[en] This constitutes studies designed to provide data for assessing the mechanisms governing the distribution, fate and pathways of entry into biological systems, as well as the ultimate hazards, associated with radon progeny and their secondary reaction products. The chemical and physical state of radon progeny are expected to influence their mobility and reactivity, and mechanisms of transport and entry into biological systems since these depend on processes such as attachment to and/or incorporation with aerosol particles. There is evidence that an appreciable fraction of the progeny are ionized and this program addresses the clustering, growth, and subsequent chemical conversion of these associated species. 2 refs