Reports tests of the effect of various air treatment devices on the radon daughter concentrations within a room. Test were carried out using an electrostatic precipitator, a humidifier and a dehumidifier. The ventilation rate of the room was measured using Krypton 85 as a tracer gas.< Finds that the use of a humidifier, dehumidifier or carbon filter gave no significant alteration to the radon concentrations in the room, but that the electrostatic precipitator reduced the concentration considerably and was equally effective when operated with or without the carbon filter.