Seventy years ago this month U.S. atomic bombs destroyed the cities of Hiroshima and Nagasaki, killing a total of roughly 200,000 Japanese people in the world's first, and so far only, use of nuclear weapons in war. Many of those who survived the initial blast died soon after from injuries, burns and radiation sickness. The scale of the devastation sparked an enduring debate over whether the use of such weapons is ever justifiable and the extent to which scientists are morally responsible for the consequences of their discoveries.

Today some 22,000 atomic bombs exist in at least eight countries, according to the United Nations Office for Disarmament Affairs. More than 65 nations support a worldwide ban on nuclear weapons. Many of the nations that own them, including the U.S., have diminished their stockpiles yet continue to upgrade their nuclear technology.

Several of the Hibakusha, or survivors of the blast, and their family members visited Scientific American's New York City office this spring during a trip to attend the 2015 review conference for the 1970 Treaty on the Non-Proliferation of Nuclear Weapons—a meeting that occurs every five years to allow its 190 signatories to evaluate progress. Edited excerpts from the interpreted conversation follow.

Words from the survivors

[After the August 6 bombing of Hiroshima], we decided to leave the town. Many people did the same thing. We took refuge in some vineyards. Because there was no food, we ate the unripe grapes, and then we developed fever and diarrhea and began to vomit. My mother thought we had got dysentery. Now I think it was from radiation poisoning…. Many of the people that I work with have had their children die of leukemia or cancer very young—in their 40s. I worry about myself, but I'm also worried about my children and their health. —Tamiko Nishimoto, age four when the bomb fell just 2.3 kilometers from her home

On August 8 the bomb was dropped on Nagasaki. I was working for Nagasaki Shipyard. At 11:02 it was like a big sun burning over the building. I was surprised, and I saw something burning outside the window. About five or six seconds later a huge explosion shook the building and sent glass flying everywhere. Those who had been standing near the windows were struck with glass. They had so many holes in them, they looked like pomegranates…. As soon as work ended at five o'clock, I went back to my dormitory in Urakami. Many people were running toward me—not so much running as slowly struggling forward. Their faces were so burned that their faces resembled rugby balls. Their hands were swollen up, and they looked like they were wearing baseball gloves. There was limp skin hanging down from their cheeks and hands. Because it was very hot, people tried to wipe their cheeks with their hands, but they ended up with the skin of both their hands and their cheeks coming down to their chin. When I finally arrived at my dorm at Urakami, it was completely burned, and all the people inside had been killed. —Takamitsu Nakayama, age 16 during bombing