomplexitiesoflivingtissue.Accordingly,itisimportanttodelveabitdeeper intothebiologicalaspectsofradiationexposureatlowlevels, sincethedevelopmentofascientifically-basedpremisetoreviewandeventuallyrevisetheLNTpremisecanonlybeaccomplishedandacceptedbythebroadradiationhealthprofessionalcommunitywhenthedetailedbiologicalprocessesare understood(Brooksetal.2023).

Detailedradiationbiologyresearchoverthepastfew decadeshasrevealedmanysecondary, “non-targeted,” effects,includingthefollowing(UNSCEAR2006b):

• Radiation-inducedgenomicinstability,inwhichifasingle cellisirradiatedandsurvives,itmayproducedaughtercells thatovergenerationshaveincreasingnumbersofmutations;

• Adaptiveresponse,whichexpressestheprovenabilityof cellsandtissuesinallorganismstorespondtoanumber ofdifferentchallengestobetterresiststressdamage,e.g., alsocausedbyradiationexposure(inthiscontext,adaptiveprotectionappearstooperatebypreventionofDNA damage,repairofdamage,andbydamageremoval);

• Bystandereffects,namelytheabilityofirradiatedcellsto conveymanifestationsofdamagetoneighboringcellsnot directlyirradiated;

• Abscopaleffects,whicharesaidtooccurifthereisasignificantresponseinatissuethatisphysicallyseparate fromtheregionofthebodyexposedtoradiation;and

Inducedclastogenicfactors,whichresultfromalargebody ofevidencethatbloodplasmafromirradiatedanimalsand humanscancontainso-called “clastogenicfactors” capable ofinducingchromosomaldamageinunexposedcells.

Intheearlyphasesofresearch,therewasaconcernfor heritableeffects,i.e.,effectsthatmightbeobservedinoffspringbornafteroneorbothparentshavebeenirradiatedprior toconception,andUNSCEARstudiedtheissueindepth (UNSCEAR2001).However,suchheritableeffectshave notbeenseeninhumans,andUNSCEARhasconcludedthat “althoughdemonstratedinanimalstudies,anincreaseintheincidenceofhereditaryeffectsinhumanpopulationscannotat presentbeattributedtoradiationexposure” (UNSCEAR2012b).

Someofthemanifestationsofthesenon-targetedand delayedeffectscaninprinciplearisespontaneouslyandafter exposuretootheragents.Theseeffectshaveledtodifferent assumptions,rangingfrom(1)thepossibilitythatlowradiationdosesproducebeneficialeffects(hormesis)ratherthan damage,to(2)thepossibilitythatlow-doseexposurebrings higherrisksthanhypothesizedfromepidemiologicalstudies.

Thecausesofthevarioustypesofresponsestoirradiationdependontheinteractionofenergydepositionevents withsensitivestructuresatthevariouslevelsofbiological organization(ICRP2015;UNSCEAR2021).Thus,exposuretofieldsofionizingradiationcreatesenergydepositionsalongtracksofsubatomicparticlesofmicroscopicdimensions.Suchenergydepositioneventsaredistributedby chanceinexposedmattersuchasbiologicaltissues.Atthe molecularlevel,theenergydepositionsoccurstochastically. Forexample,inaradiationfieldof100kVxrays,anaverage energydepositioneventpernanogramtissue(anaveragecell mass)constitutesaroundtheequivalentof1mSv.Thus,the typicallimitinglevelsofradiationdosesbeingregulatory controlledformembersofthepublic,namely1mSvperyear, wouldleadtooneenergydepositioneventpernanogramper year;namely,aroundoneinteractionpercellperyear.

Themajorconcernsofsuchexposureareanyespecially seriousdamagingchangesofthegeneticmaterial,theDNA, withimmediateattemptsofrepair,andsubsequentalterations ofcellularsignalingcontrolmechanisms.

Ingeneral,biologicalresponsestolowdosesofionizing radiationaretwofold:primaryeventsfollowingenergydeposition,andsecondaryresponsestotheprimaryevents.These latterresponsesappearwithinhoursafterexposureandmay lastoverlongtimes.Sincethetypesanddegreesofsecondaryresponsesareundergeneticcontrol,variousdegreesof radiosensitivityexistamongthehumanpopulation.

Theissueofradiosensitivityiscomplex.Theeffectsofradiationupthroughthechainofcellstructuretocomplexorganscanvaryamongindividuals,includingtheageandgenderofeachindividual.However,theICRPhasaddressedsuch effects(ICRP1998),principallythegeneticvulnerabilitiesfor

cancer,andhasconcludedthatsuchvariableshavebeenwell accountedforintheiroverall guidance.Whereasvariations inradiosensitivitycouldbeofimportanceifradiationdoses arehigh,itishighlyunlikelythattheycanbeofsignificant concernforlow-doseradiation.Anyeffectsoflow-doseradiation,ifany,arealmostcertainlyoverwhelmedbyotherfactors. Regardingthemajoreffectsfromlow-doseandlow dose-rateradiation,thefollowingreporting(Feinendegen andCuttler2018)summarizeskeyoutputsfromconsiderablebiologicalresearch:

