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What radionuclides can be found in food?
- All foods contain natural radionuclides.
- In addition, the Chernobyl accident and the atmospheric nuclear weapons tests gave rise to artificial radionuclides in food.
- The natural radioactivity in foods contributing to the radiation exposure to man is mainly caused by the potassium isotope Potassium-40 and the long-lived radionuclides of the uranium-radium decay chain and the thorium decay chain.
- Among artificial radionuclides, especially caesium-137 is important for the radiation exposure to man.
All foods contain natural radionuclides. The natural radioactivity in foods contributing to the radiation exposure to man is mainly caused by the potassium isotope potassium-40 and the long-lived radionuclides of the uranium-radium decay chain and the thorium decay chain. Relevant are
- Polonium-210 and
- The thorium isotope Thorium-230, Thorium-232 and Thorium-228.
Among artificial radionuclides, caesium-137 plays an important role as to food from forests.
How do plants and animals take up radionuclides?
Radionuclides have, in part, similar chemical characteristics as nutrients. Therefore, plants and animals also take up radioactive matter with the nutrients they need for growth. The level of specific activities in foods depends on the radioactivity content of the used source media (soils, water), the availability of the nutrients and other substances from soil and water, and on other conditions at the site of the plant or animal production. Certain plants or parts of them, such as Brazil nuts, and mushrooms, such as wood hedgehog, enrich radionuclides to a high degree.
Foods can also be radioactively contaminated via the air. From soils and rocks, for example, radon-222 gets into the atmosphere and decays into its radioactive but not gaseous decay products. These can deposit on leaf surfaces and be taken up in the leaves. Of special importance are here the long-lived decay products of radon-222, the radionuclides lead-210 and polonium-210 of which higher specific activities particularly occur in leafy vegetables. Radionuclides deposited from the atmosphere onto plant-based food and animal feed also after the Chernobyl accident and the atmospheric nuclear weapons tests.
Compared with muscle meat, offal - especially livers and kidney - has higher levels of natural radionuclides, because these organs filter the harmful substances in the metabolism. Natural radionuclides can also accumulate in fish and shell fish (among others in the soft tissue of clams, prawns, lobster and shrimp), in particular lead-210 and polonium 210.
Where do the radionuclides deposit?
Depending on the plant species and the respective development and nutritional condition of the plant at the time of harvesting, the mineral distribution in the plant parts varies. These distributions influence the specific activities in plant-based foods. For example, the specific activities of the radium isotopes radium-226 and radium-228 in cereal grains are higher than those in vegetables or fruit.
Potassium: Engine of the human metabolism
In the human body, a constant portion of potassium must be constantly present in order for the metabolism to work. Hence, the human body regulates its potassium content continuously. We can cover our potassium need entirely with our diet. The element potassium contains naturally occurring 0.0117 per cent potassium-40 with a specific activity of 30.92 becquerel per gram of potassium. Since this share is always the same, the potassium-40 activity can be calculated. Depending on age, gender and other factors, the potassium-40 activity of the human body is between about 40 and 60 becquerel per kilogram of body weight. The effective dose due to potassium-40 is on average 0.165 millisievert per year for adults and 0.185 millisievert per year for children.
Potassium is vital for all organisms and considerable amounts of it are mostly present in them. In plant-based foods, specific activities between 50 becquerel per kilogram of fresh mass can be found in fruit and 380 becquerel per kilogram of fresh mass in ripe peas or beans. The values are similar for products of animal origin (ca. 50 becquerel per litre in cow milk up to ca. 100 becquerel per kilogram of fresh mass in muscle meat, liver and kidneys of cattle). As a result of their manufacturing processes, milk powder and smoked sausage contain higher activities (above 180 becquerel per kilogram of fresh mass). The activity level in edible mushrooms varies between a few 10 and a few 100 becquerel per kilogram of fresh mass.
Caesium-137: Legacy of the Chernobyl accident and the atmospheric nuclear weapons tests
The Chernobyl accident particularly affected Southern Germany. Earlier, the atmospheric nuclear weapons tests gave rise to a widespread radioactive contamination of Germany. Today, food produced in Germany shows caesium-137 levels of only a few becquerel per kilogram of fresh mass or less. However, some species of edible wild mushrooms and game, in particular wild boars, might show significantly higher contamination levels.
State of 2017.04.12