Dr. Tsubokura's Radiation Lecture Vol.46

July 11, 2019

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

 

91. The level of radioactive cesium below the reference value is safe

The reference value for radioactive cesium in fish and sea products is determined as 100 becquerel/kilogram: This is the same value as that of agricultural products. This reference value is designed in the following manner: The total internal radiation contamination, both from radioactive cesium and other radioactive substances such as strontium, should not exceed 1 millisievert/year.

 

As with agricultural products, among other radioactive substances in sea products, we must pay the most attention to radioactive cesium. This is because radioactive cesium causes a large part of the internal exposure received from ingesting contaminated food.

 

Although it is factually incorrect, the reference value for radioactive cesium in seafood is determined with the assumption that the level of internal contamination from radioactive cesium is the same as other radioactive substances.

 

Overestimating the level of internal contamination received from radioactive substances other than cesium is meant to ensure food safety; there should be no health problems as long as the level of radioactive cesium is kept below the reference value.

 

 

92. Contamination was not detected from the surface-dwelling fish

Although the overall level of radioactive contamination in seafood continues to decrease, as with agricultural products, the absorbed contamination of radioactive substances tends to remain in some species, but this is not the case with other species.

 

Among different types of fish, radiation contamination has not been detected in small fish such as whitebait and Japanese sand lance (Konago).

 

Whitebait and sand lance reside on the ocean surface and feed on plankton. Immediately after the nuclear accident, radioactive substances leaked into the ocean and spread to broad areas. As a result, the surface-dwelling fish were severely affected by radiation contamination. Since it has been more than five years since the nuclear power plant accident (this article was published in October 2016), the radioactive substances have sunk to the ocean floor. Additionally, whitebait and sand lance have changed their generations at rapid rates. Because of these reasons, the contamination of surface-dwelling fishes is no longer detectable.

 

Some of you may remember a ton of news reports warning about the high levels of contamination exceeding the reference value detected from whitebait and sand lance immediately after the nuclear accident. Although these fish were most notorious for having a high contamination level, they now have a contamination level below the reference value in all areas of Japan, including Fukushima Prefecture.

 

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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, on October 2nd and 9th 2016 was reproduced for MRIC Global under the author's permission.

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