<P> As the map shows, the Hiroshima firestorm jumped natural firebreaks (river channels), as well as prepared firebreaks . The spread of fire stopped only when it reached the edge of the blast - damaged area, encountering less available fuel . </P> <P> Accurate casualty figures are impossible to determine, because many victims were cremated by the firestorm, along with all record of their existence . The Manhattan Project report on Hiroshima estimated that 60% of immediate deaths were caused by fire, but with the caveat that "many persons near the center of explosion suffered fatal injuries from more than one of the bomb effects ." In particular, many fire victims also received lethal doses of nuclear radiation . </P> <P> Local fallout is dust and ash from a bomb crater, contaminated with radioactive fission products . It falls to earth downwind of the crater and can produce, with radiation alone, a lethal area much larger than that from blast and fire . With an air burst, the fission products rise into the stratosphere, where they dissipate and become part of the global environment . Because Little Boy was an air burst 580 metres (1,900 ft) above the ground, there was no bomb crater and no local radioactive fallout . </P> <P> However, a burst of intense neutron and gamma radiation came directly from the fireball . Its lethal radius was 1.3 kilometres (0.8 mi), covering about half of the firestorm area . An estimated 30% of immediate fatalities were people who received lethal doses of this direct radiation, but died in the firestorm before their radiation injuries would have become apparent . Over 6,000 people survived the blast and fire, but died of radiation injuries . Among injured survivors, 30% had radiation injuries from which they recovered, but with a lifelong increase in cancer risk . To date, no radiation - related evidence of heritable diseases has been observed among the survivors' children . </P>

The development of the atomic bomb that was dropped on hiroshima