Association between mortality and residual radiation in Nagasaki atomic bomb survivors exposed at long-distance from the hypocenter

Mortality of Nagasaki A-bomb survivors exposed at 3 km or longer distance from the hypocenter was compared with that of those who were additionally exposed to residual radiation because of their entrance in the area at 1 km or closer to the center within 24 hr post explosion. The latter survivors (group I), 2,357 men with average age of 28.4 y and 2,618 women of 26.5 y at the exposure, were alive at 1970, and the former (group II, without exposure to residual radiation) was selected to match their numbers in sex, exposed distance, ages at exposure and at start of the follow-up study to those of group I. Follow-up was conducted from 1970 to 2007, and their total, malignant, cerebrovascular, cardiac and pneumonic deaths were observed. Cox proportional hazard model was used for estimation of mortality risk with covariates of sex and age at start of the study. The risk in group II was defined to be standard. Ages at start of the study were 53.3 and 51.4 y in men and women, respectively. Crude mortality tended to be higher in men of group I at ages of 40-49 and 50-59 y at start of the study. Hazard ratios of total and malignant tumor deaths in group I were 0.965 and 1.092, respectively, without statistic significance from group II and of other deaths, 0.982-0.999, also of statistic insignificance. Thus increased mortality due to residual radiation was not observed. (T.T.)