The attacks on hiroshima and nagasaki are in my opinion so horrible because of the long term consequences. Around that time Edward Teller was apparently suggesting that nuclear weopons should be used for everything from war to road clearings. So when were the long term consequences discovered? And how long did it take for people to accept it? And what did the Japanese think was happening to them when their cancer rates skyrocketed?
If you are talking about radiation effects — people knew about radiation sickness from fairly early on. The Japanese cancer rates did not "skyrocket" — people who had been within 3 km or so of the blast, and survived everything else, had a slight increase in long-term cancers (about 10% more than baseline, with the percentage going on depending on your proximity to the blast), which translates into a thousand or so excess cancers. That's not great, but it's not the dominant factor in killing people (over 100X more people were killed by blast and fire). When people got cancers after the bombing, the Japanese and American observers (the Atomic Bomb Casualty Commission) concluded correctly that they may have to do with radiation exposure, as the link between cancer and exposure was already known.
That nuclear weapons would have prompt and potentially long-term radiation effects was known from the beginning, but understanding how those effects worked took time. By the 1950s the US understanding of fallout, especially from high-yield weapons, was fairly advanced. By the 1960s this was common knowledge and led to the banning of atmospheric nuclear tests. That radiation would be an issue with "Plowshares" projects like making canals and ports (not "road clearings") was well-understood and a large part why those projects never got off the ground.