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A threshold dose of 100 mSv is assumed for radiation-induced teratogenic effectsin publication No. 90 of the ICRP (2003), which is based on the findings in the Japanese A-bomb survivors. A variety of observations about congenital malformations, foetal loss,stillbirths, and infant deaths, as well as Down ́s syndrome after the Chernobyl release makemanifestly clear the incompleteness and inadequacy of the Japanese data in these areas. Theradiation-induced developmental effects of Chernobyl, especially at great distances, areusually denied because of the low values of the estimated human exposures. Biologicaldosimetry in contaminated regions, however, shows that physical dose estimations lead toconsiderable underestimations of the true exposure. The assumptions about teratogeniceffects by incorporated radionuclides have to be revised. IntroductionThe evaluation of radiaton risks by international radiation protection committees is basedon findings concerning Japanese A-bomb survivors. The effects observed there afterprenatal exposure were mental retardation and reduced head size, while no other significantdetriment was detected. The period between the 8 and 15 week of gestation is thought tobe the period of greatest risk.As was pointed out by different, researchers the Japanese data suffer, however, fromseveral restrictions which limit their suitability as a general base for deriving radiationrisks. One point is a probable and proven severe selection bias because of the catastrophicsituation after the bombing. Another objection which must be stressed, especiallyconsidering perinatal effects, is the fact that the investigations of the Radiation EffectsResearch Foundation (RERF) in Hiroshima did not begin earlier than 5 years after thecatastrophe when the RERF research institute was established there. The completeness ofthe data must therefore be put into question.The reduced selection of assumed effects and the assertion of a threshold dose as highas 100 mSv by the ICRP contradicts former findings in experimental research (Table 1) andseveral observations in humans previous to the Chernobyl accident. Tables 2-5 list theresults of studies in regions affected by fallout from the latter. ECRR 2006: CHERNOBYL 20 YEARS AFTER 106Table 1. Minimum dose below 100 mSv which showed significant effects after in utero x-ray exposure in experimental studies (data taken from Fritz-Niggli, 1997) DosemSvDays afterconceptionEffectsReference Mice 1050505080.50.5; 1.57.5Cumulateddevelopmental defectsDeath of the embryoDeath of the embryo,polydactylyDeath of the embryo,skeletal malformationsMichel, Fritz-Niggli 1977Rugh, Grupp1959Ohzu 1965Jacobsen 1966 Rats 1050180.4; 0.7Reflex distortionsFoetal deathUNSCEAR1986Roux et al. 1983 Table 2 Observed increase of congenital malformations in utero after exposure by theChernobyl accident Country EffectsReferences BelarusNationalGeneticMonitoringRegistryAnencephaly, spina bifida, cleftlip and/or palate, polydactyly,limb reduction defects,esophageal atresia, anorectalatresia, multiple malformationsLazjuk et al. 1997 BelarusHighlyexposed regionof Gomel Checherskydistrict of theGomel region