The concentration of different radionuclides should be measured (Bq/ kg) and the "activity concentration index" calculated. The calculated value should be declared with the CE marking only when the value is above a threshold given in the specification. The threshold could for example be the same as the gamma dose present in the earth's crust, which would mean, "effectively zero" (some thresholds for natural radioactivity have been presented in a guidance developed by an expert group established under the terms of Article 31 of the Euratom Treaty).
