Physical Consideration of Internal Exposure - 1

Physical Consideration of Internal Exposure

Among fallout of the atomic bomb of Hiroshima and Nagasaki (1) 3.6􀂺1024 nuclei of
fission products, (2) (2—5) 􀂺1024 nuclei of neutron-induced radioactive matter of
bomb fragments and casing, (3) 1􀂺1026 nuclei of uranium-235 or 2.5􀂺1025 plutonium
239 which did not participate in the fission chain reaction were included respectively.
After explosion of each atomic bomb a fire ball of plasma was formed and all
radioactive nuclei listed above were included in this. The fireball turned into the
mushroom cloud. The central part of cloud rose breaking through the tropopause up to
15 km or more and the peripheral part of cloud spread along the tropopause over a
region with radius more than 15~20 km. The region where fine particles of the fallout
fell can be assumed to have been larger than the region covered by the mushroom cloud.
In the fallout a huge number of fine particles were included which had been contained
in the fireball.
The atomic bomb survivors were externally exposed by initial radiation from
outside their bodies. This exposed dose can be estimated roughly if the location of the
survivor is known. Survivors and people who entered the regions near the hypocenter
were also exposed to radiation emitted by residual radioactive matter induced by the
initial neutron beam. The doses experienced by survivors from the induced radioactive
matter can be estimated roughly by use of physical calculations and measurement data if
their actions or behaviors were known. It is difficult, however, to estimate the radiation
dose of fallout in terms of physical measurement long after the explosion because most
of the fine particles of the fallout were carried by the wind and the radioactive matter
accumulated on the surface of the earth or sank into the earth which had been brought
by so called black rain or other form of fallout were washed away by heavy rains
accompanying typhoons. It is more difficult to estimate the effects of internal exposure
by inhalation or ingestion of radioactive matter of the fallout and/or induced matter by
physical methods.

When some radioactive substances are taken into the body, if they are water or oil
soluble then they will spread throughout the whole body at the level of atom or
molecule and some radioactive materials will concentrate and/or deposit in special
organs depending on the types of chemical elements. For example, iodine concentrates
in the thyroid, and phosphorus and cobalt are concentrated in bone marrow. In this case
amounts of absorbed radioactive materials taken into body can be estimated from
excrement such as urine. By contrast, if insoluble radioactive fine particles are absorbed
there are possibilities that the fine particles are deposited in some organ while
preserving their size and that radiation emitted from these fine particles irradiate
continuously and intensively surrounding cells. In this case it is difficult to detect these
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radioactive particles from outside the body and also to estimate them from excrements.
Effects from such radioactive fine particles largely depend on their size and also on the
type of radioactive elements and type of radiation (the average life-time and alpha, beta
or gamma ray). It is difficult to represent these effects in terms of a simple factor such
as the absorbed energy per weight, the unit of absorbed dose, Gy, or by use of the
relative biological effectiveness, the unit of equivalent dose, Sv. The difference between
external uniform exposure and internal exposure by a radioactive fine particle is
illustrated in Fig. 2. Therefore the biological estimation of effective exposure dose
which includes both external and internal ones is required on the basis of analyses of the
investigation of incidence rates of the acute and clinical radiation diseases and the rate
of chromosomal aberration especially where it appears among distant survivors and
entrant survivors who were not severely exposed to the initial radiation.

initial nuclear radiation and the incidence rates of epilation based on the assumption that
epilation in the distant regions was caused by background effects other than radiation,
such as mental effects. Results obtained by Stram and Mizuno of incidence rates among
the LSS group for initial radiation dose estimated by the Dosimetry System 1986
(DS86) are shown by small closed circles in Fig. 4 (Radiation Research 117, 93-
113(1989)). As shown in Fig. 4 the incidence rate rapidly increases above 1 Gy and
exceeds 50 % at around 2.4 Gy. However, beyond 3 Gy the rates do not increase and
even decrease as dose approaches 6 Gy. This unnatural behavior of the incidence rates
in the high dose region can be explained by the fact that the LSS group contains only
people who were left alive in 1950 though they had been exposed to nearly or more than
a half-death dose of about 4 Gy as pointed out by Stewart et. al.(Health Phys. 58, 782-
735 (1990): 64, 467-472 (1993); Int J Epidemiology, 29, 708-714 (2000)) as well as
over subtraction of background incidence rates.
Incidence rates of epilation shown by open circles in Fig. 4 are those obtained by
Kyoizumi et al. (Radat Res 194, 11-18 (1998); RERF Update 7(2);4-5(1995)) by means
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of radiation exposure to transplanted human head skin onto immunodeficient mice. As
seen in Fig. 4 the incidence rates increase very slowly in the low exposure region
compared to those given by Stram and Mizuno and increase to 95.5 % and 97 %, and
almost 100 % at exposure of 4.5 Gy. From experimental studies with animals it is
known that most of dose dependence of incidence rates or death rates are represented by
a Normal(Gaussian) distribution. The incidence rates given by Kyoizumi et. al. over the
whole range of the exposure region can be fitted well by the normal distribution with an
expectation value of 2.751 Gy and standard deviation 0.794 Gy and shown by a solid
curve in Fig. 4. At this expectation value 50% of people will experience epilation.

When it is recognized that the results of Stram and Mizuno shown in Fig. 4 were
obtained from examination data of the LSS group based on the assumption that the
epilation was caused only by the exposure to initial radiation regarding the fallout
radiation as the background, it is expected that the black circles in Fig. 4 shift toward
higher dose and higher incident rates, i.e. toward the relationship obtained by Kyoizumi
et al as shown by arrows if the exposure to the fallout radiation is included. When the
exposures indicated by black circles in Fig. 4 for initial radiation dose are translated into
distance from the hypocenter by use of the DS86 estimation neglecting shielding effects,
we obtain results which are plotted by black diamonds in Fig. 3. If the shielding effects
are taken into account the diamonds shown in Fig. 3 will move to the left towards the
hypocenter and the distance between the squares will increase. It is assumed that the
systematic difference between squares and diamonds shown in Fig. 3 represents
exposed effects from fallout radiation.