Abstract. An air cleaner was installed in a room with elevated radon activity concentration, and the following parameters have been monitored: activity concentrations of 222 Rn (Rn) and its short-lived products (RnP), degree of equilibrium between Rn and RnP (F), fraction of unattached RnP (fun), and the number concentration and size distribution of aerosol particles (5-530 nm). Several hours of filtration removed the >10 nm particles almost completely, thus increasing the contribution of the <10 nm particles, associated with unattached RnP. Consequently, fun was substantially augmented with a concomitant decrease in F.