[en] The amounts of the three short-lived daughters of radon on the furnace air-cleaning filter from a house with a high level of radon were estimated to be 8.2, 33, and 38 kBq (0.22, 0.89, and 1.03 μCi) for ²¹⁸Po, ²¹⁴Pb, and ²¹⁴Bi, respectively, at the time of removal from the furnace. These data were used to calculate the airborne concentrations of the three, and the results indicated that about 70% of the daughters were lost to surfaces in the house and by impaction in the air ducts. The filter's content of ²¹⁰Pb was found to be 4.4 kBg (0.12 μCi); from this the average concentration of radon-producing filterable daughters during the time the furnace blower operated, was estimated to be 860 Bq m^-3. This indicated that there was no significant loss to surfaces or in air ducts. Possible reasons for the difference are given. The filter was also found to contain 1 kBq (27 nCi) of ²¹²Bi from the thorium series