Pre-Bombing Population Density in Hiroshima and Nagasaki: Its Measurement and Impact on Radiation Risk Estimates in the Life Span Study of Atomic Bomb Survivors.

In the Life Span Study of atomic bomb survivors, differences in urbanicity between high-dose and low-dose survivors could confound the association between radiation dose and adverse outcomes. We obtained data on the pre-bombing population distribution in Hiroshima and Nagasaki, and quantified the impact of adjustment for population density on radiation risk estimates for mortality (1950-2003) and incident solid cancer (1958-2009). Population density ranged from 4,671-14,378 and 5,748-19,149 people/km2 in urban regions of Hiroshima and Nagasaki, respectively. Radiation risk estimates for solid cancer mortality were attenuated by 5.1%, but those for all-cause mortality and incident solid cancer were unchanged. There was no overall association between population density and adverse outcomes, but there was evidence that the association between density and mortality differed by age at exposure. Among survivors 10-14 years old in 1945, there was a positive association between population density and risk of all-cause mortality (relative risk, 1.053 per 5,000 people/km2 increase, 95% confidence interval: 1.027, 1.079) and solid cancer mortality (relative risk, 1.069 per 5,000 people/km2 increase, 95% confidence interval: 1.025, 1.115). Our results suggest that radiation risk estimates from the Life Span Study are not sensitive to unmeasured confounding by urban-rural differences.