9: General Description of Damage Caused by the Atomic Explosions
<< 8: General Comparison of Hiroshima and Nagasaki || 10: Effects of the Atomic Bombings on the Inhabitants of the Bombed Cities >>
In considering the devastation in the two cities, it should be remembered
that the cities' differences in shape and topography resulted in great
differences in the damages. Hiroshima was all on low, flat ground, and was
roughly circular in shape; Nagasaki was much cut up by hills and mountain
spurs, with no regularity to its shape.
In Hiroshima almost everything up to about one mile from X was completely
destroyed, except for a small number (about 50) of heavily reinforced
concrete buildings, most of which were specially designed to withstand
earthquake shock, which were not collapsed by the blast; most of these
buildings had their interiors completely gutted, and all windows, doors,
sashes, and frames ripped out. In Nagasaki, nearly everything within 1/2
mile of the explosion was destroyed, including heavy structures. All
Japanese homes were destroyed within 1 1/2 miles from X.
Underground air raid shelters with earth cover roofs immediately below the
explosion had their roofs caved in; but beyond 1/2 mile from X they
suffered no damage.
In Nagasaki, 1500 feet from X high quality steel frame buildings were not
completely collapsed, but the entire buildings suffered mass distortion and
all panels and roofs were blown in.
In Nagasaki, 2,000 feet from X, reinforced concrete buildings with 10"
walls and 6" floors were collapsed; reinforced concrete buildings with 4"
walls and roofs were standing but were badly damaged. At 2,000 feet some
9" concrete walls were completely destroyed.
In Nagasaki, 3,500 feet from X, church buildings with 18" brick walls were
completely destroyed. 12" brick walls were severely cracked as far as
In Hiroshima, 4,400 feet from X, multi-story brick buildings were
completely demolished. In Nagasaki, similar buildings were destroyed to
In Hiroshima, roof tiles were bubbled (melted) by the flash heat out to
4,000 feet from X; in Nagasaki, the same effect was observed to 6,500 feet.
In Hiroshima, steel frame buildings were destroyed 4,200 feet from X, and
to 4,800 feet in Nagasaki.
In both cities, the mass distortion of large steel buildings was observed
out to 4,500 feet from X.
In Nagasaki, reinforced concrete smoke stacks with 8" walls, specially
designed to withstand earthquake shocks, were overturned up to 4,000 feet
In Hiroshima, steel frame buildings suffered severe structural damage up to
5,700 feet from X, and in Nagasaki the same damage was sustained as far as
In Nagasaki, 9" brick walls were heavily cracked to 5,000 feet, were
moderately cracked to 6,000 feet, and slightly cracked to 8,000 feet. In
both cities, light concrete buildings collapsed out to 4,700 feet.
In Hiroshima, multi-story brick buildings suffered structural damage up to
6,600 feet, and in Nagasaki up to 6,500 feet from X.
In both cities overhead electric installations were destroyed up to 5,500
feet; and trolley cars were destroyed up to 5,500 feet, and damaged to
Flash ignition of dry, combustible material was observed as far as 6,400
feet from X in Hiroshima, and in Nagasaki as far as 10,000 feet from X.
Severe damage to gas holders occured out to 6,500 feet in both cities.
All Japanese homes were seriously damaged up to 6,500 feet in Hiroshima,
and to 8,000 feet in Nagasaki. Most Japanese homes were damaged up to
8,000 feet in Hiroshima and 10,500 feet in Nagasaki.
The hillsides in Nagasaki were scorched by the flash radiation of heat as
far as 8,000 feet from X; this scorching gave the hillsides the appearance
of premature autumn.
In Nagasaki, very heavy plaster damage was observed in many buildings up to
9,000 feet; moderate damage was sustained as far as 12,000 feet, and light
damage up to 15,000 feet.
The flash charring of wooden telegraph poles was observed up to 9,500 feet
from X in Hiroshima, and to 11,000 feet in Nagasaki; some reports indicate
flash burns as far as 13,000 feet from X in both places.
Severe displacement of roof tiles was observed up to 8,000 feet in
Hiroshima, and to 10,000 feet in Nagasaki.
In Nagasaki, very heavy damage to window frames and doors was observed up
to 8,000 feet, and light damage up to 12,000 feet.
Roofs and wall coverings on steel frame buildings were destroyed out to
Although the sources of many fires were difficult to trace accurately, it
is believed that fires were started by primary heat radiation as far as
15,000 feet from X.
Roof damage extended as far as 16,000 feet from X in Hiroshima and in
The actual collapse of buildings was observed at the extreme range of
23,000 feet from X in Nagasaki.
Although complete window damage was observed only up to 12,000 feet from X,
some window damage occurred in Nagasaki up to 40,000 feet, and actual
breakage of glass occured up to 60,000 feet.
Heavy fire damage was sustained in a circular area in Hiroshima with a mean
radius of about 6,000 feet and a maximum radius of about 11,000 feet;
similar heavy damage occured in Nagasaki south of X up to 10,000 feet,
where it was stopped on a river course.
In Hiroshima over 60,000 of 90,000 buildings were destroyed or severely
damaged by the atomic bomb; this figure represents over 67% of the city's
In Nagasaki 14,000 or 27% of 52,000 residences were completely destroyed
and 5,40O, or 10% were half destroyed. Only 12% remained undamaged. This
destruction was limited by the layout of the city. The following is a
summary of the damage to buildings in Nagasaki as determined from a ground
survey made by the Japanese:
|Destruction of Buildings and Houses
(Compiled by Nagasaki Municipality)
|Total in Nagasaki (before atomic explosion)
|Blasted (not burned)
|Blasted and burned
|Blasted and/or burned
|Partially burned or blasted
|Total buildings and houses destroyed
In Hiroshima, all utilities and transportation services were disrupted for
varying lengths of time. In general however services were restored about
as rapidly as they could be used by the depleted population. Through
railroad service was in order in Hiroshima on 8 August, and electric power
was available in most of the surviving parts on 7 August, the day after the
bombing. The reservoir of the city was not damaged, being nearly 2 miles
from X. However, 70,000 breaks in water pipes in buildings and dwellings
were caused by the blast and fire effects. Rolling transportation suffered
extensive damage. The damage to railroad tracks, and roads was
comparatively small, however. The electric power transmission and
distribution systems were badly wrecked. The telephone system was
approximately 80% damaged, and no service was restored until 15 August.
Despite the customary Japanese lack of attention to sanitation measures, no
major epidemic broke out in the bombed cities. Although the conditions
following the bombings makes this fact seem surprising, the experience of
other bombed cities in both Germany and Japan show Hiroshima and Nagasaki
not to be isolated cases.
