When the U.S. dropped atomic bombs on Hiroshima and Nagasaki, Japan in 1945, the powerful explosions and destruction that followed were expected. But no one could say exactly what the long-term health effects would be for the people who survived.
"Radiation biology was still in its infancy," writes Gabriel Popkin for Inside Science, "and no one had ever studied the effects of an exposure even remotely on the scale of that delivered by atomic weapons."
In the decades since, the survivors have become one of the longest-studied groups in health research. Studies with 94,000 survivors through an American-Japanese partnership called the Radiation Effects Research Foundation, RERF, indicate that the risk of most cancers doubles with a doubling of radiation exposure. The one exception is leukemia, for which a doubling of the radiation dose can quadruple risk. Based on this relationship, regulators assume that even minute doses of radiation can increase cancer risk and set guidelines accordingly to limit workers’ exposure in nuclear energy facilities, uranium mines and other workplaces.
"These radiation standards are accepted worldwide," George Kerr, a consultant and health physicist, formerly of the Department of Energy's Oak Ridge National Laboratory told Paul Voosen for The New York Times. "They're kind of the Rosetta Stone."
Yet recently, scientists have started to question whether this caution is needed. “The atomic bomb happened in one crack, people were exposed, and then that was it,” Gayle Woloschak, a biologist at Northwestern University's Feinberg School of Medicine, in Chicago, Illinois told Popkin. “What would happen if you’re living close to a dump site, or something like that? You’re talking about a chronic everyday low-dose-rate exposure.”
Estimating the dose of radiation each bomb survivor received is difficult because the dose varied depending on how close they were to the hypocenters — the point of explosion. But RERF has recorded 853 cases of cancerous tumors that the researchers attribute to the bombs and 17,448 tumors from other causes such as smoking, diet and genetic factors. The survivors received an average dose of 210 millisieverts of radiation, according to The New York Times. In comparison, writes Popkin, a head CT scan delivers a dose of about 2 millisieverts and flying across the U.S. delivers a dose of about 40 microsieverts — one fiftieth that amount.
Epidemiological studies of nuclear energy workers and studies with mice, rats and other animals now suggest that low doses of radiation over time may not have a harmful effect, writes Popkin. Yet people fear radiation — an invisible, frightening force that can kill. That fact, coupled with low doses, may explain why the health effects of radiation at Fukushima have been minimal thus far: A favorable wind direction and sheer fear of radiation kept most people from receiving higher doses.
The atomic bombs’ immediate effects devastated both cities and killed between 150,000 and 246,000 people. But the psychological toll of radiation may be one of the most enduring parts of the bombs' legacy.