[en] The atmosphere contains dust particles with a radioactivity largely dependent on short lived decay products from ₂₂₂Rn. This report considers the accuracy of two methods of measuring the concentration of radioactivity. In the three step method the radioactive decay of a sample is measured three times; in the fitting method more times. It is concluded that the concentration cannot be determined with great certainty but that a reasonable determination of the equivalent equilibrium concentration is possible. As long as the measurement intervals for the three step method are well chosen, both methods are comparable. However, due to the result of the fitting method being less dependent on the measuring parameters, it is given preference. (C.F.)