Image: Officials in check for signs of radiation
Kim Kyung-Hoon  /  Reuters
Officials in protective gear check for signs of radiation on children from the evacuation area near the Fukushima Daini nuclear plant in Koriyama.
updated 3/12/2011 1:46:31 PM ET 2011-03-12T18:46:31

As the operator of an earthquake-damaged nuclear reactor in Japan raced to try and prevent the core from melting down, experts on the health effects of radiation described the situation as worrisome but not yet posing a serious threat to public safety.

Japan’s Asahi Shimbum newspaper reported early Saturday that radiation levels per hour in the area near the front entrance of the No. 1 Fukushima plant reached 0.59 micro Sievert, which is eight times the normal levels. The central control room of the reactor recorded radiation levels 1,000 times the normal level, which would be approximately 70 microsieverts per hour, or 7 millirems, according to calculations by

But prior to an explosion at the reactor late Saturday, the hourly radiation level had reached 1,015 microsievert, or 102 millirems, an amount equivalent to that permissable for a person in one year, the Kyodo News Agency quoted he Fukushima prefectural government as saying. It was unclear whether that reading was form the control room or outside the plant.

Officials of Japan's nuclear safety agency said there was no sign that radiation levels had jumped after the explosion. In fact, Japan's government spokesman Yukio Edano said the radiation around the Fukushima Daiichi plant had in fact decreased. He did not say why that was so.

Radiation exposure is often measured in units called “millirem,” which is 1/1000 of a rem. The average American is exposed to about 620 millirem each year, with about half from natural sources and half from manmade sources, according to the Nuclear Regulatory Commission.

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By comparison, normal exposure rates range from approximately 0.03 microsieverts (a microsievert is one-tenth of a millirem) per hour to 0.23 microsieverts per hour in La Paz, Bolivia, the highest city in the world. 

A chest X-ray results in an exposure of about 8 to 10 millirems per film. A cross-country airplane flight results in a dose of 4 millirems.

Exposures of less than 50 rem typically produce changes in blood chemistry, but no symptoms, according to the U.S. Environmental Protection Agency.

That makes it unlikely that someone exposed to the radiation level reported at the plant prior to the blast would be immediately affected, said Dr. Fred Mettler, emeritus professor of radiology at the University of New Mexico.

"People don't become acutely sick until they're over 50 rem and more like 100 rem," he told

Iodine distributed to residents
But virtually any increase in ambient radiation can raise long-term cancer rates, and authorities were planning to distribute iodine to residents in the area, according to the International Atomic Energy Agency. Iodine can be taken to prevent the absorption of radiation by the thyroid.

After the 1986 Chernobyl disaster, thousands of cases of thyroid cancer were reported in children and adolescents who were exposed at the time of the accident.

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Mettler noted that Japanese scientists studying health effects since Hiroshima have determined that some health effects can start to occur at exposures of 15 rem, even if the results aren't apparent for 10 years.

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There were about 80,000 survivors of the atomic bomb, for instance, with an average exposure of 23 rem, Mettler said. During the next 50 years about 9,000 of those survivors died of cancer. However, Japanese scientists concluded that the toll included about 500 excess deaths, that is, deaths that would not otherwise have been expected.

Story: Second nuclear meltdown likely under way in Japan, official says

The Associated Press, Reuters, and Kyodo News Agency contributed to this report.

Video: Meltdown fears at damaged nuclear reactor

  1. Closed captioning of: Meltdown fears at damaged nuclear reactor

    >> that. japanese officials say they have kal cau lighted that 160 people have been exposed to radio activity . 160,000 people have been evacuated from around two nuclear plants most from the one that suffered the explosion. more on this from nbc's anne thompson .

    >> reporter: nothing frightened the world more than this. an explosion at the troubled fukushima one power plant . the japanese government declared emergency when the systems failed. now fear of a meltdown. throughout the day japanese tv reporters used diagrams and maps to help a near versus nation. the blast explosioned the ex exterior building and blamed the explosion on a buildup of hydrogen. this official said after the explosion the radiation leaking from the plant actually decreased as did the pressure.

    >> i think they are being cautious and conservative and given the conditions there, i think that is wise.

    >> reporter: when the earthquake struck, the reactor shut down and systems failed. without water to cool it, the core could over heat and melt down releasing radio activity into the environment. tonight, the japanese utility took the unusual step of pumping sea water into the reactor to prevent a melt down.

    >> if you do so, you have decided that the consequences of not doing so are very serious. the evidence in this case is that they are worried about significant melting of the core.

    >> reporter: more signs of concern. people living within the evacuation zone are scanned for radiation and the interational atomic energy agency said japan is preparing to give iodine to those living near the plant.

    >> reporter: this was reported on a scale of 1 to 7. at pennsylvania's three mile island, there was a partial core melt down but not a significant release of radiation. japan now wait to see if it its fate is the same. anne thompson .


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