Eighty-seven residents participated in our study between September 2013 and March 2015; 30 individuals participated multiple times, providing 152 total participants. The data from individuals living in less affected areas and away from Fukushima at the time of the study were eliminated, leaving the data from 142 participants for analysis. Details of study participants and exposure durations are summarized in S1 Table . The participants consisted of 70 full-time farmers, 8 part-time farmers, 35 office workers, 6 housewives, 6 self-employed workers and 17 individuals with an unspecified occupation. According to the diary of each participant, during a given study period, on average the participants spent approximately 57% (SD = 13%) of the time at home, 14% (SD = 11%) of the time indoors other than their home, 16% (SD = 13%) of the time outdoors, 6% (SD = 6%) of the time in a transport vehicle, and 13% (SD = 7%) of the time in an unspecified activity. Our results indicate that people spent a substantial portion of the time at home and this agrees well with time-activity patterns in Fukushima reported elsewhere [ 10 , 16 ]. In these previous studies, some participants spent more than 50% of the time at home. For outdoor workers, the time spent outdoors was reported to be up to about 40% [ 10 ]. In our study, some famers spent approximately 50% of the day outdoors.

On average, participants spent 208 (SD = 83) hours carrying the D-shuttle and GPS for data collection. Table 1 lists summary statistics for individual external doses obtained using D-shuttle. The distributions of individual external doses for each person were generally lognormally distributed. More than 6-fold differences were observed in the mean and GM of the individual external doses of participants. The variation in the 95%:5% ratio, expressed by the ratio between the 95th percentiles to the 5th percentile, suggests a large range in inter- and intra-individual activity. The individual external dose measurements obtained using D-shuttle by the Fukushima High School students [ 11 ] demonstrated that median hourly individual external doses ranged between 0.07–0.10 μSv/h and this range falls within our results. Of the participants in our study, the person with the highest individual external dose (mean = 0.47 μSv/h) worked in a forest and his house was located in a relatively highly affected area. Other individuals with higher than average individual external doses included a retired person living in a relatively higher ambient dose area, and a farmer working at a non-remediated orchard.

Relationship between individual external dose and ambient dose rate

The relationship between the average additional external individual dose obtained using D-shuttle and the average additional ambient dose estimated based on the air-borne monitoring survey, and the relationship between the reduction factor (RF) and the average additional ambient dose, are presented in Fig 4. The RF is defined as the ratio of the additional individual external dose to the additional ambient dose. The raw data used to analyze these relationships are provided in S3 Table. Individual external dose data, making it possible to locate position and relate the ambient dose data, were used in this analysis. We observed significant positive correlation between the average additional ambient dose and the additional individual external dose for all cases (p < 0.0001). The regression line showed that the average additional individual external doses were approximately 18%, 14% and 36% of the average additional ambient doses during all study periods, while at home, and while outdoors, respectively. These results indicate that the average additional individual external doses were significantly lower than the average additional ambient doses estimated based on the air-borne monitoring survey. The exposure while at home was estimated to be the lowest mostly due to the building shielding effect. We observed no correlation between the average additional ambient dose and the reduction factor for all study periods (r = -0.11, p = 0.19) and time spent outdoors (r = 0.11, p = 0.22), and a small negative correlation for time spent at home (r = -0.26, p = 0.002). The mean values of the RFs were 0.18 (min-max: 0.02–0.42) for all study periods, 0.16 (min-max: 0.03–0.42) for time spent at home, and 0.32 (min-max: 0.01–0.80) for time spent outdoors. Previous studies also demonstrated that the levels of individual external doses directly measured using a personal dosimeter were substantially lower than levels estimated using the government-proposed equation [5, 6]. The RFs reported in these studies were on average about 0.3 and this is larger than the RFs obtained from the current study. The reasons for this discrepancy include the values of background doses used to estimate additional dose, consideration of decay in the ambient dose determination, and differences in number and type of participants.