The atomic explosion over Nagasaki affected an over-all area of
approximately 42.9 square miles of which about 8.5 square miles were water
and only about 9.8 square miles were built up, the remainder being
partially settled. Approximately 36% of the built up areas were seriously
damaged. The area most severely damaged had an average radius of about 1
mile, and covered about 2.9 square miles of which 2.4 were built up.
In Nagasaki, buildings with structural steel frames, principally the
Mitsubishi Plant as far as 6,000 feet from X were severely damaged; these
buildings were typical of wartime mill construction in America and Great
Britain, except that some of the frames were somewhat less substantial.
The damage consisted of windows broken out (100%), steel sashes ripped out
or bent, corrugated metal or corrugated asbestos roofs and sidings ripped
off, roofs bent or destroyed, roof trusses collapsed, columns bent and
cracked and concrete foundations for columns rotated. Damage to buildings
with structural steel frames was more severe where the buildings received
the effect of the blast on their sides than where the blast hit the ends of
buildings, because the buildings had more stiffness (resistance to negative
moment at the top of columns) in a longitudinal direction. Many of the
lightly constructed steel frame buildings collapsed completely while some
of the heavily constructed (to carry the weight of heavy cranes and loads)
were stripped of roof and siding, but the frames were only partially
The next most seriously damaged area in Nagasaki lies outside the 2.9
square miles just described, and embraces approximately 4.2 square miles of
which 29% was built up. The damage from blast and fire was moderate here,
but in some sections (portions of main business districts) many secondary
fires started and spread rapidly, resulting in about as much over-all
destruction as in areas much closer to X.
An area of partial damage by blast and fire lies just outside the one just
described and comprises approximately 35.8 square miles. Of this area,
roughly 1/6th was built up and 1/4th was water. The extent of damage
varied from serious (severe damage to roofs and windows in the main
business section of Nagasaki, 2.5 miles from X), to minor (broken or
occasionally broken windows at a distance of 7 miles southeast of X).
As intended, the bomb was exploded at an almost ideal location over
Nagasaki to do the maximum damage to industry, including the Mitsubishi
Steel and Arms Works, the Mitsubishi-Urakami Ordnance Works (Torpedo
Works), and numerous factories, factory training schools, and other
industrial establishments, with a minimum destruction of dwellings and
consequently, a minimum amount of casualties. Had the bomb been dropped
farther south, the Mitsubishi-Urakami Ordnance Works would not have been so
severely damaged, but the main business and residential districts of
Nagasaki would have sustained much greater damage casualties.
Calculations show that the structural steel and reinforced concrete frames
which survived the blast fairly close to X could not have withstood the
estimated peak pressures developed against the total areas presented by the
sides and roof of the buildings. The survival of these frames is explained
by the fact that they were not actually required to withstand the peak
pressure because the windows were quickly knocked out and roof and siding
stripped off thereby reducing total area and relieving the pressure. While
this saved the building frame, it permitted severe damage to building
interior and contents, and injuries to the building occupants. Buildings
without large panel openings through which the pressure could dissipate
were completely crushed, even when their frames were as strong as those
The damage sustained by reinforced concrete buildings depended both on the
proximity to X and the type and strength of the reinforced concrete
construction. Some of the buildings with reinforced concrete frames also
had reinforced concrete walls, ceilings, and partitions, while others had
brick or concrete tile walls covered either with plaster or ornamental
stone, with partitions of metal, glass, and plaster. With the exception of
the Nagasaki Medical School and Hospital group, which was designed to
withstand earthquakes and was therefore of heavier construction than most
American structures, most of the reinforced concrete structures could be
classified only as fair, with concrete of low strength and density, with
many of the columns, beams, and slabs underdesigned and improperly
reinforced. These facts account for some of the structural failures which
In general, the atomic bomb explosion damaged all windows and ripped out,
bent, or twisted most of the steel window or door sashes, ripped doors from
hinges, damaged all suspended wood, metal, and plaster ceilings. The blast
concussion also caused great damage to equipment by tumbling and battering.
Fires generally of secondary origin consumed practically all combustible
material, caused plaster to crack off, burned all wooden trim, stair
covering, wooden frames of wooden suspended ceilings, beds, mattresses, and
mats, and fused glass, ruined all equipment not already destroyed by the
blast, ruined all electrical wiring, plumbing, and caused spalling of
concrete columns and beams in many of the rooms.
Almost without exception masonry buildings of either brick or stone within
the effective limits of the blast were severely damaged so that most of
them were flattened or reduced to rubble. The wreckage of a church,
approximately 1,800 feet east of X in Nagasaki, was one of the few masonry
buildings still recognizable and only portions of the walls of this
structure were left standing. These walls were extremely thick (about 2
feet). The two domes of the church had reinforced concrete frames and
although they were toppled, they held together as units.
Practically every wooden building or building with timber frame within 2.0
miles of X was either completely destroyed or very seriously damaged, and
significant damage in Nagasaki resulted as far as 3 miles from X. Nearly
all such buildings collapsed and a very large number were consumed by fire.
A reference to the various photographs depicting damage shows that although
most of the buildings within the effective limits of the blast were totally
destroyed or severely damaged, a large number of chimneys even close to X
were left standing, apparently uninjured by the concussion. One
explanation is that concrete chimneys are approximately cylindrical in
shape and consequently offer much less wind resistance than flat surfaces
such as buildings. Another explanation is that since the cities were
subject to typhoons the more modern chimneys were probably designed to
withstand winds of high velocity. It is also probable that most of the
recently constructed chimneys as well as the more modern buildings were
constructed to withstand the acceleration of rather severe earthquakes.
Since the bombs were exploded high in the air, chimneys relatively close to
X were subjected to more of a downward than a lateral pressure, and
consequently the overturning moment was much less than might have been
Although the blast damaged many bridges to some extent, bridge damage was
on the whole slight in comparison to that suffered by buildings. The
damage varied from only damaged railings to complete destruction of the
superstructure. Some of the bridges were wrecked and the spans were shoved
off their piers and into the river bed below by the force of the blast.
Others, particularly steel plate girder bridges, were badly buckled by the
blast pressure. None of the failures observed could be attributed to
inadequate design or structural weaknesses.
The roads, and railroad and street railway trackage sustained practically
no primary damage as a result of the explosion. Most of the damage to
railroads occurred from secondary causes, such as fires and damage to
bridges or other structures. Rolling stock, as well as automobiles,
trolleys, and buses were destroyed and burned up to a considerable distance
from X. Streets were impassable for awhile because of the debris, but they
were not damaged. The height of the bomb explosion probably explains the
absence of direct damage to railroads and roads.
