Current radioactivity content of wild edible mushrooms - a candidate for an environmental reference material

After the Chernobyl accident, health and consumer protection policies have led to directives and regulations regarding maximum levels for radionuclides in foodstuffs or intervention levels to regulate food trade, but no recommendations of generic action levels for environmental materials were given. Many studies on forest ecosystems were carried out in the past; however, data on European mushroom radioactivity after 1996 and especially nowadays are limited. Presently, 90Sr and 137CS are the main anthropogenic radionuclides contributing to internal dose and the most typical radioactive contaminants of food products. Thus, because mushrooms have the tendency to accumulate and concentrate several trace elements and radionuclides, in some countries where the collection of wild fungi is tradition the consumption of wild mushrooms could still provide a substantial intake of radionuclides. The objective of the corresponding action carried out by the Institute for Reference Materials and Measurements (IRMM) is to develop and produce certified reference materials for quality assurance of radionuclide measurements in environmental samples with special emphasis on food samples. To decide on the activity concentration level required in the candidate reference material, a study was launched to evaluate the concentration of radionuclides in several mushroom samples collected in the fall of 2004 in different parts of Europe (8 countries, 18 sites). Before any chemical manipulation, the air-dried mushroom samples were measured by non-destructive low-level γ-ray spectrometry. For determination of the 90Sr activity concentration sample preparation (ashing and acid leaching) followed by radiochemical separation based on extraction chromatography and low-level liquid scintillation counting were performed. Almost 20 years after the Chernobyl accident 137CS is still confined to the upper 5-10 cm of the soil layer (and through the mushroom mycelia present in the fruiting body), while 90Sr. migrated deeper in undisturbed soils (usually being below detection limit in the mushroom samples). The levels of 137CS in mushrooms varied widely ranging from 0.5 Bq/kg to 4300 Bq/kg on dry mass basis, while those of natural 40K were relatively constant. The median concentrations of 137CS and 40K were 603 and 1087 Bq/kg dry mass, respectively. In one sample 60Co of unknown origin, was detected (24.9±2.2 Bq/kg dry mass).