The Discovery of Radiocesium-bearing Microparticles Directly Delivered to a Person in Tokyo as a Result of the Fukushima Daiichi Nuclear Disaster.

The plume from Fukushima Dai-ichi Nuclear Power Plant (FDNPP) Unit 2, which reached the Tokyo metropolitan area on 15 March 2011 immediately after the FDNPP disaster, contained insoluble radiocesium-bearing microparticles (CsMPs). This study aimed to reanalyze a mask worn during the disaster to determine the ratio of CsMPs to radiocesium that directly arrived in the Tokyo metropolitan area as a plume immediately after the disaster. CsMPs were isolated from the non-woven masks worn at the Hongo Campus of the University of Tokyo from 15-16 March 2011 that trapped radioactive material from the accident. Twenty-two CsMPs with diameters in the range of 1.9-3.6 μm were found in the mask worn outdoors by one person. The CsMP radioactive ratio to overall radiocesium content of the mask was 9.9%. The CsMPs were collected more efficiently by the mask than by a suspended particulate matter filter. A face mask might trap CsMPs in a plume more efficiently than a suspended particulate matter filter. Determining the amount of CsMPs led to a reassessment of the internal dose by CsMPs that were inhaled by the public immediately after the FDNPP accident.