Radiation doses among residents living 37 km northwest of the Fukushima Dai-ichi Nuclear Power Plant.

External and internal radiation doses were estimated for 15 residents who lived approximately 37 km northwest of the Fukushima Dai-ichi Nuclear Power Plant, which released radioactive plumes on March 11, 2011 as the result of the Tohoku earthquake and subsequent tsunami damage. Residents were interviewed on where they stayed and what they ate after the incident. To estimate external dose, the air dose rate around each person's home was measured, and cumulative effective doses up to 54 d after the deposition were calculated. To estimate committed effective dose, urinary bioassays were performed using a low-background Ge spectrometer on 54 d and 78-85 d after the deposition. The average cumulative effective dose was 8.4 mSv for adults and 5.1 mSv for children. The average committed effective dose from (134)Cs and (137)Cs was 0.055 mSv for adults and 0.029 mSv for children. Iodine-131 was observed from urinary samples of five residents, the equivalent doses for thyroid gland were 27-66 mSv at maximum. We discuss the necessity of reducing the risk of further exposure.