Dr. Tsubokura's Radiation Lecture Vol.16

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

31. The exposure dose determines the impact on the body

External exposure occurs when your body is exposed to radiation from an external source, and internal exposure occurs when you are exposed to radiation from radioactive substances that you have taken into your body. The total amount of this external and internal exposure determines its health effects on your body.Although internal radiation exposure is often assumed to be much more dangerous than external exposure is, the influence of radiation exposure is determined not by the type of exposure but by its total dose.

To compare the influence of "internal" and "external" radiation on the same measurement, we use a unit called the sievert. The magnitude of radiation as measured using this unit enables us to predict its impact on our body.When the doses of external and internal radiation exposure are 100 and 1 millisievert, respectively, as the values indicate, the level of external radiation exposure is higher.

Because radiation-contaminated foods have been strictly controlled in Fukushima since the 2011 nuclear power plant accident, external radiation has been a predominant cause of current exposures (at least 90% or more).

32. Level of external exposure is dropping year by year

External exposure occurs when your body is exposed to radiation from an external source. Because the level of internal exposure has been kept low via the strict control of contaminated food in Fukushima since the 2011 nuclear disaster, "external radiation" is known to be the predominant source of current radiation exposure.

However, the level of external exposure, a major source of current radiation exposure, has been decreasing year by year. Four and a half years after the accident (this article was published in 2015), the findings of a survey assessing the extent of external radiation exposure among school children at primary schools in the northern coastal area of Fukushima revealed that the proportion of the participants with additional annual external exposure exceeding 1 millisievert was within several percentages.

In addition, the air radiation dose rate, an important cause of radiation exposure, has gradually decreased. For example, the air dose rate in the entire Soma City, a municipality in So-so District, in 2015 was approximately 30% of that in June 2011. The reasons for this are that the half-lives of Cesium 134 and 137 are two years and 30 years, respectively, and that rain (weathering) washed the radioactive substances away from the environment. ---------------------------------------------------------------------------------------------------------------------------------------

The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, on August 8th, 16nd 2015, and was reproduced for MRIC Global under the author's permission.