社会医学研究．第 29 巻 1 号．Bulletin of Social Medicine, Vol.29(1)2011

原著

Estimation of Residual Nuclear Radiation Effects on Survivors ofHiroshima Atomic Bombing, from Incidence of Acute Radiation Disease
急性放射線症状発症率から広島原爆被爆者に対する残留放射線影響評価
Shoji Sawada
沢田昭二
Emeritus Professor of Nagoya University, Nagoya, Japan
E-mail: sawadas(at)fb3.so-net.ne.jp
名古屋大学名誉教授
Abstract
The effects of exposure due to radioactive fallout on survivors of the Hiroshima atomic bomb were estimatedby analyzing the incidence rates of acute radiation diseases, epilation, purpura and diarrhea, among survivors. It
was found that the effects of radiation exposure due to fallout exceeds, on the average, the initial nuclear radiation
effects in people who were beyond about 1.2 km from the hypocenter of the Hiroshima bomb. The average effectof radiation exposure from fallout increases with distance from the hypocenter, reaches a peak at around 1.5 km,
and then decreases gradually at farther distances but remains even at about 6 km. The peak value of estimatedhealth effects from fallout is comparable with that of acute external exposure of gamma ray doses around 1Gy.
The fact that the effects of residual nuclear radiation estimated from the incidence rate of acute diseases are
significantly larger than physically measured residual nuclear radiation doses suggests that the main effects
resulting from residual nuclear radiation were caused through internal exposure, especially intake of radioactive
small particles from fallout via ingestion and inhalation.
要旨
広島原爆被爆者の放射性降下物による被曝影響を急性放射線症状の脱毛、紫斑および下痢の発症率から評価した．

放射性降下物による人々の平均的被曝影響は爆心地から 1.2 km 以遠で、初期放射線被曝の影響を上回ることを見出した．この降下物による平均的被曝影響は，爆心地からの距離とともに増大し、約 1.5 km で最大値に達して後ゆっ
くり減少するが、6 km でも残存する．評価した降下物による最大の健康影響はガンマ線による瞬間的な外部被曝の1 Gy に相当する．急性症状発症率から推定した残留放射線の影響が物理学的に測定した線量よりかなり大きいこと
は、残留放射線の影響が内部被曝、とりわけ放射性降下物の中の放射性微粒子を呼吸や飲食で摂取したことによって起こったことを示唆する。
§1 Introduction

the bomb explosion has been not clarified as well

Doses of initial nuclear radiation emitted within one

as the initial radiation. There are two sources ofminute after the atomic bombs exploded on the cities

residual nuclear radiation. One is from fallout and theof Hiroshima and Nagasaki are well estimated by theother from neutron-induced radioactive substances.

Dosimetry System 2002 (DS02)

1)

in the regions within

Estimates of fallout radiation dose that have been

1.2 km from the hypocenter. These estimates which

made to date are based on measurements of radiation

are supported by experimental measurements on

emitted from radioactive matter produced byirradiated materials. On the other hand, the residual

radioactive fallout in rain which had been absorbed

nuclear radiation emitted one minute or more after

into soil and retained. However, these measurements－ 47 －

社会医学研究．第 29 巻 1 号．Bulletin of Social Medicine, Vol.29(1)2011

were made after heavy rains caused by a big fireby the fallout radiation. Then by subtracting the

involving the whole of Hiroshima city and also

initial nuclear radiation dose from this resulting

following major typhoon events. It follows that theseeffective dose, the effective dose due to exposure

measurements detected only a small fraction of thefrom fallout radiation alone will be obtained. This

radioactive matter which remained without having

biological dosimetry method will give only combined

been washed away. In most atmospheric nuclear

effects from both residual external and internal

explosion tests in dry deserts, there are no fallout

exposure to the survivors.

