Abstract: A method and apparatus for measuring the concentration of unattached Radon progeny in air is described. The method consists of measuring the conductivity of the air or alternatively the concentration of fast ions in the air, and converting the measurement to a concentration of unattached Radon progeny by applying a predetermined ratio found to exist between conductivity and the concentration of unattached Radon progeny on the one hand, or between the concentrations of fast ions and unattached Radon progeny on the other hand. Apparatus for implementing the method includes means for measuring conductivity (or fast ion concentration), means for calculating the concentration of unattached Radon progeny using the predetermined ratios and output means for displaying the result of the calculation.