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Comment on the Treatment of Dose-Response Relationship for the Epidemiological Data of Atomic Bomb Survivors

Tatsuo Matsuura Radiation Education Forum, Sanwa Daiichi Building, 1-17-2, Nishi-Shinbashi, Minato-ku, Tokyo, 105 Japan

1. INTRODUCTION

At present, the general public in all over the world have an excessive fear and concern for the risk of radiation and radioactivity (1). If things will be left as they are, not only the sound development of peaceful uses of nuclear technology and ionizing radiations in various fields will be obstructed and may lead to earlier global energy crisis, but also not a few innocent people who are ignorant of true scientific facts will never cease to suffer from the psychological stress, frequently resulting in various kinds of tragedy due to the unnecessary fear for very minor amount of radiation (2,3). As a scientist who has been engaged in the use of radiation and radioisotopes for many decades and knows from his own experience their usefulness and the degree of accompanied risk, we cannot look on this perception gap between experts and general public and that many people are being worried by incorrect information (4). Several experts nowadays point out that ICRP recommendations (5), which employ the linear non- threshold (LNT) model for the effect of low level radiation as the basic principle, are responsible to the "" of the general public (6-10). And both animal experiment and epidemiological studies are now being intensively made, and whether or not there is a threshold dose at low level in the dose-response relationship for the cancer incidence is the most controversial issue (11). Although recent experimental research shows that a quite different mechanism can be working at low level radiation contrary to the harmful effect at medium or high level radiation (6,15,16), it seems that the authorities such as ICRP and UNSCEAR (12) give too much emphasis on the epidemiological data of atomic bomb survivors provided by the Radiation Effects Research Foundation (RERF) for the risk estimation of low dose (11). In this paper, a criticism is given for the present RERF’s treatment of the epidemiological data of atomic bomb survivors, as an extension of the idea which was presented before by the author (1). The author hopes that the present consideration will be accepted in its basic concept and be able to contribute to cease the ever-lasting controversy on the credibility of the LNT model.

II. CRITICISM ON THE TREATMENT OF THE EPIDEMIOLOGICAL DATA OF ATOMIC BOMB SURVIVORS

There are two fundamental questions as to the present analysis of the dose-response relationship (DRR) of atomic bomb (A-bomb) survivors: one is for the credibility the exposed dose value of the survivors, another is for the validity of the treatment of “zero dose group.”

A. Necessity of Exact Assessment of the Exposed Dose of the Survivors by Adding the Chronic Dose

The DRR for solid cancer incidence for A-bomb survivors at low dose has been reported to be on a linear line as schematically depicted as the solid line (a) in Fig.1, by smoothing some ruggedness. (Please note here that the line does not start from zero point but from a point in abscissa in the middle of 0 and 0.01 Sv (or 0.005 Sv), because this is the “zero exposed group”.) The data of this form have been regarded to be very important, and the precision or statistical significance of several plots in the direction of ordinate has been the hot topic of controversy (19,20,21). However, there is a question as to the accuracy of the values in the direction of abscissa, i.e., the values and precision of the exposed dose of the survivors. Although the present data have elaborately been estimated by the very sophisticated calculation called "T65D" or "DS86" (22), They are only for the acute dose exposed to the survivors at the instant of bomb explosion. Actually, many survivors had lived at the same places as they lived before soon after the mishaps, mostly in temporarily constructed small barracks. Thus they had been obliged to receive some additional chronic exposure from the contaminated circumstances, either by fallout and/or by neutron-induced radioactivity. The radiation level was extremely high just after the explosion of the bomb, due to the short life radionuclides of fission product in the fallout and/or of induced activities in the ground, and the high level lasted at least for a few weeks. Although the exact estimation of this cumulative chronic dose is very difficult task, probably depending from individual to individual, this contribution should be estimated even as an approximate value and added to the already estimated acute dose for the purpose of obtaining the correct DRR. To understand the importance of this contribution, let us introduce a few anecdotes about the “in-city

