This part of ISO 11665 describes continuous measurement methods for radon-222. It gives indications for continuous measuring of the temporal variations of radon activity concentration in open or confned atmospheres.This part of ISO 11665 is intended for assessing temporal changes in radon activity concentration in the environment, in public buildings, in homes and in work places, as a function of infuence quantities such as ventilation and/or meteorological conditions.The measurement method described is applicable to air samples with radon activity concentration greater than 5 Bq/m3.