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dangerous"

levelofradiation?

Howdoesradiationdamagehealth?

Whataretheconsequencesofacuteandlong-termlow-doseradiation?

Thoughradioactivesteamhasbeenreleasedtoreducepressurewithinthewreckedcomplex'

sreactorsandtherehasbeenadditionalradiationleakagefromthethreeexplosionsthere,theresultingspikesinradiationlevelshavenotbeensustained.Thehighestradiationlevelreportedthusfarwasapulseof400millisievertsperhouratreactorNo.3,measuredat10:

22A.M.localtimeMarch15.（Asievertisaunitofionizingradiationequalto100rems;

aremisadosageunitofx-rayandgamma-rayradiationexposure.）Thelevelofradiationdecreasesdramaticallyasdistancefromthesiteincreases.RadiationlevelsinTokyo,about220kilometerstothesouthwest,havebeenreportedtobeonlyslightlyabovenormal.

"

Wearenowherenearlevelswherepeopleshouldbeworried,"

saysSusanM.Langhorst,ahealthphysicistandtheradiationsafetyofficeratWashingtonUniversityinSaintLouis.

AccordingtoAbelGonzalez,vicechairmanoftheInternationalCommissiononRadiologicalProtectionwhostudiedthe1986Chernobyldisaster,currentinformationcomingfromJapanaboutlevelsofradiationleakageareincompleteatbestandspeculationsabout"

worst-casescenarios"

areasofyetirrelevant.

Thehealtheffectscausedbyradiationexposuredependonitslevel,typeandduration.

Radiationlevel:

Theaveragepersonisexposedto2to3millisievertsofbackgroundradiationperyearfromacombinationofcosmicradiationandemissionsfrombuildingmaterialsandnaturalradioactivesubstancesintheenvironment.

TheU.S.NuclearRegulatoryCommissionrecommendsthatbeyondthisbackgroundlevel,thepubliclimittheirexposuretolessthananadditionalonemillisievertperyear.TheU.S.limitforradiationworkersis50millisievertsannually,althoughfewworkersareexposedtoanythingapproachingthatamount.Forpatientsundergoingmedicalradiationthereisnostrictexposurelimit—itistheresponsibilityofmedicalprofessionalstoweightherisksandbenefitsofradiationusedindiagnosticsandtreatment,accordingtoLanghorst.AsingleCTscan,forexample,canexposeapatienttomorethanonemillisievert.

Radiationsickness（oracuteradiationsyndrome）usuallysetsinafterawhole-bodydoseofthreesieverts—3,000timestherecommendedpublicdoselimitperyear,Langhorstsays.Thefirstsymptomsofradiationsickness—nausea,vomiting,anddiarrhea—canappearwithinminutesorindays,accordingtotheU.S.CentersforDiseaseControlandPrevention.Aperiodofseriousillness,includingappetiteloss,fatigue,fever,gastrointestinalproblems,andpossibleseizuresorcoma,mayfollowandlastfromhourstomonths.

Radiationtype:

Ofconcerninthecurrentsituationisionizingradiation,whichisproducedbyspontaneouslydecayingheavyisotopes,suchasiodine131andcesium137.（Isotopesarespeciesofthesameelement,albeitwithdifferentnumbersofneutronsandhencedifferentatomicmasses.）Thistypeofradiationhassufficientenergytoionizeatoms（usuallycreatingapositivechargebyknockingoutelectrons）,therebygivingthemthechemicalpotentialtoreactdeleteriouslywiththeatomsandmoleculesoflivingtissues.

