Mikiso Iwasa, who had taken refuge behind his house, returned days later to search for his mother. She had been burnt beyond recognition.
"That wasn't a human, it was a thing. My mother was killed as a thing. Not as a human."
For a month afterward, he says he walked through the streets of Hiroshima looking for his sister and any help he could find.
"After that month, I started showing symptoms of illness – red spots appeared on my body, my throat hurt, I couldn't eat, I had a temperature, my gums bled, and my hair fell out. For 20 days I remained in bed, on the verge of death," said Iwasa.
"For 12 years the Hibakusha were left to themselves. So we helped each other. Especially because we were sick, we [couldn't] work. "If we do get a job, we get sick again. We lived with our sickness."
Many who survived the bomb, died later because of a lack of medical infrastructure and assistance from the Japanese government, which was struggling to recover from the attacks. Radiation sickness was not understood in the years immediately following the bombings, and many of the Hibakusha found themselves ostracized from society.
But in 1957, 12 years after the explosions, a law was enacted, formally recognizing the survivors as the Hibakusha, and they were finally able to receive the medical treatment they deserved.
The Bulletin of Atomic Scientists estimates there are nearly 8,000 operational nuclear weapons around the world.
After a year of intense negotiations, the United States and Russia agreed in March to the most comprehensive arms control agreement in nearly two decades. If the START treaty is ratified by the Senate and Russian Parliament, it will cut the nuclear arsenals of both countries from around 2,200 deployed warheads each to 1,550.
The Hibakusha are quick to remind the world that it took merely two bombs, considered weak by today's standards, to wreak absolute devastation.
As they gather in the now rebuilt cities of Hiroshima and Nagasaki, on they are using the 65th anniversary of the bombings to warn the world of what they suffered.
"We have overcome and in order to not let the human race experience such atrocities, we have come to say abolish nuclear weapons and never create Hibakusha again," said Iwasa. "This is not something that we're doing for ourselves, but for you and all people to never have to experience this again."