wi exposure standard!"tor inhaled plutonium. It also’ts possible that the eritical health effects for inhaled alpha emitting particles are the incidence of atherosclerosis and other degenerative diseases of the cardiovascular system. The published evidence supporting chese conclusions is briefly reviewed below. 2. Tumor Production: The interactions of varicus types of radiation with living cells and their mutagenic effects have been widely investigated, with results which have been reviewed and summarized by Lea (15) » Muller (16) and others. When alphas interact with the chromosome or its genes in the nucleus of a cell, the dense ionization in the track of the alphaparticles give rise to closely spaced breaks which bring about a 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 spaced 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 of 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 cell division (17,18) . However, as Lea (15) and others have pointed out, some cell nuclei experience only minor structural changes “ne vs ew eg ep . .- oy fees agit * 2 . s : , “gah sai! " toe . i on