Intake ratio of 131I to 137Cs derived from thyroid and whole-body doses to Fukushima residents.

This study deals with the intake ratio of (131)I to (137)Cs that allows for the utilisation of late whole-body measurements to reconstruct the internal thyroid doses to Fukushima residents. The ratio was derived from the thyroid dose distribution of children and the effective dose distribution of adults based on the assumption that various age groups of persons inhaled the two nuclides at the same activity ratio and at around the same time, while taking into account age-dependent ventilation rates. The two dose distributions were obtained from residents of Iitate village and Kawamata town, located northwest of Fukushima Daiichi nuclear power plant (FDNPP). As a result, the intake ratios for the residents were 2-3, which was much smaller than the activity ratio observed in air sampling. A main reason for this discrepancy presumably lies in the relatively smaller thyroid uptake for iodine in the Japanese subjects than that in the reference persons on whom the biokinetic model promulgated by International Commission on Radiological Protection is based. The actual intake ratio of the two nuclides is believed to have been higher south of the FDNPP; however, this would depend on which of three significant plume events dominantly contributed to the intake for individuals. Further studies are needed to clarify this issue as a part of the reconstruction of early internal doses related to the FDNPP accident.