• Lowdosesofionizingradiationtoexposedcells,quite differentfromhighdoses,causeinexperimentalbiologicalsystemspotentialprotectivecellularsignalingchanges thatarefairlylargeincomparisonwithrelativelyveryfew potentiallyseriousDNAdamages,andmostsuchdamage isbasicallyrepairedrelativelyrapidly.Asdoseratesincrease,thenumberof “hits” increaseinalinearfashion. Butthesignalingchangesthatfollowaremanifestina non-linearfashionincomplexbiologicalsystems.Inparticular,adaptiveresponse-protectionswithbeneficialconsequenceshavebeenmeasuredtoreducespontaneous damageintheexposedsystem.Othersecondaryresponses(referredtoearlier)includegenomicinstability, bystandereffects,andabscopaleffectsthathavebeenobservedathigherdoses(Brooksetal.2016);

• Atchroniclowdose-rateexposure,repairmechanisms andsecondaryresponsesreduceorpreventcellulardamageaccumulation.Anycelldamagethatdoesoccurneeds tobecomepropagatedthroughtheincreasinglycomplex levelsofbiologicalorganizationinordertocauselatedetrimentalhealtheffects.Such damage,ifany,andthepropagationofsuchdamage,isaninherentlynon-linearprocess(Feinendegenetal.2010);

• Secondaryresponsestolowdosesofradiationhavebeen unequivocallyobservedinthehumanbody.Yet,thesignificanceforthemtocauseclinicalhealtheffectsisstill beingdebated.Whereassomeconjectureremains,ithas beenarguedthatessentiallyallbiologicalsystemscontributepositivehealthresultsatlowdoses(e.g.,medications, vaccinations,traceelements,etc.)butresultinconsiderable damageathighdoses;simplyput,thepoisonisinthedose;

• Radiationbiologydataindicatethattheprobabilityofaclinicalmalignancyinthehumanbodyfollowinglow-doseexposure,ifitoccursatall,issmall.Spontaneousdamagefrom physiologicalreactiveoxygenspecies(ROS),thekeyreactionprovidingessentialenergyforlifeitself,isfar morefrequent.Yetourbodilyprotectivesystem(immune system)clearlyaccommodatesandeffectivelyrepairs suchdamage.Furtherdiscussionofthispowerfulimmune systemisincludedbelow;

• Contrarytoearlierconcernsthatcellulardamagecanoccurevenatlowlevelsofradiation,recentexperimental

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evidencehasrevealedjusttheopposite;namely,thatbiologicalsystemsapparentlyrequireacertainlowlevelof radiationtosurviveandthrive(WaltarandFeinendegen 2020);and

• Ingeneral,livingsystemsprotectthemselvesagainstdamagebypreventingdamagetoarise,repairingdamagethat occurs,andremovingdamagethatescapesrepair.Theremovalisakeycomponentoftheimmunesystem.

Anissuethathasbeenwidelydiscussedbytheradiobiologycommunityistheeffectofionizingradiationonthe immunesystem.Theissuehasbeendeeplyreviewedby UNSCEARsince1972(UNSCEAR1972).Theimmune systemisoneofthemostcomplexsystemsofthehuman body.Itreliesonhighlyspecializedcellswiththeirspecific functionofprovidingaverycomplexsetofbiochemicalreactionsthatyieldseveraltypes(lymphocytesandaccessory cells)strategicallyspreadthroughoutthebody,perfectlypositionedtorecognizeantigens(non-selforforeignsubstancesandcells)andtoneutralizeordestroythem.This systemprotectsagainstinfectionsandcancer.Thereare twodifferentbutinterrelatedformsofimmunity:innate andacquiredimmunity.Innateimmunityisfullyfunctional beforeanyforeignagententersthebodyandtherebyprovidesarapiddefense.Acquiredimmunitydevelopsaftera pathogenhasenteredthebodyandmaintainsmemoryof previousexposures,yieldingastrongerresponsefollowing subsequentexposuretothesameantigen.Acquiredimmune responsesaremainlyexecutedbyB-lymphocytes(humoral responses)andT-lymphocytes (cell-mediatedresponses) (Feinendegenetal.2011).

Theeffectsofionizingradiationontheimmunesystem canbeassessedbyestimatingchangesincellnumbersorby usingavarietyoffunctionalassays.Theimpactofsuchalterationsinimmuneresponsedependsonfactorssuchasthedose ofradiation,itstemporalrelationtoimmunization,andgenetic disposition.Thereisnodoubtthathighdosesofradiationproduceimmunosuppression,mainlyduetothedestructionof cells.Lymphocytesareveryradiosensitive,andtheirreduction iscurrentlyusedasanearlyindicatorofthelevelofanaccidentalacuteexposure.Radiation-inducedchangesinimmune parametersseemtobemoredependentontotaldosethanon doserate(Liuetal.2020).Persistingeffectsontheimmune systemhavebeenobservedafterexposuretohighdoses.

