*3.3. Dietary Intake and CED Values of 40K*

The minimum and maximum total dietary intake values of 40K between 2006 and 2011 were 65 Bq/person/day and 94 Bq/person/day, respectively (Table 5), and the mean was calculated to be 79 Bq/person/day (SD = 7.0). Since 40K was found in almost all TDS samples, the upper limit was in accordance with the lower limit in each MB. The intake values of 40K showed smaller variation than those obtained for Cs, and no significant difference was observed before or after the FDNPP accident.


**Table 5.** Total daily dietary intake values and committed effective doses of 40K for adults in Japan, estimated via total diet studies from 2006 to 2011.

\*<sup>1</sup> FDNPP: Fukushima Daiichi nuclear power plant. \*2 Mean and standard deviation value before the Fukushima Daiichi nuclear power plant accident.

The present study was based on food consumption data derived from the NHNS between 2002 and 2004, as previously described. According to the NHNS, the daily K intake for adults was 2.452 g/person/day in 2002 [41], 2.426 g/person/day in 2003 [42], and 2.372 g/person/day in 2004 [43]. Since 1 g of K corresponds to 30.3 Bq of 40K, the intake of this radionuclide can be estimated to be 74 Bq/person/day in 2002, 74 Bq/person/day in 2003, and 72 Bq/person/day in 2004; this is in close agreement with the mean 40K intake value obtained in the present study. On the other hand, 40K intake between 2003 and 2008 varied from 5 to 150 Bq/person/day, according to a DPS conducted by the NRA [32]. The mean 40K intake was calculated to be 57 Bq/person/day (SD = 16), and was clearly different from our finding. The DPS was performed on five consecutive days from June to January, whereas the NHNS was conducted for only a single day in November, to ensure the participation of a sufficient number of households. Thus, it should be noted that seasonal changes in food item availability and individual daily variations in food intake cannot be assessed using the NHNS and this TDS [7,44].

The minimum, maximum, and mean total CED values of natural occurring radionuclide 40K between 2006 and 2011 were 150 μSv, 210 μSv, and 180 μSv (SD = 16), respectively, which are in close agreement with a previous TDS [13]. The mean value obtained was approximately ten times higher than the maximum CED obtained for artificial radioactive Cs in the present study, and was comparable to the worldwide average of 170 μSv [45]. Figure 4 shows the percent contributions of the contributing food groups to the CED values of 40K in 2011. The food group "other vegetables, mushrooms, and seaweeds" had the greatest contribution to the CED values in Sendai City (31%), Fukushima City (26%), and Tokyo (32%). These results were similar to those of Tsutsumi et al. [33] and Miyazaki et al. [34].

**Figure 4.** Percentage contributions of food groups to the committed effective doses of 40K in 2011. Distance from the Fukushima Daiichi nuclear power plant to each city is denoted in parentheses. This figure was modified from [19].

#### *3.4. TDS of Radionuclides Conducted by Other Countries*

The second International Workshop on Total Diet Studies compiled a list of core (screening), intermediate, and comprehensive (refined assessment) priority chemicals that should be considered for inclusion in a TDS [46]. Radionuclides are categorized into the intermediate list, and there are three countries that routinely conduct TDSs, including Japan [2].

The United States has been performing TDSs continuously since 1961, as mentioned previously. The FDA routinely analyzes the following radionuclides in their TDS analysis: 137Cs, 90Sr, 106Ru, 131I, and 40K. According to the FDA's report [47], the following three of 2984 samples were above the reporting limit of 5 Bq/kg for 137Cs between 2006 and 2014: "baby foods, squash" in 2007 (93.3 Bq/kg), "raisin bran cereal" in 2009 (10.8 Bq/kg), and "salad dressing, creamy/buttermilk type, low calorie" in 2014 (40.5 Bq/kg).

