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Preliminary assessment of ecological exposure of adult residents in Fukushima Prefecture to radioactive cesium through ingestion and inhalation

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This study estimated the ecological exposure of adult residents of Fukushima Prefecture to cesium-134 (134Cs) and cesium-137 (137Cs) through dietary ingestion and inhalation during the first week of July 2011, approximately four months after the Fukushima Daiichi nuclear power plant accident. Fifty-five whole-day meal sets were purchased from local grocery stores at multiple sampling sites at varying distances from the plant.

Key numbers

Dietary intake (whole-day meals):

  • 134Cs or 137Cs detected above the method detection limit (MDL) in 36 of 55 meal sets (65.5%) from Fukushima Prefecture; control meals from Kyoto showed detection in only 1 of 19 (5.3%) (Table 1, p. 3).
  • Median estimated combined dose by ingestion: 3.0 µSv/year (range: not detectable to 83.1 µSv/year) across Fukushima Prefecture (Table 1, p. 3).
  • Mean ± SD combined dose by ingestion: 6.4 ± 12.5 µSv/year (Table 1, p. 3).
  • Highest mean ingestion dose by site: Souma 17.4 ± 25.3 µSv/year; Iwaki 8.6 ± 7.8 µSv/year (Table 1, p. 3).

Cow’s milk (Fukushima Prefecture, n = 21):

  • Median total activity (134Cs + 137Cs): 4.1 Bq/kg (range: not detectable to 10.1 Bq/kg); well below the recommended limit of 200 Bq/kg (Table 2, p. 4).

Vegetables and fruit (Fukushima Prefecture, n = 43):

  • No vegetable exceeded 100 Bq/kg except shiitake mushrooms (Lentinula edodes) (Results, p. 5).
  • Shiitake at Kawamata: 140.4 Bq/kg 134Cs + 164.2 Bq/kg 137Cs = 304.6 Bq/kg total; Minamisouma: 127.1 + 154.7 = 281.8 Bq/kg total; up to 60% of the 500 Bq/kg limit (Table 2, p. 4-5).
  • Shiitake at Iwaki: not detectable for both isotopes (Table 2, p. 5).

Air sampling (Table 3, p. 6):

  • Maximum inhalation dose: Namie town 76.9 µSv/year; Iitate village 14.7 µSv/year; Katsurao village 27.7 µSv/year (at the edge of the 20-km evacuation zone).
  • Inland and coastal sites outside the 20-km radius: estimated inhalation doses below 3 µSv/year at 9 of 12 locations (Table 3, p. 6-7).
  • Maximum combined inhalation + ingestion dose in areas outside the 20-km radius: approximately 160 µSv/year (83.1 by ingestion + 76.9 by inhalation); well below the publicly permissible dose of 1 mSv/year (Results, p. 7).

Effective dose coefficients used (ICRP Publication 72):

  • Ingestion: 0.019 µSv/Bq for 134Cs; 0.013 µSv/Bq for 137Cs (Methods, p. 3).
  • Inhalation: 0.02 µSv/Bq for 134Cs; 0.039 µSv/Bq for 137Cs (Methods, p. 3).

Detection limits: 0.05 Bq/kg for food; 0.2 Bq/kg for vegetables and milk; 0.2 mBq/m3 for air dust (Methods, p. 3).

Methods (brief)

Whole-day meal sets were purchased by five male researchers from popular grocery stores at sampling sites in four Fukushima cities or towns (Iwaki, Souma, Nihonmatsu, Fukushima City) and from a Kyoto control site. Each set comprised prepackaged breakfast, lunch, dinner, desserts, snacks, and beverages homogenized with locally collected tap water. Freeze-dried homogenates and separately purchased vegetables and milk were measured with a high-purity germanium detector at characteristic gamma energies (134Cs: 604.7 and 795.9 keV; 137Cs: 661.7 keV). Air sampling used a high-volume sampler (minimum 50 m3 per site) and an eight-stage Andersen cascade impactor at Fukushima City for size-fraction analysis. All activities were decay-corrected to March 15, 2011.

Evidence fitness

This paper supports documentation of radioactive cesium concentrations in Fukushima Prefecture foods and air approximately four months after the 2011 nuclear accident. It characterizes the matrix-level variation in contamination (shiitake mushrooms substantially higher than other vegetables) and provides dose estimates for residential exposure under the conditions of the survey. It does not address stable cesium concentrations in food, long-term dietary exposure trends, or exposure in populations outside eastern Japan.

Limitations

  • Survey conducted during the rainy season when aerosol resuspension is relatively low; inhalation doses may be underestimates in drier conditions.
  • All meal samples purchased outside the 20-30 km planned evacuation zone; conditions within heavily restricted areas are not represented.
  • Consumer food sourcing patterns from outside the contamination zone (imported foods) were not assessed; actual resident exposure may differ.
  • Sample sizes per site are small (n = 10 per city for meals).
  • Cross-sectional measurement; no longitudinal trend data.

Update history

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