rains and radioactive fine particles in the fallout

In this article, the incidence rates of acute radiationthat fill the air after the atomic cloud disappears are

diseases among survivors of the Hiroshima atomic

measured by equipment set up before the explosion

bomb are analyzed in order to clarify the effects

over wide area. In Hiroshima and Nagasaki, theof residual nuclear radiation from fallout. The factradioactive fine particles that would have filled thethat the obtained effects of residual nuclear radiation

air under the atomic clouds were carried away by

estimated from the incidence rate of acute diseases

the wind long before measurements. In addition, theare significantly larger than would be expected from

effects of residual nuclear radiation are the result of

the physically measured residual nuclear radiation

both external and internal exposure through intake

doses suggests that the main effects from residualof radioactive microscopic particles by inhalation andnuclear radiation were caused through internal

ingestion. In general, physical measurements of these

exposure, especially the intake of radioactive small

exposure effects were not done and quantification

particles from fallout via ingestion and inhalation, as

through measurement is now difficult. These facts

well as external exposure from radioactive particles

imply that there are severe limitations to estimationclinging to the skin or clothes. The results of theof the residual nuclear radiation doses delivered byeffects of residual nuclear radiation obtained here

the Hiroshima atomic bomb using physical methods.

from the incidence rates are consistent with studies

There have been many investigations of acuteof the frequency of chromosomal aberration andradiation diseases among atomic bomb survivors,

mortality and incidence rates of chronic diseases,

both from immediately after the bombing and later

obtained from comparison with people who wereon, and all results of these investigations show that

truly unexposed from the nuclear radiation.

acute diseases such as epilation, purpura and diarrhea
appeared even in regions 2 km or more from the§2

Relation between Incidence Rate of Epilationand Exposed Dosehypocenter. The fact that the diseases have occurred
systematically among survivors who were present inThis section explores the relation between theregions where the initial nuclear radiations scarcely

exposure dose and the incidence rates of epilation, areached suggests that they should be explained intypical acute radiation disease. Stram and Mizuno2)

terms of fallout radiation. In order to understand thefirst derived a relation between absorbed dose ofeffects of residual nuclear radiation comprehensively,

acute external exposure from the initial nucleari t i s n e c e s s a r y t o s t u d y t h e r e s u l t s o f suchradiation of the atomic bomb and the incidence rates

investigations of acute diseases as well as the risksof epilation. They employed the results of the Life-of chronic diseases and frequency of chromosomal

Span-Study (LSS, 58,500 Hiroshima people and 28,132

aberration, i.e. biological effects caused by radiation

Nagasaki people) group of the Atomic Bomb Casualty

exposure among survivors.

Commission (ABCC, the predecessor of Radiation

If one can determine the relation between radiation

Effect Research Foundation, RERF), obtained aroundexposure and incidence rates of a specific acute1950 for the heavy epilation (above 67% ) that appeared

disease, then it is possible to obtain the effective

within 60 days after the detonation of the bomb.

dose from the sum of external exposure by the initial

Their results are shown by small black circles in Fig.

nuclear radiation and external and internal exposure

1. The horizontal axis of Fig. 1 is scaled according－ 48 －

社会医学研究．第 29 巻 1 号．Bulletin of Social Medicine, Vol.29(1)2011

to the initial nuclear radiation dose estimated by3)near zero dose, represented by black circles, are too

the Dosimetry System 1986 (DS86) . As shown in

rapid in comparison with the normal distribution.

Fig. 1 the incidence rate increases slowly up to 0.85

This different behavior between the incidence rates

Gy of the initial nuclear radiation dose, then rapidly

found by Stram and Mizuno and those of normal

increases above 1 Gy and exceeds 50% at around 2.4

distribution can be explained by adding the increase

Gy. However, beyond 3 Gy the rates do not increaseof fallout exposure to their analysis.

and even decrease as dose approaches 6 Gy. This

Taking account of the above observations, theunnatural behavior of the incidence rates in the high

KSRF relation is adopted as the relation between

dose region can be explained by the fact that the LSS

incidence rates of epilation and exposed dose ingroup contains only survivors who could survive eventhe following analysis.

though they had been exposed at levels of near or

acute disease in the region below 1 km from themore than a half-death-dose of about 4 Gy, as pointed

Hiroshima hypocenter are excluded from analysis

4-6)

out by Stewart et al.

