In the present work the effect of an HEPA filter and an electrostatic precipitator on the behaviour of Rn decay products has been studied in laboratory conditions. Both filters were found to decrease the equilibrium factor of progeny and increase the unattached fraction of decay products. In a clean air they also decreased the activity of unattached progeny. An electrostatic precipitator was found to produce condensation nuclei which were observable when no other particle sources were present. The ozone produced by the corona discharge of the filter probably has a great effect on the production of these submicron particles. The impurities of the air were found to grow the panicles and thus their influence on the behaviour of Rn decay products became in some cases significant.