The Rn and thoron daughter concentrations in different atmospheres are discussed. If the daughter concentrations are expressed in WL [working levels], calculations show that the thoron daughter concentration may well exceed the Rn daughter concentration. This was also verified by a few experimental results in basements. In dwellings, lining materials on walls and floor act as a shield for the thoron exhalation, but even in such houses the thoron daughter concentration may be significant.