Contamination of mushrooms and wild boar with radioactive caesium-137

For many years there have been reports of radioactive caesium-137 (Cs-137) that is sometimes detected in large quantities in mushrooms and wild boar, especially in regions of southern Germany. Just in time for the start of the mushroom season, there have again been more reports of this in recent weeks. Where does the substance come from and why is it still detectable in larger quantities in mushrooms and wild boar, while it is decreasing overall in the ecosystem?

What is caesium-137?

Cs-137 is a so-called artificial radionuclide. This refers to radioactive types of atoms that have not formed naturally but have been created through technical processes. For example, Cs-137 is produced during the nuclear fission of uranium-235 in a nuclear reactor or during the explosion of an atomic bomb. It has a half-life of about 30 years and emits beta radiation as it decays.

Where does the caesium-137 come from?

Cs-137 entered the atmosphere i.a. during the nuclear accident at Chernobyl but also in the course of above-ground nuclear weapons tests in the 1950s and 1960s. Depending on the weather conditions during the release (e.g. wind direction and strength), the isotope could move over long distances in the atmosphere and thus be washed out of the air with precipitation even in more distant areas. In the case of Chernobyl, for example, Cs-137 reached Western Europe and was deposited on the ground there. The contamination was very different locally, depending on the respective weather conditions. For example, significantly more Cs-137 was deposited in south-eastern Bavaria than in the rest of Germany (see BfS overview map of soil contamination with Cs-137 in 1986).

How does the caesium-137 get into mushrooms and wild boar?

In forest soils, Cs-137 remains bound in upper soil layers for a longer time than is the case with soils that are used for agriculture. This is due, among other things, to the fact that the soils are mineralised differently. Some types of fungi can absorb and store the Cs-137 present in the forest soil very easily, and so the isotope can enter the human food chain either through the fungi themselves or via wild boars, which have such fungi on their menu.

Over the years, the caesium in the soil migrates deeper and deeper, making it harder for it to find its way into fungi and plants. An exception are mushrooms such as the deer truffle, which grows underground and accumulates Cs-137 in its flesh. Wild boars dig it up in the winter months in search of food. This in turn increases the likelihood that they will ingest more Cs-137 - compared to the summer months with other food components.     

Recent study: Relevance of nuclear weapons tests as origin of caesium-137 greater than previously assumed

So far, it has been assumed that the Cs-137 contamination detected in wild boar is mainly the result of the Chernobyl nuclear accident. A recent study by researchers from the University of Hanover and the Vienna University of Technology, published in the journal Environmental Science & Technology, now suggests that a larger proportion of the Cs-137 (up to 68 per cent) probably originates from above-ground nuclear weapons tests in the 1950s and 60s.

To find out the origin of Cs-137, the researchers had determined the ratio of the caesium isotopes Cs-135 and Cs-137. The two types of atoms differ in the number of neutrons they have in their nucleus, i.e. they have different weights. The quantitative ratio of these isotopes represents a kind of "radiological fingerprint" that makes it possible to prove the origin of the caesium (nuclear accident or atomic bomb explosion).

How dangerous is caesium-137 for humans?

Radioactive caesium-137 is found i.a. in certain wild mushrooms and can accumulate in the muscle tissue of wild animals and enter the human food chain via these pathways. A maximum value of 600 becquerels per kilogram fresh mass (Bq/kg) applies to foodstuffs intended for trade in Germany (see also Federal Office of Consumer Protection and Food Safety). This maximum value applies throughout Europe. For dairy products and for food for infants and young children, a maximum value of 370 Bq/kg applies.

In its latest survey, the Federal Office for Radiation Protection (BfS) determined values of more than 1,000 Bq/kg for some wild mushroom species (see Mushroom Report). Significantly higher values of up to 15,000 Bq/kg were also detected in wild boar meat in the above-mentioned survey. According to the BfS, the consumption of 200 grams of mushrooms with 2,000 Bq/kg caesium-137 leads to a radiation dose of about 0.005 millisievert. To put this into perspective, a flight from Frankfurt to New York results in an effective dose of 0.032 to 0.075 millisieverts.  

The measured values on mushrooms collected by the BfS differ considerably depending on the region. On the one hand, Cs-137 was deposited more strongly in some areas than in others due to weather conditions. For example, higher concentrations of Cs-137 can be detected in parts of the Bavarian Forest or the Alpine region. In addition, the composition of the soil and the respective type of fungus also play a role in the uptake of Cs-137. Accordingly, the exposure of game can also vary. In addition to the particularly polluted south of Germany, measurements in other federal states have also shown elevated values in the past, as shown e.g. in a study from Rhineland-Palatinate. Data collected by the University of Göttingen in Lower Saxony and Hesse in 1986-89 also showed that wild boar meat was contaminated with between 6 and 5,425 Bq/kg.

Since the above-mentioned limits of 600 and 370 Bq/kg, respectively, only apply to products intended for trade and not to mushrooms and game collected and hunted by the consumers themselves, the BfS advises: "Anyone wishing to reduce their personal radiation dose should refrain from excessive consumption of game and mushrooms collected and hunted by the consumers themselves in the more highly contaminated areas of Germany." In normal quantities, however, the additional radiation dose from eating wild mushrooms and game is comparatively low.