3.1.3. 40K Levels

Potassium is an essential element in food products and humans. Potassium-40, whose natural abundance is 0.0117%, is the only naturally occurring radionuclide of potassium, and has a very long half-life of 1.248 billion years. Accordingly, 40K in foods contributes considerably to internal exposure in the general public. The activity concentration values of 40K in the TDS samples are listed in Table 3. In contrast to 137Cs and 134Cs, 40K was detected in all food groups, and the levels were in the range of 10–100 Bq/kg in most of the TDS samples analyzed. The mean 40K level values between 2006 and 2011 measured for individual food groups were the highest in "green and yellow vegetables", at 90 Bq/kg, followed by "pulses and their products" (84 Bq/kg), and "fish and shellfish" (83 Bq/kg). In contrast, mean 40K values were relatively low in "fats and oils", "drinking water", "rice and rice products", and "beverages". The 40K activity concentration was not significantly different before and after the FDNPP accident.


**Table**


#### *IJERPH* **2020**, *17*, 8131



**Table**

After the Fukushima Daiichi nuclear power plant accident. \*6 Calculated under the assumption that the 40K activity

categorized as "Seasonings and beverages" in 2006. \*8 This food group was categorized as "Others" in 2006.

concentrations

 of the non-detected

 were zero. \*7 This food group was

*IJERPH* **2020**, *17*, 8131

#### *3.2. Dietary Intake and CED Values of Radioactive Cs*

#### 3.2.1. Before the FDNPP Accident

Table 4 presents the total daily dietary intake values and CEDs of radioactive Cs and 40K for adults in each MB. Between 2006 and 2010, the lower and upper limits of radioactive Cs intake varied from 0.0047 to 0.0320 Bq/person/day and from 0.060 to 0.122 Bq/person/day, respectively. The mean values obtained for the lower and upper limits during the same period were 0.020 Bq/person/day (SD = 0.0082) and 0.085 Bq/person/day (SD = 0.019), respectively. Similar results were obtained in a previous study [13]. The lower and upper limits in each MB were significantly different because of the low detection rates of 137Cs and 134Cs before the FDNPP accident (30% and 0%, respectively).

**Table 4.** Total daily dietary intake values and committed effective doses of radioactive Cs (137Cs + 134Cs) for adults in Japan, estimated via total diet studies from 2006 to 2011.


\*1 Assuming that the activity concentrations of the non-detected radionuclides were zero. \*2 Assuming that the activity concentrations of non-detected radionuclides were equal to the limits of detection. \*3 FDNPP: Fukushima Daiichi nuclear power plant. \*4 Mean and standard deviation values before the Fukushima Daiichi nuclear power plant accident.

As previously described, the NRA conducted DPS studies of radionuclides until 2008, and compiled the results from 47 prefectures in Japan into a database [32]. According to the database, the daily intake of 137Cs for adults between 2003 and 2008 ranged from below the detection limit to 0.56 Bq/person/day, and the mean was calculated to be 0.018 Bq/person/day (SD = 0.031), assuming that the activity concentration value of ND radionuclides was zero. Although a larger variation was observed, this finding is similar to the mean lower limit of radioactive Cs intake obtained in the present study.

The lower and upper limits of the CEDs of radioactive Cs were in the range of 0.022–0.150 μSv and 0.33–0.69 μSv, with mean values of 0.098 μSv (SD = 0.041) and 0.47 μSv (SD = 0.11), respectively. The current standard limits for radionuclides in food in Japan were established on the basis of an annual CED of 1 mSv, representing the maximum permissible dose due to food consumption [27,31]. Although the upper limit of the CEDs obtained were overestimated, due to the low detection rate of 137Cs and 134Cs, the highest CED of 0.69 μSv is approximately 1400 times lower than the maximum permissible dose, and thus would seem to pose no health risk.

#### 3.2.2. After the FDNPP Accident

Since 137Cs and 134Cs were detected in most of the food groups after the FDNPP accident, the lower and upper limits of radioactive Cs intakes were calculated using actual activity concentrations of these radionuclides, rather than zero or LODs. Thus, the lower and upper limits were almost the same in each MB after the FDNPP accident, while they were significantly different before the accident. The lower limits of total radioactive Cs intake were estimated to be 2.9 Bq/person/day in Fukushima City, 2.2 Bq/person/day in Sendai City, and 0.67 Bq/person/day in Tokyo, after the FDNPP accident (Table 4).

