in activity median aerodynamic diameter (AMAD) with a density of 1). For a pure plutonium oxide particle of density 11, this would reduce to 2.4 microns diameter, and if the plutonium were adsorbed on other materials so as to give an effective density of 2.5, the relative diameter would be about 5 microns. In a given deposition field these particles are relatively rare, as fine particles are not efficiently deposited from a cloud and tend to be dispersed at large distances with high dilution factors. Once on the ground they tend to become fixed or bound to larger particles by water action. Those remaining on the surface are relatively difficult to resuspend. If plutonium becomes resuspended, the deposition in human lung then becomes a function of the larger size of the carrier soil particles. Approximately 85. percent of all plutonium detected in fallout following ROLLER COASTER experiments was detected on particles above 10 microns in diameter. Resuspension Factors. Resuspension studies conducted following Project 57, and confirmed by other studies, have shown that once deposited on the ground, the plutonium oxide tends to remain in place and very quickly reaches a vertical distribution in the soil. Daily air con- centration measurements for a considerable period of time following an original plutonium deposition indicates that there is an approximately 35 day half-time of reduction for resuspension. Data from Project 57 and from ROLLER COASTER indicate resuspension factors * in the range *The resuspension factor is the ratio of the air concentration above 4 con- taminated surface (in ug/m3) to the surface deposition level (in yg/m?). Wet - wae aja U8