A large part of the electric supply was interrupted by the bomb blast
chiefly through damage to electric substations and overhead transmission
systems. Both gas works in Nagasaki were severely damaged by the bomb.
These works would have required 6-7 months to get into operation. In
addition to the damage sustained by the electrical and gas systems, severe
damage to the water supply system was reported by the Japanese government;
the chief damage was a number of breaks in the large water mains and in
almost all of the distributing pipes in the areas which were affected by
the blast. Nagasaki was still suffering from a water shortage inside the
city six weeks after the atomic attack.
The Nagasaki Prefectural report describes vividly the effects of the bomb
on the city and its inhabitants:
"Within a radius of 1 kilometer from X, men and animals died almost
instantaneously and outside a radius of 1 kilometer and within a radius of
2 kilometers from X, some men and animals died instantly from the great
blast and heat but the great majority were seriously or superficially
injured. Houses and other structures were completely destroyed while fires
broke out everywhere. Trees were uprooted and withered by the heat.
"Outside a radius of 2 kilometers and within a radius of 4 kilometers from
X, men and animals suffered various degrees of injury from window glass and
other fragments scattered about by the blast and many were burned by the
intense heat. Dwellings and other structures were half damaged by blast.
"Outside a radius of 4 kilometers and within a radius of 8 kilometers
living creatures were injured by materials blown about by the blast; the
majority were only superficially wounded. Houses were only half or
The British Mission to Japan interpreted their observations of the
destruction of buildings to apply to similar construction of their own as
A similar bomb exploding in a similar fashion would produce the following
effects on normal British houses:
Up to 1,000 yards from X it would cause complete collapse.
Up to 1 mile from X it would damage the houses beyond repair.
Up to 1.5 miles from X it would render them uninhabitable without extensive
repair, particularly to roof timbers.
Up to 2.5 miles from X it would render them uninhabitable until first-aid
repairs had been carried out.
The fire damage in both cities was tremendous, but was more complete in
Hiroshima than in Nagasaki. The effect of the fires was to change
profoundly the appearance of the city and to leave the central part bare,
except for some reinforced concrete and steel frames and objects such as
safes, chimney stacks, and pieces of twisted sheet metal. The fire damage
resulted more from the properties of the cities themselves than from those
of the bombs.
The conflagration in Hiroshima caused high winds to spring up as air was
drawn in toward the center of the burning area, creating a "fire storm".
The wind velocity in the city had been less than 5 miles per hour before
the bombing, but the fire-wind attained a velocity of 30-40 miles per hour.
These great winds restricted the perimeter of the fire but greatly added to
the damage of the conflagration within the perimeter and caused the deaths
of many persons who might otherwise have escaped. In Nagasaki, very severe
damage was caused by fires, but no extensive "fire storm" engulfed the
city. In both cities, some of the fires close to X were no doubt started
by the ignition of highly combustible material such as paper, straw, and
dry cloth, upon the instantaneous radiation of heat from the nuclear
explosion. The presence of large amounts of unburnt combustible materials
near X, however, indicated that even though the heat of the blast was very
intense, its duration was insufficient to raise the temperature of many
materials to the kindling point except in cases where conditions were
ideal. The majority of the fires were of secondary origin starting from
the usual electrical short-circuits, broken gas lines, overturned stoves,
open fires, charcoal braziers, lamps, etc., following collapse or serious
damage from the direct blast.
Fire fighting and rescue units were stripped of men and equipment. Almost
30 hours elapsed before any rescue parties were observable. In Hiroshima
only a handful of fire engines were available for fighting the ensuing
fires, and none of these were of first class type. In any case, however,
it is not likely that any fire fighting equipment or personnel or
organization could have effected any significant reduction in the amount of
damage caused by the tremendous conflagration.
A study of numerous aerial photographs made prior to the atomic bombings
indicates that between 10 June and 9 August 1945 the Japanese constructed
fire breaks in certain areas of the cities in order to control large scale
fires. In general these fire breaks were not effective because fires were
started at so many locations simultaneously. They appear, however, to have
helped prevent fires from spreading farther east into the main business and
residential section of Nagasaki.
There has been great difficulty in estimating the total casualties in the
Japanese cities as a result of the atomic bombing. The extensive
destruction of civil installations (hospitals, fire and police
department, and government agencies) the state of utter confusion
immediately following the explosion, as well as the uncertainty regarding
the actual population before the bombing, contribute to the difficulty of
making estimates of casualties. The Japanese periodic censuses are not
complete. Finally, the great fires that raged in each city totally
consumed many bodies.
The number of total casualties has been estimated at various times since
the bombings with wide discrepancies. The Manhattan Engineer District's
best available figures are:
Estimates of Casualties
The relation of total casualties to distance from X, the center of damage
and point directly under the air-burst explosion of the bomb, is of great
importance in evaluating the casualty-producing effect of the bombs. This
relationship for the total population of Nagasaki is shown in the table
below, based on the first-obtained casualty figures of the District:
Relation of Total Casualties to Distance from X
from X, feet
||Killed per square
|0 - 1,640
|1,640 - 3,300
|3,300 - 4,900
|4,900 - 6,550
|6,550 - 9,850
No figure for total pre-raid population at these different distances were
available. Such figures would be necessary in order to compute per cent
mortality. A calculation made by the British Mission to Japan and based on
a preliminary analysis of the study of the Joint Medical-Atomic Bomb
Investigating Commission gives the following calculated values for per cent
mortality at increasing distances from X:
Per-Cent Mortality at Various Distances
|Distance from X, in feet
|0 - 1000
|1000 - 2000
|2000 - 3000
|3000 - 4000
|4000 - 5000
|5000 - 6000
|6000 - 7000
|7000 - 8000
|8000 - 9000
|9000 - 10,000
It seems almost certain from the various reports that the greatest total
number of deaths were those occurring immediately after the bombing. The
causes of many of the deaths can only be surmised, and of course many
persons near the center of explosion suffered fatal injuries from more than
one of the bomb effects. The proper order of importance for possible
causes of death is: burns, mechanical injury, and gamma radiation. Early
estimates by the Japanese are shown in D below:
Cause of Immediate Deaths
||Cause of Death
||Percent of Total|
The Nature of an Atomic Explosion
The most striking difference between the explosion of an atomic bomb and
that of an ordinary T.N.T. bomb is of course in magnitude; as the President
announced after the Hiroshima attack, the explosive energy of each of the
atomic bombs was equivalent to about 20,000 tons of T.N.T.