The lower limits of the total CED of radioactive Cs in 2011 were 12 μSv in Sendai City, 17 μSv in Fukushima City, and 3.8 μSv in Tokyo. The high value obtained for Fukushima City is expected, as it is located the closest to the FDNPP incident. Figure 2 displays the lower limits of CED values of radioactive Cs from 2003 to 2011, and shows that values obtained after the FDNPP accident were about two orders of magnitude higher than before the accident. Nevertheless, the highest CED of radioactive Cs (17 μSv), which was the exposure level in half a year after the FDNPP accident, is approximately 60 times lower than the maximum permissible dose.

**Figure 2.** Committed effective doses of radioactive Cs (137Cs + 134Cs) obtained from each market basket (2003–2011). The closed circles show the results of each market basket. The doses were estimated with the assumptions of one-year intake values of the total diet samples, and the activity concentration values of non-detected radionuclides to be zero.

Figure 3 summarizes the CED values of radioactive Cs, organized by food group, in 2011. The three food groups with the highest contributions to the CEDs were "milk and dairy products" (66%), "fish and shellfish" (17%), and "rice and rice products" (8.2%) in Sendai City; "rice and rice products" (33%), fruits (32%), and "milk and dairy products" (15%) in Fukushima City; and "milk and dairy products" (69%), "rice and rice products" (7.6%), and "fish and shellfish" (7.0%) in Tokyo.

**Figure 3.** Committed effective doses of radioactive cesium (137Cs + 134Cs) by food groups in 2011. Distance from the Fukushima Daiichi nuclear power plant to each city is denoted in parentheses. This figure was modified from [19].

In 2011, TDSs of radioactive Cs and 40K were also carried out by Tsutsumi et al. [33] and Miyazaki et al. [34]. Tsutsumi et al. analyzed MBs from Sendai City, Fukushima City, and Tokyo in September and November 2011, and revealed that the lower limits of total CED values of radioactive Cs were 17 μSv in Sendai City, 19 μSv in Fukushima City, and 2.1 μSv in Tokyo. They also found that "fish and shellfish", "fruits", "green and yellow vegetables", and "milk and dairy products" were the main contributors to the CED values obtained. Although the contributing food groups varied to some extent, their findings are similar to the total CED values of radioactive Cs obtained in the present study. Miyazaki et al. collected an MB from Nagoya City in August 2011, and measured CED values for radioactive Cs of 1.5 μSv, which was lower than that obtained in both our study and that of Tsutsumi et al. This might be because of the distance between the FDNPP accident site and Nagoya City, which at 446 km, is much larger than that to Sendai City (95 km), Fukushima City (62 km), or Tokyo (227 km).

After 2011, the TDSs have been performed by other institutions in Japan. The National Institute of Health Sciences have been conducting the TDS, which was targeting 15 areas, including Fukushima Prefecture, Miyagi Prefecture, and Tokyo, twice a year since 2011. Uekusa et al. [35] revealed that the maximum CEDs due to radioactive Cs decreased to 9.4 μSv (March 2012), 3.8 μSv (September 2012), and 7.1 μSv (March 2013) from 19 μSv in 2011 [33]. In the past two years, the results were as follows: 1.1 μSv (February–March 2018) [36], 1.1 μSv (September–October 2018) [37], 1.0 μSv (February–March 2019) [38], and 1.0 μSv (September–October 2019) [39]. The Tokyo Metropolitan Institute of Public Health also have been carrying out the TDS in Tokyo once a year. Their results between 2012 and 2019 showed that the CEDs due to radioactive Cs in Tokyo ranged from 0.20 μSv (2017) to 1.3 μSv (2012) [40]. All the results mentioned above indicated that the CEDs due to radioactive Cs clearly decreased from those in 2011, as expected. In 2019, the maximum CED of 1.0 μSv was 17 times lower than that from the present study, and was one-thousandth of the maximum permissible level. However, the maximum CED between 2012 and 2019 was still higher than the maximum upper limit of the CED, due to radioactive Cs before the FDNPP accident (0.69 μSv).