Atlowdosesandlow-doserates,lowdose-ratechanges, andtheeffectsofionizingradiationontheimmunesystem mightbesuppressiveorstimulatoryornone.Someexperimentaldataappearstorevealthatusinglow-levelradiation tostimulatetheimmunesystemwouldlimitflusymptoms (CalabreseandDhawan2013),butthelong-termimpacts oftheselowradiationdosesontheimmunefunctionsinrelationtohumanhealthhavenotbeenevaluated.Manypeople throughouttheworldroutinelyvisitradiationhealthspas(e.g.,

mineswheretheradiationleveliswellabovenaturalbackground) andclaimconsiderablerelieffromnagginghealthissues.

UNSCEARhasconcludedthatwhilethesuppressive effectsofhighdosesofionizingradiationarewelldocumented(UN2014),uncertaintyexistsregardingtheeffects oflowradiationdosesontheimmunesystemsinceboth stimulatoryandsuppressiveeffectshavebeenreported.

Inspiteofthisnewinformation,therecontinuestobe considerabledebateregardingthecausalrelationshipbetween thenon-targetedeffectsandtheobservedhealtheffectsattributabletoradiation.UNSCEARconcludedseveralyearsago (UNSCEAR2006c)thattheestimationofthehealtheffects ofradiationshouldbebasedonepidemiologicalobservations wherethereisastatisticallysignificantdose-relatedincreasein diseaseincidence.Thesedirectobservationsofadversehealth outcomeswouldimplicitlytakeaccountofmechanisticelementsrelatingnotonlytothetargeted(direct)effectsofirradiationbutalsotothenon-targetedanddelayedeffects.

RecentUNSCEARestimates(UNSCEAR2019)onthe biologicalmechanismsrelevantfortheinferenceofcancer risksfromlow-dose,lowdose-rateradiationacknowledges theexistenceofsecondaryresponsesfollowinglow-doseradiationexposureasdescribedabove.Butbecauseoftheuncertaintiesofthesesecondaryresponses,whicharesmall anddifficulttodetectaccuratelyforlow-dosesituations, UNSCEARremainscautiousregardingtheirconsistency, disposition,andreproducibility.

Giventheseuncertainties,UNSCEARjudgesthereis stillinsufficientjustificationtorecommendchangesinthe currentradiobiologicalparadigmonthebasisofthesesecondaryresponses.Here,UNSCEARclaimsthatsofar,there isaconsiderabledegreeofuncertaintyregardingthemechanismsbridgingsecondaryresponsestoclinicalcancer.In thiscontext,itshouldbeemphasizedagainthattheincidenceofcanceratverylowdosescannotbemeasuredfor epistemologicalreasonsunlesslargepopulations(enough tobestatisticallysignificant)arebeingstudied.Further, giventheheterogeneityofindividualsforvariousaspects, theriskofcanceratverylowdosescouldnotbeevidenced evenifalargenumberofindividualswerestudied.

Insummarizingtherecentresearchwithintheradiationbiologycommunity,despiteremaininguncertainties astowhetherandtowhatdegreeadaptiveprotectionalways operatesagainstradiogenicandnon-radiogenicdamage, anyradiationhealtheffectsatlowdose,iftheyexistatall, arealmostalwaysdwarfedbyothertoxinsthatwelivewith everyday,suchasthenaturalbodilyburningofoxygenso essentialtolife.

MISUSEOFLNTANDRADIATIONFEAR

ThewideandimpreciseuseoftheacronymLNT,withoutclarificationofitsrealmeaning,haslikelybeenacause

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ofseriousconfusionaboutthehealtheffectsattributableto radiationexposuresituationsinvolvinglowdose,lowdose rate,andlowdose-ratechange.Thisismostdisturbingbecausethesearesomeradiationexposuresituationsassociatedwiththemanybeneficialusesofradiation.

Thewiderbutepistemologicallywrongconnotationof LNThasbeentheimproperuseofthequantitytermed collectivedose.Thecollectivedoseisanextensivequantity (differentlyfromtheintensivequantitydose),whichisdefinedasthetotalradiationdoseincurredbyapopulation. Animproperuseofthisquantityhasbeentointegratethe lowdoseofanindividualcohortmemberandmultiplyit bythetotalnumberofpeopleinthatcohort resultingina largehypothesizednumberofcasualties.BothUNSCEAR andICRPhavewarnedthatthisapproachisimproper.This misuseofcollectivedosehasbeendoneevenatacademic levels,e.g.,wronglyattributingtotheChernobylaccident aroundonemilliondeaths(Yablokovetal.2009)!Thisabsurdcalculationhasbeenwithdrawn,butthedamagewas done,andpublicfearofradiationwasincreased.

Giventhemisuseofsuchradiationriskcalculations, UNSCEAR,followingaspecificrequestfromtheUnited NationsGeneralAssembly,addressedtheissueofattributionofhealtheffectstodifferentlevelsofradiationexposure.Itreachedanumberofconclusions,significantlynotingthatincreasesintheincidenceofhealtheffectsinpopulationscannotbeattributedtolow-doseradiationexposure situations.Only notionalrisksfromplannedsituations maybeprospectivelyinferredforpurposesofradiationprotectionandallocationofresources.

EventheHolySeehasweighedinonthisissue.Areportonbiologicalimplicationsofoptimizationinradiation protectionfromthePontificalAcademyofSciencesindicatedthat “Therearereasonstobelievethattheassumptions inherentintheLNTmodelarelikelytooverestimatethereal riskatthelowdosesofinterest” (PontificalAcademyof Sciences1983).

