The casualties sustained by the inhabitants of both cities were due to:
A. "Flash" burns, caused directly by the almost instantaneous
radiation of heat and light at the moment of the explosion.
B. Burns resulting from the fires caused by the explosion.
C. Mechanical injuries caused by collapse of buildings, flying debris,
and forceable hurling--about of persons struck by the blast pressure
waves.
D. Radiation injuries caused by the instantaneous penetrating
radiation (in many respects similar to excessive X-ray exposure) from
the nuclear explosion; all of these effective radiations occurred
during the first minute after initiation of the explosion, and nearly
all occurred during the first second of the explosion.
No casualties were suffered as a result of any persistent radioactivity
of fission products of the bomb, or any induced radioactivity of
objects near the explosion. The gamma radiations emitted by the
nuclear explosion did not, of course, inflict any damage on structures.
The number of casualties which resulted from the pure blast effect
alone (i.e., because of simple pressure) was probably negligible in
comparison to that caused by other effects.
The central portions of the cities underneath the explosions suffered
almost complete destruction. The only surviving objects were the
frames of a small number of strong reinforced concrete buildings which
were not collapsed by the blast; most of these buildings suffered
extensive damage from interior fires, had their windows, doors, and
partitions knocked out, and all other fixtures which were not integral
parts of the reinforced concrete frames burned or blown away; the
casualties in such buildings near the center of explosion were almost
100%. In Hiroshima fires sprang up simultaneously all over the wide
flat central area of the city; these fires soon combined in an immense
"fire storm" (high winds blowing inwards toward the center of a large
conflagration) similar to those caused by ordinary mass incendiary
raids; the resulting terrific conflagration burned out almost
everything which had not already been destroyed by the blast in a
roughly circular area of 4.4 square miles around the point directly
under the explosion (this point will hereafter in this report be
referred to as X). Similar fires broke out in Nagasaki, but no
devastating fire storm resulted as in Hiroshima because of the
irregular shape of the city.
In both cities the blast totally destroyed everything within a radius
of 1 mile from the center of explosion, except for certain reinforced
concrete frames as noted above. The atomic explosion almost completely
destroyed Hiroshima's identity as a city. Over a fourth of the
population was killed in one stroke and an additional fourth seriously
injured, so that even if there had been no damage to structures and
installations the normal city life would still have been completely
shattered. Nearly everything was heavily damaged up to a radius of 3
miles from the blast, and beyond this distance damage, although
comparatively light, extended for several more miles. Glass was broken
up to 12 miles.
In Nagasaki, a smaller area of the city was actually destroyed than in
Hiroshima, because the hills which enclosed the target area restricted
the spread of the great blast; but careful examination of the effects
of the explosion gave evidence of even greater blast effects than in
Hiroshima. Total destruction spread over an area of about 3 square
miles. Over a third of the 50,000 buildings in the target area of
Nagasaki were destroyed or seriously damaged. The complete destruction
of the huge steel works and the torpedo plant was especially
impressive. The steel frames of all buildings within a mile of the
explosion were pushed away, as by a giant hand, from the point of
detonation. The badly burned area extended for 3 miles in length. The
hillsides up to a radius of 8,000 feet were scorched, giving them an
autumnal appearance.