Removal of radioactive radon daughters from air

In the method for removing radioactive radon daughters from air which comprises filtering air contaminated with radon daughters through a water-wetted filter to capture the radon daughters in the water, the improvement which comprises: circulating the water contaminated with the captured radon daughters through a filter medium comprising a waterinsoluble granular substrate wherein the surface of the substrate has been dried of water and then wetted with a normally liquid hydrocarbon; and returning the filtered water to again wet the air filter and capture additional radon daughters