Combined effects of space flight factors and radiation on humans

The probability that a dose of ionizing radiation kills a cell is about 10, 000 times the probability that the cell will be transformed to malignancy. On the other hand, the number of cells killed required to significantly imp act health is about 10,000 times the number that must be transformed to cau se a late malignancy. If these two risks, cell killing and malignant transf ormation, are about equal, then the risk that occurs during a mission is mo re significant than the risk that occurs after a mission. The latent period for acute irradiation effects (cell killing) is about 2-4 weeks; the laten t period for malignancy is 10-20 years. If these statements are approximate ly true, then the impact of cell killing on health in the low-gravity envir onment of space flight should be examined to establish an estimate of risk. The objective of this study is to synthesize data and conclusions from thr ee areas of space biology and environmental health to arrive at rational ri sk assessment for radiations received by spacecraft crews: (1) the increase d physiological demands of the space flight environment; (2) the effects of the space flight environment on physiological systems; and (3) the effects of radiation on physiological systems. One physiological system has been c hosen: the immune response and its components, consisting of myeloid and ly mphoid proliferative cell compartments. Best-case and worst-case scenarios are considered. In the worst case, a doubling of immune-function demand, ac companied by a halving of immune capacity, would reduce the endangering dos e to a crew member to around 1 Gy. (C) 1999 Published by Elsevier Science B .V. All rights reserved.