RERF Report No. 7-14

Paper published in Radiation Research§“Long-Term Effects of the Rain Exposure Shortly after the Atomic Bombings inHiroshima and Nagasaki”
Ritsu Sakata, Eric J. Grant, Kyoji Furukawa, Munechika Misumi, Harry Cullings, Kotaro Ozasa,and Roy E. Shore
Radiat Res 2014 (November); 182(6):599-606

Study Findings
Using a population of individuals responding to surveys from 1949 to 1961, we comparedmortality and cancer incidence between those reporting rain exposure shortly after the atomicbombing and those reporting no such rain exposure. No increase in mortality or cancer
incidence was observed in those reporting rain exposure.
Explanation
1. Objectives
The objective of the study was to determine whether mortality or cancer incidence washigher among individuals reporting rain exposure shortly after the atomic bombing thanindividuals reporting no such rain exposure.
2. Methods
The study included 86,609 Life Span Study cohort members who had an estimatedDosimetry System 2002 (DS02) radiation dose. The Atomic Bomb Casualty Commission, thepredecessor to the Radiation Effects Research Foundation, conducted surveys through personalinterviews from 1949 to 1961. Based on these survey results, the respondents were categorized
into three groups according to their response to the question on rain exposure (i.e., “Yes,” “No,”“Unknown”). The excess relative risks (ERRs) of mortality between 1950 and 2005 and cancer
incidence between 1958 and 2005 were estimated among those reporting rain exposure relative
to those reporting no rain exposure. The data were adjusted for city, sex, year of birth, attainedage, and individual DS02 dose estimates from direct radiation exposure.
3. ResultsOf the individuals interviewed, 42,050 (72%) in Hiroshima and 25,064 (89%) in Nagasaki
responded to the question on rain exposure. Of these, 11,661 (20%) in Hiroshima and 733(2.6%) in Nagasaki reported rain exposure.

The ERR of all-cause mortality from 1950 to 2005 among those who reported rain
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exposure in Hiroshima was 0.01 (95% confidence interval (CI): -0.02, 0.04). The ERRs ofmortality for solid cancer and leukemia for the same period were -0.02 (95% CI: -0.06,0.04) and 0.06 (95% CI: -0.15, 0.32), respectively. No significant increase in risk wasobserved.
A weak association was observed between all-cause mortality and rain exposure inNagasaki (ERR = 0.08, 95% CI: 0.00006, 0.17) for the entire period of observation (1950to 2005). However, the association was not statistically significant for the period after allsurveys had been completed (1962 to 2005). The ERRs of mortality for solid cancer andleukemia between 1950 and 2005 were 0.14 (95% CI: -0.01, 0.33) and -0.03 (95% CI:-0.07, 0.02), respectively, suggesting no significant increase in risk.

No increase in cancer incidence in those who reported rain exposure was observed in eithercity.
The individuals reporting rain exposure in Hiroshima were not at significantly higher riskof mortality from all cause, solid cancer, or leukemia or of developing solid cancer or leukemia.
The findings, however, were inconsistent for those reporting rain exposure in Nagasaki;
all-cause mortality risk was higher for certain observation periods and statistical models but notothers. In addition, risk estimates for cancer mortality and incidence were inconsistent with eachother. These inconsistencies could be attributable to the low number of individuals who reported
rain exposure in Nagasaki (733), which is too small to provide reliable statistical inference. In
addition, answers to the question on rain exposure would more likely be “unknown” when the1949–1961 surveys, including the question on rain exposure, were conducted after individuals
died and questions were answered by their proxies who had no knowledge regarding whether or
not that individual had been caught in rain (outcome-dependent bias). It was also possible when
the surveys were conducted after individuals had developed cancer or died that the respondents
might have associated this health outcome with their rain-exposure experience (recall bias). For
the observation period after completion of the surveys (i.e., starting in 1962), analysis results
were probably less affected by these biases. For this observation period, therefore, no significant
increase in risk was observed in Nagasaki. No increased risk was observed in either city formortality from or the occurrence of leukemia, which is often taken to be a “sentinel” indicatorof radiation effects. For these reasons, the association observed in Nagasaki is unlikely to beattributable to exposure to radiation from rain. In this study, a large percentage of individuals
have missing data on rain exposure. In addition, we did not obtain any information on rain
exposure from a considerable proportion of the subjects and no details were available on thecircumstances surrounding the rainfall and how individuals experienced the rain from those
providing the information on rain exposure. The results should thus be interpreted with caution.

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The Radiation Effects Research Foundation has studied A-bomb survivors and their offspring inHiroshima and Nagasaki for more than 60 years. RERF’s research achievements are considered theprincipal scientific basis for radiation risk assessment by the United Nations Scientific Committee on theEffects of Atomic Radiation (UNSCEAR) and for recommendations regarding radiation protection
standards by the International Commission on Radiological Protection (ICRP). RERF expresses its
profound gratitude to the A-bomb survivors and survivors’ offspring for their cooperation in our studies.
§Radiation Research, which is an official monthly journal of the Radiation Research Society, publishes
original, peer-reviewed papers and review articles on radiation effects and related issues in the fields ofphysics, chemistry, biology, and medicine. (Impact factor in 2013: 2.445)

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