Ionizingradiationtakesdifferentforms:

Ingammaandx-rayradiationatomsreleaseenergeticlightparticlesthatarepowerfulenoughtopenetratethebody.Alphaandbetaparticleradiationislowerenergyandcanoftenbeblockedbyjustasheetofpaper.Ifradioactivematerialisingestedorinhaledintothebody,however,itisactuallythelowerenergyalphaandbetaradiationthatbecomesthemoredangerous.That'

sbecausealargeportionofgammaandx-rayradiationwillpassdirectlythroughthebodywithoutinteractingwiththetissue（consideringthatattheatomiclevel,thebodyismostlyemptyspace）,whereasalphaandbetaradiation,unabletopenetratetissue,willexpendalltheirenergybycollidingwiththeatomsinthebodyandlikelycausemoredamage.

IntheFukushimasituation,theradioactiveisotopesdetected,iodine131andcesium137,emitbothgammaandbetaradiation.Theseradioactiveelementsareby-productsofthefissionreactionthatgeneratespowerinthenuclearplants.

TheJapanesegovernmenthasevacuated180,000peoplefromwithina20-kilometerradiusoftheFukushimaDaiichicomplex.Theyareurgingpeoplewithin30kilometersoftheplanttoremainindoors,closeallwindows,andtochangeclothesandwashexposedskinaftergoingoutside.Thesemeasuresaremainlyaimedatreducingthepotentialforinhalingoringestingbeta-emittingradioactivematerial.

Exposuretime:

Averyhighsingledoseofradiation（acquiredwithinminutescanbemoreharmfulthanthesamedosageaccumulatedovertime.AccordingtotheWorldNuclearAssociation,asingleone-sievertdoseislikelytocausetemporaryradiationsicknessandlowerwhitebloodcellcount,butisnotfatal.Onefive-sievertdosewouldlikelykillhalfofthoseexposedwithinamonth.At10sieverts,deathoccurswithinafewweeks.

Theeffectsoflong-term,low-doseradiationaremuchmoredifficulttogauge.DNAdamagefromionizingradiationcancausemutationsthatleadtocancer,especiallyintissueswithhighratesofcelldivision,suchasthegastrointestinaltract,reproductivecellsandbonemarrow.Buttheincreaseincancerriskissosmallastobedifficulttodeterminewithoutstudyingaverylargeexposedpopulationofpeople.Asanexample,accordingtoLanghorst,10,000peopleexposedtoa0.01-sievertwhole-bodydoseofradiationwouldpotentiallyincreasethetotalnumberofcancersinthatpopulationbyeight.Thenormalprevalenceofcancer,however,wouldpredict2,000to3,300cancercasesinapopulationof10,000,so"

howdoyouseeeightexcesscancers?

"

Langhorstasks.

Chernobyl'

slessons:

AccordingtoGonzalez,someoftheemergencyworkersatChernobylreceivedseveralsievertsofradiation,andmanywereworking"

basicallynaked"

duetotheheat,allowingcontaminatedpowdertobeabsorbedthroughtheirskin.Incomparison,theJapaneseworkersaremostlikelyverywell-equippedandprotectedatleastfromdirectskindoses.

TheTokyoElectricPowerCo.（TEPCO）,theplant'

sowners,hasevacuatedmostofitsworkers,but50remainatthesitetopumpcoolingseawaterintothereactorsandpreventmoreexplosions.Theseworkersarelikelyexposingthemselvestohighlevelsofradiationandbravingsignificanthealthrisks."

Asamatterofprecaution,Iwouldlimittheworkers'

exposureto0.1sievertandIwouldrotatethem,"

Gonzalezsays.Theworkersshouldbewearingpersonaldetectorsthatcalculateboththerateandtotaldoseofradiationandthatsetoffalarmswhenmaximumdosesarereached."

Ifthedoseoftheworkersstarttoapproachonesievertthenthesituationisserious,"

hesays.

ThethousandsofchildrenwhobecamesickintheaftermathoftheChernobyldisasterwerenotharmedfromdirectradiationorevenfrominhalationofradioactiveparticles,butfromdrinkingmilkcontaminatedwithiodine131.Theisotope,releasedbytheChernobylexplosion,hadcontaminatedthegrassonwhichcowsfed,andtheradioactivesubstanceaccumulatedincows'

milk.Parents,unawareofthedanger,servedcontaminatedmilktotheirchildren."

