In environmental monitoring the measurement of RaA activity concentrations is a reliable indicator of radon activity concentrations. Following a brief review of established methods of radon daughter activity concentration measurement, a method is given which enables activity concentrations as low as 0.05 pCi 1^-1 (50 pCi m^-3) of RaA to be measured with simple readily transportable equipment. The method also measures RaB (²¹⁴Pb) and RaC (²¹⁴Bi) activity concentrations and working levels with improved precision compared with established methods.