THELNTMODELASCURRENTLYUSEDFOR REGULATORYPURPOSES

Asnotedearlier,thetermLNTusedbytheradiation protectioncommunitysimplyreferstoapracticalmodel formanagingoperationalradiationprotection.Thismodel assumesradiationexposureshouldbelimitedbycertain doselevels.Belowtheselevels,protectionshouldfollowa processofoptimizationofprotectionbyselecting(among theavailableprotectionalternatives)thebestprotectionoptionundertheprevailingcircumstances.Itsimplyassumes thatradiationrisksmightbeconjecturedandinferredat anylevelofdose.

However,iftheseconjecturalinferenceswithinthe practicalLNTradiationprotectionmodelareunderstood tobeprovenfacts,agapinunderstandingiscreatedthatis

likelyresponsibleforcreating publicfear.Theseriousepistemologicallimitationsofthebiologicalandepidemiological sciencesforvalidatingtheLNTmodelmustberecognized.

Despitetheseclearreservations,theLNTradiationprotectionmodelisrecognizedasrathereasytoadministerand servesasthegenerallyacceptedradiationprotectionmodel inmostnationsontheglobe.However,aswesumuptheevidencefromtheperspectiveofeitherradiationepidemiologyorradiationbiology,thereisnovalidreasontofear low-doseradiationexposures inspiteofthevariousinterpretationsoftheLNT.

Manyprofessionalsintheradiationcommunitynow arguethattheLNTmodelasusedbytheregulatorycommunityshouldberevisedanditspurposeclarified atleastfor radiationlevelsinthelow-dose,lowdose-ratedomain.They basetheirargumentsalongthelinesoutlinedabove.

Anewresearchanddevelopmentprogramiscurrently beingdesignedbytheUSNationalAcademiesofScience, Engineering,andMedicine(NASEM2022)toseekthedata neededtoevolveamorescientificapproachtotheregulationofradiationexposure.Hopefully,sufficientdatawill bederivedtoprovideawidelyacceptablemodeltobeembeddedinregulationapproachesworldwide.Butthismay takeseveralyears,anditisdependentonasustained fundingbasethatisoftendifficulttomaintainforthedurationneeded.

So,whatshouldwedointhemeantimetohelpremovethe unnecessaryanddestructivepublicfearoflow-doseradiation?

POTENTIALREGULATORYSOLUTIONFORTHE LOW-DOSECONUNDRUM:EXCLUSIONS

ANDEXEMPTIONS

Theconundrumofprotectionagainstlow-doseexposure,oftencausedbymisunderstandingsandmisapplicationsderivedfromtheLNTmodel,couldbesolvedthrough abetterformulationofthecurrentregulatoryapproach withoutenteringintocontroversialbiologicaland epistemologicaldiscussions.

Ontheissueofthedoselimits

Forinstance,takingintoaccountthecurrentdoserestrictionsestablishedininternationalprotectionstandards,itisalreadyestablished(albeitnotnecessarilyclear)thattheannual doseofsomeindividualsshouldnotexceed100mSvunder anycircumstance(namely,thefactualdose “limit”).ThecurrentvaluesforprotectioncriteriarecommendedbyICRP andestablishedininternationalsafetystandardsincludesuch afactuallimitforsomeemergencyexposuresituations.But theword “limit” isusedratherdifferentlywhenreferringto plannedexposuresituations.Itthenreferstoa “limit” ofanincrementofdoseaboveabackgrounddosethatishigherthan thestated “limit.” Tosuchapeculiar “limit,” thepublished lowvalueof1mSvy 1 forthepublichasbeenassigned.That

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isacauseofmajorconfusion.Itisquitereasonablethatthis leadsthepublictobelievethatreceivingadoseaboveornear 1mSy 1 isverydangerous.Theybecomesurprised(andperhapsrelaxed)whentheylearnthattheyarecontinuallyexposedtobackgroundnaturalradiationatdosesconsiderably higherthansucha “limit.”

Itshouldbenotedthatthe1mSvlevelwassomehow arbitrarilyselectedasanannualdoselimit.Ittracesback toapreviouslevelof5mSv,whichwasreducedbyafactor offivefollowingachangeinthedosimetryofthecohortof HiroshimaandNagasakivictims.Butthat5mSvhadbeen subjectivelyselectedasonetenthoftheoccupationaldose limitatthetime,whichwas50mSvy 1;namely,1order ofmagnitudelowerthanavalueconsideredsafeforanindividualworker.The50mSvy 1,inturn,hadbeenderived froman “indexofharm” developedbyICRP(ICRP1977).

Overtimethe “1mSvlimit” wasperceivedasapurposelylowextradoserestrictionofferedforthesakeofthe precautionaryprincipleduringa plannedexposuresituation involvingmembersofthepublic.Itshouldbenotedthata plannedexposuresituationisasituationofexposurethat arisesfromtheplannedoperationofaradiationsourceor fromaplannedactivitythatresultsinanexposureduetoa source.Sinceprovisionforprotectionandsafetycanbe madebeforeembarkingontheactivityconcerned,associatedexposuresandtheirprobabilitiesofoccurrencecanbe restrictedfromtheoutsettolowprecautionarylevels.In fact,theprimarymeansofcontrollingexposureinplanned exposuresituationsisbygooddesignofinstallations, equipment,andoperatingprocedures.

