1 For these reasons, the insoluble alpha emitting smoke particle, uranium oxide, thorium oxide and other alpha emitting particles of moderate to low svecific activity may be expected to give rise to a higher tumor risk per alpha disintegration or for a given cumulative dose. Similarly plutonium-239 in mixed fallout particles may be expected to produce more tumors per disintegration cnan is the case for pure *°*Pu0, and **%pu0, + However although larger burdens of hot particles will be required for a given tumor risk, such risks can be expected to increase with both alpha specific activity and with particle surface area, and the effects should occur earlier for a given burden of smaller particles of higher specific activity. The above considerations make it obvious that the present practice of averaeine the ainha dose over the whole lunge or some arbitrary fraction thereof (10-13) is a highly questionable and grossly misleading procedure -at best. It also should be noted that americium-241 is present in association with plutonium contamination in the Rocky Flats area and in nuclear test areas. In addition, curium isotopes as well as americium-241 will be present in high concentration in the nuclear fuel mixture from fission and breeder reactors which use plutonium fuel. The chemical behavior of americiuu and curium in the environment will give rise to their substantial uptake in the biosphere and the food chain. Thus the ingestion of americium and curium, their uptake from the gastrointestinal tract, and their accumulation in the liver and skeletal tissue of mammals and man will give rise to additional serious health risks. These contaminants will be relatively more serious than plutonium inhalation in some environments, particularly in vegetated areas of moderate to high rainfall, where soil resuspension processes are not effective.