It's long been a vexing issue in the scientific community, spurred in large part by public panic over nuclear power, waste and radiation: Why the terror?
Compared to notorious killers like driving, smoking or drinking, nuclear risks -- though objectively carrying little danger in their modern deployments -- stir the deepest fears in Americans, a terror that is surfacing again as engineers strive to contain the crisis at Japan's crippled Fukushima Daiichi power station.
For nuclear workers and residents near the reactors, of course, the crisis at Fukushima is a very tangible health threat. But the overwhelming global public and media response to the Japan power plant's problems -- where no one is known to have died from radiation exposure -- is set in sharp relief when compared to the reaction to thousands who perished in the earthquake and tsunami.
Public fear of radiation released halfway around the world -- the chasm between actual and perceived risk -- has helped give rise over the past three decades to a new field of social science, aptly named "risk perception." Through diligent questioning, study and an improved understanding of how humans make decisions, these scientists have been able to categorize why people instinctively fear certain risks.
And nuclear radiation, it turns out, hits all the buttons.
Radiation is difficult to comprehend, incredibly complex in its varieties and jargon -- gamma rays, ionization, sieverts -- and that alone makes it scary. Despite the inability of reactors to cause nuclear explosions, they have obvious associations with nuclear weapons. And, at high doses, radiation can cause cancer and death, and not an ordinary kind of death, said Brian Zikmund-Fisher, a health behavior professor at the University of Michigan School of Public Health.
"Radiation can be deadly, and it's deadly in a kind of gruesome way," he said. "The emotionality of someone with radiation sickness, their hair falling out, is a gruesome image. ... It's icky in the way that being poisoned is icky. How awful -- how bad it feels."
It is important to note that at all times we are being bombarded with cosmic radiation, and Americans are, on average, exposed to 6.2 millisieverts (mSv) of radiation a year, to use a standard measure. Evidence of direct health effects from radiation exposure only begin between 100 and 200 mSv, when a very slight increase in the chance of developing cancer later in life first appears. Radiation at the Fukushima plant has reached these levels at times, but beyond the plant's boundaries, exposure has remained at low doses.
Compared with the earthquake, which while devastating was also local, people around the world can imagine being affected by the radiation at Fukushima. That makes it personal. And despite its sweeping power, they can ultimately wrap their heads around tsunami, too, Zikmund-Fisher said.
"It's a scale that I can imagine," he said. "Even the tsunami. It's huge, it's catastrophic, but I can imagine a giant wall of water. That would be bad. But it's not so strange."
Radiation is invisible. It feels like a lingering trap.
As the sociologist Kai Erikson put it, "Invisible contaminants remain a part of the surroundings -- absorbed into the grain of the landscape, the tissues of the body and, worst of all, into the genetic material of survivors. An 'all clear' is never sounded. The book of accounts is never closed."
There are still more buttons. Unlike X-rays used for medical therapy or imaging, the radiation from nuclear power is not a risk people directly choose. When we choose risk, it's less scary. It's created by humans, which inherently feels more threatening than sources like the sun or radon gas. It cannot simply be extinguished, like a fire -- uncontrollable. It is rare, alien. Familiar risks like car accidents, though far more likely to kill people, are still better accepted.
The emotional stirrings like those caused by radiation are incredibly important to how people judge risk. At all times, humans evaluate risk on cognitive and emotional levels, the parallel processes weaving in unpredictable ways. It's not about ignorance. Even someone -- say, a science reporter -- aware that flying increases radiation exposure yet does no harm can still, at the same time, feel very worried about Fukushima Daiichi, Zikmund-Fisher said.
"That's not inconsistent with how our bodies are wired," he said.
These fears are deeply human, and many of them stretch back to the original scientists who split the atom during the Manhattan Project. These researchers exercised abundant caution, more than any scientist before them, as they believed "they had powers that transcended anything humans had done before," said Spencer Weart, a science historian and author of Nuclear Fear and The Discovery of Global Warming.
"Just about everything you find with nuclear fears originated with a nuclear physicist," he said.
The risk perception field has its origins in the works of scientists like Paul Slovic and Baruch Fischhoff.
About 35 years ago -- before the Three Mile Island and Chernobyl accidents -- the nuclear industry approached Fischhoff, who is now a professor of decision science at Carnegie Mellon University, to study why the public opposed nuclear power. He found an industry that struggled to explain, without condescension or untoward complexity, how it operated.
