Masao Tomonaga was aged 2 and in bed with a fever at his home in Nagasaki when the second atomic bomb used in war exploded above the city on Aug. 9, 1945.
The blast that killed nearly 74,000 in the southwestern Japan city by the end of 1945 led the now 81-year-old doctor on a path to studying radiation's effect on leukemia and other cancers in survivors.
Now an honorary director of the Japanese Red Cross Nagasaki Genbaku Hospital, he also fulfills roles including as chairperson of the survivors' group Nagasaki Prefecture Hibakusha Association and running a care home for those affected by the bomb.
He has given his accounts of the bombing by traveling overseas many times, most recently to the United States in November when he organized dialogue sessions with ordinary U.S. citizens, but his telling of the day itself comes from his mother's memories.
"She described this great white light, and that she was blinded. Then came the blast wave, half-destroying the house," he recalled. "Beams and pillars holding up the ceiling came down, but fortunately, my futon was right between them. I was lucky to have no injuries when the bomb exploded."
About 24 hours before, he had been in Nagasaki University Hospital, located near the hypocenter of the bomb.
"A day's difference saved me. One day later, and I'd have been fully exposed, just 600 meters from the blast," Tomonaga reflected.
Their home, some 2.5 kilometers from the hypocenter, was quickly consumed by fire, and Tomonaga evacuated to nearby Omura with his mother and grandparents, where the family stayed for around three years.
They were rejoined by his father, Masanobu Tomonaga, a physician who returned from Taiwan about a year and a half after the war. Together, the family went back to Nagasaki, where the elder Tomonaga treated atomic bomb survivors who had developed leukemia.
Rates of the blood cancer were high among survivors in the years immediately after the bombings of Hiroshima and Nagasaki, and occurrences were greater than in the general population.
His father transferred to the school of medicine at Hiroshima University in 1959 for his expertise, and Tomonaga enrolled in high school in the western Japan city, which suffered the first U.S. atomic bombing three days before Nagasaki.
"The Hiroshima newspapers often reported new leukemia cases, and I gradually felt concerned because I had been relatively close to the hypocenter," he said.
Tomonaga eventually returned to his home city to enroll in medicine at Nagasaki University. There he began researching leukemia and, at his father's suggestion, spent three years gathering information in Nagasaki for the then Atomic Bomb Casualty Commission, a U.S.-Japan project into the bomb's effects.
"I wasn't conscious of the bomb initially, but the research showed again how terrible it was," he said. "It was clear radiation from the atomic bomb has an inhumane, extremely appalling effect on the human race. I felt the world needed to know."
Tomonaga began his activism in earnest in 1983, joining the International Physicians for the Prevention of Nuclear War.
"The explosion itself was intense, of course, and it was a cruel weapon, but it didn't end there. The atomic bomb left people in Hiroshima and Nagasaki with radiation sickness that affected their cells' ability to reproduce. That is a scientific certainty," he said.
In 2013, he spoke at a conference in Norway on the effects of nuclear arms on humans, a gathering credited as leading to the United Nations' 2017 adoption of the Treaty on the Prohibition of Nuclear Weapons.
His leukemia research, too, has continued despite rates of the disease having fallen after peaking in the first 10 years following the bombings, with occurrences of other cancers rising instead.
In 2005, he published a paper showing survivors with high levels of radiation exposure were more likely to develop myelodysplastic syndromes, a blood cancer in which the bone marrow stops producing healthy blood cells.
But by Tomonaga's reckoning, prospects of ridding the world of nuclear arms are at a historic low. He cited nuclear threats in Russia's invasion of Ukraine and the Israel-Hamas war and reports of increasing arsenals in nuclear-possessing states.
"The situation is reversing. We have to speak out, but how? As Nagasaki survivors, we concluded that if the people in nuclear countries don't do their part, governments won't change."
To help them speak up, Tomonaga traveled to the United States in November with a group of atomic bomb survivors, descendants and others to give their accounts and exchange views with citizens at 20 locations in three American cities. He said he felt a shift in dialogue during the visit.
"It used to be harder to argue in America for abolishing nuclear weapons on grounds of inhumanity. It invited responses about Pearl Harbor and Japan starting the war," he said. "But now, the big change I see is people are open to that argument."
Partly, he credits the Oscar-winning film "Oppenheimer," about the life of the bomb's creator Robert Oppenheimer, as helping to show younger generations the inhumanity of nuclear weapons. So impressed by the film was Tomonaga that he lent his voice to its promotional campaign upon its Japan release in March.
"Oppenheimer was in a dilemma, thinking he was a God of Death. Such a person is of no use to a nuclear-possessing country, so the government cast him out. The fact that a film like that can come from the United States is amazing."
Nearly 80 years on, the number of survivors dwindles, with 106,825 government-recognized hibakusha alive at the end of March. Tomonaga is pessimistic about the prospects for abolition and anti-nuclear efforts after survivors, who have played central roles, have all died.
"Honestly, there's nothing to be done. We (survivors) don't have an idea either, perhaps apart from asking for research into the effects on DNA to continue," he said.
"What we survivors can do at this final stage is travel the world and tell our stories. Researchers can continue showing their work. There's still a lot we can do to appeal to people's common sense," he said.
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