But in addition to its vastly greater power, an atomic explosion has
several other very special characteristics. Ordinary explosion is a
chemical reaction in which energy is released by the rearrangement of the
atoms of the explosive material. In an atomic explosion the identity of
the atoms, not simply their arrangement, is changed. A considerable
fraction of the mass of the explosive charge, which may be uranium 235 or
plutonium, is transformed into energy. Einstein's equation, E = mc^2,
shows that matter that is transformed into energy may yield a total energy
equivalent to the mass multiplied by the square of the velocity of light.
The significance of the equation is easily seen when one recalls that the
velocity of light is 186,000 miles per second. The energy released when a
pound of T.N.T. explodes would, if converted entirely into heat, raise the
temperature of 36 lbs. of water from freezing temperature (32 deg F) to
boiling temperature (212 deg F). The nuclear fission of a pound of uranium
would produce an equal temperature rise in over 200 million pounds of
The explosive effect of an ordinary material such as T.N.T. is derived from
the rapid conversion of solid T.N.T. to gas, which occupies initially the
same volume as the solid; it exerts intense pressures on the surrounding
air and expands rapidly to a volume many times larger than the initial
volume. A wave of high pressure thus rapidly moves outward from the center
of the explosion and is the major cause of damage from ordinary high
explosives. An atomic bomb also generates a wave of high pressure which is
in fact of, much higher pressure than that from ordinary explosions; and
this wave is again the major cause of damage to buildings and other
structures. It differs from the pressure wave of a block buster in the
size of the area over which high pressures are generated. It also differs
in the duration of the pressure pulse at any given point: the pressure from
a blockbuster lasts for a few milliseconds (a millisecond is one thousandth
of a second) only, that from the atomic bomb for nearly a second, and was
felt by observers both in Japan and in New Mexico as a very strong wind
The next greatest difference between the atomic bomb and the T.N.T.
explosion is the fact that the atomic bomb gives off greater amounts of
radiation. Most of this radiation is "light" of some wave-length ranging
from the so-called heat radiations of very long wave length to the
so-called gamma rays which have wave-lengths even shorter than the X-rays
used in medicine. All of these radiations travel at the same speed; this,
the speed of light, is 186,000 miles per second. The radiations are
intense enough to kill people within an appreciable distance from the
explosion, and are in fact the major cause of deaths and injuries apart
from mechanical injuries. The greatest number of radiation injuries was
probably due to the ultra-violet rays which have a wave length slightly
shorter than visible light and which caused flash burn comparable to severe
sunburn. After these, the gamma rays of ultra short wave length are most
important; these cause injuries similar to those from over-doses of X-rays.
The origin of the gamma rays is different from that of the bulk of the
radiation: the latter is caused by the extremely high temperatures in the
bomb, in the same way as light is emitted from the hot surface of the sun
or from the wires in an incandescent lamp. The gamma rays on the other
hand are emitted by the atomic nuclei themselves when they are transformed
in the fission process. The gamma rays are therefore specific to the
atomic bomb and are completely absent in T.N.T. explosions. The light of
longer wave length (visible and ultra-violet) is also emitted by a T.N.T.
explosion, but with much smaller intensity than by an atomic bomb, which
makes it insignificant as far as damage is concerned.
A large fraction of the gamma rays is emitted in the first few microseconds
(millionths of a second) of the atomic explosion, together with neutrons
which are also produced in the nuclear fission. The neutrons have much
less damage effect than the gamma rays because they have a smaller
intensity and also because they are strongly absorbed in air and therefore
can penetrate only to relatively small distances from the explosion: at a
thousand yards the neutron intensity is negligible. After the nuclear
emission, strong gamma radiation continues to come from the exploded bomb.
This generates from the fission products and continues for about one minute
until all of the explosion products have risen to such a height that the
intensity received on the ground is negligible. A large number of beta
rays are also emitted during this time, but they are unimportant because
their range is not very great, only a few feet. The range of alpha
particles from the unused active material and fissionable material of the
bomb is even smaller.
Apart from the gamma radiation ordinary light is emitted, some of which is
visible and some of which is the ultra violet rays mainly responsible for
flash burns. The emission of light starts a few milliseconds after the
nuclear explosion when the energy from the explosion reaches the air
surrounding the bomb. The observer sees then a ball of fire which rapidly
grows in size. During most of the early time, the ball of fire extends as
far as the wave of high pressure. As the ball of fire grows its
temperature and brightness decrease. Several milliseconds after the
initiation of the explosion, the brightness of the ball of fire goes
through a minimum, then it gets somewhat brighter and remains at the order
of a few times the brightness of the sun for a period of 10 to 15 seconds
for an observer at six miles distance. Most of the radiation is given off
after this point of maximum brightness. Also after this maximum, the
pressure waves run ahead of the ball of fire.
The ball of fire rapidly expands from the size of the bomb to a radius of
several hundred feet at one second after the explosion. After this the
most striking feature is the rise of the ball of fire at the rate of about
30 yards per second. Meanwhile it also continues to expand by mixing with
the cooler air surrounding it. At the end of the first minute the ball has
expanded to a radius of several hundred yards and risen to a height of
about one mile. The shock wave has by now reached a radius of 15 miles and
its pressure dropped to less than 1/10 of a pound per square inch. The
ball now loses its brilliance and appears as a great cloud of smoke: the
pulverized material of the bomb. This cloud continues to rise vertically
and finally mushrooms out at an altitude of about 25,000 feet depending
upon meteorological conditions. The cloud reaches a maximum height of
between 50,000 and 70,000 feet in a time of over 30 minutes.
It is of interest to note that Dr. Hans Bethe, then a member of the
Manhattan Engineer District on loan from Cornell University, predicted the
existence and characteristics of this ball of fire months before the first
test was carried out.
To summarize, radiation comes in two bursts - an extremely intense one
lasting only about 3 milliseconds and a less intense one of much longer
duration lasting several seconds. The second burst contains by far the
larger fraction of the total light energy, more than 90%. But the first
flash is especially large in ultra-violet radiation which is biologically
more effective. Moreover, because the heat in this flash comes in such a
short time, there is no time for any cooling to take place, and the
temperature of a person's skin can be raised 50 degrees centigrade by the
flash of visible and ultra-violet rays in the first millisecond at a
distance of 4,000 yards. People may be injured by flash burns at even
larger distances. Gamma radiation danger does not extend nearly so far and
neutron radiation danger is still more limited.