Constructingtheideathatareal “limit” [i.e.,apointbeyondwhichanindividualdoseshallnotpass(aterminal pointorboundaryforindividualdoses;namely,therealregulatoryrestrictiononthelevelofdoseincurredbyindividuals)]shouldbe,andinfactalreadyis,somethingcloser tothe100mSvy 1 level.Thisisrecognizedincurrentstandardstobeanacceptablesafelimitbecauseitisallowedto beincurredundersomeconditions,whicharespecifiedin theinternationalstandards.Adoptingthisinterpretation wouldconstituteahugestepingreatlyamelioratingtheunnecessarypublicfearoflow-levelradiation.

Ontheissueofoptimizingprotection

Inaddition,thestandardsrequirethatprotectionbeoptimized;namely,thatthebestoption(amongtheavailableprotectionoptions)beselectedundertheprevailingcircumstances. Optimizationinthegeneralsenseisintendedtofindtheprotectionsolutionthatprovidesthemaximumbenefitwiththeminimumdisadvantages.Unfortunately,mainlyduetohistoricalreasons,optimizationofprotectionissometimesconfusedwith minimizationofindividualdoses.Thisissimplywrong.

Amostunfortunateexampleofplacingunduefocuson radiationdosewastheevacuationproceedingsfollowingthe

Fukushimaaccident.Asnotedinthenextsectiondealingwith socialconsequences,thedeathtollduetothelong-termevacuationprocessresultedinahighnumberofactualfatalities, butnonewereduetotheradiationexposureitself.

Ontheissueofregulatoryscope

Thelow-doseconundrumcouldwellbesolvedthrough acleardefinitionofthescopeofwhathastoberegulatedby introducingthekeyconceptsofexclusionfromregulations andexemptionsfromregulating.Theconceptofexemption fromregulatorycontrolwascarefullyconsideredmanyyears ago(LinsleyandGonzalez1988),anditwasintroducedin Europeveryearly(EC1993);however,aninternationalconsensushasbeenelusive.

Regulationofradiationexposurehasnotincludeda cleardefinitionoftheregulatoryscope.ICRPhasmadeclear recommendationsinthisregard(ICRP2007b),buttheyhave beengenerallyignoredbyregulators.Forinstance,thereis notauniversal,homogeneous,coherent,andconsistentincorporationofthecrucialconceptofexclusionandexemptionrecommendedbyICRPintonationalregulations.

Legislativeandregulatoryauthoritiesshouldexercise someeffortstowarddefiningthescopeofradiationprotectioncontrolmeasuresthroughlegislationandregulations. Forthispurpose,theycouldusethewell-establishedand universallyacceptedradiationprotectionprinciplesofjustificationandoptimization.

Someradiationexposuresituationsmaybeconsidered for exclusionbythelegislationbecausetheirregulatorycontrolisdeemedtobeunamenableorunjustified.Newprudent legislationcouldthendevelopunambiguous exclusion criteriafordefiningthescopeofradiationprotectionlegislationbyusingtheoldlegislativeprincipleof deminimisnon curatlex.Iflegislationwithexclusioncriteriawereclearlyestablished,itwouldallowtacklingsomecontroversialissues ofspecificexposuresituations,suchasexposuretocosmic raysatgroundlevelandothernaturaloccurrences.

Inaddition,regulatorsmay exemptradiationlow-level exposuresituationsfromregulatorycontrolonthebasisthat deregulationistheoptimumprotectionoption(deminimis noncuratprætor).Suchactionwouldprovidethegeneral publicagreatservice.Thisguidancewouldprovidethebasis forrecognizingthat,inmanycases,exemptionsprovidethe optimalsolution.Thiswouldresolvetheproblemsofregulatinglow-energyornon-penetratingradiation,somenaturally occurringradioactivesubstances,lowlevelsofradioactivity inconsumergoods,andlow-levelradioactiveresidues.

Thus,theconceptsof exclusionand exemptionshould becomemodernparallelstotheancientlegalprinciplesof deminimisnoncuratlex and deminimisnoncuratprætor, respectively,whichoriginatedinRomanlawtwomillennia agoandsincethenhavegovernedthelegalproblemofregulatingtrifles:namely,regulatingwhatisinconsequential,

unfeasible,unimportant,orirrelevantfromthepointofview eitherofthelegislatorortheregulator(trifleinthissenseis notnecessarilyasynonymoftrivial).The deminimisnon curatlex principleaddressesthesituationsthatthelawshould (orshouldnot)takeaccountof,orcover.The deminimisnon curatprætor principleaddressesthesituations,amongthose coveredbythelaw,thatcanbefreedbytheregulatorfrom someorallregulatorycontrols.

SOCIALCONSEQUENCESOFRADIATIONFEAR

Beingfearfulofsomethingsisbothunderstandable andharmless doingnodamagetothegeneralpublic.Beingafraidofthedarkorbeingfearfulofsnakesmayimpact thelifestyleofanindividual,butithasnoimpactonothers. Butbeingafraidoflow-levelradiation,wherethereisno justificationforsuchfear,doescausedamagetosociety. Followingarejustafewexampleswheresuchunfounded fearoflow-levelradiationhasresultedinconsequences quitedetrimentaltomodernlife.

