A new method to determine the air concentration of (220)Rn, commonly referred to as thoron, has been developed. The method is insensitive to the presence of any concentration of (222)Rn. Air turbulence, humidity and temperature do not influence the measurement. This method was primarily developed for field measurement of (220)Rn and (212)Pb concentrations at workplaces in underground environments with high ((222)Rn) concentration, but has the potential to be used for reference measurement of (220)Rn concentration in a calibration facility. The sampling and measurement devices are traceable to international standards. The proposed measuring technique has the potential to contribute to easy and accurate measurements of (220)Rn. This paper describes the method and an example of its use in underground workplaces.