New estimates of radiation lethality ....

A preliminary analysis of data from a new survey of acute deaths among Japanese residents who had lived within 1,300 meters of the atomic-bobm hypocenter in Hiroshima suggests that the radiation dose required to kill 50 percent of those exposed -- the LD-50 -- may be four times lower than previously thought. "My thesis is that the deaths that occurred after the first day were nearly all due to radiation exposure," as opposed to the explosion itself or its resulting heat, explains Joseph Rotblat, of the University of London, in England. He used data collected by two Japanese teams of researchers. The data list when individuals died, how far they were from ground zero at the time of the blast and the nature of any building materials that might have provided shielding from radiation.

Half of the acute deaths -- those between 1 and 60 days after the blast -- occurred within a distance of 892 meters from the point on the earth's surface that was directly below the blast. Rotblat computed radiation doses likely throughout this region for the various types and quantities of radiation that are estimated to have been emitted by the bomb. (These figures were based on preliminary calculations suggested at a U.S.-Japan joint workshop on atomic-bomb dosimetry earlier this year.) His calculations result in an LD-50 for human bone marrow of 154 rads -- or one-quarter of the 600-rad bone-marrow dose that he reports "is being used in estimates of radiation casualties in a nuclear war." Rotblat says the 600-rad figure had been derived partly from animal data and partly from data on the few human radiation-accident victims (many of whom had received medical treatment); it was not derived from data on Japanese bomb victims, he points out -- largely "because of the alleged difficulty in separating [their] radiation mortality from that caused by blast or heat."