aren defects, . that @ complete appruach to risk assessment at the time of exposure must involve the HSEF for cells, Since the HSEF approach could replace the present approaches using LET, it has significance with respect to differences in “track structure” seen with radiations of different “quality". Some of the severity of cellulac effect that has been ascribed to LET and track structure differences, may well be due to a difference in dose to the cells. With most, particularly planned transfers of chromosomal agents, it has been more or less generally * accepted that a larger dose will he more effective per unit dose than a smaller one, apparently with Little or no necessary requirement being percelved to investigate why. The interpretation of a “linear, non-threshold” curve (for exposure and not dose) also changes with the HSEF apprdach. following any amount of population exposure, there of course can be stochastic interactions with health consequences. amount”, i.28., as little as a single encounter, the conditions for this are 1) one must That is to say, It is true that “any could be lethal. However, first have experienced such an encounter, and 2) It must be of a size such that the dose . transferred is large enough to have some tangible probability of causing a quantal response, REFERENCES 1. Parker, H. and Roesch, W.C, X Rays and Gamma Rays. 2. In Clark, G.L. (€d.) The Encyclopedia of Chapman and Wall, London; Reinhold, NY, 1963. Skarsgard, L.D., Kihlman, B.A. et al. ‘Survival, chromosome abnormalities and recovery in heavy fun and X-irradtated cells. Rad... Res., Suppl. 7, 2U8-221, 1967. 3. Morstin, K., et al. , A probabilistic approach to relate microdosimetry, biological effects and radiation quality. Submitted Se to Radiation Research, 4, Rossi, H.H. Specification of radiation quality. Radiat. Res. 10, 522-531, 1959. De Rossi, H.H. Energy distribution in the adsorption of radiation. Advances in Biological and Medical Physics, Vol. I1, pp. 27-85, 1967. “% 6. Rossi, H.W. Radiation Dosimetry, Vol. Eds., Academic Press, NY) 1968, -226- I (FH. Attix, W.C. Roesch, Fe