**5.13. Healing earths**

Minerals and sediments consisting mainly of silicon dioxide (quartz) are known as siliceous earths. These fine, floury mineral mixtures are deposits of the silica shells of diatoms, the main constituent of marine phytoplankton. The dead cells sink to the ocean floor and form sedi‐ ments. These layers of sediment are extracted in numerous mines all over the world. Siliceous earths have a wide variety of uses in, for example, the pharmaceutical and food industries (e.g., as a food supplement). Due to their special structural properties, foreign atoms and ions are incorporated during sediment formation, such as radionuclides of the natural decay series of uranium and thorium [86].

In 2008, we collected some siliceous earth products on the Swiss market and analysed them with γ-spectrometry. In two products, the limit value for natural radionuclides was exceeded (50 Bq/kg). Furthermore, the annual dose by regular consumption of one product reached half of the permitted yearly dose of 1 mSv. The Federal Office for Health Products Control, Swiss Medic, complained about these products. The company involved then withdrew the product from the market [86]. An inspection of the products in 2010 showed that two products from one producer in Germany slightly exceeded the limit value. Higher levels of 226Ra and 228Ra were the reason for this. The estimation of the received annual dose from the consumption of the product according to recommended amount per day as advised on the information leaflet enclosed would lead to 0.1 mSv/year. Healing earths, and also silica-based chemicals of chemical laboratories, remain a source for natural radionuclides [87].
