|| Misasa Spa is one of the most highly radioactive hot springs in Japan, the waters of which contain mainly (222)Rn (437±132 Bq/liter). Radon contents of indoor air of private houses and health resort hotels (built of wood) at Misasa Spa range from 18.5 to 55.5 mBq/liter and 22.2 to
129.5 mBq/liter, respectively. Radon contents in the air of facilities using spring waters at Misasa Branch Hospital of Okayama University were measured to be ; bathroom 807±78 mBq/liter; Hubbardtank bathroom 5306±2568 mBq/liter ; the drinking hall 1491±178 mBq/liter. The environmental and dose rate inside private houses has been measured to be 14.0±1.8 μR/h. Chromosome aberrations (dicentrics) in the peripheral blood lymphocytes of residents of Misasa Spa were investigated in 14 persons; the mean value of aberration frequencies were 0.21%.