This report covers most of the first year of the current grant period of Ciarkson University's continuing study of the chemical and physical behavior of the 21Spoatom immediately following its formation by the alpha decay of radon. Because small changes in size for activity in the sub-10 nm size range result in large changes in the delivered dose per unit exposure, this behavior must be understood if the exposure to radon progeny and it dose to the cells in the respiratory tract are to be fully assessed. In order to pursue this general goal, two areas of radon progeny behavior are being pursued; laboratory studies under controlled conditions to better understand the fundamental physical and chemical processes that affect the progeny's atmospheric behavior and studies in actual indoor environments to develop a better assessment of the exposure of the occupants of that space to the size and concentration of the indoor radioactive aerosol. Thus, two sets of specific goals have been established for this project. The specific tasks of the controlled laboratory studies are 1.) Determine the formation rates of .OH radicals formed by the radiolysis of air following radon decay, 2.) Examine the formation of particles by the radiolytic oxidation of substances like SO2, ethylene, and H2S to lower vapor pressure compounds and determine the role of gas phase additives such as H20 and NH3 in determining the particle size, 3.) Measure the rate of ion-induced nucleation using a thermal diffusion cloud chamber, and 4.) Measure the neutralization rate of 218p°O+xin 0 2 at low radon concentrations. The specific tasks of the exposure studies in occupied indoor spaces are 1.) Initiate measurements of i the activity size distributions in actual homes with occupants present so that the variability of the indoor activity size distributions can be assessed with respect to indoor aerosol sources and general lifestyle variations of the occupants, 2.) Initiate a prospective study of the utility of measurement of deposited 21°pb embedded in glass surfaces as a measure of the long-term, integrated exposure of the population to radon, and 3.) Develop the methodology to determine the hygroscopicity of the indoor aerosol so that the changes in deposition efficiency of the radioactive indoor aerosol with hygroscopic growth in the respiratory tract can be assessed. This report describes the progress toward achieving these objectives.