High levels of the unattached 212Pb fraction of the 220Rn decay products in the atmosphere of the monazite storage facility were found: f1 = 0.3-0.5 at the aerosol concentration of 20 000 cm-3 and 0.1-0.16 at aerosol concentration ∼140 000 cm-3. It is shown that the sampling of aerosols in an atmosphere with a high concentration of thoron result in the local change of the equilibrium between the attached and unattached fractions near the sampling point. As a result, the measured value of the unattached fraction of thoron decay products may differ by several orders of magnitude from the mean value of unattached fraction the in the indoor atmosphere. A local increase of the unattached fraction 212Pb can take place in the worker's breathing zone in the atmosphere with a high concentration of thoron and it should be taken into account in the assessment of the inhalation intake of the thoron decay products for workers.