"The nuclear industry had done a terrible job communicating the facts of the industry," Fischhoff said, a trend that never changed. "It continues to do a horrible job of communicating."
Fischhoff's pioneering surveys looked at three groups of residents in Eugene, Oregon -- college students, the League of Women Voters, club members -- and compared them to experts from the risk assessment community, probing their perceived risks for 30 activities and technologies. The students and women's group rated nuclear power No. 1. The experts ranked it No. 20.
Fischhoff's study and further work by Slovic, a psychology professor at the University of Oregon, have found that there are two simplified scales that can track how people judge risks: dread and the unknown. The unknown includes risks that aren't observable, aren't known to those exposed; risks that carry uncertainty, or delayed effects. Dread, meanwhile, draws on associations with uncontrollable, catastrophic and fatal consequences; involuntary risks that seem inequitable, carrying dangers for future generations.
"Almost everything with nuclear goes very, very highly on that dread scale," Zikmund-Fisher said.
'Good memory for scary stuff'
The parallel cognitive and emotional understandings of risk are not a precisely settled science, though, Fischhoff added.
Often, poorly understood technologies provoke feelings of dread, and it can be difficult to tease out whether it is the emotional or cognitive response that ultimately causes the risk perception, he said. People are complicated.
However raised, these fears would exist even without the historical associations tacked onto reactors. But those exist -- in coverage of Japan, the call-backs to Hiroshima and Nagasaki are frequent, for good reason -- and they seriously stigmatize the industry, in the psychological sense of the word, said David Ropeik, a former television reporter and the author of How Risky Is It, Really? which examines risk-perception science.
"When we only have partial information to make sense of it, we compare it to what we already know," he said. Imagery of terrifying events like atomic explosions remain more readily available in the mind, and easy to call on, a dynamic known as the availability heuristic.
"We have a particularly good memory for the scary stuff," Ropeik added.
Ropeik, in a way, is atoning for past sins. During his work as a television reporter, he reported on nuclear power plants like they were a "second Satan" for two decades, he said. But during that time, he never reported the actual epidemiology of cancer risk from radiation.
Compared with many other carcinogens, radiation is relatively weak at limited exposures -- thyroid exposure to iodine being the most important concern at Fukushima -- making it difficult to sort out of the existing likelihood that a large minority of people is already bound to develop cancer in their lifetime.
Even plutonium -- deadly if inhaled -- can be outdone in risk, added Weart, the historian.
"Pound for pound, plutonium is by no means the most dangerous substance on earth," he said.
Authorities lost public trust
Weart pegs the development of nuclear fear to the huge amount of atmospheric nuclear testing that occurred during the late 1950s.
Hiroshima and Nagasaki had been frightening, but contained. There was a balance between nuclear hopes and terror. But reasonable fears about testing fallout, and then the Cuban missile crisis, Weart said, finally sealed nuclear's fate as a "dirty word."
Americans went through a process likely to be seen in Japan in the future, where their nuclear regulatory agencies have underplayed nuclear accidents. The U.S. Atomic Energy Commission was responsible for both promoting and regulating the nuclear power industry until the early 1970s, and came under strong fire for what many saw as weak regulation. Congress ultimately abolished the agency, dividing into the Energy Department and Nuclear Regulatory Commission.
When the public trust is lost by these agencies, it is difficult to recover.
"Concerns about radiation have always been [tied to] worry about authority," Weart said.
Questions of authority are rising, and will soon spike, in the United States, as negligible bits of radioactive particles are detected from Fukushima. And authorities speaking to the public must appreciate that the public has mental models -- say, a model of being poisoned -- that they may association with radiation, Michigan's Zikmund-Fisher said.
"If I apply that mental model to radiation, which is probably what a lot of people are doing, than I'm probably not going to feel very safe," he said.
Despite the decades the government and industry have had to improve how they talk to the public about radiation risks, not much has improved. "We have very little by way in stock of tested messages that will make it clear to people what's going on in a credible way," Fischhoff said.
The dragging uncertainty at Fukushima has grown more powerful in its own way, too.
"Part of the reason why this is so emotionally powerful is that it doesn't feel like we know when it will end," Zikmund-Fisher said. "And that's very scary. Sometimes knowing that the bad thing is going to happen, and then it's going to be done -- it's easier to tolerate."
More on radiation:
What it is.
How it works.
How it spreads.
Click here for a special report on Japan's nuclear crisis.