The incidence rates of

because most of the people within 1 km were

Incidence rates of epilation, shown by open circles

killed and the reported rates are statistically7)

unreliable. Furthermore, the total sums of gamma

based on radiation exposure to human head skin

ray and neutron doses at 1 km are 4.48 Gy from thetransplanted onto immunodeficient mice. As seen

estimation by DS02, by which the calculated incidence

in Fig. 1 the incidence rates increase very slowly inrate reached almost 100% on the basis of the assumed

the low exposure region compared to those given

normal distribution of the KSTS relation.

in Fig. 1, are those obtained by Kyoizumi et al.

by Stram and Mizuno, and increase to 95.5% and 97% ,almost 100% at exposure of 4.5 Gy. From experimental§3 E
 stimation of Fallout Exposure from Incidence
Rate of Epilation among LSS Group

studies with animals it is known that most of thedose dependence of incidence rates or death rates

In this section the radiation exposure effects

is represented by a normal (Gaussian) distribution7).

from the fallout of the Hiroshima bombing are

The inci}dence rates given by Kyoizumi et al. overestimated on the basis of incidence rates of epilation

the whole range of the exposure region can be fitted

among the LSS-Hiroshima group. Preston et al. 9)well by a normal distribution with an expectation

reported separately the incidence rates of epilation

value of 2.751 Gy and standard deviation of 0.794 Gy,

in Hiroshima and Nagasaki survivors among the LSS

i.e. N (2.751 Gy, 0.794 Gy), and shown by a solid curvegroup, which was analyzed by Stram and Mizuno.

in Fig. 1. The normal distribution N (2.751 Gy, 0.794

The dependence of the incidence rates of epilation ofGy) will be referred to as the KSTS relation. When

58,500 Hiroshima survivors among the LSS is shown4)

one recognizes that the results of Stram and Mizuno

by square of distance from the hypocenter in Fig. 2.

shown in Fig. 1 were obtained from examination

The incidence rate of 100% at 0.75 km is scaled out ofdata of the LSS on the basis of the assumption thatthe frame. The black circles and dashed line in Fig.

the epilation was caused only by the exposure to

1 for the initial nuclear radiation dose dependence

initial nuclear radiation, and regarding the fallout

are translated into distance dependence by use of theradiation as the background, there is a possibility thatDS86 estimation neglecting shielding effects and are

the black circles in Fig. 1 will shift toward higher

plotted by black diamonds and a broken line in Fig.

dose and higher incident rates, i.e. toward the KSTS

2. If the shielding effects are taken into account, therelation if the exposure to the fallout radiation isdiamonds shown in Fig. 2 will move left toward theincluded. The incidence rates of epilation below the 3

hypocenter and the difference between the squares

Gy exposure region given by Stram and Mizuno canwill increase. In the following it is assumed thatbe fitted by a normal distribution N (2,404 Gy, 1,026

the systematic difference between the squares andGy) except for the region near zero dose. However,diamonds shown in Fig. 2 represents exposed effects

the increases of the incidence rates in the region

from fallout radiation.

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社会医学研究．第 29 巻 1 号．Bulletin of Social Medicine, Vol.29(1)2011

It is known that the Hiroshima bomb at the time

In the analysis of the incidence rate of epilationof detonation was not oriented vertically but inclined

it is assumed that the cylindrically symmetric total

about 15 degrees, and the initial nuclear radiation

exposed dose D(r) at distance r km from hypocenter

was not uniform in angle because radiation was quite

is given by the sum of the initial nuclear radiation

suppressed in the bomb nose direction. But thisexposure cP(r), with shielding effects represented

cylindrical asymmetry is limited to within 1 km fromby a parameter c, and exposure F(r) from fallout

1)

the hypocenter . On the other hand, at the time ofradiation as

explosion there was ambient wind blowing at 1 to   D(r) = cP(r) + F(r).

