The use of induced time-dependent 222Rn behavior to determine source rate magnitudes, ventilation rates (air change rates), and other parameters that affect 222Rn and progeny levels and exposure to building occupants is investigated. When the subject space is purged and the buildup (seepage) back into the space is measured, theoretical, normalized equations show a unique, one-to-one correspondence of the 222Rn and particulate progeny temporal levels to the air change rate in the space. The Bateman equations have been solved in closed form for 222Rn and progeny in air and trapped on a filter under these conditions. A total of 28 measurements of the time-dependent behavior of radon progeny for two test facilities [one with a constant air change rate and one with a constant National Institute of Standards and Technology (NIST)-calibrated source] and four residential dwellings were made. The results were compared with theory and with air change rate measurements made by anemometer flow rates and by the conventional method (SF6 decay). For a factor of 2 range in air change rates in the NIST constant source case, the agreement with the SF6 method air change rate was within ±10.6% standard deviation and agreement with the NIST source magnitude of 37.0 ± 1 kBq was within ±4.9% standard deviation. Agreement to within ±17.7% standard deviation was obtained on determination of air change rates for the residential dwellings. Based on the airborne concentrations and air change rates, source emanation rate magnitudes were obtained. Analyses of the results are presented in detail, and factors affecting the accuracy and feasibility of the method are identified.