Although there is considerable spread Mass-Loading Approach method is based upon measured or in these data, there is no indication of a 239, 2405, preferential association of 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 material is derived from the contaminated aerosol, the specific activity is lower soil, than that of the soil. The other approximate prediction For fresh deposits this approach is If we assume that this is generally not valid because the freshly deposited debris is much more likely to be resus~ true, a general and conservative. method pended than the remainder of the of predicting resuspended air concentra- weathered soil surface, After many trons 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 for-the 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 maybe highly 239, 2405, were preferentially associated correlated with the source in the sense with the smaller particle sizes more that children playing in sand, adults cul- 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!* and by the more mass than would be recorded by a high-volume cascade impactor study. remote stationary sampler, The latter study found an AMAD of 3.0 um for 239, 240 Pu, whereas the total mass The lower and upper bounds of ambient air mass loading can be fixed rather median aerodynamic diameter was 1,7 um. easily for any site. There has been con- The specific activity of the material col- siderable interest in establishing a 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 ug/m 3 (15) Average data from all established in a reasonable wav by the five samplers are: size, um levels found in mine atmospheres which 239, 24054, dpm/g >7 930 3.3 to 7 700 2.0 to 3.3 1030 1,1 to 2.0 1300 0.01 to 1.1 460 All stages 830 (Soil) The upper bound can be have led to a considerable prevalence of pneumoconicsis in the affected ¥corkers > Examination ci these data indicate that current stundards for occupational dust exposure {(~ 1-i0 me! im) have avery Small, or perhaps no margin of safety, Such that a reasonable upper bound can 17 (2700) be taken as 1 mg/ m*, tty British data