TheThreeMilenuclearaccident

Thisaccident,whichoccurredon28March1979near Harrisburg,PA,resultedinapartialcoremeltdownanda hugeeconomicloss(Rosztoczy2019).However,theradioactivesubstancesreleasedintotheatmospherewereminiscule,andthetheoreticallycalculatedradiationdoseswere lowerthanthenaturalbackgrounddosesincurredbyanairlinepassengerflyingfromLosAngelestoNewYork.Yet thesensationalnegativepublicityandfeargeneratedbythis eventresultedinalargesocialimpactonthepopulationand, moreover,inamajorhaltinthelicensingandconstruction ofplannednuclearreactorexpansions especiallyinthe UnitedStates.Anexponentialnumberofnewlicensingregulationsandbureaucraticpaperworkwasgenerated,drivingupthe costofnewplantstothepointofeconomicunacceptability.

TheChernobylnuclearaccident

Thisaccident,whichoccurredon26April1986in Ukraine(thenpartoftheformerSovietUnion),wasundoubtedlytheworstnuclearaccidenttoeveroccurintheworld. Therewasaworldwidereactionfollowingtheaccident, mainlytriggeredbythefearofradiation.Manypublishedarticles,evenbyrenownedacademies,containedpredictionsof ahumancatastropheassigningmillionsofvictimstotheaccident.However,thefactuallyobservedhealtheffectsattributabletoradiationexposurefromtheaccidenthavebeen assessedtobe muchlowerbytheinternationalscientific community(IAEA1996b;Gonzalez1996).

UNSCEARreportedthefollowingradiation-relatedconsequencesfromtheChernobylaccident(UNSCEAR2008a):

• 134plantstaffandemergencyworkers,manyofwhomalso incurredskininjuriesduetobetairradiation,receivedhigh dosesofradiationthatresultedinacuteradiationsyndrome.

• Thehighradiationdosesprovedfatalfor28ofthese people.

• 19survivorshavesincedied(followingthereportingof 2008),buttheirdeathshavebeenforvariousreasons andusuallynotassociatedwithradiationexposure.

• Severalhundredthousandpeoplewereinvolvedinrecoveryoperations,butthereisnoevidenceofhealtheffectsthatcanbeattributedtoradiationexposure,with theexceptionofasubstantialnumberofthyroidcancers attributedtodrinkinguncontrolledmilkcontaminated with 131I.Nopromptcountermeasuresweretaken,which resultedin15fatalities(uptotheyear2005)amongchildren,whoaremuchmoresusceptibletothyroidcancer thanadults.

• Unsurprisingly,theInternationalConferenceon Chernobyl:LookingBacktoGoForward (IAEA2005)concludedthat “Themajorityofworkerswhoparticipated inthecleanupefforts,themanythousandsofpersons evacuatedduringtheearlydaysfollowingtheaccident, andallthosewhocontinuedtoliveincontaminatedareas receivedradiationdosesfromChernobyl-releasedradionuclidesthatwererelativelylowandunlikelytoleadto widespreadandserioushealtheffects.” Thedosesto theseindividualsarecomparabletothosecausedbynaturallyoccurringradionuclidesthatproducearangeof backgroundlevelsroutinelyexperiencedbyeveryone ontheplanet.

Somenotableregionsofhighbackgroundradiationexistinseveralcountriesthatarecausedbyhigherconcentrationsofnaturalradionuclidesinbeachsandsorinsoilor water.TheChernobylexposuresarenotunlikethesenaturallyoccurringareasthatarenotassociatedwithdiscernible radiationhealtheffects.

Buttherewasalsoasocialtragedy mainlycausedby thefearoftheradiationfalloutfromtheunshieldedaccident (itshouldberecalledthatChernobyldidnothaveanycontainmenttomitigatetheconsequencesoftheaccident,instark contrasttothestandardsinnuclearreactorsworldwide).Such radiationawayfromtheimmediatesiteoftheplant,though low,wasmeasurablethroughoutmostofthenorthernregions oftheworld.Asaresult,peoplereactedwithunderstandable fear.Forinstance,themediahysteriainlargepartsofwestern Europeledtoanestimatedadditional40,000abortionsamong marriedwomenduringthepost-Chernobylmonths,allof whichweretheresultoffear(IAEA1996c).Theradiation levelswereneverhighenoughtojustifyevenoneofthose abortions.Somefoodsupplies,manyonlyshowingamodest presenceofradioactivesubstances,wereoftenabandoned, andevendrinkingwaterwassometimeslabeledhazardous forthelocalcitizenry,basedsimplyonfear.

Theinternationalconferencesumminguptheconsequencesoftheaccident(IAEA1996b)includedapopulation

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surveyonnon-radiationrelatedsymptomscarriedoutin areasdirectlyaffectedbytheaccidentvs.thoseareasnotdirectlyaffected.Thepopulationsinbothareassufferedabout thesamefearofradiation.Theseresultsshowedthatthefear ofradiationisnotdirectlyrelatedtowhetherpeoplearelivinginareasaffectedornot.