3 meters per second toward the north-west. The

The formula for exposure from fallout radiationstrong updraft of the column part of the atomic cloud(1)

F(r) is assumed to becaused rapid extension of the wing part of the cloud   F(r) = a r exp( – r2 / b2 ) + d

along the tropopause (surface between troposphere

where parameters a, b, and d represent magnitude,

and stratospheric) with a radius 10 to 15 km within

extension and distance independent components of

one hour. Under this widely extended atomic cloud,fallout exposure effects, respectively. Theoreticalstrong wind blew toward the hypocenter.

Small

incidence rates are calculated from the exposure dose

falling raindrops of the extended parts of the atomic

D(r) given in (1) by use of the KSTS relation between

cloud were drawn down by this wind and theseincidence rate and exposed dose.

raindrops returned the small radioactive particles.

parameters in (1) and (2) is determined so that the x2

The atmosphere under the atomic cloud was thenvalue is minimum, which represents the fitness of thefilled with floating radioactive small particles carriedcalculated incidence rates to those of the LSS groupby wind toward the hypocenter. Heavy fallout rains

and obtained as c = 0.522, a = 0.808 Gy/km, b = 2.062

fell from the central part of the atomic cloud. The

km, and d = 0.786 Gy. The resulting fitted incidence

atomic cloud moved toward the west-northwest along

rates are shown with bold lines in Fig. 2. The doses

the ambient wind and heavy fallout rains fell in theof total, initial nuclear radiation and fallout exposure,regions located west-northwest from the hypocenter.

D(r), cP(r) and F(r), with the obtained parameter set

The rains scavenged radioactive floating particles toare shown by a bold dashed curve, a thin dasheda certain extent. It is supposed that the fallout finecurve and a bold solid curve, respectively, and theparticles were moved toward northwest direction

initial nuclear radiation doses estimated by DS02

by the ambient wind and toward the hypocenter by

are shown by a thin solid line in Fig. 3. As seen in

the centripetal wind. To clarify these complicatedFig. 3, the effects of fallout exposure increases withbehaviors of particles, which were inferred to be

distance from hypocenter up to 1 km, but this hastaken in and to be an important cause of acutelarge ambiguity because the incidence rates in the

diseases among survivors, it is necessary to carryregion below 1 km were not employed in the present

out a direction dependent analysis. However, data

analysis. Exposure from the initial nuclear radiationon acute diseases have so far only been examined

rapidly decreased with distance from the hypocenter

separately by distance and not by direction fromand at about 1.2 km the fallout effects cross over thethe hypocenter.

In the following analysis, theeffect of initial nuclear radiation. Beyond this distancedistance dependence of radiation exposure means

the fallout effects become dominant. The estimatedthe average of the directions with the same distance.

exposure from fallout radiation reaches about 1.5

In these averages the northwest regions beyond 2

Gy at around 1.45 km, and then decreases slowly.

km from the hypocenter are not included though

Beyond 4 km the exposure effect of fallout takes an

the fallout particles moved in this direction because

almost constant value of 0.79 Gy. This result fromthe population northwest of Hiroshima was blocked

the incidence rates of epilation, one of the actual

by the mountains located about 2 km from theaccepted and universally agreed conditions of the

hypocenter.

bomb survivors, indicates overwhelming effects of－ 50 －(2)

A set of four

社会医学研究．第 29 巻 1 号．Bulletin of Social Medicine, Vol.29(1)2011

fallout beyond about 1.5 km from the hypocenter of§５ E
 stimation of Fallout Exposure from Incidence

