On the other hand, more detailed studies indicate the soils and dust not to be important. For example, Ter Haar (1974) used 7!°Pb in the If one is to use this model it should be applied as given unless specified changes are made. The results of one such application is given in Table 1. environment as a tracer for lead and noted that the ?'°Pb content of the where the inhalation to result in 500 mrem/yr to each organ at the end two towns with natural lead levels of about 900 ug/g and 400 ug/g. skeleton is the limiting organ in both class Y and class W material with only a factor of about 5 of 6 difference with particle size for particles from 0.1 to 50 um AMAD. This is due to the increased deposition of larger particles in the upper respiratory tract and the significant rapid excreta from children with signs of exposure and those with low exposure was identical. He concluded that dust and air-suspended particles were not the source of lead in these urban children. Barltrop (1974) measured the soil, blood levels of mother and child, and feces from children in Ina further preliminary study he compared two villages where the soils were 10,000 ppm and 500 ppm. He concluded, "The results of our studies to date suggest that local soil lead levels of the order of 10,000 ppm are without major significance and that on present evidence the recent con- cern with regard to contaminated soils in cities is not wet] founded." These results combine the direct ingestion and inhalation pathways. It is, however, encouraging to note that those who have directly studied the relation between quantity in the children and quantity in the soil, in- dicate that there appears to be no great problem. However, we will only be able to tel? this unequivocally with plutonium when greater attention is paid to this problem and appropriate measurements are made. In the foregoing, I have considered the models in a somewhat idealized fashion in that I have ignored many factors of importance in the actual application. For example, in the general resuspension model, I have ignored the very important factors of soil condition, coverage of the soil by vegetation or rocks, terrain influences on dispersion, and the estimation of deposition. The latter two are problems which relate to the estimation of dispersion and are receiving attention but will result in more complicated analyses. The first two require more studies of re- suspension under various conditions and, perhaps, more theoretical modeling of the attachment forces and the influence of soil particle distribution on probability of the particles being removed from the ground. In the local resuspension and transfer resuspension considerably of 70 years continuous exposure is presented. absorption prescribed for these regions. Interestingly enough, the Obviously one can prescribe a third class in which the movement from the lung is slower and the absorption from the upper regions smaller so that the lung becomes critical. Regardless of the constants used, it is apparent that one cannot com- pletely ignore those particle sizes above the “respirable” range unless it has been shown that the material in the soil has a solubility such that movement from the upper respiratory tract is negligible. Thus, in any detailed analysis, it is not proper to use the current MPC’s in air for soluble material based on only absorption to the blood stream and for lung based only on dose to lung. The true situation is a combination of both. The particte size question has been intensified by Johnson et al. (1976) who advocate applying the Colorado limit for plutonium in soils only to the soil particles less than 5 um obtained by completely breaking up the aggregates. In an earlier report (Healy, 1974) I recommended that the limit be applied to the very surface of the soil and to particles less than 100 um. This was done in an attempt to use data on resuspension rate from one experiment in a more general manner. Now that actual re- suspension rate data are available, this assumption is no longer necessary. One objection to the particle size proposal is given by the interpreta- tion of the ICRP lung model in Table 1. Unless it is shown that the plutonium in the soil is more insoluble than the ICRP assumes, then the more work is required in defining and measuring appropriate parameters contribution of the larger particles from either general resuspension or local resuspension is of importance, soil source. When one selects a particular particle size to estimate a hazard, it is and in making use of opportunities to assess the overall intake from a There are two other items in the overall resuspension-inhalation model which could be of some importance in a particular situation. The first is the application of the ICRP Task Group (ICRP, 1966) on lung dynamics model for inhaled material and the second is the importance of sizes of contaminated particles in the soil. The ICRP Task Group mode? accounts for the deposition of individual particle sizes (AMAD) in three compartments of the respiratory tract - the nasopharyngeal region, the tracheo-bronchial region, and the pulmonary region. It then describes the movement from these regions to the blood stream and the lymph with different retention times for each compartment depending on the class or solubility of the substance. The later retention and distribution factors have been revised by the ICRP Committee on Plutonium Metabolism (ICRP, 1972). 216 assumed that only this particle size is of importance and all others may be neglected. Let us see the effect of this on a local resuspension situation which occurs for 40 hours per year with a disturbance for which a resuspension factor of 5 x 107’ m-! is appropriate. We will use d as the depth of soil appropriate to the resuspension factor, p as the soil density, and f as the fraction of the particles of interestby weight in the soil. I have used an air limit of 3.5 x 107!" uCi/ml or 0.035 pCi/m? averaged over the year. If we further assume that all of the plutonium in the soil is in the small particles, then the minimum limit for the small particles and the soil considered separately is: 1500 pCi/g -= ofd Some representative values are given in Table 2. 21