Natural radioactive substances can be found everywhere in our environment – for example, in the rocks of the earth’s crust, in soils and water, and in the atmosphere.

The – harmless – dietary contribution to human radiation dose stems principally from potassium-40, in addition to carbon-14, rubidium-87, uranium-238, uranium-234, radium-226, radium-228, lead-210 and polonium-210, as well as thorium-230, thorium-232 and thorium-228.

In Europe, the Chornobyl reactor accident (Chernobyl in Russian) in 1986 and above-ground nuclear weapons testing until 1980 led to the dispersal of artificially generated radioactive substances by wind and weather over large parts of Europe.

Today, caesium-137 in particular is still a factor in foodstuffs from Germany, with regional variations. Especially food from the forest, such as mushrooms or wild boar, can have elevated levels of caesium-137 in some regions. Agricultural products exhibit a consistently low specific activity (i.e. the ratio of the activity of a radionuclide to the mass of the material in which the radionuclide is distributed).

Plants also absorb radioactive substances contained in the soil via their root system together with the nutrients they need to grow. Along with the water needed for life, plants also absorb radioactive substances that are dissolved in it.

Foods of plant origin can also be radioactively contaminated via the air. For example, radon-222 is released into the atmosphere from soils and rocks and decays into its radioactive decay products, which attach themselves to suspended matter. These can be deposited on leaf surfaces and partially absorbed into the leaves. Deposition on above-ground parts of plants can also be caused by authorised discharges of radioactive substances with exhaust air, the irrigation of agricultural crops, and dust stirred up from the soil.

Artificial radionuclides from the atmosphere were deposited on food and animal feed in Germany following the Chornobyl reactor accident and above-ground nuclear weapons testing.

Foods of animal origin

Depending on the plant species and the respective developmental and nutritional state of the plants at the time of harvest, the minerals are distributed differently in different parts of the plant. These distributions influence the specific activities in foods of plant origin. Examples include:

• The specific activities of radium-226 and radium-228 are higher in cereal grains than in vegetables or fruit.
• Long-lived decay products of radon-222, and the radionuclides lead-210 and polonium-210, may be present at higher specific activities due to deposits on leaf surfaces, especially in leafy vegetables.
• The specific activities of potassium-40 are between 50 becquerels per kilogram of fresh mass in fruit and 380 becquerels per kilogram of fresh mass in ripe peas or beans. In edible mushrooms, the potassium-40 content varies between just over 10 and several hundred becquerels per kilogram of fresh mass.
• Certain plants or parts thereof, such as Brazil nuts and some species of fungi (e.g. Hydnum repandum), can accumulate high levels of specific radionuclides.

Foods of animal origin Source: Elena Schweitzer/Stock.adobe.com

• Natural radionuclides can also accumulate in fish and seafood (including in the soft tissue of mussels, prawns, lobsters and shrimp) – particularly in the case of lead-210 and polonium-210.
• The specific activity of potassium-40 in products of animal origin is between about 50 becquerels per litre in cow's milk and about 100 becquerels per kilogram of fresh mass in cattle's muscle meat, livers and kidneys. Milk powder and long-life sausage products contain higher activities (over 180 becquerels per kilogram of fresh mass) due to their manufacturing processes.
• Innards, and especially livers and kidneys, have higher levels of natural radionuclides compared to muscle meat, as these organs filter out and accumulate pollutants during metabolism.
• High levels of caesium-137 (several thousand becquerels per kilogram or more) can be measured in the meat of wild boar in some regions of Germany. However, caesium-137 is subject to a limit of 600 becquerels per kilogram in food, meaning that food contaminated to a higher level must not be sold.