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Safety_Series_025_1968 - gnssn - International Atomic Energy ...

Safety_Series_025_1968 - gnssn - International Atomic Energy ...

This publication is no

This publication is no longer validPlease see http://www.ns-iaea.org/standards/1.4. M ETABO LISM AND T O X IC ITY OF RAD IOACTIVEM ATERIALS1.4.1. GeneralR adion uclides can enter the human body by v ariou s paths andcause internal irradiation , the im portan ce of which is p articu la rlyem phasized by the fact that certain radionuclides are deposited m oreor less permanently in the body, leading to contamination which mayhave seriou s con seq u en ces fo r the affected individual. F o r thisrea son it is n e ce s s a ry to have a detailed know ledge of a ll thechannels through which ra d ioa ctiv e m a teria ls can be absorbed , ofthe fa ctors which a ccelera te this absorption, of the manner and siteso f deposition in the body, and of the rates o f elim in ation . Muchvaluable inform ation in this respect is contained in the ICRP reports [11].The physical and ch em ical properties of the absorbed m aterialsa re o f great im p orta n ce. The length of exp osu re is in flu enced bythe physical h a lf-life and other b iological param eters, and the patternof in ju ry depends on the nature of the em itted radiation. Alpha andbeta rays are absorbed lo ca lly and d e liv e r the dose to sm a ll areaso f tissu e, while gam m a rays in flu en ce la rg e p ortion s o f the bodyor may even escape the body. As already stated, the chem ical natureof the absorbed m aterial plays an important role in defining the ultimatebiologica l effect. R adioisotopes of.elem ents norm ally presentin the body (phosphorus, carbon, calcium , sulphur, iron, strontium)participate in the m etabolic p rocess and behave in the body like stableisotop es of the sam e elem ents. They may be incorporated into im ­portant b iologica l com pounds and changed during disintegration intoother elem ents which are foreign to the compound.D iffe re n ce s betw een lo ca liz e d and d iffu sed in tern al radiationm ay be of extrem e im p ortan ce. Som e radion u clides m ay be d ep o­sited and aggregated in to'h ot s p o t s '. The lo ca l doses and d ose ratesin such cases may attain high values. Hot spots com plicate also theconcept of Relative B iological E fficien cy (RBE) fo r internally depositedbon e-seekin g m aterials in the sense that not only linear energytran sfer (LET) but also factors like dose, dose rate, biological endpointetc. must be considered.1.4.2. Absorption of radionuclides1. 4. 2. 1. Absorption from the gastro-intestinal tractIngestion is im portant m ainly fo r m aterials solu ble in bodyflu id s. Some solu ble com pounds m ay be con verted to insoluble hydroxid es at the pH of body fluids and v ice v ersa .4 4

This publication is no longer validPlease see http://www.ns-iaea.org/standards/The in gestion o f ra d ioactiv e m a teria ls by m an can take p la ceeither with drinking w ater, with food, by sw allow ing of inhaled p a r­tic le s , o r by acciden tal penetration into the mouth cavity.F ood -ch ain s are of great im portance in evaluating potential con ­tam ination o f the body by ra d ioa ctiv e m a teria ls p resen t in the environm en t. One of the m ost com m on routes to man fo r ra d ioactivefa ll-ou t is as follow s:F a ll-ou t ---------* plant ---------* cattle ---------* meat (m ilk)-------- > manMilk is a v ery im portant food -ch ain path, esp ecially fo r radioactivestrontium and iodine.1.4. 2. 2. A bsorption through inhalationIn the atom ic industry the m ost usual entry of radioa ctive m a­te r ia ls in the hum an bod y du rin g rou tin e op era tion s is that o f in ­halation. The inhaled radioactive p a rticles may be tran sferred intothe circu la tion and deposited in a c r itic a l organ . They m ay in ju red ire ctly the r e s p ira to ry su rfa ce s o f the lungs o r be ab sorb ed bybronchial lymph nodes. The depth of penetration into the respiratorysystem depends on p a rticle s iz e . S m all p a rticle s fr e e ly enter thelo w e r p ortion s o f the lungs; la rg e p a rticle s a re d ep osited m ainlyin the upper re s p ir a to r y tra ct and a re ea sily c le a r e d .Usually p a rticles are heterogeneous in size. V ery often radioactivem aterial is attached to p a rticles of inert m aterial.The fu rth er fate o f inhaled p a rticle s depends upon th eir solu ­bility in body fluids. It determ ines to a large extent their depositionand subsequent excretion (see F ig. 15).1.4. 2. 3. A bsorption through the skin ^The intact skin absorbs v ery little radioactive m aterial. Tritium ,h ow ever, and som e oth ers can be so ab sorb ed . The p erm ea b ilityo f skin w hich has been in ju red even by su rfa ce a b ra sion s is v e rycon sid era b ly in cre a se d . Wounds p erm it open entry of ra d ioa ctiv em a teria l. The p re s e n ce o f org a n ic solven ts on the su rfa ce o f theskin a ls o a c c e le r a t e s the p e n e tra tio n o f r a d io a c t iv e m a t e r ia ls .When la rge quantities of rad ioactive m aterial com es in contactwith the skin, e.g. during an industrial accident, the skin absorptionshould be taken into consideration.45'

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