Journal of Radiation Research, Vol. 58, No. 5, 2017, pp. 745–754 doi: 10.1093/jrr/rrx029 Advance Access Publication: 6 July 2017 Rediscovery of an old article reporting that the area around the epicenter in Hiroshima was heavily contaminated with residual radiation, indicating that exposure doses of A-bomb survivors were largely underestimated Shizuyo Sutou* School of Pharmacy, Shujitsu University, 1–6-1 Nishigawara, Naka Ward, Okayama City 703–8234, Japan *Corresponding author. Tel/Fax: +81-86-273-6369; Email: [email protected] Received December 30, 2016; Revised April 13, 2017; Editorial Decision May 18, 2017 ABSTRACT The A-bomb blast released a huge amount of energy: thermal radiation (35%), blast energy (50%), and nuclear radiation (15%). Of the 15%, 5% was initial radiation released within 30 s and 10% was residual radiation, the majority of which was fallout. Exposure doses of hibakusha (A-bomb survivors) were estimated solely on the basis of the initial radiation. The effects of the residual radiation on hibakusha have been considered controver- sial; some groups assert that the residual radiation was negligible, but others refute that assertion. I recently dis- covered a six-decade-old article written in Japanese by a medical doctor, Gensaku Obo, from Hiroshima City. This article clearly indicates that the area around the epicenter in Hiroshima was heavily contaminated with residual radiation. It reports that non-hibakusha who entered Hiroshima soon after the blast suffered from severe acute radiation sickness, including burns, external injuries, fever, diarrhea, skin bleeding, sore throat and loss of hair—as if they were real hibakusha. This means that (i) some of those who entered Hiroshima in the early days after the blast could be regarded as indirect hibakusha; (ii) ‘in-the-city-control’ people in the Life Span Study (LSS) must have been irradiated more or less from residual radiation and could not function properly as the negative control; (iii) exposure doses of hibakusha were largely underestimated; and (iv) cancer risk in the LSS was largely overestimated. Obo’s article is very important to understand the health effects of A-bombs so that the essence of it is translated from Japanese to English with the permission of the publisher. KEYWORDS: atomic bomb, black rain, exposure dose estimation, Gensaku Obo, hibakusha, Life Span Study INTRODUCTION A-bomb radiation. Exposure doses were reexamined and Dose At first, the exposure doses of hibakusha (A-bomb survivors) were System 1986 (DS86) was established [2]. In the period around the estimated using data collected from A-bomb tests on the ground in 1990s, DS86 was revised again and Dose System 2002 (DS02) was the Nevada desert. A-bombs dropped on Hiroshima and Nagasaki established [3]. DS02 is the current system used to estimate the were detonated at 600 m and 503 m heights, respectively. To obtain exposure doses of hibakusha. Thus, T65D was the basic dosimetry more accurate data, the ICHIBAN project was planned, in which a system and DS86 and DS02 are modified versions. The ICHIBAN 510 m high tower was constructed in the Nevada desert [1]. A project was a large-scale simulation model of A-bombs, but fallout nuclear reactor or other radiation source was placed on the top of was not produced, as it is by real A-bombs, creating a major under- the tower and data were collected. The dosimetry of the ICHIBAN estimation of exposure doses of hibakusha. project was named tentative dose 1965 (T65D). Around the 1980s, The energy of a typical A-bomb is divided into three compo- it was found that T65D did not correctly reflect the intensity of nents: 35% thermal radiation (heat and light), 50% blast energy © The Author 2017. Published by Oxford University Press on behalf of The Japan Radiation Research Society and Japanese Society for Radiation Oncology. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact [email protected] • 745 746 • S. Sutou (pressure shock wave) and 15% nuclear radiation [4]. Of the 15%, water from the many waterways in Hiroshima. One half of an 5% is initial radiation (released within 30 s) and 10% is residual A-bomb energy is blast energy (pressure shock wave), and this radiation, which consists largely of fallout, plus a small amount of energy leads to the whirling of soil and debris up to form a Mach induced radioactivity [5]. Induced radioactivity is produced by the wave mass. A mushroom cloud was formed by the action of joint action of neutrons in making non-radioactive substances into radio- actions of the thermal and blast energy (Fig. 1, left). The 15% active ones, but its lifespan is very short and the activity becomes nuclear radiation comprised of initial (one-third of the nuclear