Dr. Tsubokura's Radiation Lecture Vol.153

Author: Masaharu Tsubokura M.D., PhD.

Editor: Yudai Kaneda


305 Potassium, existing for a long time

November 21, 2020


Radioactive potassium (potassium-40) is found in the mineral potassium, which is essential for sustaining life, at a rate of 1/10,000. Radioactive potassium is always found in the body in a certain amount, and we are exposed to about 0.18 millisieverts of radiation per year.


In addition to potassium-40, carbon-14 and tritium (also called hydrogen-3) are other naturally existing radioactive substances in our bodies.


Carbon-14 is created when neutrons produced by the collision of cosmic rays and the atmosphere encounter nitrogen, which comprises about 80% of the air. Carbon-14 is converted back into nitrogen by emitting beta rays. Tritium is a radioactive substance that is both natural and artificial.


Each of these radioactive substances has its own half decay period. Tritium’s is about 12 years, carbon-14’s is 5,730 years, and potassium-40’s is 1.3 billion years. Potassium-40 is a radioactive substance introduced to the Earth from outer space when the Earth was born, so it has existed for a long time. The amount of potassium-40 has remained almost the same throughout our existence.




306 Radioactive substances in food

November 28, 2020


Radioactive potassium, a radioactive substance in nature, is always found in our bodies at a certain level, and we are exposed to about 0.18 millisieverts of radiation per year. Since we are exposed to radiation from radioactive substances in our bodies, we are internally exposed to radioactive potassium. In addition to this radioactive potassium, we ingest radioactive substances such as carbon-14 and tritium (also called hydrogen-3) through our mouths and are exposed internally.


Among the various radioactive substances in nature, lead-210 and polonium-210 account for a large percentage of the internal exposure of Japanese people.


These are radioactive substances created when radioactive radon in the atmosphere changes, but they sink into rivers and oceans and are taken into our bodies through seafood. This is unique to the Japanese diet, which is rich in seafood, and differs from that of Western countries.


We are exposed to about 0.8 millisieverts of radiation per year from lead-210 and polonium-210. This is more than we are exposed to from radioactive potassium and accounts for about 40% of the radiation we receive from our surroundings, including both external and internal exposures.

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