Exposure to radiation can have a dramatic and immediate effect on the human body.
The gastrointestinal system is very sensitive to radiation, leading to nausea and vomiting immediately after exposure. The blood system is often the hardest hit, although antibiotics and transfusions may allow a recovery.
But severe radiation damage to the immune system can cause overwhelming infections. And although nerves and the brain are most resistant to radiation, acute exposure usually results in damage to the central nervous system. High doses can kill outright.
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Restrictions lifted near site of Japanese nuclear accident
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Medical Correspondent Eileen O'Connor looks at the medical lessons learned from the Chernobyl nuclear reactor accident.
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Medical Correspondent Dr. Steve Salvatore looks at the effects of radiation exposure.
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The long-term effects of radiation exposure can include sterility, cancer and genetic damage that can be passed to children.
However, experts say there are three ways to minimize the risk of radiation exposure:
Time: Radioactive materials decompose and lose strength over time. For some materials, the process is quick, but for others, it takes centuries.
Distance: The further away from the source of radiation, the better.
Shielding: In an exposed area, heavy, dense materials such as lead offer protection.
As officials in Japan cope with radiation exposure from Thursday's accident, they will also try to avoid some of the mistakes made 13 years ago when a serious accident occurred at the Chernobyl nuclear power plant in Ukraine.
People in the affected area were actually told to gather outside, despite high levels of radiation still coming from the plant. The pictures were part of an effort by the Soviet government, then in power, to convince the outside world that all was well.
Thousands of cases of thyroid cancer and leukemia resulted.
By contrast, in Japan, people in the area near the accident were being ordered to stay inside their homes, to keep them shielded from potential radiation.
