However, two approximate methods may be used — the resuspension factor approach and an argument based upon ambient air particulate concentrations, “under quiescent conditions, or after administrative control has been established in the case of an accident, " A value of 107°/m is suggested under conditions of with the assumption that the particulates moderaie activity. are derived from the contaminated sur- ever, that exceptionally higher values face. The former method has been fre- Stewart states, how- (mean of 10° Ym) were observed during quently used, but almost always in the the Hurricane Trial (Monte Bello Islands) context of a fresh surface deposit. and credited this to the nature of the The latter method is inappropriate to the small islands exposed to sea breezes. fresh deposit situation, but should be Values approaching 10°3/m when dust is reasonably valid after enough time has raised by pedestrians and vehicles are elapsed for the surface-deposited material to become fairly well mixed witha few centimeters of the soil surface. also reported by Stewart, Kathren® has also considered the resuspension factor approach and has recommendedthe use of 10°4/m asa ResusSpension Factor Approach conservative but appropriate value for The resuspension factor, K, is defined setting standards for Pu, surface con- as * K = Airconcentration (Ci/m>) Surface deposition (Ci/m2) ’ ta ang thus-has units of m -1 "., : Nis almost - always implied that both measurements are made at the same location. The diffi- culties with this approach are fairly _,obvious — no allowance is made for the geometrica! configuration of the source, the particle-size distributions of the contaminant and the soil surface, vegetation cover, etc. Stewart! and Mishima” have tabulated values of K from many experiments including those involving laboratory floors as well as native soils, As would be expected, the tabulated values cover an enormous range and vary from 1077 to i938 /m. Most of the high values, however, are derived from exveri- ments with laboratory floor surfaces and/ or with artificial disturbance. For outdoor situations, Stewart! sug- tamination, Lenghem* 5 has suggested that 2 value of 10°°/m is a reasonable average value to use in estimating the potential hazard of occupancy of a piutoniumcontaminated area, At the same time, however, Langham notes that many measured values lie in the range of 10°° to 10°! /m and reports that his own measurements in 1956 produced a value of 7X 10°°/m, These recommended values, however, are all intended for application during the time period immediately following deposition. Numerous studies!’ 5-8 have shown that air concentrations of resuspended materials decrease witn time. With the assumption that this decrease can be represented by a single exponential func- tion, half-times of 35 to 70 days have been reported”? 7, 8 This decrease in air activity is not explainable by the gests as a guide for planning purposes relatively minor loss of material from that a value for K of 1078 /m be used the initial site of deposition» °, but is a 14-23