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Dr. Tsubokura's Radiation Lecture Vol.31

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

61. Dosimeters are designed to prioritize safety

External radiation exposure occurs when the body is exposed to penetrating radiation from an external source. Some of you may have experience measuring this using cumulative dosimeters.

Cumulative dosimeters are set to display the same or higher air dose rate as the actual level of radiation exposure one would experience in one place. These devices are designed in such a way as to prioritize the safety of those who carry them, as the health effects of radiation on specific individuals are slightly different depending on their age and body shape as well as the direction from which radiation reaches them.

For example, radiation exposure from the front side has greater adverse health effects on the body than exposure from the back side since we have more muscles on the posterior side to protect us from the penetrating radiation.

As such, the actual level of daily radiation exposure is smaller than the value obtained by a simple calculation of the values obtained by the dosimeters.

62. The reference value is not the border line between safe and unsafe levels of radiation exposure

External exposure refers to radiation exposure from sources outside of our bodies. Some of you may have heard the value of 0.23 microsieverts/hour.This is used to determine whether to conduct decontamination work or radiation contamination surveillance.

This value was estimated to meet the annual radiation dose limit, which is 1 millisievert (= 1000 microsieverts) per year. The annual dose limit is converted into a limit dose per hour, assuming that people spend 16 hours of their time inside of buildings and 8 hours outside.

Although it is appropriate to use the reference value as a criterion for conducting decontamination and radiation surveys, this 0.23 microsievert value is not a border line that determines safe and unsafe radiation levels.

Moreover, the annual radiation dose limit of 1 millisievert/year is a significantly low level and will not cause any health problems such as cancer.


The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, on Mearch 6th and 13th 2016 was reproduced for MRIC Global under the author's permission.

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