Mr. Taniguchi, who was 16 at the time, was about a mile from the center of the explosion of the second atomic bomb dropped by U.S. forces on Japan. The city of Hiroshima had been nearly leveled three days earlier. More than 200,000 people were estimated to have been killed in the two blasts.
Within a week of the Nagasaki bombing, Japan surrendered, bringing an end to World War II. Mr. Taniguchi's struggles were just beginning.
After a long, painful recovery, he devoted the rest of his life to peace and disarmament, often baring his scars as a symbol of the horrors of nuclear war.
"I realized that I must live on behalf of those who died unwillingly," he told author Susan Southard for her 2015 book "Nagasaki."
Mr. Taniguchi died Aug. 30 in Nagasaki at age 88, according to a statement from a group he helped lead, the Japan Confederation of A- and H-Bomb Sufferers Organizations. The cause was cancer.
As Mr. Taniguchi tried to climb to his feet after the explosion, "the skin of my left arm, from the shoulder to the tip of my fingers, was dripping like rags," he said. "I put my hand to my back, but there was no clothing. I could only feel something slimy."
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He retrieved the scattered letters from his mailbag.
"I didn't feel any pain and there was no blood," he said. "But all my energy seemed to vanish."
He was carried to a grassy spot on a hill and placed alongside other victims.
"When the morning came," Mr. Taniguchi said in 1994, "no one lying with me was still alive."
He was not rescued for three days. He was eventually taken to a Japanese military hospital. His skin was stripped away from his back, exposing his muscles. He spent almost two years lying on his stomach, while his back was suppurating with infections.
"The doctors were clueless about how to treat me," he said.
In January 1946, a film crew from the U.S. Strategic Bombing Survey came to the hospital and recorded Mr. Taniguchi being treated for his wounds. The three minutes of silent color film were so gruesome that they were not shown in public for more than 25 years.
"From shoulders to waist, his raw, bloodred tissue glistens under the lights," Southard wrote in "Nagasaki."
Burns and blisters covered much of the rest of his body.
"He cried every time he heard the instrument cart approaching," Southard wrote, "and when the nurses removed the gauze from his back, he screamed in pain and begged the nurses to let him die. 'Kill me, kill me,' he cried."
Mr. Taniguchi was not released from the hospital until 1949. He later went back to his job as a mail carrier and was not considered completely healed until 1960, although he continued to have medical problems throughout his life. He dealt with keloid scars and tumors and, despite his ramrod straight posture, never went a day without pain.
At a 2010 United Nations conference to review terms of a treaty on the nonproliferation of nuclear arms, Mr. Taniguchi held up a picture of himself as a young man, with his back exposed on the hospital bed.
"I am not a guinea pig, nor am I an exhibit," he said. "But you who are here today, please don't turn your eyes away from me. Please look at me again."
Mr. Taniguchi became one of several prominent hibakusha, or "atomic-bomb-affected people," who spoke out about their suffering, often despite public ridicule of their disfigurement.
"We never received any professional psychological counseling," Mr. Taniguchi told the Guardian newspaper of Britain in 1988, "but in our group of 60 people we've tried to do it for each other — at least to make the survivors talk about that day. We've saved some people from killing themselves."
Mr. Taniguchi became a determined advocate for the elimination of nuclear arms. He often traveled overseas to speak at conferences, including in the United States, and called for the Japanese government to pay the medical expenses incurred by the survivors.
He noted that the United States had never shown remorse for the damage caused by atomic weapons, but he was even harsher toward his own country.
"No one in the Japanese government has ever apologized about getting involved in that war, either," he said.
After the end of World War II, Japan adopted a constitutional provision renouncing war and prohibiting the deployment of military forces outside the country's borders. In 2015, amid 70th anniversary observances of the attacks, legislation was passed and signed by Prime Minister Shinzo Abe allowing Japanese forces to take part in international conflicts.
Mr. Taniguchi denounced the change in policy, calling it a betrayal of the country's pacifist principles.
"I am worried about what will happen to the world," he said, "when there are no more atomic bomb survivors."
Sumiteru Taniguchi was born Jan. 26, 1929, in Fukuoka, Japan. According to Japanese news reports, his mother died when he was an infant. His father worked for the railroad before being conscripted into the military.
Mr. Taniguchi spent much of his childhood with his maternal grandparents in Nagasaki before going to work for the postal service at 14.
When he was 24, Mr. Taniguchi had an arranged marriage that was put together by friends and family members.
"My wife never saw me before the wedding and was not told about my injuries," he told the Chicago Tribune in 2001. "She cried a lot on our honeymoon. It wasn't the scars so much that frightened her, but fear how long I would survive."
His wife, Eiko, applied lotion to her husband's scars and massaged his back. She died last year. Survivors include two children; four grandchildren; and two great-grandchildren.
In 1986, Southard was enlisted as a translator when Mr. Taniguchi came to Washington. She often visited him in Nagasaki and once asked him to describe the significance of his survival amid such suffering.
"Just that I lived," he said. "That I have lived this long. I have sadness and struggle that goes with being alive, but I went to the very last edge of life, so I feel joy in the fact that I'm here, now."
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