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larger image TIFF original image Download: Fig 4. Relationship between average additional individual external dose obtained using D-shuttle and average additional ambient dose estimated based on the 7th air-borne monitoring survey (left) and the relationship between RF (the ratios of individual external doses to ambient doses) and average additional ambient dose (right). The first, second and third rows represent the results for data from the total study period, while at home, and while outdoors, respectively. https://doi.org/10.1371/journal.pone.0158879.g004

Individual external dose, expressed as Hp (10) values, measured in affected areas in Fukushima have previously been reported to be comparable with the ISO or ROT irradiation geometries [13, 14]. Earlier published evidence showed that the values of Hp (10) were about 0.7 times that of the ambient dose rate, expressed as H*(10), if the measurements were conducted in outdoor field conditions in affected areas in Fukushima [17]. In the current study, most of the RFs were derived from outdoor data in which the mean value was 0.32 (min-max: 0.01–0.80), which is much lower than the reported value of 0.7. There can be several reasons for such discrepancies and variabilities among the RFs. The airborne monitoring-based ambient dose rates, which used in the current study, are considered as an aerial average, and not the ambient dose rates based on the point locations of individuals. The airborne monitoring data were obtained using high-sensitivity radiation detectors installed on helicopters: second-by-second measurements are taken of gamma rays emitted from radioactive substances deposited in circles of a diameter approximately twice the flight altitude (target altitude: 150 to 300 m) and centered around ground locations directly below the flight trajectory [18]. If a helicopter flies at an altitude of approximately 300 m above the ground, the system measures the average value of the radiation in a circle in 300 m radius on the ground [18]. A dedicated software program was used to determine the hourly ambient dose rate 1 m above the ground surface at each location based on the value of gamma rays measured in the air and the reading from the survey meter on the ground [18]. Therefore, the airborne monitoring-based ambient dose rates, therefore, may be over or under estimations of the actual ambient dose rates measured on the ground near the locations of individuals. The reasons for very low RFs obtained in this stu294dy can be due to several factors such as decontamination activity, and the behavior and locations of individuals. If an individual stays or works in a decontaminated area and the airborne monitoring system does not detect the decrease in radiation level of the decontamination area, the values for individual external dose can be much lower than the airborne monitoring-based ambient dose rates. In addition, an individual working for a long period of time on tarmacked surfaces or in farm trucks could be exposed to a low individual external doses. Quantifications of the effects of these factors on RFs remain a future challenges.

People are not typically standing motionless outdoors, but rather spend time both outdoors and indoors where there is some sort of radiation shielding. The mean RF for time spent at home obtained in this study was 0.16 (min-max: 0.03–0.42). The median reduction factor with an interquartile range for wooden houses, which is the ratio of indoor ambient dose rate to outdoor ambient dose rate, has been reported to be 0.43 (0.34–0.53) [19]. Since the methods for calculating home RFs in our study and the reported reduction factors are different, it is not possible to directly compare our home RFs and the reported reduction factors. The reported reduction factors are defined as the ratio of indoor ambient dose rate to outdoor ambient dose rate, while the home RFs in our study are defined as the ratio of additional individual dose rate to airborne-based additional ambient dose rate. Since decontamination works has been conducted in the most of houses in the affected areas in Fukushima, the outdoor ambient dose rate measured at decontaminated points around a house are considered to be lower than the airborne-based ambient dose rate. Moreover, the values of Hp (10) were about 0.7 times that of the ambient dose rate [17]. Taking these relationships and variability of RFs into account, our estimated home RFs are comparable to those reported in previous studies [10, 19] and can be used to estimate individual external dose during time spent at home based on the airborne-based ambient dose.

In the current study, we used publicly available ambient dose rate data estimated from the airborne monitoring survey provided by the Japanese government. More accurate predictions of individual external doses could be possible if ambient dose rates and reduction factors corresponding to an individual’s activity locations were prepared, but this would be impossible to perform for a large number of individuals. The current study relates the individual external doses to the publicly available ambient dose rates and with the activity-patterns of the individuals. The results provide, from a practical viewpoint, valuable information for understanding and estimating realistic individual external doses in the affected areas in Fukushima, especially for evacuees who want to know their individual external doses after returning to their original homes.