The radioactivity of air particles in samples collected from 20 to 30 municipalities in the National Air Sampling Network, established in 1953, indicated an increase of beta radioactivity of 1000 fold or more immediately following known atomic weapon tests. It was suspected that substantial amounts of the total radioactivity might be associated with discrete particles which might find their way into the respiratory tract of man. The purpose of the study was to determine the radioactivity distribution of collected particles. Samples collected at the Robert A. Taft Sanitary Engineering Center, Cincinnati, Ohio, during the spring of 1955 provided the test material for this study, the results of which are reported in the document.