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But first what science is still learning from the survivors of the Hiroshima and Nagasaki bombings.
This week, President Obama becomes the first sitting president to visit Hiroshima.
Miles O'Brien looks at how much of what we know about the impact of radiation comes from a study about the long-term effects there. It's part of our weekly series on the Leading Edge of science.
Some images might be disturbing to some viewers.
The scientists who designed the first atomic weapons didn't spend much time researching the long-term health consequences of their devastating creation.
EVAN DOUPLE, Radiation Effects Research Foundation:
The Manhattan Project focused so much on physics, and really didn't know very much about the biological effects and health effects in particular.
When I met him in 2012, radiation biophysicist Evan Douple was the associate chief of research at the Radiation Effects Research Foundation. The joint Japanese and U.S. study has been following the survivors of the atomic bombings of Hiroshima and Nagasaki for nearly 70 years.
There isn't anything comparable to this kind of study.
The work began almost immediately after the war, when President Truman ordered the creation the Atomic Bomb Casualty Commission in 1946.
The scientists have recreated through interviews and questionnaires the locations of where each individual survivor was exposed, and so they could reconstruct by line of sight what the radiation quantities would be at different distances.
Ninety-four thousand irradiated bomb survivors volunteered to become subjects. Another 26,000 who lived in other Japanese cities during the bombings, but were not exposed to radiation, were included as a comparison or control group.
Today, about a third of the exposed and comparison subjects are still alive, many making routine pilgrimages up a hill overlooking the once devastated city of Hiroshima to undergo medical exams. Their blood is routinely analyzed, researchers on the lookout for damaged chromosomes and other signs of disease.
Over the years, they have frozen thousands of blood and tissue samples.
If that survivor comes down with a cancer or a health effect today, one can go back into the freezer and perhaps find what changed in that individual that eventually ended up resulting in a cancer.
Researchers here have published hundreds of papers documenting the links between radiation and its ill health effects, primarily cancer. They have written the book that is the basis for the standards for acceptable radiation exposure.
Among the findings over the years: Thyroid cancer and leukemia are the first to strike. Solid cancers come 10 to 30 years later. Young people are more susceptible to developing cancer than adults, and women are more susceptible than men. Perhaps the most important, a single exposure increases cancer risk for life.
The bottom line? Of 94,000 survivors studied over 70 years, about 1,000 additional cases of cancer can be attributed to radiation from the bombs.
JOHN BOICE, National Council on Radiation Protection and Measurements: So, the atomic bomb survivors study, which is a remarkable study — it's the gold standard in terms of radiation epidemiology — it has limitations, and one of the limitations is, the dose was given all at once.
Radiation epidemiologist John Boice is hoping to address that limitation. He is spearheading a study of workers from the Manhattan Project, atomic veterans who witnessed atmospheric bomb tests, utility employees at nuclear power plants, and medical professionals, one million in all.
Radiation exposure data captured by the dosimeters routinely worn by these workers over the years is compared with their health history and cause of death.
So, when we complete this study, that we will be able to say what the risks are from these various levels of radiation. Are they the same as Hiroshima and Nagasaki? Are they lower? Are they higher?
Some studies suggest our bodies can fight back and repair damage caused by low-level, yet chronic, exposure to radiation. But no one is sure.
Biophysicist David Brenner is also hoping to fill in some of this uncharted territory. He is director of the Center for Radiological Research at Columbia University Medical Center.
DAVID BRENNER, Columbia University Medical Center:
Any level of radiation can produce DNA damage, which ultimately can produce a cancer.
But when the doses get very low, scientists are hard-pressed to quantify the link between radiation and cancer. There is just too much cancer out there already. Four in 10 Americans will eventually battle the disease anyway.
So how do you detect a very small increase in that 40 percent? And if there was an increase from 40 percent to 40.1 percent, how would we ever detect that? And that's really our problem.
So could there be a line below which there is no risk?
But what we're trying to do is to figure out, what is the best way to extrapolate the risks that we know from high doses to low doses? In order to do that, we really want to know the mechanism on how exactly does radiation produce cancer.
Brenner employs a particle accelerator to try and find some answers. In experiments, he aims high-energy protons at a single cell.
What we want to do is to be able to expose a cell to radiation, but not expose all the neighbor cells to radiation, because that's actually what — the real-life situation at very, very low doses.
This isn't just an academic pursuit. The atomic bomb survivors study is the basis of risk estimates for everything from doses in medical care, to work rules for radiation workers, to evacuation orders in the wake of a nuclear power plant meltdown.
But these rules, which can affect so many lives in so many devastating ways, are based solely on educated guesswork. In January 2015, the U.S. House passed the low-dose radiation bill which would provide funds to begin a large study. But the bill stalled in the Senate, frustrating scientists who believe the nation that unleashed the nuclear age is obligated to never stop trying to fully comprehend its long-term impact.
Miles O'Brien, the "PBS NewsHour," Hiroshima, Japan.
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