In general, it takes a pretty high dose of radiation to increase cancer risk, Higley said. For instance, there were reports that one Japanese worker was exposed to 10 rem (100 millisievert, mSV), a measurement of radiation dose. From that exposure, his lifetime cancer risk would go up about half a percent, Higley said. According to Higley, the dose is the equivalent of about five CT scans . Americans are exposed to about 0.3 rem (3 mSv) each year from natural sources, such as the sun.
Potentially, exposure to any type of radiation can increase cancer risk, with higher exposure increasing the risk, Bouville said.
No increases in cancer rates were observed after the release of radioactive from a power plant on Three Mile Island, Pa., in 1979, Zablotska said.
A person’s risk of getting sick depends on how much radiation the body absorbs. Those exposed to high levels of radiation, about 200 rem, (2000 millisievert ) could develop radiation sickness, Bouville said. A chest X-ray is about 0.02 rem, (0.2 millisieverts mSv), according to the Interational Atomic Energy Agency.
People are exposed to about 0.24 rem (2.4 mSv) per year from natural background radiation in the environment, the IAEA says.
Radiation sickness is often fatal and can produce such symptoms as bleeding and shedding of the lining on the gastrointestinal tract, Zablotska said. About 140 people suffered from it as a result of the Chernobyl accident, Zablotska said.
A radiation dose of 40 rem, (400 mSv) per hour was reported at one of the Japanese power plants at one point following the March 11 earthquakes and tsunami that damaged their cooling systems, according to the IAEA. This is a high dose but was isolated to a single location, the IAEA says.
“”That is definitely an area where you do not want to stay for prolonged period,”” Higley said. She notes that a total dose of 400 to 600 rem can be lethal. But the radiation levels have been decreasing after the observed spike, she said. She speculates the spike may have been due to the release of a puff of radioactive material when pressure dropped at the facility.
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