Post-bombing radiation levels in Hiroshima/Nagasaki?

The residents of Hiroshima and Nagasaki in August 1945 were subjected to large doses of radiation when those cities were attacked with nuclear weapons, as well as in the hours and days afterwards as nuclear fallout rained from the sky. Presumably the levels of exposure have been dropping ever since, but how fast did they drop?

Current recommendations from the US Nuclear Regulatory Commission on safe levels of radiation exposure are as follows:

…to protect health and safety, the U.S. Nuclear Regulatory Commission (NRC) has established standards that allow exposures of up to 5,000 mrem per year for those who work with and around radioactive material, and 100 mrem per year for members of the public (in addition to the radiation we receive from natural background sources).

How long after the bombings did radiation levels in Hiroshima and Nagasaki fall to either of those listed levels?

Rather rapidly apparently.
The amount of radiation was massive for those that were caught in the blast, but residual radiation , although feared, seems to not have been detrimental.
Link to a page on the very subject:

There are two kinds of radiation from nuclear weapons.

The first is called “prompt radiation”, and is the burst of radiation caused by the actual nuclear reactions that cause the explosion- the fusion or fission in other words. This is intense, but has a relatively short radius of effect, and as bombs get larger, the radiation radius doesn’t scale with the heat and blast.

The second is the radiation from fallout- the radioactive elements of the bomb casing, fission byproducts, and random environmental stuff that got incorporated into the fireball. This is both longer lived and spreads- it’s generally airborne, and settles out of the atmosphere. The big kicker with fallout is how close the bomb is to the ground when it detonates. Sub-surface and surface bursts (i.e. the bomb goes off below or at ground level generate a LOT more fallout than what are called air bursts, where the bomb is detonated in the air above the target. Since it’s blast overpressure and heat that do most of the damage, detonating it at some specific altitude maximizes both of those effects.

In Hiroshima and Nagasaki’s case, they were both airbursts. So very little overall fallout- just the material that the bombs themselves were composed of, and IIRC, the winds blew that mostly out to sea in both cases.

In other cases, like the Castle Bravo test, they were done at what amounts to ground level in coral atolls, and huge amounts of reef material were incorporated in the blast and rained back down on other parts of the Pacific.

In 1945, it was feared that Hiroshima and Nagasaki would remain uninhabitable for about 75 years (which would, incidentally, mean that this period would end about now). This did not turn out to be true; reconstruction in both began almost immediately, and they have been flourishing cities for decades.

In 1945 they didn’t really know how all this stuff worked exactly either.

I’m skeptical that the physicists actually thought they’d be uninhabitable for 75 years; they did know enough to refute that. I suspect that it was probably just speculation from uninformed people.

This was helpful, thanks. It sounds like radiation levels faded relatively quickly after the bombing. From your link:

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.

This page has a table indicating that doses up to 100 rem can cause a temporary reduction in white blood cells, and an increased risk of cancer later in life. But beyond that, not much. So if you lived in Hiroshima/Nagasaki but were out of town on business when the bombs hit, you were probably fine even if you came home later that day.

The rapid reduction of ambient radiation levels after the bombing is consistent with what I found on a website maintained by the city of Hiroshima:

Research has indicated that 24 hours after the bombing the quantity of residual radiation a person would receive at the hypocenter would be 1/1000th of the quantity received immediately following the explosion. A week later, it would be 1/1,000,000th. Thus, residual radiation declined rapidly.

The US military did try to downplay the radiation effects, but as mentioned, the airburst meant that the horrible radiation burns were mostly as a result of initial exposure to the blast. I think it was John Hersey’s book where he does mention not being allowed to report the effects that were completely different from conventional bombing.

Another article I recall mentioned the US military’s initial “hairy-chested disregard for invisible dangers” that they could not see, before it was made abundantly clear what the risks were. When warned about the risk of fallout from tests, one officer had the men in his command brush each other down with brooms. The crew of a Japanese fishing boat caught in the drift of fallout from a surface test apparently got very sick and some died.

My dad was in Nagasaki In late September 1945 as part of UDT 11 surveying the harbor in preparation for the US occupation. He said the stench from the dead bodies made several team members sick.

The first British A-bomb test had the bomb located in the hold of a ship, below the waterline. The British were concerned about the possibility of a bomb being smuggled into a British port and detonated, so they tested it that way.

The fallout was apparently severe:

Two Dragonfly helicopters flew in to gather a sample of contaminated seawater from the lagoon. Scientists in gas masks and protective gear visited points in pinnaces to collect samples and retrieve recordings. Tracker controlled this aspect, as it had the decontamination facilities. Air samples were collected by RAAF Avro Lincoln aircraft. Although the feared tidal surge had not occurred, radioactive contamination of the islands was widespread and severe. It was clear that had an atomic bomb exploded in a British port, it would have been a catastrophe worse than the bombing of Hiroshima and Nagasaki.[92][101] The fallout cloud rose to 3,000 metres (10,000 ft) and was blown out to sea, as intended; but later reversed direction and blew over the Australian mainland. Very low levels of radioactivity were detected as far away as Brisbane.[102]

The photo of the bomb blast showed a “cauliflower cloud” instead of a mushroom cloud, because of all the mud mixed up from the ocean floor.

(The Brits apparently originally planned to do their tests at Churchill Manitoba, but chose Australia instead for the better climate. Lucky Aussies!)

Too bad. Churchill really needs it’s harbour dredged better…

Yeah, I remember reading that the Sedan test in 1962 had significant fallout in Iowa of all places, considering that the test was at the National Test Site in southern Nevada.

It also left a crater that is 100 meters deep and about 390 in diameter.