Radiocesium-bearing microparticles discovered on masks worn during indoor cleaning.

A decade has passed since the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on March 11, 2011. However, radioactive particles have recently been detected in the indoor air of some residences near the FDNPP. Following the recommendations of previous research, we determined the presence of radiocesium-bearing microparticles (CsMPs) and measured the radioactivity of radiocesium that adhered on non-woven face masks worn by six persons during the indoor cleaning of 59 residences in Namie, Futaba, Okuma, and Tomioka towns in Fukushima Prefecture. Of the 284 masks worn in this study, significant 137Cs radioactivity was detected in 268, and 44 new CsMPs were discovered in 28. The results of this study also suggest the presence of highly concentrated soluble radiocesium particles or soluble radioactive cesium aerosols adhered to house dust. This implies that the CsMPs constituted a large proportion of radioactivity in the indoor air contamination for particles in the 1.0-2.5 µm size range due to the radioactive radiocesium particles. It is desirable to wear masks during cleaning to prevent inhalation of CsMPs.

Comparison of DNA sequencing and morphological identification techniques to characterize environmental fungal communities.

Culture-independent DNA sequencing of fungal internal transcribed spacer 2 (ITS2) region was compared to a culture-dependent morphological identification technique to characterize house dust-borne fungal communities. The abundant genera were Aspergillus, Wallemia, Cladosporium, and Penicillium. Statistically significant between-method correlations were observed for Wallemia and Cladosporium (Spearman's ρ = 0.75 and 0.72, respectively; p < 0.001). Penicillium tended to be detected with much higher (averaged 26-times) relative abundances by the culture-based method than by the DNA-based method, although statistically significant inter-method correlation was observed with Spearman's ρ = 0.61 (p = 0.002). Large DNA sequencing-based relative abundances observed for Alternaria and Aureobasidium were likely due to multicellularity of their spores with large number of per-spore ITS2 copies. The failure of the culture-based method in detectiing Toxicocladosporium, Verrucocladosporium, and Sterigmatomyces was likely due to their fastidiousness growth on our nutrient medium. Comparing between the two different techniques clarified the causes of biases in identifying environmental fungal communities, which should be amended and/or taken into consideration when the methods are used for future fungal ecological studies.

Resuspension and deposition of PM2.5 and PM10 containing radiocesium during and after indoor cleaning of uninhabited houses in Fukushima, Japan.

Radiocesium contamination in homes could be a serious concern following Japan's 2011 Fukushima Daiichi Nuclear Power Plant accident, including exposure to radiocesium during cleaning when residents return home after the lifting of evacuation orders. This study measured PM2.5 and PM10 concentrations containing radiocesium during cleaning (dusting, vacuuming with a cordless cyclone unit, and vacuuming with a corded paper-pack unit), as well as air exchange rates, in 12 residential houses in Fukushima. Surface dusting of walls, shelves, and furniture significantly increased concentrations of PM2.5 and PM10 by up to 6.3 and 16 times the background (outdoor) level, respectively. Vacuuming with a paper-pack unit increased levels by 2.2 and 3.3 times, while vacuuming with a cordless cyclone unit increased these by 1.3 and 1.5 times, respectively. Measurements in 11 houses revealed an average air exchange rate of 0.22/h and dry deposition rates for PM2.5 and PM10 of 0.13/h and 0.32/h, respectively. Dry deposition rates were not correlated with building age, although the air exchange rates showed statistically significant increases with increasing building age. Dry deposition rates of PM2.5 significantly decreased with increasing air exchange rates.

Estimated internal exposure doses due to indoor radiocaesium contamination in residential houses after the Fukushima nuclear accident.

This work first reports the estimation of the internal exposure from ingestion of house dust and inhalation of aerosol, by employing a measured data on 137Cs activities, bioaccessibility (solubility to water and 1 M HCl), and particle size distribution. The house dust and aerosol samples were collected during the actual indoor cleaning by vacuuming and dusting, from 65 houses and buildings in proximity to the Fukushima Daiichi nuclear power plant (FDNPP) (1.6-16.1 km from the FDNPP) during a period from April 2016 to January 2019. Committed effective doses for an adult owing to the ingestion of house dust of 20 mg per day, which adheres to one's hands through the hand-to-mouth, and those owing to inhalation of aerosol during dusting for 1.5 h while wearing a mask, were calculated using DCAL software for each house or building, as 1.13 µSv and 4.55 µSv as maximum doses, respectively (as of March 2011). Both the committed effective doses, owing to ingestion and inhalation, were inversely correlated with the distance from the FDNPP, and positively correlated with the indoor surface contamination.

