The victims of the nuclear attack on Hiroshima received a double-deadly dose of radiation, according to a new study published in PLOS ONE. The study is the first to use human tissue samples from victims of the bombing to precisely measure how much radiation the people of Hiroshima received.

On August 6, 1945, the US dropped an atomic bomb, nicknamed Little Boy, on the city of Hiroshima in Japan, killing an estimated 66,000 people, and injuring a further 69,000, although many estimate these figures to be higher. The bomb devastated the city, and just three days later, the city of Nagasaki suffered a similar fate at the hands of the “Fat Man” atomic bomb.

To work out just how much lethal radiation the victims of Little Boy suffered, researchers from the University of São Paolo analyzed the jaw bone of someone who was just a kilometer (0.6 miles) away from the heart of the explosion.

They used a method known as paramagnetism, where bones become weakly magnetic following exposure to X-ray or gamma radiation. This serves as a marker for how much radiation has been absorbed, and allowed the scientists to perform a technique known as radiation dosimetry. The study’s lead author originally used this to determine the age of ancient bones in Brazil.

The researchers made this process more accurate using a technique known as electron spin resonance spectroscopy and found that the victim had suffered a radiation dose of 9.46 grays – just 4-5 grays of radiation is enough to kill you. Using their technique, the researchers were able to isolate the radiation dose, separating it from background radiation caused by things like the heat of the explosion.

The bombings of Hiroshima and Nagasaki are thought to have brought an end to World War II, although many argue that it was the Soviet Union’s sudden invasion that led Imperial Japan to surrender. While the new research provides insight into these past bombings, the team also believes it could be relevant in the future.

“Currently, there's renewed interest in this kind of methodology due to the risk of terrorist attacks in countries like the United States," explained study author Oswaldo Baffa in a statement.

"Imagine someone in New York planting an ordinary bomb with a small amount of radioactive material stuck to the explosive. Techniques like this can help identify who has been exposed to radioactive fallout and needs treatment."