1 P-2a-S1 hibakusha.” According to a report telling the story of (23), it is said that there were many people who were not in the city when the bomb was dropped, but they died because of their frequent visits to the center of the city to search for his relatives and friends, or to rescue injured people. The fatal exposure for such cases must have been as much as about a few Gy. Another case (24) is a boy, who, a son of Mr. M. Uda, a staff of Hiroshima Meteorological Observatory, was not in Hiroshima when the bomb was dropped and came back to his home located 4.2 km west from the site of explosion (hypocenter) after two months of bombing, showed loss of his hair, as a symptom of radiation disease soon after his coming back to his home. And it was proved that this was caused by the high radioactivity found in the black–colored deposit of rain attached on a sliding door near his bedroom. (The measurement was done by Dr. Y. Miyazaki, a personnel of Riken.) We can easily imagine that many survivors may have been more or less in similar circumstances. For this type of chronic dose, the author proposes to call it "in-city walking-around dose" for the survivors. As the first approximation, we may assume that all survivors, belonging to every cohort, received an equal amount of chronic dose on average by this way, say, about 0.1 Gy. Then the revised DRR should shift rightward by this value as shown by the line (b) in Fig. 1. Then the extrapolation of the line will clearly give a threshold value of at least 0.1 Gy.

B. Classification of Chronic Dose

As stated above, there are several types in chronic dose, and it is hoped to be able to estimate each contribution for all cohorts of survivors. First, let us consider the characteristics of the chronic dose due to the fallout and the induced activity. Both have a localized nature, but their ways are different. As for the contribution of the induced radioactivity due to neutron bombardment, it much depends on the distance from the hypocenter. Assumed a survivor lived in the same place as before, the nearer he/or she was to the hypocenter and received the higher instantaneous dose, the higher must have been the level of the induced activity, thus the larger the additional integrated exposure. Then, if this contribution is added to the "in-city walking-around dose," the pattern of DRR will schematically become the line (c) of Fig. 1. As shown here, the net contribution by this factor for the cohort of low acute dose, i.e., at distant place from the hypocenter, is negligible. As for the fallout, it is a well-known historical fact that a heavy black-colored rain, which was highly radioactive, fell in a wide range of the city, about 20 or 30 minutes after the bomb explosion (23,24,25). The area where the radioactive rain fell depended on the direction of wind of that day. In Hiroshima, it is reported that the area covered a considerable wide area in the direction of northwest from the hypocenter, as shown in Fig. 2 (24), which had an oval shape with a long diameter and a short one of 29 km and 15 km, respectively (23,25). In , the direction was east. Because the wide area of this rainfall was concerned with the districts of every cohort, the way of contribution of this factor may be regarded to be as the first approximation the same to each cohort, although within a cohort the value must differ considerably depending on the direction as to the hypocenter. By adding this contribution, the pattern of DRR will become the line (d ) in Fig.1. We should note that the total dose should increase even for "zero dose group" by this contribution. There is some additional information about the fallout called "Black rain." Recently, it has been reported (26) that the trace of the black rain is still remaining on the wall of an old house in Hiroshima, and that the chemical analysis of the black residue revealed that the black color was due to the colloidal iron oxide, which must be from the vessel of the bomb.

C. Information on the Fallout in Nagasaki and on the Residual Radioactivity

There is an important information as to the effect of fallout. That is a case for the residents at Nishiyama District, located 3 km from the hypocenter in Nagasaki. There, the instantaneous radiation was shielded by a mountain, but the "black rain" containing the fallout radioactivity gave the 280 residents a chronic exposure of as much as 0.2 Sv (6) or 0.7 Sv (22) and caused an abnormal increase in leukocytes of most residents for several months (6, 27). For these area in Nagasaki, maps are available, which illustrate the intensity of residual radioactivity, as shown in Fig. 3 (28,29). This shows that the considerably high intensity of radioactivity remained in this district even a few months after the bomb explosion, and that within Nishiyama District there is a wide range of variation, and there are several hot spots, where the activity was extraordinarily high. It is worthy of note also that by comparing the values of activity in Nishiyama District and the hypocenter of Nagasaki, the contribution of fallout is far more higher than that of induced activity after several months from bomb explosion. Thus it is possible that the additional dose due to fallout may not be regarded to be uniform but may be as shown as a dotted line (e) in Fig.1, which includes an erratic, extraordinarily high additional dose value, corresponding to the presence of hot spots in fallout. For this pattern of DRR, the threshold can appear with a little higher probability than the case of the line (d).