UNSCEARreportedthosepsychologicaltraumasand otherrelatedeffectsattributabletothefearofradiation,indicatingthat “TheChernobylaccidentisknowntohavehad majoreffectsthatarenotrelatedtotheradiationdose.They includeeffectsbroughtonbyanxiety…anddistress…and areessentiallyunrelatedtoanyactualradiationexposure” (UNSCEAR2008b).

Tragically,themostundesirableconsequenceofthehorrificaccidentatChernobylwasthefearofradiationrather thantheaccidentitself.

TheFukushimanuclearaccident

TheFukushimaaccident,occurringon11March2011 inJapan,wastriggeredbyanextremelylargeearthquakeunderthePacificOcean,whichcausedatsunami-generated wallofwater.Thetsunamiinundationhighapproached 33mandtherunuphighwasupto39m,killingtensofthousandsofpeople(IAEA2015a).TheFukushimaDaiichisite consistedofsixnuclearpowerplants.Atthetimeoftheaccident,Units1,2,and3wereoperating,butUnits4,5,and6 wereinplannedshutdown.Theearthquakedestroyedmost electricpowerlinesintheregion,andthethreeoperating plantslosttheirbackuppowersupplieswhentheensuing floodswipedouttheinternalelectricalsupply,includingtheir standbydieselgenerators.Theresultinglossofcoolingtothe threeoperatingreactorsledtocoremeltdownsandasizable releaseofradioactivematerialsintotheatmosphere,resulting indepositsonlandandintheocean.

Authoritiesmandatedtheevacuationandresettlement ofsomethousandsofresidentsandimposedarelocation ofthemfor,insomecases,severalyears.Theresultwasseriousdetrimentaleffectstothosepeople,includingthedeath ofsomeofthemostvulnerable.ButNONEoftheseimpacts weretheresultofradiationexposure.Theradiationlevels werejusttoosmall.

TheassessmentoftheFukushimaaccidentundertaken bytheIAEArevealedthat “dosesincurredbymembersofthe publicwerelowandgenerallycomparablewiththerangeof effectivedosesincurredduetogloballevelsofnaturalbackgroundradiation.” Unsurprisingly,itconcludedthat “noearly radiation-inducedhealtheffectswereobservedamongworkers ormembersofthepublicthatcouldbeattributedtotheaccident.” ThisconclusionagreedwiththeUNSCEARassessmentthat “nodiscernibleincreasedincidenceof radiation-relatedhealtheffectsisexpectedamongexposedmembersofthepublicandtheirdescendants” (IAEA2015b).

UNSCEARestimatedthat “Themostimportanthealth effect[fromtheaccident]isonmentalandsocialwellbeing,relatedtotheenormousimpactoftheearthquake, tsunamiandnuclearaccident,andthefearandstigmarelatedtotheperceivedriskofexposuretoionizingradiation” (UNSCEAR2014).Similarconclusionsandlessonswere reportedbyanICRPTaskGroup(Gonzalezetal.2013). Again,thefearofradiationhasbeenamajorculpritin theFukushimaconsequences.

Globalnuclearpoweradvances

Theadventofnuclearpower,startingwiththefirstemploymentofcommercialnuclearpowerinthe1960s,now providesapproximately10%oftheworld’selectricity.Itis generallyrecognizedbyresponsiblelong-termplannersthat theglobalneedsforelectricitywillcontinuetoclimb especiallyasthepushfortheelectrificationofthetransportationindustrymatures.Butthefearof “anythingnuclear” isimpedingthesociallicensingofnuclearprograms mainlybecauseoffearofradiation.

Nuclearwastecleanup

Oneoftheissuesthatconcernsmanyabouttheacceptabilityofnuclearpoweristhequestionofwhattodowith theso-called “nuclearwaste;” namely,radioactiveusednuclearfuelremovedfromthereactoronceitsusefullifetime togeneratepowerisover.First,weneedtorecognizethat about96%ofthis “waste” isnotwasteatall.Itcanbeconvertedtousefulnuclearfuelsinceitiscomprisedmostlyof uraniumorplutonium.Manycountrieshavedeclaredthat thisisnota “waste” butratheran “asset” tobepreserved. Unfortunately,thecurrentregulationsforthedesignof anundergroundnuclearwasterepositoryintheUnited Statesrequiretheradiationlevelatthesurfaceofsucharepositorytobelessthan0.15mSvy 1 forthefirst10,000y afterrepositoryclosure,whichisabout20timeslessthan naturalbackground.Thisabsurdrestriction,basedonthe unnecessaryfearengenderedbyradiation,makesitalmost impossibletodesignandbuildanuclearwasterepository. Myriadofbenefitsfromradiationasaby-productof nuclearenergy

