A method of analyzing filter samples by alpha spectroscopy was developed for accurately determining the airborne concentrations of RaA, RaB, and RaC in atmospheres contaminated with radon. The method was used to determine the concentration of radon daughter products in a uranium mine atmosphere and to evaluate respiratory protective devices worn by miners. Measurements of the radon daughter activity deposited in the respiratory tract of person exposed to a representative mine atmosphere indicated average percent depositions of 50.7%, 43.9% and 41.4% for the RaA, RaB and RaC components. The fraction of the activity deposited depended on the properties of the aerosol inhaled and on the breathing pattern of the subject. Increased deposition was observed for large tidal volumes and low breathing rates. Several commercial and experimental respirator filters were tested for their ability to remove the radon daughter activity encountered in a mine atmosphere. High removal efficiencies were exhibited by densely packed fiber filters and by resin-impregnated wool filters. An in vivo counting system capable of detecting the RaC activity present in the respiratory tract of working miners was used to estimate the protection afforded by respirators under mining conditions. Protection factors of 5 to 10 were achieved by severalmore » commercial respirators.« less