Radiocesium concentration in indoor air during residential house cleaning in Fukushima Dai-ichi nuclear power plant evacuation areas.

After the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, outdoor decontamination was conducted by the Japanese Government, whereas indoor contamination was the responsibility of individual residents. However, no data exist regarding 137Cs exposure during indoor cleaning. Therefore, indoor 137Cs radioactivity concentrations in indoor air were determined during cleaning in 60 houses within the evacuation area near the FDNPP, Fukushima Prefecture, Japan. Radiocesium activity concentrations per cubic meter of indoor air during dusting (mean ±â€¯SD (median)) was 6.8 ±â€¯7.9 (4.7) and 1.6 ±â€¯2.7 (0.78) Bq/m3 for all aerodynamic diameter of aerosol particle and PM2.5, respectively. Radiocesium activity concentrations in indoor air decreased with decreasing the aerodynamic diameter of aerosol particles (mean: 0.099, 0.22, 0.41, 0.92, 2.2, and 2.9 Bq/m3 for the aerodynamic diameter of <0.25, 0.25-0.5, 0.5-1.0, 1.0-2.5, 2.5-6.6, and >6.6 µm, respectively) and was inversely proportional to the square of the distance from the FDNPP. Indoor 137Cs radioactivity concentrations were significantly higher during dusting than during vacuuming with either a cyclone handy cleaner or a normal vacuum cleaner. The mean deposited activities in tracheobronchial and alveolar regions was estimated to be 1.9 and 2.8 Bq, respectively, during a 2-h dusting period in the studied houses. For values below the detection limit, half of the detection limit was used for calculating the means and SDs.

Review of reduction factors by buildings for gamma radiation from radiocaesium deposited on the ground due to fallout.

In order to estimate residents' external dose due to radionuclide exposure resulting from fallout deposit on the ground, the shielding and dose reduction effects provided by structures such as houses and workplaces are taken into account as most individuals spend a large portion of their time indoors. Many works on both calculation and measurement for European and American settlements have been reported and factors such as, shielding factors, protection factors, reduction factors, and location factors have been determined. However, measurement data for Japanese settlements are lacking. Thus, the Japanese government used reduction factors given in the International Atomic Energy Agency documents from American and European settlements when Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident occurred. The United Nations Scientific Committee on the Effects of Atomic Radiation used location factors from European settlements for the same reason. Soon after the FDNPP accident, several measurements and calculations were performed to obtain specific reduction factors for Japanese settlements due to this lack of data. This research reviews previous studies that determined factors such as, shielding factors, protection factors, reduction factors, and location factors and summarizes specific results for Japan. We discuss the issues in determining these factors and in applying them to estimate indoor dose. The contribution of surface contamination to the indoor ambient dose equivalent rate is also discussed.

Radiocesium contamination in house dust within evacuation areas close to the Fukushima Daiichi nuclear power plant.

Outdoor decontamination efforts have been ongoing since the Fukushima Daiichi nuclear power plant (FDNPP) accident; however, little is known about indoor contamination. Therefore, house dust was sampled based on particle size in 21 wooden buildings (19 residential houses and 2 community centers) within the evacuation area close to the FDNPP, Fukushima Prefecture, Japan. Activities of radiocesium (137Cs) per gram of house dust increased with decreasing particle size (mean: 6.1 × 103, 2.6 × 103, 1.6 × 103, 7.5 × 102, 5.0 × 102, and 4.6 × 102 Bq/g for <4-20, 20-63, 63-180, 180-500, 500-1000, and 1000-2000 µm, respectively). The 137Cs concentrations in house dust were inversely related to the square of distance from the FDNPP for <4-1000 µm particles. From the results of the multiple linear regression analysis, distance from the FDNPP and direction from the FDNPP (northwest) were significantly related to the radioactivity of house dust. It was found that 19%, 33%, and 48% of 137Cs in house dust were extracted in water, 1 M HCl, and not extracted, respectively. Considering the bioaccessibility and assuming a 20 mg/day daily intake of house dust, the daily doses would be 7.2 Bq/day (mean) and 18 Bq/day (95th percent quantile). These results provide valuable insight into indoor radioactive Cs contamination in the area around the FDNPP and possible oral exposure to indoor radioactive Cs after returning home.