The high skin temperatures result from the first flash of high intensity
radiation and are probably as significant for injuries as the total dosages
which come mainly from the second more sustained burst of radiation. The
combination of skin temperature increase plus large ultra-violet flux
inside 4,000 yards is injurious in all cases to exposed personnel. Beyond
this point there may be cases of injury, depending upon the individual
sensitivity. The infra-red dosage is probably less important because of its
Characteristics of the Damage Caused by the Atomic Bomb
The damage to man-made structures caused by the bombs was due to two
distinct causes: first the blast, or pressure wave, emanating from the
center of the explosion, and, second, the fires which were caused either by
the heat of the explosion itself or by the collapse of buildings containing
stoves, electrical fixtures, or any other equipment which might produce
what is known as a secondary fire, and subsequent spread of these fires.
The blast produced by the atomic bomb has already been stated to be
approximately equivalent to that of 20,000 tons of T.N.T. Given this
figure, one may calculate the expected peak pressures in the air, at
various distances from the center of the explosion, which occurred
following detonation of the bomb. The peak pressures which were calculated
before the bombs were dropped agreed very closely with those which were
actually experienced in the cities during the attack as computed by Allied
experts in a number of ingenious ways after the occupation of Japan.
The blast of pressure from the atomic bombs differed from that of ordinary
high explosive bombs in three main ways:
A. Downward thrust. Because the explosions were well up in the air, much
of the damage resulted from a downward pressure. This pressure of course
most largely effected flat roofs. Some telegraph and other poles
immediately below the explosion remained upright while those at greater
distances from the center of damage, being more largely exposed to a
horizontal thrust from the blast pressure waves, were overturned or tilted.
Trees underneath the explosion remained upright but had their branches
B. Mass distortion of buildings. An ordinary bomb can damage only a part
of a large building, which may then collapse further under the action of
gravity. But the blast wave from an atomic bomb is so large that it can
engulf whole buildings, no matter how great their size, pushing them over
as though a giant hand had given them a shove.
C. Long duration of the positive pressure pulse and consequent small
effect of the negative pressure, or suction, phase. In any explosion, the
positive pressure exerted by the blast lasts for a definite period of time
(usually a small fraction of a second) and is then followed by a somewhat
longer period of negative pressure, or suction. The negative pressure is
always much weaker than the positive, but in ordinary explosions the short
duration of the positive pulse results in many structures not having time
to fail in that phase, while they are able to fail under the more extended,
though weaker, negative pressure. But the duration of the positive pulse
is approximately proportional to the 1/3 power of the size of the explosive
charge. Thus, if the relation held true throughout the range in question,
a 10-ton T.N.T. explosion would have a positive pulse only about 1/14th as
long as that of a 20,000-ton explosion. Consequently, the atomic
explosions had positive pulses so much longer then those of ordinary
explosives that nearly all failures probably occurred during this phase,
and very little damage could be attributed to the suction which followed.
One other interesting feature was the combination of flash ignition and
comparative slow pressure wave. Some objects, such as thin, dry wooden
slats, were ignited by the radiated flash heat, and then their fires were
blown out some time later (depending on their distance from X) by the
pressure blast which followed the flash radiation.
Calculations of the Peak Pressure of the Blast Wave
Several ingenious methods were used by the various investigators to
determine, upon visiting the wrecked cities, what had actually been the
peak pressures exerted by the atomic blasts. These pressures were computed
for various distances from X, and curves were then plotted which were
checked against the theoretical predictions of what the pressures would be.
A further check was afforded from the readings obtained by the measuring
instruments which were dropped by parachute at each atomic attack. The
peak pressure figures gave a direct clue to the equivalent T.N.T. tonnage
of the atomic bombs, since the pressures developed by any given amount of
T.N.T. can be calculated easily.
One of the simplest methods of estimating the peak pressure is from
crushing of oil drums, gasoline cans, or any other empty thin metal vessel
with a small opening. The assumption made is that the blast wave pressure
comes on instantaneously, the resulting pressure on the can is more than
the case can withstand, and the walls collapse inward. The air inside is
compressed adiabatically to such a point that the pressure inside is less
by a certain amount than the pressure outside, this amount being the
pressure difference outside and in that the walls can stand in their
crumpled condition. The uncertainties involved are, first, that some air
rushes in through any opening that the can may have, and thus helps to
build up the pressure inside; and, second, that as the pressure outside
falls, the air inside cannot escape sufficiently fast to avoid the walls of
the can being blown out again to some extent. These uncertainties are such
that estimates of pressure based on this method are on the low side, i.e.,
they are underestimated.
Another method of calculating the peak-pressure is through the bending of
steel flagpoles, or lightning conductors, away from the explosion. It is
possible to calculate the drag on a pole or rod in an airstream of a
certain density and velocity; by connecting this drag with the strength of
the pole in question, a determination of the pressure wave may be obtained.
Still another method of estimating the peak pressure is through the
overturning of memorial stones, of which there are a great quantity in
Japan. The dimensions of the stones can be used along with known data on
the pressure exerted by wind against flat surfaces, to calculate the
Long Range Blast Damage
There was no consistency in the long range blast damage. Observers often
thought that they had found the limit, and then 2,000 feet farther away
would find further evidence of damage.
The most impressive long range damage was the collapse of some of the
barracks sheds at Kamigo, 23,000 feet south of X in Nagasaki. It was
remarkable to see some of the buildings intact to the last details,
including the roof and even the windows, and yet next to them a similar
building collapsed to ground level.
The limiting radius for severe displacement of roof tiles in Nagasaki was
about 10,000 feet although isolated cases were found up to 16,000 feet.
In Hiroshima the general limiting radius was about 8,000 feet; however,
even at a distance of 26,000 feet from X in Hiroshima, some tiles were
At Mogi, 7 miles from X in Nagasaki, over steep hills over 600 feet high,
about 10% of the glass came out. In nearer, sequestered localities only 4
miles from X, no damage of any kind was caused. An interesting effect was
noted at Mogi; eyewitnesses said that they thought a raid was being made on
the place; one big flash was seen, then a loud roar, followed at several
second intervals by half a dozen other loud reports, from all directions.
These successive reports were obviously reflections from the hills
The ground shock in most cities was very light. Water pipes still carried
water and where leaks were visible they were mainly above ground.
Virtually all of the damage to underground utilities was caused by the
collapse of buildings rather than by any direct exertion of the blast
pressure. This fact of course resulted from the bombs' having been
exploded high in the air.
Shielding, or Screening from Blast
In any explosion, a certain amount of protection from blast may be gained
by having any large and substantial object between the protected object and
the center of the explosion. This shielding effect was noticeable in the
atomic explosions, just as in ordinary cases, although the magnitude of the
explosions and the fact that they occurred at a considerable height in the
air caused marked differences from the shielding which would have
characterized ordinary bomb explosions.
