RERF Report No. 1-15

Paper published in Radiation Research§“Association of Acute Radiation Syndromes and Rain after the Bombings inthe Atomic-bomb Survivors”
Kotaro Ozasa, Ritsu Sakata, Harry M. Cullings, Eric J. Grant
Radiat Res 2016 (June); 185(6):604-15(doi: 10.1667/RR14038.1)

Study Findings
Association between exposure to rain that fell soon after the atomic bombingsand development of acute symptoms was examined based on responses to interview
surveys of Life Span Study (LSS) members conducted during the 1950s. Although
the frequency of development of such acute symptoms was slightly higher in those
who reported exposure to rain, the association was not thought to be caused by
uniform radioactivity in the rain with potential effects equivalent to a level ofseveral hundred milligray (mGy); rather, recall bias, and other factors, were
thought to be more plausible explanations. Nevertheless, such issues as
insufficient data available for use in the study limited the effectiveness of the study
analysis.
Explanation
1. Study purpose
Based on various experiences of atomic bomb (A-bomb) survivors, it has beenthought that acute radiation syndrome, which includes symptoms such as epilation
and bleeding that appeared after the bombings, was associated with exposure toradiation from rain that fell soon thereafter. However epidemiological analysis had
not been conducted. The association was examined by comparison of information
involving exposure to rain soon after the bombings and development of acutesymptoms. This information was obtained in interviews of Life Span Study (LSS)
members in the 1950s, with direct radiation effects from the A-bombings taken intoconsideration.
2. Study methods
Several interviews of LSS members were conducted in the late 1940s to collectinformation regarding their situation at the time of the bombings, and in the 1950s
the interviews included questions about exposure to rain that fell soon after thebombings and development of acute radiation symptoms. The responses obtained
consisted of: “was exposed” (in this case, information on location at exposure wasrequested), “not exposed,” and “unknown.” This study analyzed typical acuteradiation symptoms such as epilation, bleeding, and oropharyngeal lesions inrelation with responses to questions regarding exposure to rain.
3. Study results
The frequency of the development of acute symptoms differed significantly
among levels of direct radiation from the A-bombings : 30–80% at the level of 3 Gyor higher, 1–6% at around 100 mGy, and 0.04–2% at less than 5 mGy, for a varietyof symptoms. At the same level of direct radiation, the reported frequency of thedevelopment of acute symptoms was slightly higher in those who indicated they
had been exposed to rain than in those who did not, both in Hiroshima and1

RERF Report No. 1-15Nagasaki. The frequency of many acute symptoms, however, was significantly
lower in those who were exposed to rain and to direct radiation of <5mGy than inthose who were not exposed to rain but exposed to direct radiation of 100–500 mGy.
Thus, it is unlikely that the rain contained uniformly high levels of radiation toproduce effects equivalent to those experienced at a level of several hundred mGy.
Odds ratio (OR) was used to evaluate, in this study, the association between
exposure to rain and frequency of development of acute symptoms (i.e., whatpercentage increase), and a larger OR indicates a stronger association. The OR ofreporting exposure to rain somewhere in Hiroshima was 1.19 (p = 0.08) among
those who developed severe epilation (defined as >2/3 of scalp), 1.23 (p < 0.01) incases of at least slight epilation (>1/4 of scalp), 1.48 (p < 0.01) for bleeding, and 1.47(p < 0.01) for oropharyngeal lesions. The ORs in Nagasaki were 1.76 (p = 0.11), 2.10(p < 0.01), 2.01 (p < 0.01), and 2.46 (p < 0.01), respectively. As for survivor location
at exposure to rain in Hiroshima, the OR was largest for those who reported
exposure to rain in an area southeast of the hypocenter. The ORs for those whoindicated they had been exposed to rain in the areas where rainfall was reported,
i.e., clockwise from west-southwest to northeast, including the west-southwest area
beyond 2 km that included the Koi/Takasu districts where high residual radiation
was detected, were similar to the OR for exposure to rain somewhere in Hiroshima.
In Nagasaki, the ORs tended to be bigger for those who reported exposure to rainin the western half of the area centered on the hypocenter than for those in theeastern half. The frequency of acute symptoms was not high among those who hadreported exposure to rain in the area beyond 2 km from the hypocenter to the eastthat included the Nishiyama district, where heavy rainfall and high residual
radiation were observed soon after the bombing.
Study SignificanceThe association between radiation exposure and development of acute
symptoms was, in general, strongest for severe epilation and weakest for epilationof lesser degrees, as well as bleeding and oropharyngeal lesions, because the latter
symptoms contained a larger number of cases not caused by radiation exposure. In
this study, reports of exposure to rain were associated with development ofsymptoms other than severe epilation; this association was similar to, or slightly
stronger than, the association with severe epilation. Also, the association tended to
be stronger in those who were exposed to rain in areas where there were few
reports of rainfall than in those who were exposed to rain in areas where rainfall
was frequently reported. The association was not stronger in the areas whereground residual radiation levels were high in post-war surveys than in other areas.
This trend in association is thought to be somewhat inconsistent with thehypothesis that acute symptoms developed due to radiation exposure from rain.
Potential reasons for the observed association between exposure to rain anddevelopment of acute symptoms included: a strong radioactive fallout contained in
the rain and a high radiosensitivity of the individuals exposed to the rain;synergistic effects between radiation and poor hygienic conditions; as well as
inaccuracies and recall bias in memories and in reports of locations at the time of
the bombings and exposure to rain and timing related to the development ofsymptoms (recall bias is the tendency for people to believe they might havedeveloped acute symptoms because they had been exposed to rain, or vice versa).
Although there may have been some individual cases of development of acute
symptoms caused by radioactive fallout in rain, the overall trend in the results is2

RERF Report No. 1-15

inconsistent with the conclusion that acute symptoms were generally caused byradioactive fallout in rain. Rather, recall bias, and other factors, were thought to be
more plausible explanations for the association. However, insufficient data imposed
limits on the assessment of whether the reports of rainfall after the A-bombingsand development of acute symptoms were attributed to actual radioactive fallout oruncertain memories and other such factors.
The Radiation Effects Research Foundation has studied A-bomb survivors and theiroffspring in Hiroshima and Nagasaki for around 70 years. RERF’s research achievements
are considered the principal scientific basis for radiation risk assessment by the United
Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and forrecommendations regarding radiation protection standards by the InternationalCommission on Radiological Protection (ICRP). RERF expresses its profound gratitude tothe 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 2014: 2.911)3