Dr. Tsubokura's Radiation Lecture Vol.152

Author: Masaharu Tsubokura M.D., PhD.

Editors: Yudai Kaneda


303 Radioactive substances are always found in our bodies.

November 7, 2020

Exposure to radiation emitted from radioactive substances outside the body is called “external exposure,” and exposure to radiation emitted from those taken into the body by inhalation or eating is called “internal exposure.”


The whole body counter is used to test for internal exposure. When there are radioactive substances in our bodies, a little bit of the radiation emitted from them is released from the body. This is detected by a machine, and the amount of radioactive substances in the body is estimated based on the amount of radiation coming out of the body. Various types of machines have been used to do this, such as those that allow the patient to stand or sit and even those specially designed for infants.


In fact, radioactive substances can always be detected from any person tested. This is because our bodies contain a naturally occurring radioactive substance called radioactive potassium. Some people are surprised to hear that radioactive potassium was detected when an infant was tested with a whole body counter.


The amount of radioactive potassium in the body depends on the person’s body size and muscle mass. It is about 60 times the body weight. A person who weighs 60 kg will have about 4000 becquerels of radioactive potassium in their body.


304 Our body has a certain amount of potassium.

November 14, 2020

Potassium is a mineral essential for sustaining life. By interacting with the sodium contained in salt and such, potassium plays a role in regulating the amount of water in cells, muscle contractions, and nerve excitation.


Potassium is crucial to our bodies, so too much or too little can be a problem. The kidneys regulate the amount of potassium in our bodies to keep it constant, so people with renal function disorders should be careful not to take in too much potassium. On the other hand, since potassium is found in many foods, running short of it is not very common in normal life, except in cases of repeated diarrhea or vomiting.


Radioactive potassium (potassium-40) is contained in potassium at a proportion of 1/10,000. The amount of potassium in the body is regulated by our diet and excretion to remain constant. Therefore, a certain amount of radioactive potassium will also always be in the body. The annual radiation exposure we experience from this ever-present radioactive potassium is 0.18mSv.


Radioactive cesium does not remain in the body forever. Even if radioactive cesium is detected in the body, it will be excreted in the urine and gradually disappear.

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