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D. An Additional Consideration of “Zero-Dose” Group

An additional aspect as to the dose of A-bomb survivors, which has been pointed out by Prof. Emeritus Tsutomu Sugahara (30), is the fact that the exposed group below 0.01 Gy has until quite recently been treated as "zero dose group". Since 1996, such as in ref. 12, "zero" has become below 0.005 Gy. Because this “zero dose” group is treated as the control for comparison with other cohorts, if a hormetic effect actually had occurred in this group, every other value at higher doses will naturally show only positive risk. It should be noted that this is probable, because the exact dose range for this "zero dose" group is not beween 0 and 0.01 Gy (or 0.005 Gy) but between (0 + x) and (0.01 (or 0.005 Gy) + x) Gy, where x is the additional chronic dose and can be the order of cGy. What is stated here can be expressed in dotted line (f) in Fig.1, which shows that a threshold value will appear in much higher position than the line (e).

III. CONCLUSION AND PROPOSAL

(1) For the determination of presence or absence of threshold at low dose radiation in the cancer incidence from epidemiological data, it is necessary to re-estimate the individual dose of all atomic bomb survivors including the “zero dose group”, by paying much attention to the chronic dose from various origins. This may come from the fallout called “black rain”, from the induced activity by their living at the contaminated area, and from the possibility of "walking around" of survivors in highly contaminated area just after the explosion of the bomb. In this connection, It is urgently needed to collect the information and data of the exposed dose of “in-city hibakusha.” It is very probable that a threshold value exists in dose response relationship, by positively assuming the various contributions stated above and estimating reasonable non-zero values for chronic dose for all survivors. Thus we believe the LNT model is no longer valid also from the epidemiological data. (2) The recommendations of ICRP should be amended, both by honestly adopting the several recent scientific findings and plausible reasoning (31), and by carefully considering social effects. The policy makers should try to establish a reasonable and acceptable policy of radiation protection, so as not to make the majority of people feel uneasy nor the society waste too much money, for the wholesome development of the peaceful uses of radiation and nuclear energy. We support the basic idea of recommendation summarized in BRPS Conference which was recently held (32).

SUMMARY There are two important questions as to the treatment of epidemiological data of atomic bomb survivors, which have been the foundation of the LNT hypothesis for the effect of low level radiation exposure. First point is the necessity of re-assessment of individual radiation dose of the survivors, by taking account of the chronic dose due to various origins, i.e., the fallout called "black rain", induced activity near the residence, and the possibility of survivors' additional exposure after bombing just as "in-city hibakusha", in addition to the instantaneous acute dose. The second is the fact that the “zero dose” group, which is used as the control of more exposed group, is the group, which acute dose was estimated to be below 0.01 Gy until 1996, but actually very possibly received much more dose, thus the hormetic effect might be hidden in this dose range. After all, there is a positive possibility for the existence of a threshold value in dose response relationship. ICRP should reconsider its recommendation, both from the scientific facts as described here and from the social responsibility of proliferating correct knowledge about the risk of radiation.

ACKNOWLEDGEMENTS The author wishes to express his sincere thanks to Professor Emeritus Tsutomu Sugahara, Professor Emeritus Sohei Kondo, Professor Emeritus Nobufusa Saito, Professsor Emeritus Masanobu Sakanoue, Professor Emeritus Hikoyuki Yamaguchi, Dr. Hiroshi Tanooka, Dr. Masato Kaneko, and several colleagues of Radiation Education Forum, for their warm encouragement. Special thanks are due to Professor Emeritus T. Sugahara, for his continuous interest in this work and for providing the author the most recent information on BRPS Conference.

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