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  • 1 Kobe Pharmaceutical University, 4-19-1 Motoyamakita-machi, Higashinada-ku, Kobe-shi, Hyogo-ken, 658-8558 Japan
  • 2 National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba-shi, Chiba-ken, 263-8555 Japan
  • 3 Fukuoka Institute of Health and Environment Sciences, 39, Mukaizano, Dazaifu, 818-013 Japan
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Indoor airborne radon concentration released from water was estimated over several months based on experimental measurements. When heated water including radon was used, the radon level became high in the entire house. Filling the bathtub with hot water had a strong effect on the indoor radon level. In winter, the indoor radon concentration was high due to windows being closed. The radon transfer coefficient was estimated 2.5·10−4 in this test house. This is 2.5 times higher than the average radon transfer efficiency estimated by UNSCEAR.