Hiroshima. For example at 2.25 km and 2.75 km from

Rates of Epilation Examinations Other than LSS

the hypocenter, the dose estimations of the initial

In Fig. 4 incidence rates of epilation examined by

nuclear radiation by DS02 are 0.0302 Gy and 0.0053

the Joint Commission for the Investigation of theGy, while the incidence rates of epilation among the
LSS-Hiroshima at these distances are 3.5 % and 2.1 % .
The estimated fallout exposure effects from theseincidence rates is 1.34 Gy and 1.16 Gy, about 44 and219 times of the DS02 initial nuclear radiation.
The maximum cumulative exposure from falloutof the Hiroshima bomb has been considered hitherto

Atomic Bomb 9) and Tokyo Imperial University 10)
in 1945, and investigated by O-ho 11) in 1957, areshown together with those of LSS-Hiroshima. In theinvestigation by O-ho, all survivors were classified
into four types according to whether they were
exposed indoors or outdoors and did or did not enter
the central region within 1 km from the Hiroshima
hypocenter within 3 months. The O-ho examination

to be between 0.006 and 0.02 Gy in the Koi-Takasu

of the No Entrance case is very important because

region mentioned in the DS86 report, which are

the exposures from the induced radioactive matter inshown by cross marks in Fig. 3. These absorbed

the central region were not included.

doses were obtained from measurement of radiation

Except for two incidence rates at 2 km and 4 km*

from fallout matter retained in the soil of theseby O-ho all the examined incidence rates of epilation

regions, which are located between 2 and 4 km to theamong the Hiroshima survivors closely coincidewest of the hypocenter where light radioactive fallout
rain fell but heavy rain caused by the whole city
fire did not. As seen in Fig. 3, exposure from fallout
estimated from epilation incidence rates in the 2 to
4 km region are 1.4 Gy to 0.85 Gy, which are 40 to
230 times the physically obtained values. This large
discrepancy suggests that the physically measured
values are only measurements of part of the fallout,
and that large effects of internal exposure which can
only be deduced by biological methods should betaken into account. It is noteworthy that the values
obtained here are average exposures in the same
distant regions from the hypocenter, irrespective of
direction. This result supports the understandingthat fallout particles were distributed in the air oververy wide regions under the expanded atomic bomb.

with each other, indicating the reliability of all theseinvestigations. That the rates of the LSS-Hiroshima
between 1.75 km and 2.75 km are slightly lower
systematically than the others may be explainedby the fact that in the LSS examination epilation isdefined as only heavy epilation with 67% loss of hair
within 60 days from the atomic bombing.
The same fitting method used in the LSS groupis applied for these incidence rates of epilation. The
resulting sets of parameters from application of
formulae (1) and (2) are given in Table I. The fitted
incidence rates curves are shown by thin dashed lines
in Fig. 4. The calculated doses of total, initial nuclear
and fallout exposure, D(r), cP(r) and F(r), obtained byfitting to the reported incidence rate curves of epilation,
are shown by a bold dashed curve, a thin dashed curveand a bold solid curve, respectively, in Fig. 5.

*1 These data at 2 km and 4 km given by O-ho are omitted in the x2 fitting.

－ 51 －

社会医学研究．第 29 巻 1 号．Bulletin of Social Medicine, Vol.29(1)2011

The peak values of exposure by fallout are found tosimilar to those of epilation shown by squares. The

lie between 1.58 Gy and 1.78 Gy, slightly higher thansame normal distribution of epilation is then usedthat of the LSS, as expected from the small difference

for the incidence rate-exposure relation of purpura.

among incidence rates. In the region beyond 3 km

Incidence rates of diarrhea, shown by triangles, arefrom the hypocenter the fallout exposure estimation

very large compared to epilation or purpura in thefrom O-ho’s incidence rates is similar to those fromdistant regions beyond 1.5 km where the fallout

the LSS. A rapid decrease of fallout exposure dose

exposure gave significant effects. The incidence rates

is seen beyond 3 km in the examination by the Jointof diarrhea were rather small in the short distanceCommission, where incidence rates of epilation in theregions where the initial nuclear radiation exposure

region 4-5 km and beyond 5 km are zero based on

dominated. Therefore, in the case of diarrhea, a larger

examination of very few survivors compared withexpectation value for the normal distribution thanLSS.

those for epilation and purpura is required for external
exposure from the initial nuclear radiation, and a smaller§6 C omparison of Fallout Exposure Estimated fromexpectation value is required for the fallout exposure.