Anotherfactorgenerallyunknowntothegeneralpublic relatestothemyriadofservicesandproductsthatradiation technologyhasprovidedtomodernlife.Anearlybookaddressingthistopic(Waltar2004)summarizedtheenormous contributionsthatradiationtechnologyisalreadyprovidinginthefieldsofmedicine,agriculture,modernindustry, transportation,spaceexploration,combatingterrorismand crime,artsandsciences,andenvironmentalprotection.This workhasbeenexpandedconsiderablyanddocumentedin therecentElsevierencyclopediaonnuclearenergy (Greenspan2021),whichcontains26chaptersintheSectiontitled “TheMedical,Agricultural,andIndustrialApplicationsofNuclearTechnology.” Thissectionisfocusedon

thesehighlybeneficialcontributionstoourglobalsociety.Already,thevalueofthesecontributionswellexceedsthatof thecommercialnuclearpowerindustry bothintermsof jobsandtheeconomy.Yettherearestillpeoplewhowill notsubmittoacomputerizedtomography(CT)scanbecause ofradiationfear,eventhoughthescienceclearlyshowsthat thebenefitsfaroutweightherisks(McCollough2018). Thereappearstobenoendinsightforthefurtheradvancementofsuchradiationtechnologies,butonlyiftheunsubstantiatedfearoflow-levelradiationcanbecurbed.

CONCLUSION

Thisarticlehasdescribedviewsonthehealtheffectsattributabletoradiationexposuresituationsinvolvinglowdose,lowdose-rate,andlowdose-ratechanges,takinginto considerationtheprincipalscientificprofessionsstudying thistopic;namely,radiationpathology,radiationepidemiology,radiationbiology,andradiationprotection.Itsgoalhas beentopresentanoverviewofthescientificevidence gainedoverthepasthalfcenturythatprovidessufficientassurancethatthereisnovalidreasontofearlowlevelsof radiationexposure.

Thecurrentapproachthatismostusedtoguideregulatorsinprotectingthegeneralpublicfromradiationexposure isreferredtoasLinearNo-Threshold(LNT).Itiscurrently deeplyembeddedintheinternationalradiationregulatory system,yetitisunderstoodverydifferentlybythevarious professionsinvolved,someasapremise,someasahypothesis, andsomeasapracticalmodel.Asaresult,theappropriateness ofhowitisusedishighlydebated.BecauseLNThasbeenunderstoodbymanyasimplyingthatradiationatanylevel,even downtozero,canpotentiallybeinterpretedtobeakillerof peopleandpotentiallymanypeopleoverspaceandtimeifthey areexposedtoeveninfinitelysmalllevelsofradiation,itis nowonderthatmanymembersofthepublicandtheir policy-makersarestillfearfuloftheword “radiation.”

Mostpolicymakerswillagreethatendeavorsinvolvingradiationhavemajorbenefits(e.g.,medical,agricultural,andindustrialapplications)iftheirradiationexposure canbeproperlyharnessed,butitcanbeharmfulifuncontrolled.Thereissolidagreementamongthefullradiation healthcommunitythatunrestrictedradiationexposureat highlevelsisdangerousforindividualsandclearlyshould beprevented.Thereisalsosolidagreementthatexposure atmediumlevelscanincreasethenormalincidenceof healtheffectsassociatedwithradiation(suchasmalignancies,whichcannotbeattributedindividuallybutonlycollectively)andshouldalsobelimited.

Butregardinglowlevelradiation,theconundrumis thattheeffects,eitherdetrimentalorbeneficial,ifany,are fromtheclinicalperspective verysmallanddifficulttobe evidenced.Anyradiationhealtheffectsatlowdose,ifthey

existatall,arealmostalwaysdwarfedbyothertoxinsthat welivewitheveryday,suchasthenaturalbodilyburning ofoxygensoessentialtolife.

AnerredinterpretationoftheLNTradiationprotection modelfeedsdirectlyintopublicfears.Unfortunately,such unsubstantiatedfearisnotwithoutconsequences.Suchfear hasalreadycauseddetrimentaleffectsonfrightenedpeople. Moreover,itisseriouslyrestrictingtheuseofradiationtechnologyinthefieldsofmedicine,agriculture,modernindustry,andenergy.Withtheever-growingneedtoliftbillionsof ourfellowcitizensoutofpovertyandprovidethemwith readyaccesstotheadvancesnecessarytoexperiencea higherqualityoflife,theunsubstantiatedfearofradiation isbecomingamatterofsevereethicalconsideration.

Enormousprogresscanandshouldbemadebyencouragingradiationregulatorybodiestorecognizetheshortcomingsofthepresentlyinterpretedandappliedregulation oflow-doseradiationexposuresituationsandtheneedtorevisitit allowingthemtotakeintoaccountthefactthatradiationdamage(ifany)atlowlevelsisessentiallyinsignificant.Themostsignificantcontributionofsuchregulatory agencieswouldbetoemploytheirworkinamannerclearly demonstratingtoacautiousandsuspectingpublicthatlow levelsofradiationaresimplynottobefeared.Welivein anatmosphereofsuchradiationeverydayofourlivesand shouldnotbealarmedwhensmallamountsofadditionalradiationexposure,usuallysimilartothevariationsinbackground naturalexposure,areintroducedintoourenvironment.

Inconclusion,thereisaclearscientificbasisforremovingtheunnecessaryanddebilitatingfearoflow-level radiation.Oncethisconclusionisacceptedbyapresently confusedpublic,enormousprogressinharnessingradiation processescanbeachievedtobringintobeingthebenefits desiredbyourgrowingglobalhumanity.

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