Experiments in the Rocky Flats area also have shown that about one-third of the airborne plutcnium which has been resuspended from soil surfaces by wind action falls within the respirable particle size range. However only a very small fraction of the bulk surface soil is made up of insoluble particles of respirable size. For this reason, surface soils with one © picocurie of plutonium per gram (the Colorado interim soil standard) should contain an estimated 10 to 100 pCi of plutonium per gram of insoluble soil particles of respirable size. Such a soil level should lead to plutonium lung burdens of 5 to 50 picocuries by age 20, or 15 to 150 picoecurtes by age 6C, with correspoidingly higher concentrations in the iymph nodes, liver, and bone. Thus the Colorado interim soil standard is hardly ‘a safe or acceptable standard unless it can be shown that such levels of plutonium have no serious long term health effects. There are, of course, a number of considerations which make it inappropriate to equate the effects of a given burden of low specific activity alpha emitting cigarette smoke particles with the same amount of alpha activity in hot particles. The Los Alamos experiments (12,13) make it evident that rost of the alpha dose from “hot" particles of Pud, is wasted in the excessive irradiation of cells within the alpha range of the hot particle surface. Thus the high tumor risk for the hot *?*Puo -particles (11) can be variously attributed to (a) the mobility of the 2 smaller particles (b) the recoil ablation and/or dissolution rates which increase with specific activity and with surface area of hot particles and (c) the irradiation of larger numbers of cells with scattered protons ~ (an effect that may be significant fer very hot particles). ~ me °