The outstanding example of shielding was that afforded by the hills in the
city of Nagasaki; it was the shielding of these hills which resulted in the
smaller area of devastation in Nagasaki despite the fact that the bomb used
there was not less powerful. The hills gave effective shielding only at
such distances from the center of explosion that the blast pressure was
becoming critical - that is, was only barely sufficient to cause collapse -
for the structure. Houses built in ravines in Nagasaki pointing well away
from the center of the explosion survived without damage, but others at
similar distances in ravines pointing toward the center of explosion were
greatly damaged. In the north of Nagasaki there was a small hamlet about
8,000 feet from the center of explosion; one could see a distinctive
variation in the intensity of damage across the hamlet, corresponding with
the shadows thrown by a sharp hill.
The best example of shielding by a hill was southeast of the center of
explosion in Nagasaki. The damage at 8,000 feet from X consisted of light
plaster damage and destruction of about half the windows. These buildings
were of European type and were on the reverse side of a steep hill. At the
same distance to the south-southeast the damage was considerably greater,
i.e., all windows and frames, doors, were damaged and heavy plaster damage
and cracks in the brick work also appeared. The contrast may be
illustrated also by the fact that at the Nagasaki Prefectural office at
10,800 feet the damage was bad enough for the building to be evacuated,
while at the Nagasaki Normal School to which the Prefectural office had
been moved, at the same distance, the damage was comparatively light.
Because of the height of the bursts no evidence was expected of the
shielding of one building by another, at least up to a considerable radius.
It was in fact difficult to find any evidence at any distance of such
shielding. There appeared to have been a little shielding of the building
behind the Administration Building of the Torpedo Works in Nagasaki, but
the benefits were very slight. There was also some evidence that the group
of buildings comprising the Medical School in Nagasaki did afford each
other mutual protection. On the whole, however, shielding of one building
by another was not noticeable.
There was one other peculiar type of shielding, best exhibited by the
workers' houses to the north of the torpedo plant in Nagasaki. These were
6,000 to 7,000 feet north of X. The damage to these houses was not nearly
as bad as those over a thousand feet farther away from the center of
explosion. It seemed as though the great destruction caused in the torpedo
plant had weakened the blast a little, and the full power was not restored
for another 1,000 feet or more.
As already stated, a characteristic feature of the atomic bomb, which is
quite foreign to ordinary explosives, is that a very appreciable fraction
of the energy liberated goes into radiant heat and light. For a
sufficiently large explosion, the flash burn produced by this radiated
energy will become the dominant cause of damage, since the area of burn
damage will increase in proportion to the energy released, whereas the area
of blast damage increases only with the two-thirds power of the energy.
Although such a reversal of the mechanism of damage was not achieved in the
Hiroshima and Nagasaki bombs, the effects of the flash were, however, very
evident, and many casualties resulted from flash burns. A discussion of
the casualties caused by flash burns will be given later; in this section
will be described the other flash effects which were observed in the two
The duration of the heat radiation from the bomb is so short, just a few
thousandths of a second, that there is no time for the energy falling on a
surface to be dissipated by thermal defusion; the flash burn is typically a
surface effect. In other words the surface of either a person or an object
exposed to the flash is raised to a very high temperature while immediately
beneath the surface very little rise in temperature occurs.
The flash burning of the surface of objects, particularly wooden objects,
occurred in Hiroshima up to a radius of 9,500 feet from X; at Nagasaki
burns were visible up to 11,000 feet from X. The charring and blackening
of all telephone poles, trees and wooden posts in the areas not destroyed
by the general fire occurred only on the side facing the center of
explosion and did not go around the corners of buildings or hills. The
exact position of the explosion was in fact accurately determined by taking
a number of sights from various objects which had been flash burned on one
To illustrate the effects of the flash burn, the following describes a
number of examples found by an observer moving northward from the center of
explosion in Nagasaki. First occurred a row of fence posts at the north
edge of the prison hill, at 0.3 miles from X. The top and upper part of
these posts were heavily charred. The charring on the front of the posts
was sharply limited by the shadow of a wall. This wall had however been
completely demolished by the blast, which of course arrived some time after
the flash. At the north edge of the Torpedo works, 1.05 miles from X,
telephone poles were charred to a depth of about 0.5 millimeters. A light
piece of wood similar to the flat side of an orange crate, was found
leaning against one of the telephone poles. Its front surface was charred
the same way as the pole, but it was evident that it had actually been
ignited. The wood was blackened through a couple of cracks and nail holes,
and around the edges onto the back surface. It seemed likely that this
piece of wood had flamed up under the flash for a few seconds before the
flame was blown out by the wind of the blast. Farther out, between 1.05
and 1.5 miles from the explosion, were many trees and poles showing a
blackening. Some of the poles had platforms near the top. The shadows
cast by the platforms were clearly visible and showed that the bomb had
detonated at a considerable height. The row of poles turned north and
crossed the mountain ridge; the flash burn was plainly visible all the way
to the top of the ridge, the farthest burn observed being at 2.0 miles from
Another striking effect of the flash burn was the autumnal appearance of
the bowl formed by the hills on three sides of the explosion point. The
ridges are about 1.5 miles from X. Throughout this bowl the foliage turned
yellow, although on the far side of the ridges the countryside was quite
green. This autumnal appearance of the trees extended to about 8,000 feet
However, shrubs and small plants quite near the center of explosion in
Hiroshima, although stripped of leaves, had obviously not been killed.
Many were throwing out new buds when observers visited the city.
There are two other remarkable effects of the heat radiated from the bomb
explosion. The first of these is the manner in which heat roughened the
surface of polished granite, which retained its polish only where it was
shielded from the radiated heat travelling in straight lines from the
explosion. This roughening by radiated heat caused by the unequal
expansion of the constituent crystals of the stone; for granite crystals
the melting temperature is about 600 deg centigrade. Therefore the depth
of roughening and ultimate flaking of the granite surface indicated the
depth to which this temperature occurred and helped to determine the
average ground temperatures in the instant following the explosion. This
effect was noted for distances about 1 1/2 times as great in Nagasaki as in
The second remarkable effect was the bubbling of roof tile. The size of
the bubbles and their extent was proportional to their nearness to the
center of explosion and also depended on how squarely the tile itself was
faced toward the explosion. The distance ratio of this effect between
Nagasaki and Hiroshima was about the same as for the flaking of polished
Various other effects of the radiated heat were noted, including the
lightening of asphalt road surfaces in spots which had not been protected
from the radiated heat by any object such as that of a person walking along
the road. Various other surfaces were discolored in different ways by the
As has already been mentioned the fact that radiant heat traveled only in
straight lines from the center of explosion enabled observers to determine
the direction toward the center of explosion from a number of different
points, by observing the "shadows" which were cast by intervening objects
where they shielded the otherwise exposed surface of some object. Thus the
center of explosion was located with considerable accuracy. In a number of
cases these "shadows" also gave an indication of the height of burst of the
bomb and occasionally a distinct penumbra was found which enabled observers
to calculate the diameter of the ball of fire at the instant it was
exerting the maximum charring or burning effect.