radi- one hundredth and one thousandth one hour and one day after the ation energy) and residual radiation (two-thirds of the nuclear radi- blast, respectively. Thus, induced radioactivity is mostly negligible. ation energy). Most of the residual radiation consisted of fallout On the other hand, a large fraction of fallout, 40–70%, is believed to that was sucked into the mushroom cloud. Vapors and water went fall down onto the ground within a day (although the situation up into the sky and cooled to form raindrops containing fallout, depends largely on weather and geographical features). This makes soot and other debris; the resultant black rain started to pour down calculation of the effects of fallout on the exposure doses difficult. 20–30 min after the detonation and covered almost all of Hiroshima Black rain never falls in the Nevada desert, because water and City (Fig. 1, right). organic sources are scarce. At Hiroshima and Nagasaki, thermal radiation incinerated or scalded plants, animals (including humans), Abstract of Obo’s article and reproduction of seven houses, and a variety of organic substances. From the many water- tables from that article ways in Hiroshima and Nagasaki, a large volume of water was eva- Introductory remarks by Obo porated, and even water itself seemed to have been sucked up as if There are several reports indicating the occurrence of disorders by a tornado. The vapor and water went up into the sky and cooled, such as reduced leukocyte counts and radiation sickness in non- thereafter, to form raindrops containing soot and other debris; the hibakusha who entered the central area around the epicenter after resultant black rain started to pour down 20–30 min after the deton- the A-bomb blast. Approximately the same number of reports sup- ation. It is well known that the heavy black rain was highly radio- port no hazardous effects from residual radiation. Therefore, a active. Therefore there is the possibility that the black rain included decisive conclusion has not been reached as to the effects of residual most fallout (two-thirds of the nuclear radiation energy), i.e. twice radiation. as much radiation as the initial radiation (one-third of the nuclear Obo had been studying the relationship between the A-bomb radiation energy). However, accurate estimation of exposure doses radiation and the death rate by malignant neoplasm. During the from residual radiation is quite difficult, in spite of long years of study, he wanted to learn the effects of the residual radiation on research, and the findings remain disparate and inconclusive [6]. A human health, but he found that no statistical observation had been report indicated that rain exposure shortly after the A-bombings in conducted. Thus, he decided to carry out a personal survey of all Hiroshima and Nagasaki was unlikely to increase cancer risk [7]. hibakusha within a certain area concerning their exposure condi- However, exposure to the rain itself was not the main issue; what tions, their radiation sickness (and degree if applicable), and their was critically important was whether or not people entered black behavior during the 3 months after the blast. Examinees were classi- rain–affected areas, as Obo’s article shows [8]. His article is entitled fied into two groups: those who entered the central area and those ‘Statistical observation of disorders induced by residual radiation of who did not. He also found 629 non-hibakusha who were not in A-bomb.’ It was reported in a rather minor medical journal, ‘Nihon Hiroshima City during the A-bomb blast, but who entered the City Iji Shinpo (Japan Medical Journal)’, in Japanese, 60 years ago. He after the blast. They were also classified into two groups, as for wrote that all people who lived in the 7 km radius from the epicen- hibakusha. ter were personally interviewed, but the total population in the area was not reported; thus, the percentage of responders is not known. Students of Hiroshima University took part in this study, but the Definitions and methods number of students is not reported. Self-report symptoms 12 years ‘Hibakusha’ are people who were in Hiroshima City at 8:15 a.m. on after the A-bomb were surveyed; however, people’s memories might 6 August 1945. ‘Non-hibakusha’ are people who entered Hiroshima not be always reliable. No data on radiation doses were available at City within the period of 3 months after the blast. ‘Exposure condi- the time when Obo carried out his research. Thus, his article con- tions’ include locations within the blast, whether indoors or outdoors tains some weaknesses from the present viewpoint, but is still during the blast, presence or absence of shields, structure of buildings worthy of being widely read.