Canada also continuously carries out TDSs of radionuclides. Health Canada has been monitoring natural (40K and 210Pb) and artificial (137Cs, 134Cs, 131I, 241Am, 57Co, and 60Co) radionuclides in foods since 2000. Their results between 2015 and 2017 showed that all 480 samples tested contained 137Cs and 134Cs levels below the minimum detection limit, showing a value of approximately 1.3 Bq/kg [48].

The result obtained from the American study was comparable to our finding, in that only one of the 266 TDS samples exceeded 137Cs levels of 5 Bq/kg. Similarly, the Canadian study was consistent with respect to 137Cs levels, specifically for those observed before the FDNPP accident in the present study. However, such a comparison should be interpreted with caution, since the number of composite samples analyzed in each MB differed between the three countries (approximately 280 for the United States, approximately 160 for Canada, and 14 for the present study) [5,49]. In the present study, individual food items with high levels of 137Cs may be underrepresented, due to the grouping of a greater number of food items within each category, which could have lower levels of the radionuclide [1,50]. This dilution effect might have caused the 137Cs levels after the FDNPP accident in the present study to be lower than those of "baby foods, squash", etc., as mentioned above. Meanwhile, the detection rate of 137Cs in the present study was much higher than that in the United States and Canada, owing to the lower LOD values of 137Cs. The present study evaluated the exposure level to 137Cs as previously described, whereas the United States and Canada did not, because 137Cs was not detected in nearly any samples.

China [51] and Lebanon [52] also conducted TDSs of radionuclides in 1990 and 2004, respectively. For each MB, both countries collected 12 composite samples, similar to the approach used in the present study. Cs-137 activity concentration in their TDS was found to be below 0.1 Bq/kg for most of the food

groups, with the exception of potatoes, which had a value of 10.21 Bq/kg in China; that is consistent with our findings prior to the FDNPP accident.

#### **4. Conclusions**

The present study provides an estimate of the average dietary exposure of 137Cs, 134Cs, and 40K for adults in Japan. Before the FDNPP accident, activity concentrations of 137Cs were in the range of those reported in other countries, and dietary intake values and CEDs were consistent with our previous TDS. Similarly, after the FDNPP accident, the activity concentration and exposure levels of radioactive Cs were well below the regulatory levels, despite an increase in two orders of magnitude being measured. The exposure levels of 40K did not differ before and after the FDNPP accident. The mean CED of 40K was comparable to the international average, and was 10 times higher than the highest CED of radioactive Cs obtained in the present study.

On an international level, TDSs of radionuclides are scarce, and currently, only Japan evaluates trends in exposure levels to these contaminants. Thus, our findings provide invaluable information with regard to the radiological safety of foods. However, the present study has some limitations. First, the exposure levels immediately after the FDNPP accident could not be estimated, because the TDS in 2011 was performed approximately six months after the accident. Second, seasonal variation cannot be assessed, as the NHNS and this TDS were performed in only one season each year. Finally, the present study did not examine the exposure of children, or exposure to other radionuclides, such as 90Sr, Pu, and 106Ru, which are targets of standard limit regulations in foods. Therefore, further studies are required to determine the dietary exposure levels of these radionuclides for both adults and children.

**Author Contributions:** Conceptualization, H.T., I.I., S.M., K.I. and H.S.; methodology, H.T., I.I., S.M., K.I. and H.S.; formal analysis, H.T. and H.S.; investigation, H.T., I.I., S.M., K.I. and H.S.; resources, I.I., S.M., K.I. and H.S.; data curation, H.T. and H.S.; writing—original draft preparation, H.T.; writing—review and editing, H.T., H.S.; visualization, H.T.; supervision, H.S.; project administration, H.S.; funding acquisition, H.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the MHLW HLSRG (Research on Food Safety) (grant nos. H16-Shokuhin-Ippan-015, H19-Shokuhin-Ippan-003, and H22-Shokuhin-Ippan-017).

**Acknowledgments:** The authors would like to thank Michiko Koyano for her technical support in sample preparation.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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International Journal of *Environmental Research and Public Health*
