Although there is considerable spread Mass- Loading Approach The other approximate prediction in these data, there is no indication of a method is based upon measured or preferential associaticn of 239, 2405, assumed levels of particulate matter in with a particular particle size; as would ambient air with the assumption that this be expected as a result of dilution by inert materia! is derived from the contaminated aerosol, the specific activity is lower soil, than that of the soil. For fresh deposits this approach is not valid because the freshly deposited If we assume that this is generally debris is much more iikely to be resus- true, a generai and conservative.method pended than the remainder of the of predicting resuspended air concentra- weathered soii surface, After many tions of contaminants would be to simply years of weathering since the initial multiply the ambient air mass loading by deposition, however, the contaminating the contaminant concentration in soil, material should be reasonably well mixed factor of some uncertainty for a specific with a centimeter or two of soil, such calculation is what value to use forthe that the contaminant activity per gram of ambient air mass loading in the absence airborne particulate should approximate of specific data, that in the upper soil. more uncertain because of the possibility However, a major A This becomes even difficulty could arise if, for example, that the people involved may be highly 239, 2405, were preferentially associated correlated with the source in the sense with the smaller narticte sizes mare that children plaving in sand. adultes enl- likely to become airborne, tivating crops, etc,, may generate their For the Nevada Test Site, such is not the case as own "ambient air" which contains much determined by soil analyses! 4 and by the more mass than would be recorded bya high-volume cascade impactor study. remote stationary sampler, The lower and upper bounds of ambient The latter study found an AMAD of 3.0 wm for 239, 240 air mass loading can be fixed rather median aerodynamic diameter was 1.7 um, easily for any site. The specific activity of the material col- siderable interest in establishing a Pu, whereas the total mass There has been con- lected on each stage can also be examined "background level" of mass loading, and for a preferential association of plutonium this is generally believed to be about with particle size, 10 we/m 3 (5) Average data from all established in a reasonable way bythe five samplers are: 239, 240 Pu, Size, um >7 950 3,3 to 7 700 2.0 to 3.3 1030 1,1 to 2.0 1300 0.01 to 1.1 480 All stages (Soil) | The upper bound can be 890 (2700) dpm/g levels found in mine atmospheres which have led to a considerable prevalence of pneumoconiosis in the affected workers ©, Examination of these data indicate that current standards for occupationa! dust exposure (~ 1-10 mg/m?) have a very small, or perhaps no margin of safety, such that a reasonable upper bound can be taken as 1 mg/m°, British data’ i11-24