a Ff oe 4. Critical Yealth ~ffects: bets te It 1s widely recognized that inhaled insoluble _, alpha emitting particles deposited in the lung are, in part, translocated via the phagocytic action of macrophages te the lymph nodes and to other sites in the reticuloendothelial system, and also via blocd leucocytes to the liver, spleen and bone marrow. Recent experiments with inhaled plutonium make it evident that the pattern and rate of translocation of plutonium from the lung to other sites is highly dependent on particle size and specific activity, with more rapid transport of the smaller and more active particles. Thus, it is far from obvious whether the lung, lymph nodes, liver, bone or other organ, or fraction thereof, should be taken as the critical organ or critical tissue site. It has long been known that those tissues in which there is more active cell division suffer the earliest and most severe radiation damage effects, and that this includes the blood forming cells in lymphatic glands and in bone marrow (16537) ¢cneffects include the destruction of rapidly multiplying celis that produce the blood platelets which assist in the control of blood clotting. Similarly the population of leucocytes is reduced with a corresponding reduction in resistance to disease. These effects plus the accompanying chromosome structural changes can give rise to the eerlier incidence not only of cancers, but the whole pattern of diseases of the cardiovascular and renal systems ‘97 »38)Let us review the mounting evidence which suggests that inhaled insoluble alpha emitting particles may be the agent of atherosclerosis and thus give rise to an increased risk of death by early coronaries and strokes. Atherosclerosis is reported to be present in every instance of partial or complete arterial occlusion and every case of coronary thrombosis (39),