Theory of protection of man in the region of the primary cosmic radiation.

T H E P O T E N T I A L hazard to humans from exposure to the pr imary cosmic radiation at very high altitude centers upon the heavy nuclei component of the pr imary radiation. I t is not the total ionization dosage of these rays which creates the potential danger but their extremely high specific ionization. I t has been shown 6'7 that living cells which happen to be in or close to the central core of the ionization column of a heavy nucleus track are exposed in their full cell volume to ionization dosages as high as several thousand roentgens. Moreover, these dosages are administered in times far shorter than one millionth of a second and this circumstance is apt to increase the destructiveness of the heavy nucleus hit further. There is no doubt that such an exposure must be considered an above-threshold injury for the cells affected and that is putting it mildly. However, the percentage of cells of a human organism exposed to such extremely high dosages is very small even in a prolonged stay of many hours in the heavy nuclei region. I f one attempts to assess this novel type of radiation exposure with regard to its damage two questions arise. (1 Is this peculiar type of radiation in-