and the results expressed per area with the assumption that the top one em is available for resuspension. ’ The agreement appears to be quite good between all the measurements. It is generally accepted that freshly deposited material will produce air concentrations of resuspended materials that decrease with time. This decrease has a half-time of about 35 to 70 days and is presumably caused by the migration of the initially surface-deposited material into the soil. The resuspension factor should eventually reach a steady-state condition when the source has "aged" sufficiently. Calculations by Anspaugh et ai. (1974) of 20-year-old plutonium at the Nevada Test Site give a resuspension factor of 3 x 107!9/m and 2 x 10-9/m. The thorium and uranium should represent the ultimate aged source and their resuspension factors, the equilibrium condition. If it is assumed that most of the fallout plutonium was deposited in the early 19608, the agreement between the plutonium, uranium, and thorium indicates that in 15 years the plutonium has reached an aged condition. The data in Table l, therefore, represents resuspension factors based on long-term averages of many measurements. In order to examine the particle size distribution of fallout plutonium, samples were collected using an Anderson 2000 high-volume cascade impac- tor. The sampler was operated at 20 cfm and the effective cutoff diam- eters were assumed to be those quoted by the manufacturer. height was at one meter. The sampling The impactor was run from July 5, 1977, to August 2, 1977, and gave a total air volume of 22,800 m>. was weighed and radiochemically analyzed. Each stage The results for particulate concentration, plutonium-239,240 air concentration, and plutonium—-239,240 concentration per gram of suspended material for each stage is collected in Table 2. The general trend of the fallout plutonium is that the concentration increases with decreasing particle size. About 67% of fallout plutonium has a particle size of less than 1.1 wm AMAD, whereas Myers et al. (1975) found only 50% of the soil resuspended by rototilling a sludge- contaminated field to be in this fraction. For comparison, data is included from the GMX site (Anspaugh and Phelps, 1976) and is for highvolume cascade impactor #4. No similar trend in the GMX data exists. About 20% of the resuspended plutonium at GMX would be expected to undergo pulmonary deposition, based upon the ICRP Task Group of Lung Dynamics model (Morrow, 1966), as opposed to 28% of the fallout plutonium. This compares to 25% measured by Volchok and Knuth (1972) at Rocky Flats. In either the fallout or GMX case, there is no obvious correla- tion of specific activity with particle size. Thorium and uranium chemical analyses were also performed on the filter papers from the Anderson 2000 Impactor. The thorium results were unusable primarily because of the high thorium blank from the paper. Each paper had about 0.5 pCi of thorium. The uranium results are presented in Table 3. The concentrations per volume of air decrease regularly with 733