One more interesting feature connected with heat radiation was the charring
of fabric to different degrees depending upon the color of the fabric. A
number of instances were recorded in which persons wearing clothing of
various colors received burns greatly varying in degree, the degree of burn
depending upon the color of the fabric over the skin in question. For
example a shirt of alternate light and dark gray stripes, each about 1/8 of
an inch wide, had the dark stripes completely burned out but the light
stripes were undamaged; and a piece of Japanese paper exposed nearly 1 1/2
miles from X had the characters which were written in black ink neatly
Characteristics of the Injuries to Persons
Injuries to persons resulting from the atomic explosions were of the
A. Burns, from
1. Flash radiation of heat
2. Fires started by the explosions.
B. Mechanical injuries from collapse of buildings, flying debris, etc.
C. Direct effects of the high blast pressure, i.e., straight
D. Radiation injuries, from the instantaneous emission of gamma rays and
It is impossible to assign exact percentages of casualties to each of the
types of injury, because so many victims were injured by more than one
effect of the explosions. However, it is certain that the greater part of
the casualties resulted from burns and mechanical injures. Col. Warren,
one of America's foremost radioligists, stated it is probable that 7 per
cent or less of the deaths resulted primarily from radiation disease.
The greatest single factor influencing the occurrence of casualties was the
distance of the person concerned from the center of explosion.
Estimates based on the study of a selected group of 900 patients indicated
that total casualties occurred as far out as 14,000 feet at Nagasaki and
12,000 feet at Hiroshima.
Burns were suffered at a considerable greater distance from X than any
other type of injury, and mechanical injuries farther out than radiation
Medical findings show that no person was injured by radioactivity who was
not exposed to the actual explosion of the bombs. No injuries resulted
from persistent radioactivity of any sort.
Two types of burns were observed. These are generally differentiated as
flame or fire burn and so-called flash burn.
The early appearance of the flame burn as reported by the Japanese, and the
later appearance as observed, was not unusual.
The flash burn presented several distinctive features. Marked redness of
the affected skin areas appeared almost immediately, according to the
Japanese, with progressive changes in the skin taking place over a period
of a few hours. When seen after 50 days, the most distinctive feature of
these burns was their sharp limitation to exposed skin areas facing the
center of the explosion. For instance, a patient who had been walking in a
direction at right angles to a line drawn between him and the explosion,
and whose arms were swinging, might have burns only on the outside of the
arm nearest the center and on the inside of the other arm.
Generally, any type of shielding protected the skin against flash burns,
although burns through one, and very occasionally more, layers of clothing
did occur in patients near the center. In such cases, it was not unusual
to find burns through black but not through white clothing, on the same
patient. Flash burns also tended to involve areas where the clothes were
tightly drawn over the skin, such as at the elbows and shoulders.
The Japanese report the incidence of burns in patients surviving more than
a few hours after the explosion, and seeking medical attention, as high as
95%. The total mortalities due to burns alone cannot be estimated with any
degree of accuracy. As mentioned already, it is believed that the majority
of all the deaths occurred immediately. Of these, the Japanese estimate
that 75%, and most of the reports estimate that over 50%, of the deaths
were due to burns.
In general, the incidence of burns was in direct proportion to the distance
from X. However, certain irregularities in this relationship result in the
medical studies because of variations in the amount of shielding from flash
burn, and because of the lack of complete data on persons killed outright
close to X.
The maximum distance from X at which flash burns were observed is of
paramount interest. It has been estimated that patients with burns at
Hiroshima were all less than 7,500 feet from the center of the explosion at
the time of the bombing. At Nagasaki, patients with burns were observed
out to the remarkable distance of 13,800 feet.
The mechanical injuries included fractures, lacerations, contusions,
abrasions, and other effects to be expected from falling roofs, crumbling
walls, flying debris and glass, and other indirect blast effects. The
appearance of these various types of mechanical injuries was not remarkable
to the medical authorities who studied them.
It was estimated that patients with lacerations at Hiroshima were less than
10,600 feet from X, whereas at Nagasaki they extended as far as 12,200
The tremendous drag of wind, even as far as 1 mile from X, must have
resulted in many injuries and deaths. Some large pieces of a prison wall,
for example, were flung 80 feet, and many have gone 30 feet high before
falling. The same fate must have befallen many persons, and the chances of
a human being surviving such treatment are probably small.
No estimate of the number of deaths or early symptoms due to blast pressure
can be made. The pressures developed on the ground under the explosions
were not sufficient to kill more than those people very near the center of
damage (within a few hundred feet at most). Very few cases of ruptured ear
drums were noted, and it is the general feeling of the medical authorities
that the direct blast effects were not great. Many of the Japanese
reports, which are believed to be false, describe immediate effects such as
ruptured abdomens with protruding intestines and protruding eyes, but no
such results were actually traced to the effect of air pressure alone.
As pointed out in another section of this report the radiations from the
nuclear explosions which caused injuries to persons were primarily those
experienced within the first second after the explosion; a few may have
occurred later, but all occurred in the first minute. The other two
general types of radiation, viz., radiation from scattered fission products
and induced radioactivity from objects near the center of explosion, were
definitely proved not to have caused any casualties.
The proper designation of radiation injuries is somewhat difficult.
Probably the two most direct designations are radiation injury and gamma
ray injury. The former term is not entirely suitable in that it does not
define the type of radiation as ionizing and allows possible confusion with
other types of radiation (e.g., infra-red). The objection to the latter
term is that it limits the ionizing radiation to gamma rays, which were
undoubtedly the most important; but the possible contribution of neutron
and even beta rays to the biological effects cannot be entirely ignored.
Radiation injury has the advantage of custom, since it is generally
understood in medicine to refer to X-ray effect as distinguished from the
effects of actinic radiation. Accordingly, radiation injury is used in
this report to mean injury due only to ionizing radiation.
According to Japanese observations, the early symptons in patients
suffering from radiation injury closely resembled the symptons observed in
patients receiving intensive roentgen therapy, as well as those observed in
experimental animals receiving large doses of X-rays. The important
symptoms reported by the Japanese and observed by American authorities were
epilation (lose of hair), petechiae (bleeding into the skin), and other
hemorrhagic manifestations, oropharyngeal lesions (inflammation of the
mouth and throat), vomiting, diarrhea, and fever.