Review of reduction factors by buildings for gamma radiation from radiocaesium deposited on the ground due to fallout.

In order to estimate residents' external dose due to radionuclide exposure resulting from fallout deposit on the ground, the shielding and dose reduction effects provided by structures such as houses and workplaces are taken into account as most individuals spend a large portion of their time indoors. Many works on both calculation and measurement for European and American settlements have been reported and factors such as, shielding factors, protection factors, reduction factors, and location factors have been determined. However, measurement data for Japanese settlements are lacking. Thus, the Japanese government used reduction factors given in the International Atomic Energy Agency documents from American and European settlements when Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident occurred. The United Nations Scientific Committee on the Effects of Atomic Radiation used location factors from European settlements for the same reason. Soon after the FDNPP accident, several measurements and calculations were performed to obtain specific reduction factors for Japanese settlements due to this lack of data. This research reviews previous studies that determined factors such as, shielding factors, protection factors, reduction factors, and location factors and summarizes specific results for Japan. We discuss the issues in determining these factors and in applying them to estimate indoor dose. The contribution of surface contamination to the indoor ambient dose equivalent rate is also discussed.

Indoor radiocaesium contamination in residential houses within evacuation areas after the Fukushima nuclear accident.

Indoor contaminants were investigated from July 2013 to January 2015 within ninety-five residential houses in five evacuation zones, Iitate village, Odaka district, and the towns of Futaba, Okuma, and Tomioka. A dry smear test was applied to the surface of materials and structures in rooms and in the roof-space of houses. We found that (134)Cs and (137)Cs were the dominant radionuclides in indoor surface contamination, and there was a distance dependence from the Fukushima Daiichi nuclear power plant (FDNPP). For surface contamination in Iitate village (29-49 km from the FDNPP), 24.8% of samples exceeded the detection limit, which is quite a low value, while in Okuma (<3.0 km from the FDNPP), 99.7% of samples exceeded the detection limit and surface contamination levels exceeded 20 Bq/cm(2) (the value was corrected to March 2011). In residential houses in Okuma, Futaba, and Tomioka, closer to the FDNPP than those in Odaka district and Iitate village, surface contamination was inversely proportional to the square of the distance between a house and the FDNPP. In the houses closest to the FDNPP, the contribution of surface contamination to the ambient dose equivalent rate was evaluated to be approximately 0.3 µSv/h.

Reduction factors for wooden houses due to external γ-radiation based on in situ measurements after the Fukushima nuclear accident.

For estimation of residents' exposure dose after a nuclear accident, the reduction factor, which is the ratio of the indoor dose to the outdoor dose is essential, as most individuals spend a large portion of their time indoors. After the Fukushima nuclear accident, we evaluated the median reduction factor with an interquartile range of 0.43 (0.34-0.53) based on 522 survey results for 69 detached wooden houses in two evacuation zones, Iitate village and Odaka district. The results indicated no statistically significant difference in the median reduction factor to the representative value of 0.4 given in the International Atomic Energy Agency (IAEA)-TECDOC-225 and 1162. However, with regard to the representative range of the reduction factor, we recommend the wider range of 0.2 to 0.7 or at least 0.2 to 0.6, which covered 87.7% and 80.7% of the data, respectively, rather than 0.2 to 0.5 given in the IAEA document, which covered only 66.5% of the data. We found that the location of the room within the house and area topography, and the use of cement roof tiles had the greatest influence on the reduction factor.

Evaluation of personal dose equivalent using optically stimulated luminescent dosemeters in Marumori after the Fukushima nuclear accident.

The personal dose equivalent for 36 Marumori residents living in wooden houses was evaluated using optically stimulated luminescent dosemeters after the Fukushima nuclear accident. The cumulative doses to residents over 200 d from 1 September 2011 ranged from 536 to 1490 µSv. In each resident's house, the indoor and outdoor ambient dose equivalents were measured with an NaI scintillation survey meter. A linear relationship with a slope of 0.5 was found between them; however, the ratio of the indoor to the outdoor ambient dose equivalent varied from 0.36 to 1.0. A better linear relationship was observed between the personal dose equivalent and the indoor ambient dose equivalent compared with that between the personal dose equivalent and the outdoor ambient dose equivalent. These results indicate that the individual exposure dose depends on the indoor ambient dose equivalent rather than the outdoor ambient dose equivalent. These results should be very helpful in obtaining the countermeasures to reduce residents' exposure dose.