. a 6 ot WASH-1535. a oh} és ~ -8- There, by introducing a fictitiously large mass of exposed tissuc, the calculated dose bacemes covmensurately small. In passing from the real situation in which a hot particle irradiates 10 to 100 alveoli, to the fictional situation in which the ionizing radiation from the hot particle is averaged cver 10° alveoli, the dose scale has decreased by roughly a factor of a million. . Living tissue shows extensive intra-cellular and inter-cellular organization. Several regimes of biological response would be expected as physical characteristics of exposure are varied. Carcinogenic response to whole organ exposure by non acute doses of radiation will fall in one of these regimes, and this will be a regime in which there is human experience. From the physical characteristics of plutonium aerosols, from the lung deposition experience with aerosols, and from the lung clearance " experience with plutonium particulates, it can be inferred that at least one class of particles exist which subject lung tissue to an exposure associated with a different carcinogenic response regime. This is because other biological phenomenon has intervened. For hot particle exposure that phenomenon is mitotic death of cells, j.e., loss of the cell's ability to divide. A literature on the subject. There is an extensive Radiologically induced mitotic death is, in fact, the basis for treating malignant tissue with ionizing radiation, and is the cause of most acute symptoms consequent to radiation exposure. Even though the intercession of extensive mitotic death of cells must inevitably place certain particulate exposures in a different response - regime from whole lung, nan acute exposures, a compelling argumant might see be made that the carcinogenic response in the former case is necessarily