Japan’s National Institute of Health Sciences measured radioactive strontium-90 in 53 foods collected after the Fukushima Daiichi accident, split by whether they also carried accident-derived radiocesium. Strontium-90 was detected at low activities in both groups and stayed within ranges recorded before the accident, so the authors found no clear accident-related increase. Observed strontium-90 to cesium-137 ratios were below those assumed when Japan set its radionuclide food limits, except in whole freshwater fish.
Key numbers
- Strontium-90 was detected in 25 of 40 radiocesium-positive samples and 8 of 13 radiocesium-negative samples (Abstract; Results, p. 5).
- Detected strontium-90 in positive samples: 0.028 to 0.87 Bq/kg fresh weight and 0.42 to 3.7 Bq/kg dry weight; lowest LOD 0.02 Bq/kg (Results, p. 5; Table 2, p. 3).
- Brown rice 0.036 to 0.071 Bq/kg; rice bran 0.27 Bq/kg; crude wheat 0.057 and 0.16 Bq/kg; wheat bran 0.57 Bq/kg; soybean 0.28 Bq/kg; beef 0.028 and 0.081 Bq/kg (two of five detected) (Table 2, p. 3; Results, p. 9).
- Whole freshwater fish (pond smelt, loach) 0.073 to 0.15 Bq/kg in positive and 0.028 to 0.17 Bq/kg in negative samples (Table 2 and 3; Results, p. 8).
- Dried mulberry leaf tea 3.7 Bq/kg dry weight (positive) and 2.0 to 5.5 Bq/kg dry weight (negative); dried shiitake harvested in 2010, before the accident, 0.88 Bq/kg dry weight (Tables 2 and 3, pp. 3 and 5).
- Observed strontium-90 to cesium-137 ratios were 0.00044 to 0.0013 in brown rice and 0.000043 to 0.000067 in beef, below the estimated ratios behind the limits; whole freshwater fish ratios of 0.0039 to 0.025 exceeded the estimated 0.000024 (Table 5, p. 7; Results, pp. 8 to 9).
- Using an adult dose coefficient of 2.8 x 10^-5 mSv/Bq for strontium-90, the maximum committed effective dose ratio of strontium-90 to radiocesium was 0.063, below the 0.5 assumed for the limits (Results, p. 8).
- Japanese standards from 1 April 2012 limit radiocesium to 100 Bq/kg in foods other than drinking water, milk and infant foods, set so that dose from the target radionuclides would not exceed 1 mSv per year (Introduction, p. 1 to 2).
Methods (brief)
Radiocesium was screened by HPGe gamma spectrometry. Samples were ashed at 600 degrees C, strontium was separated by cation exchange or ferric chloride co-precipitation following Japanese radioactivity measurement series methods, and strontium-90 was quantified from its yttrium-90 daughter on a low-background gas-flow counter, corrected for strontium recovery (mostly above 75 percent). IAEA reference materials were analyzed to validate the method.
Evidence fitness
Supports measured strontium-90 activities in Japanese staple foods after Fukushima, their comparison with pre-accident survey ranges, and the design logic of Japan’s radionuclide food limits. It does not address stable strontium or establish dose-response relationships.
Limitations
Most radiocesium-positive samples were non-commercial and most negative samples commercial, so the groups differ in more than contamination. Many results sit near the detection limit, and the flatfish result is a reference value only because of low recovery. Whole-body fish include bone, which concentrates strontium. Pre-accident comparison data come from government databases without stated detection limits.
Related evidence
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