Epilation was one of the most spectacular and obvious findings. The
appearance of the epilated patient was typical. The crown was involved
more than the sides, and in many instances the resemblance to a monk's
tonsure was striking. In extreme cases the hair was totally lost. In some
cases, re-growth of hair had begun by the time patients were seen 50 days
after the bombing. Curiously, epilation of hair other than that of the
scalp was extremely unusual.
Petechiae and other hemorrhagic manifestations were striking findings.
Bleeding began usually from the gums and in the more seriously affected was
soon evident from every possible source. Petechiae appeared on the limbs
and on pressure points. Large ecchymoses (hemorrhages under the skin)
developed about needle punctures, and wounds partially healed broke down
and bled freely. Retinal hemorrhages occurred in many of the patients.
The bleeding time and the coagulation time were prolonged. The platelets
(coagulation of the blood) were characteristically reduced in numbers.
Nausea and vomiting appearing within a few hours after the explosion was
reported frequently by the Japanese. This usually had subsided by the
following morning, although occasionally it continued for two or three
days. Vomiting was not infrequently reported and observed during the
course of the later symptoms, although at these times it generally appeared
to be related to other manifestation of systemic reactions associated with
Diarrhea of varying degrees of severity was reported and observed. In the
more severe cases, it was frequently bloody. For reasons which are not yet
clear, the diarrhea in some cases was very persistent.
Lesions of the gums, and the oral mucous membrane, and the throat were
observed. The affected areas became deep red, then violacious in color;
and in many instances ulcerations and necrosis (breakdown of tissue)
followed. Blood counts done and recorded by the Japanese, as well as
counts done by the Manhattan Engineer District Group, on such patients
regularly showed leucopenia (low-white blood cell count). In extreme cases
the white blood cell count was below 1,000 (normal count is around 7,000).
In association with the leucopenia and the oropharyngeal lesions, a variety
of other infective processes were seen. Wounds and burns which were
healing adequately suppurated and serious necrosis occurred. At the same
time, similar ulcerations were observed in the larynx, bowels, and in
females, the gentalia. Fever usually accompanied these lesions.
Eye injuries produced by the atomic bombings in both cities were the
subject of special investigations. The usual types of mechanical injuries
were seen. In addition, lesions consisting of retinal hemorrhage and
exudation were observed and 75% of the patients showing them had other
signs of radiation injury.
The progress of radiation disease of various degrees of severity is shown
in the following table:
Summary of Radiation Injury
Clinical Symptoms and Findings
|Day after Explosion
||1. Nausea and
||1. Nausea and
||after 1-2 hours.
||after 1-2 hours.
|| No Definite
|| 3. Vomiting
||4. Inflammation of
the mouth and throat
|| 5. Fever
||6. Rapid emaciation
|| 2. Beginning
|| 3. Loss of appetite
||and general malaise.
||2. Loss of appetite|
||of the mouth and
||3. Sore throat.|
||7. Petechiae, diarrhea
||and nose bleeds
||8. Rapid emaciation
Or Death (Mortality
||poor health or
injuries or infection)|
It was concluded that persons exposed to the bombs at the time of
detonation did show effects from ionizing radiation and that some of these
patients, otherwise uninjured, died. Deaths from radiation began about a
week after exposure and reached a peak in 3 to 4 weeks. They practically
ceased to occur after 7 to 8 weeks.
Treatment of the burns and other physical injuries was carried out by the
Japanese by orthodox methods. Treatment of radiation effects by them
included general supportative measures such as rest and high vitamin and
caloric diets. Liver and calcium preparations were administered by
injection and blood transfusions were used to combat hemorrhage. Special
vitamin preparations and other special drugs used in the treatment of
similar medical conditions were used by American Army Medical Corps
officers after their arrival. Although the general measures instituted
were of some benefit no definite effect of any of the specific measures on
the course of the disease could be demonstrated. The use of sulfonamide
drugs by the Japanese and particularly of penicillin by the American
physicians after their arrival undoubtedly helped control the infections
and they appear to be the single important type of treatment which may have
effectively altered the earlier course of these patients.
One of the most important tasks assigned to the mission which investigated
the effects of the bombing was that of determining if the radiation effects
were all due to the instantaneous discharges at the time of the explosion,
or if people were being harmed in addition from persistent radioactivity.
This question was investigated from two points of view. Direct
measurements of persistent radioactivity were made at the time of the
investigation. From these measurements, calculations were made of the
graded radiation dosages, i.e., the total amount of radiation which could
have been absorbed by any person. These calculations showed that the
highest dosage which would have been received from persistent radioactivity
at Hiroshima was between 6 and 25 roentgens of gamma radiation; the highest
in the Nagasaki Area was between 30 and 110 roentgens of gamma radiation.
The latter figure does not refer to the city itself, but to a localized
area in the Nishiyama District. In interpreting these findings it must be
understood that to get these dosages, one would have had to remain at the
point of highest radioactivity for 6 weeks continuously, from the first
hour after the bombing. It is apparent therefore that insofar as could be
determined at Hiroshima and Nagasaki, the residual radiation alone could
not have been detrimental to the health of persons entering and living in
the bombed areas after the explosion.
The second approach to this question was to determine if any persons not in
the city at the time of the explosion, but coming in immediately afterwards
exhibited any symptoms or findings which might have been due to persistence
induced radioactivity. By the time of the arrival of the Manhattan
Engineer District group, several Japanese studies had been done on such
persons. None of the persons examined in any of these studies showed any
symptoms which could be attributed to radiation, and their actual blood
cell counts were consistently within the normal range. Throughout the
period of the Manhattan Engineer District investigation, Japanese doctors
and patients were repeatedly requested to bring to them any patients who
they thought might be examples of persons harmed from persistent
radioactivity. No such subjects were found.
It was concluded therefore as a result of these findings and lack of
findings, that although a measurable quantity of induced radioactivity was
found, it had not been sufficient to cause any harm to persons living in
the two cities after the bombings.
Shielding from Radiation
Exact figures on the thicknesses of various substances to provide complete
or partial protection from the effects of radiation in relation to the
distance from the center of explosion, cannot be released at this time.
Studies of collected data are still under way. It can be stated, however,
that at a reasonable distance, say about 1/2 mile from the center of
explosion, protection to persons from radiation injury can be afforded by a
layer of concrete or other material whose thickness does not preclude
Radiation ultimately caused the death of the few persons not killed by
other effects and who were fully exposed to the bombs up to a distance of
about 1/2 mile from X. The British Mission has estimated that people in
the open had a 50% chance of surviving the effects of radiation at 3/4 of a
mile from X.
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