CertainlythiswillnothappeninJapan,"

Gonzalezsays.

Whenitcomestoradiationexposure,professionalswhofrequentlyworkwithradioactivematerials,whetherinahospitaloranuclearpowerplant,abidebytheALARAprinciple:

aslowasreasonablyachievable"

.Radiationexposurelimitsareconservativelysetwellbelowthelevelsknowntoinduceradiationsicknessorsuspectedofcausinglong-termhealtheffects.Temporaryexposuretodosagesmanytimestheselimits,however,isnotnecessarilydangerous.

NewsoftheU.S.Navyrepositioningitswarshipsupwindofthereactorsite,thedistributionofpotassiumiodidepillsbytheJapanesegovernment,andimagesofofficialsinhazmatsuitsusingGeigercounterstomeasureradiationlevelsamongbabiesmaystokethepublic'

sfears—but,fornow,thesemeasuresareALARAinaction,or"

goodextraprecautions,"

Gonzalezsays.Theideahereistoalwayserronthesideofcaution.

辐射如何威胁健康

随着福岛辐射泄漏的担忧增加，是否有可能衡量辐射暴露的直接和持久的健康影响？

以下是日本辐射病和其他威胁背后的科学

3月11日地震和海啸之后福岛第一核电站发展中的危机引起人们对辐射照射对健康影响的担忧：

什么是“危险的”辐射水平？

辐射如何损害健康？

急性和长期低剂量辐射的后果是什么？

虽然放射性蒸汽已经释放，以减少在被毁坏的综合体反应堆内的压力，并且在那里发生的三次爆炸还有额外的辐射泄漏，但由此造成的辐射水平峰值并未得到维持。

迄今为止报告的最高辐射水平是在上午10点22分测量的3号反应堆每小时400毫升的脉冲。

当地时间3月15日。

（西弗特是等于100雷姆斯的电离辐射的单位;

雷姆是x射线和伽马射线辐射暴露的剂量单位。

）随着离场所的距离增加，辐射水平急剧下降。

据报道，距离西南约220公里的东京的辐射水平仅略高于正常水平。

“我们远离人们应该担心的水平，”圣路易斯华盛顿大学的健康物理学家兼辐射安全官员苏珊M.朗格斯特说。

根据研究1986年切尔诺贝利灾难的国际辐射防护委员会副主席阿贝尔冈萨雷斯的说法，目前来自日本的关于辐射泄漏水平的信息最多是不完整的，对“最坏情况”的猜测仍然不相关。

辐射照射引起的健康影响取决于其水平，类型和持续时间。

辐射水平：

一般人每年接触2至3毫西弗的背景辐射，从宇宙辐射和环境中的建筑材料和天然放射性物质的排放物的组合。

美国核管理委员会建议，除了这个背景水平之外，公众将其暴露限制在每年少于1毫西弗。

美国辐射工作者的年限为每年50毫西弗，尽管几乎没有工人接触任何接近这一数量的东西。

根据Langhorst的说法，对于接受医疗辐射的患者，没有严格的暴露限制-医疗专业人员有责任权衡用于诊断和治疗的辐射的风险和益处。

例如，单次CT扫描可以使患者接触到超过一毫西弗。

Langhorst说，放射病（或急性放射综合征）通常在全身三次三倍体剂量后达到-每年推荐的公共剂量限制的3,000倍。

根据美国疾病控制和预防中心的数据，放射病的首要症状-恶心，呕吐和腹泻可能在几分钟或几天内出现。

包括食欲减退，疲劳，发热，肠胃问题以及可能的癫痫发作或昏迷在内的一段时间的严重疾病可能持续数小时至数月。

辐射类型：

在当前情况下，值得关注的是电离辐射，它是通过自发腐蚀重同位素（如碘13