KIEV, Ukraine—In 1986 the worst nuclear accident in history took place when reactor No. 4 in the power plant at nearby Chernobyl exploded, spewing large amounts of radiation into the atmosphere. Now, almost 25 years later, the lesson that scientists are learning is that radiation might not be the only cause of this disaster's long-term medical effects, and perhaps not even the main one.

The explosion immediately killed two workers, and 28 firemen and nuclear power plant staff died from acute radiation syndrome in the three months afterward. The disaster also released a plume of radioactive fallout that contaminated more than 200,000 square kilometers of Europe, roughly three quarters of which lies in the former Soviet republics of Belarus, Russia and Ukraine. Hints of fallout were also detected elsewhere in the Northern Hemisphere, reaching Japan in six days and the U.S. in 10, but in most cases only negligible amounts were found.

Investigators of the medical consequences of the disaster must contend with both the very real effects of the tragedy and the desire to blame every bad medical outcome on Chernobyl. A major challenge lies in figuring out how much fallout was released during the event. Estimates range from roughly 5 percent of the radioactive material in the reactor, which is what is usually cited, to 95 percent, says civil and environmental engineer Eric Schmieman of Battelle Memorial Institute. It is difficult to make a firmer assessment because the contemporary hazards at the Chernobyl site prevent safe surveys.

Therefore, it remains difficult to know who was exposed to radiation from the event and how large a dose they received, and then to figure out the fallout's real effects. A 2005 report from the Chernobyl Forum, which is made up of the International Atomic Energy Agency (IAEA) and seven United Nations agencies, along with Belarus, Russia and Ukraine, estimated that five million people currently live in contaminated areas of those three countries. In addition, 350,000 workers helped contain and clean the accident, and roughly 240,000 of these "liquidators" worked in key activities at the reactor and in the 30-kilometer "exclusion zone" surrounding the accident. (Later, the number of registered liquidators rose to 600,000, although only a small fraction of these were exposed to high levels of radiation, the report noted.)

There is heated debate about how many deaths should be linked to the tragedy and its fallout. The IAEA's 2005 Chernobyl Forum report estimated that 4,000 such casualties might occur among the 600,000  people they considered . In response, the European Green Party commissioned an alternative study released in 2006, "The Other Report on Chernobyl," or the TORCH report, which estimated 30,000 to 60,000 extra cancer deaths.

"Radiation is the obvious and even exaggerated culprit with Chernobyl," says health physicist Vadim Chumak at the Academy of Medical Sciences of Ukraine's Research Center for Radiation Medicine in Kiev. It's not that simple, he says, adding: "One must consider a whole variety of factors when it comes to Chernobyl."

The problem with studying the effects of this fallout is that the world is bathed with naturally occurring low-level radiation, with sources ranging from long-distance air travel to plasma televisions. There are other confounding factors researchers must cope with as well, such as industrial pollution and differences in lifestyle and health care. In addition, they have to deal with the fact that looking for Chernobyl-linked problems might turn up ones that might have escaped medical attention otherwise but had nothing to do with the disaster. For instance, when it comes to victims of the atomic bombs dropped on Hiroshima and Nagasaki, those who did not die of acute radiation syndrome or the widespread lack of medical care after the attacks lived longer than other members of their generation because they received government-sponsored medical attention for the rest of their lives, Chumak says.

Scientists had to invent new techniques to determine how much radiation people might have actually received. For instance, Chumak and his colleagues developed a way to estimate doses people received by analyzing tooth enamel for effects of ionizing radiation. The Kiev-based team also helped establish ways to three-dimensionally simulate the radiation fields surrounding radioactive material from the disaster.

In the end there are only a handful of rigorous studies linking Chernobyl to disease, Chumak explains. For instance, there have been at least 1,800 documented cases of thyroid cancer in children up to 14 years of age when the disaster happened, far more than normal. Children's thyroid glands are especially vulnerable, because they are prone to absorbing radioactive iodine, a by-product of the meltdown. Also, there seem to be increased levels of leukemia and cataracts at statistically significant levels among liquidators. "We're desperately looking for the effects of radiation, but we have not found more than thyroid cancer, leukemia and cataracts that is convincing," Chumak says.

There is no doubt that people who lived in the exclusion zone have suffered problems. In addition to the cancer and cataracts reported, one also sees issues with the cardiovascular, lung, digestive and kidney systems. "We see early aging in the group from the Chernobyl disaster—if we see a man from that group who is, say, 50, the medical examination might suggest he is 10 or 15 years older compared with a member of the population of Ukraine not influenced by the disaster," says Anver Gasanov, deputy chief medical officer at the Research Center for Radiation Medicine.

Whereas the obvious culprit might be radiation, however, other factors might be at play. For instance, elderly people who were allowed to resettle inside the exclusion zone actually live longer than those who were not, Chumak says.

The issue might be stress. "You have all kinds of stress connected with the disaster that can lead to bad habits, such as smoking, drinking, drugs as well as add to disorders such as depression, and that then influences other diseases they can get," Gasanov explains.

More than 350,000 people from the most severely contaminated areas were relocated, and the move from a rural or village home to a completely unfamiliar life in a city apartment for many could have been traumatic, especially for the elderly, Chumak says. And when it comes to liquidators, "there is 'victim syndrome,' where they think they are damaged beyond repair and there is no point in carrying on with their lives," he adds.

 Scientists are continuing to research other potential effects of the disaster, such as whether increased incidence of mental retardation is occurring in subsequent generations due to their progenitors' radiation exposure. "We have looked at the children of victims, and now we are looking at a next generation of victims, the grandsons and granddaughters," Gasanov says.

The work done on Chernobyl is now finding use far beyond the disaster, and may be useful in assisting researchers addressing any health impacts of the latest nuclear plant disaster in Fukushima, Japan. At the same time, here in Kiev physicist Elena Bakhanova at the Research Center for Radiation Medicine is adapting the principles used to three-dimensionally model radiation fields at Chernobyl to help investigate the health effects of medical radiation.

"There are many doctors now with no training in radiology who are dealing with radiation, such as minimally invasive procedures carried out by regular cardiologists," Chumak says. "We want to learn more about the harm there might be to patients and to doctors."


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