If nuclear fallout lasts thousands of years, how did Hiroshima and Nagasaki recover so quickly?

August 29, 2003

Dear Cecil:

If nuclear fallout takes thousands of years to dissipate, how did the Japanese return to Hiroshima and Nagasaki three months after the nuclear bombs exploded? Doesn't the area stay radioactive and uninhabitable for thousands of years?

Not necessarily, obviously. As nuclear explosions go, the blasts at Hiroshima and Nagasaki were pretty clean. In fact, in the weeks following the bombings, American authorities trying to keep a lid on the deteriorating PR situation portrayed A-bomb damage as being just like that from conventional weapons, except that there was more of it. Those already dying of "atomic sickness" knew better. Only gradually did the world realize that, even if you can safely walk through the ruins of a bombed city soon afterward, the effects of a nuclear attack continue to show up for years.

Atom bombs like the ones dropped on Japan produce two types of radiation: initial and residual. (I'm getting this from Hiroshima and Nagasaki: The Physical, Medical and Social Effects of the Atomic Bombings, an exhaustive Japanese study, published in English in 1981.) Initial radiation is released by the explosion itself. Residual radiation comes later from radionuclides, radioactive isotopes either generated by the explosion or else induced in soil, building materials, bodies, etc, by neutron bombardment unleashed by the blast.

The bombs that destroyed Hiroshima and Nagasaki produced their share of residual radiation, but it didn't stick around long, for two reasons. First, both bombs were detonated more than 500 meters above street level so as to wreak maximum destruction (surrounding buildings would have blocked much of the force of ground-level explosions). That limited surface contamination, since most of the radioactive debris was carried off in the mushroom cloud instead of being embedded in the earth. There was plenty of lethal fallout in the form of "ashes of death" and "black rain," but it was spread over a fairly wide area.

Second, most of the radionuclides had brief half-lives--some lasting just minutes. The bomb sites were intensely radioactive for the first few hours after the explosions, but thereafter the danger diminished rapidly. American scientists sweeping Hiroshima with Geiger counters a month after the explosion to see if the area was safe for occupation troops found a devastated city but little radioactivity. Water lilies blackened by the blast had already begun to grow again, suggesting that whatever radioactivity there had been immediately following the blast had quickly dissipated.

U.S. military authorities touted these findings to an apprehensive world as proof that A-bombs really weren't so bad. A rumor widespread among Japanese civilians--evidently based on comments made by an American science writer in an interview published shortly after the bombings--held that Hiroshima and Nagasaki would be uninhabitable for 70 or 75 years. To quell such talk, American military leaders held a press conference at which they suggested that the explosions had been massive but otherwise ordinary, denied any lingering danger, and predicted there would be no further deaths.

None of this turned out to be true. Although residual radiation was a relatively minor threat, many of those who survived the blasts had already absorbed the initial radiation doses that would eventually kill or cripple them. Radiation deaths began a week after the bombings and peaked three or four weeks later. People with few apparent injuries would suddenly develop ghastly symptoms--hair loss, purple skin blotches, and bloody discharge from various orifices were among the more obvious--and die soon after. Of the 103,000 people estimated by the U.S. military to have been killed by the bombs, 36,000 died a day or more after the blasts. (Granted, many had multiple injuries and didn't die of radiation poisoning alone.)

Radiation deaths subsided after seven or eight weeks but latent effects continued to appear for a long time. Fetuses irradiated in the wombs of their mothers were subject to high rates of miscarriage, stillbirth, and birth defects--many kids were retarded or had unusually small heads (microcephaly), stunted growth, or other afflictions. Cases of leukemia surged in 1947 and peaked in the early 1950s. Additional problems included other cancers and blood disorders, cataracts, heavy scarring (keloid), and male sterility. However, no genetic damage was detected in children conceived after the blasts. Oddly enough, notwithstanding all the calamities visited on the Japanese by the bombs, the two things everybody now expects to happen in a nuclear war, mutant kids and the land glowing blue forevermore, didn't.

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