Radioactive caesium in wild mushrooms: the quantity is decisive

Wild mushrooms may contain radioactive caesium. Source: pikselstock/stock.adobe.com

Almost 40 years after the reactor catastrophe in Chornobyl (Russian: Chernobyl), many people still ask whether it is once again safe to collect wild mushrooms in summer and autumn. From the point of view of radiation protection: yes, it is.

In some regions of Germany, wild mushrooms may still have elevated levels of radioactive caesium-137. However, the radiation dose depends on the quantity of mushrooms consumed: As the Federal Office for Radiation Protection (BfS) explained on the occasion of the publication of its current mushroom report, a moderate consumption is safe everywhere in Germany.

According to the BfS, some mushroom varieties may still exceed the limit value of 600 Bq caesium-137 per kilogram of fresh mass – especially those found in the Bavarian Forest and the adjacent areas, in Donaumoos south-west of Ingolstadt, in the Mittenwald region, and in Berchtesgadener Land. This limit applies to mushrooms sold commercially but not to mushrooms collected in the wild.

The high levels of caesium-137 are due mainly to the reactor accident at Chornobyl in 1986. At that time, large quantities of radioactive materials were spread over Europe with the air. Compared with the rest of Germany, the aforementioned regions were affected by particularly high levels of caesium-137 fallout. In addition, wild mushrooms contain caesium-137 that was released in the aboveground nuclear weapons tests of the 20th century.

Measured values vary greatly

In its mushroom report, the BfS publishes current measured values annually. To this end, experts investigate the caesium-137 content of wild-growing edible mushrooms from selected locations. Depending on the type of mushroom and the caesium contamination of the soil at the collection site, there are clear differences.

In the measurements of the past three years (2021–2023), the highest values were recorded in the hedgehog mushroom (Hydnum repandum) and the terracotta hedgehog (Hydnum rufescens). Individual samples of these mushrooms contained over 4,000 Bq of caesium-137 per kilogram of fresh matter.

The BfS detected values of more than 1,000 Bq caesium-137 per kilogram at the investigated locations in various woodwax species (Hygrophorus), yellowfoot (Craterellus tubaeformis), bay bolete (Imleria badia), tawny grisette (Amanita fulva), dove-coloured tricholoma (Tricholoma columbetta), blackening brittlegill (Russula nigricans), and Lepista glaucocana.

Moderate consumption leads only to a low radiation dose

Dr Inge Paulini

"Even though some mushrooms collected in the wild can still exceed the limit for mushrooms sold in markets or supermarkets, as long as they are consumed in moderation, they lead only to a small additional dose of radiation", explains BfS President Inge Paulini. "In addition to the amount of caesium contamination of the mushroom, the quantity you eat plays a major role."

Paulini stresses that the additional radiation dose considered acceptable is a highly personal decision. An example calculation could help: "An adult who consumes 200 g of mushrooms containing 2,000 Bq caesium-137/kg every week receives an additional radiation dose of 0.27 mSv/year. That's about as much as about 20 flights from Frankfurt am Main to Gran Canaria."

Potentially highly contaminated mushroom varieties can be avoided

"If you want to keep your additional radiation dose as low as possible when collecting mushrooms, you should avoid potentially heavily contaminated mushroom varieties in the areas of Germany that were particularly affected by Chornobyl", says Paulini. "There are many tasty alternatives."

For example, even at the most contaminated sampling sites, all measured values of the following species remained below 5 Bq per kilogram of fresh matter during the investigation period:

• the prince (Agaricus augustus)
• bleeding agaricus (Agaricus fuscofibrillosus)
• chestnut bolete (Gyroporus castaneus)
• woodear (Auricularia auricula-judae)
• giant polypore (Meripilus giganteus).

Soil contamination with caesium-137 in 1986. The current values can be determined by multiplying the figures by 0.41.

Measurements of other mushroom varieties can be read in the mushroom report of the BfS. The report also provides information on which regions of Germany are still particularly affected by the reactor accident at Chornobyl. The report is available for download at www.bfs.de/pilzbericht in the digital on-line repository and information system (DORIS) of the BfS.

Hardly caesium-137 in farmed mushrooms

All mushrooms sold commercially must comply with the limit value of 600 Bq caesium-137 per kilogram. Because they are grown on substrates that have hardly any radioactive caesium, mushrooms from commercial mushroom farms (e.g. button, oyster, and shiitake mushrooms) generally contain little caesium-137.

Caesium-137 is a radioactive isotope of caesium that does not occur naturally. Among other things, it is produced during nuclear fission in nuclear power plants. It has a half-life of approx. 30 years. This means that the amount of caesium-137 deposited on the ground in Germany in 1986 has decreased by more than half.