expusure standards for inhaled plutonium. It also is possible that the eritical health effects for inhaled alpha emitting particles are the dncidence of atherosclerosis and other degenerative diseases of the cardiovascular system. The published evidence supporting these conclusions is briefly reviewed below. 2. Tumor Production: The interactions of various types of radiation with living cells and their mutagenic effects have been widely investigated, with results which have been reviewed and summarized by Lea end others. (15) | mutter 26) when 2lphas interact with the chromosome or its genes in the nucleus of a cell, the dense ionization in the track of the alpha particles give rise to closely spaced breaks which bring abouta wide variety of irreversible chromosome structural changes, or mutations. X-ray and Y-ray interactions give rise to a diffuse distribution of ions, resulting in widely speced individual breaks, most of which can undergo repair by recombining without structural change. Thus permanent structural changes for X-rays and Y-rays are proportional to the square of the dose, with greatly reduced incidence at low dose rates. By contrast, structural changes resulting from alpha interactions are directly proportional to the number of interactions and are independent vf alpha interaction rates. Thus, with regird to the production of irreversible structural changes in cells the relative biological effectiveness of alpha radiation, compared | to X-rays and Y-rays, increases markedly at lower dose rates and over ‘longer periods of exposure. For alpha interactions with cell nuclei, most of the structural changes are lethal and lead to the mitotic death of the cell, at the next or subsequent celi division (17,18) . However, as Lea (15) and others have pointed out, some cell nuclei experience only minor structural changes