Incidence Rates of Three Different Acute Diseases

The adapted expectation values and standard deviations

In the following it will be examined whether the

are listed in Table II and were obtained by multiplying

incidence rates of three different acute diseases,

the ratios shown there. Using the normal distributions

epilation, purpura and diarrhea can be explainedwith expectation values and standard deviations givenby the same exposure dose. The incidence rates ofin Table II, the incidence rates of epilation, purpura andepilation, purpura and diarrhea among Hiroshimadiarrhea are fitted in Fig. 6 and the resulting incidence

survivors who were exposed indoors and did notrates are displayed by thin dashed, solid and chain

enter the central region found by O-ho11) are shown

curves for epilation, purpura and diarrhea, respectively,

in Fig. 6. As is seen in this figure, the incidence

whose parameters for formulae (1) and (2) are listed inrates of purpura shown by closed circles are of

Table III.

Table II Normal distributions of incidence rate-exposure dose relations of acute radiation diseases

Table III Parameters in formulae (1) and (2) of exposed doses from incidence rates of epilation,
purpura and diarrhea－ 52 －

社会医学研究．第 29 巻 1 号．Bulletin of Social Medicine, Vol.29(1)2011

In this analysis of epilation and purpura theatomic bombing estimated from incidence rates

shielding effect parameter c is fixed at 0.5 and theof acute diseases among survivors are very large

data on incidence rates of these diseases at 1 km areand extended to a wide area. Effects of exposure

omitted because when these data for epilation andto fallout radiation were greater than the effects of

purpura are used, unnaturally small values of c, 0.22

initial nuclear prompt radiation beyond about 1.2

and 0.23 are obtained. These unnatural small values

km from the hypocenter and decreased slowly withat 1 km may be explained by a similar reason to that

distance, remaining at about 0.7-0.8 Gy even at 6 km.

for the incidence rates of epilation among the LSS in

The maximum exposure effects from fallout of 0.02

the large exposed region given by Stram and Mizuno,

Gy at Takasu, the special region located 3 km westi.e., many died.

from the Hiroshima hypocenter were obtained from

The results of exposure doses calculated from thephysical measurement of radiation emitted from

calculated parameters listed in Table III are shown

radioactive nuclei brought by fallout rain and retainedin Fig. 7. As seen in this figure, incidence rates ofin the soil. Fallout exposure effects estimated fromthree entirely different acute diseases are reproduced

acute diseases were between 1.1 Gy and 1.3 Gy atwith high accuracy by almost similar exposure doses.

3 km from the hypocenter, irrespective of direction

This fact tells us that epilation and diarrhea as wellfrom the hypocenter. This large difference between

as purpura occurred in the regions where the initialphysical measurements and biological estimations

nuclear radiation could scarcely reach and wereof fallout exposure implies that the main exposure

caused by fallout radiation, not by mental shock or byeffects were either caused by widely distributed finepoor sanitary conditions.

fallout particles, resulting in internal exposure due to

The fact that the expectation value of the normal

their intake, or by an error in the currently accepted

distribution of diarrhea incidence is small for fallout

radiobiological effectiveness of certain ingested or

exposure but large for the initial nuclear exposure

inhaled isotopic components of the fallout.

can be explained by a difference between external
and internal exposures, as follows.

