Estimation of the radiation dose via indoor dust in the Ibaraki and Chiba prefectures, 150–200 km south from the Fukushima Daiichi Nuclear Power Plant

Abstract The radiation dose effects due to the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident are of public concern. Although indoor dust is an important exposure route for some hazardous chemicals, the radiation exposure dose from the radioactive caesium (Cs) in it has not been well investigated. Hence, in this study, we measured the radioactive Cs in the indoor dust of private houses in order to estimate the resulting human exposure. We collected vacuum cleaner dust twice (in April and June 2012, n = 250 and 241, respectively) in the prefectures of Chiba and Ibaraki, from houses located 150–200 km south from the FDNPP. The activity concentrations of 134Cs and 137Cs in the indoor dust were measured using germanium γ-ray detectors, and the exposure dose was calculated and the household-to-household variations were analysed. The median activity concentrations of 134Cs + 137Cs in the indoor dust collected in April and June were 1,470 and 1,340 Bq kg−1, respectively. The median committed effective doses via indoor dust ingestion were estimated as 0.73, 0.66, and 0.43 × 10−3 mSv year−1, and those via inhalation were estimated as 0.03, 0.04, and 0.06 × 10−3 mSv year−1 for one- and ten-year-old children and adults, respectively. Those were much smaller than the annual dose limit of 1 mSv year−1, which was set as a goal to be achieved in the a few years or decades (clear period has not been set). Within the detached houses, multiple regression analysis showed that wet wiping of floor was a significant reduction factor of the Cs concentration in dust; however, the small determination coefficient implies the presence of factors that were not considered in this study.