nonuniform alpha radiation dose distribution in the lung. The authors of this study take exception to the conclusions and reccmmendations of Geesaman, Tamplin and Cochran (6,8) and conclude that "the nonuniform dose distribution of plutonium particles in the lung is not more hazardous and may be less hazardous than 4£ the plutonium were uniformly distributed and that the mean dose lung model is a radisbiologically sound basis for establishment of plutonium standards." . Bair et a1. (29) fail to take into account the full implications of some of the recent published results: in particular, the observed higher tumor risks for 73°Pud, than for 28%u0, ON) the apparently limited biological response of mammal lung cells from 75*Pu and ?2°Pu incorporated into ceramic microspheres results 2”). (12,13) and the tobacco smoke radioactivity The latter results imply that as little as a few picocuries ‘of insoluble alpha emitting particles in the lung may give rise to a significant risk of lung cancer and othec ou.ious héalth effects in tthe chronic exposure case. . On the basis of a brief review of the known effects of alpha interactions with cells (below) it will become evident that alpha radiation induced cancer in mammals and man must be brought about by subjecting a large number of living cells to a limited number of alpha interactions. Thus, in principle, the highest riek would be associated with a uniform distribution of the alpha dose, in accordance with the conclusion of Bair et al. However, in fact, we are almost always concerned with a highly ‘drregular tissue distribution of alpha emitting particles. For hot particles, the tumor incidence must be due to the low dose irradiation of a large number of cells by a very small fraction of the hot particle burden. And for long term exposures, unacceptably high tumor risks appear to be associated with picocurie burdens of internal alpha emitters. Tuis serious possibility calls for a drastic downward revision of permissible