Since the various examinations of incidence rates

In the caseof acute radiation induced diseases analyzed hereof fallout exposure, radioactive fine particles andgive almost the same results for fallout exposure, theradionuclides with specific affinity for biologicalgreatest ambiguity of obtained exposure doses arises

materials and tissues among fallout were taken intofrom ambiguity in the relations between the incidence

body, directly reached the intestinal wall and wererates and exposure dose used here; that is, ambiguity

retained there for a period of time. The emitted

in the expectation values and values of the standard

radiation, which had weak penetration power, thendeviation of the generated normal distributions.

caused dense ionization and heavy damage to the thin

However, if the fallout radiation exposure of 1.0 to

membrane, and diarrhea followed. The exposure was

1.5 Gy obtained here is added to the initial nuclear

chronic as the particulate and chemical radioisotopic

prompt radiation exposures in the region between

material (e.g. Sr-90, Cs-137) was retained for some

1 Gy and 3 Gy, which corresponds to exposure

time. On the other hand, in cases of instantaneous

distances between 1.0 km and 1.2 km, and to epilation

acute initial nuclear radiation exposure only strong

incidence rates of about 10 % , which correspond topenetrative radiation such as gamma rays could

the difference between the solid line and the broken

reach from outside of the body to the intestinal wall,line in the region between 1km and 1.2 km in Fig.

but passed away through the thin membrane leaving

2, then the broken line in Fig. 1 shifts to the higher

scarcely any damage.

dose direction and higher incidence rate direction
and almost overlaps with the solid line obtained by§7

Summary and Discussion

Kyoizumi et al., as shown by black squares in Fig. 8.

As described in the foregoing sections, theThe unnaturally rapid increase in the incidence rate

exposure effects of fallout from the Hiroshimaof epilation in the near zero dose region of the Stram-－ 53 －

社会医学研究．第 29 巻 1 号．Bulletin of Social Medicine, Vol.29(1)2011

Mizuno relation, shown in Fig. 1, can be correlated

cancer of the respiratory system and the incidence

to the decreases of incidence rates in the regionof thyroid and female breast cancer in the control

between 1.5 km and 3 km from the hypocenter wheregroup showed that they had been affected by fallout

the initial nuclear radiation exposures were between

radiation.

0 to 1 Gy. This fact supports the conclusion that theon the mortality of the LSS Hiroshima group from

relation between the incidence rates and exposure

all diseases and various cancers compared withdose among survivors is not much different fromthose of all inhabitants of Hiroshima prefecture andthat used here on the basis of the relation given bywith those of all Okayama prefecture, a neighbor

Kyoizumi et al.

prefecture of Hiroshima, indicates comparable effects

A recent study by Watanabe et al. 14)

The results obtained here do not contradictof fallout exposure with the present estimation

results of investigations of chromosomal aberration

among extremely low exposure groups (exposed

among survivors. The frequency of chromosome

from initial nuclear radiation less than 0.005 Sv) andaberrations in circulating lymphocytes of survivors

low exposure groups (exposed from initial nuclearof the Hiroshima bombing was compared with 11

radiation between 0.005 Sv and 0.1 Sv) of the LSS.

non-irradiated healthy controls visiting the Japan

Using the same method employed here, similar

Red Cross Central Hospital in Tokyo between April

effects from the residual nuclear radiation exposure

12)1967 and March 1968 by Miyata and Sasaki . It was

can be estimated for the survivors of Nagasaki asfound that more than 1.6 km from the hypocenter,

well as for the ‘entrant survivors’ who entered

the effects of exposure from fallout estimated fromregions about 1 km from the hypocenter after thefrequency of chromosomal aberration exceeded the

explosion of atomic bomb and who were exposed

effects of the initial nuclear irradiation. If we note

to residual nuclear radiation emitted by induced

that the estimated dose based on the frequency ofradioactive matter. The estimated results from thesechromosomal aberrations in circulating lymphocytes

cases will be reported elsewhere.

represents the effects averaged over the whole
body, and that local effects from insoluble radioactive

Acknowledgements

particles or other internal isotopic exposures, which

The author thanks Dr. Chris Busby for reading thisare considered in the incidence rates of acutemanuscript and providing useful comments, and for

diseases, are not included, then the present results

his kind improvement of the author’s English.

from acute diseases do not contradict those obtained
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