The human body always contains radioactive substances
Our environment contains radioactive substances of both natural and artificial origin. They enter the human body in food, drinking water or inhaled air. In the body, radioactive substances expose a person to internal radiation. The radioactivity in the human body can be measured directly with a whole body counter or indirectly from human secretions. The monitoring of radioactivity in the human body is part of environmental radiation monitoring.
Radioactive substances enter the human body in food, drinking water and inhaled air. The body always contains naturally occurring radioactive substances. Potassium-40 is an example of naturally occurring radioactive substances in the human body. Naturally occurring potassium always includes a standard portion of the radioactive potassium-40 isotope. An adult person usually contains 3,000–6,000 becquerels (Bq) of potassium-40. In addition, naturally occurring uranium and thorium from the bedrock, as well as their decay products, are found in the human body. The best known and, with regard to the radiation exposure of Finns, the most important decay product is radon, a substance that belongs to the decay series of uranium. Radioactive substances enter the human body in food, drinking water and inhaled air.
In addition, we receive radioactive substances originating from the nuclear fallout accidents in food. The long-lived radioisotopes cesium-137 (half-life 30 years) and strontium-90 (28 years), originating mainly from the long-distance exposure of atmospheric nuclear weapon tests before 1963, are still present in small quantities in the environment. As a consequence of the 1986 Chernobyl nuclear accident, people are still receiving cesium-137 in food. Cesium-134, originally also present in the fallout, has nearly disappeared by now, due to its short half-life (two years). Immediately following the accident, Finns received small amounts of radioactive iodine-131 in milk and inhaled air.
Small amounts of radioactive substances are also released into the environment from controlled releases of nuclear power plants and reprocessing plants. However, the amounts of radioactive substances released from normally functioning nuclear power plants are so low that they are insignificant for people.
Radioactive substances behave differently
The absorption, retention and distribution of radioactive substances in various organs and tissues as well as their elimination from the body are influenced by their chemical form, solubility and particle size. In the event of an accident, some of the radioactive substance that has come into contact with the skin may be absorbed into the body through the skin or wounds and may also cause an internal radiation dose.
Absorption of ingested radionuclides occurs mainly in the small intestine. The transport and sticking of inhaled particles to different parts of the respiratory system depends on the nature and size of the particles. Some of the particles in the lungs are transported by cilia to the throat, after which they behave like swallowed material.
Of the radioactive substances, cesium and potassium are mainly transported to human muscles. Strontium, like calcium, enters the skeleton and radioactive iodine enters the thyroid gland. The elimination of substances from the body is affected by their circulation in the body. The rate of leaving the body is described by the biological half-life. It is the time in which half of the substance is excreted. For example, the biological half-life of cesium-137 in an adult is on average 110 days. Biological half-lives are shorter in children than in adults.
In addition to excretion, the amount of radioactive substance in the body decreases as a result of radioactive decay. It is difficult to remove radioactive substances from the body. The substances used for removal are quite ineffective and, in addition, toxic in large quantities. The exception is radioactive iodine, the amount of which in the thyroid gland can be effectively reduced by pre-administration of iodine tablets containing non-radioactive iodine.