Image: JoNel Aleccia
By JoNel Aleccia Health writer
updated 3/14/2011 5:07:12 PM ET 2011-03-14T21:07:12

Amid dire reports of melting fuel rods and sickened workers at Japan’s beleaguered Fukushima Dai-ichi nuclear reactor, the public health risk from radiation exposure remains very low in that country — or abroad, experts say.

“In general, right now, the citizens of Japan have far more other things to worry about than nuclear power,” said Richard L. Morin, a professor of radiologic physics at the Mayo Clinic and chair of the safety committee of the American College of Radiology.

“There’s not a significant risk to anybody in the United States, including Hawaii,” he added.

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Though talk of a nuclear “meltdown” raises specters of acute radiation sickness and long-term cancers, such as those seen after the 1986 Chernobyl accident in which the reactor blew up, the radiation levels detected outside the Japan plant remain within legal limits, Japanese officials told reporters.

Exposure lower than CT scans, air flight
American experts monitoring the situation agreed, saying that reported radiation exposure remains far lower than normal exposure from background radiation in the environment, from medical procedures such as CT scans, or even from transatlantic air flights.

“I haven’t seen anything so far that seems to indicate that people are being exposed to levels of radiation that are acutely dangerous,” said G. Donald Frey, a professor of radiology at the Medical University of South Carolina.

Experts measure exposure to ionizing radiation in several ways, but the most current and accepted is based on the sievert, a unit named after R.M. Sievert, a 20th century Swedish physicist. The average person in the United States is exposed to about 6.2 millisieverts annually, with about half from background radiation and about half from medical sources.

For comparison, some experts refer to rem and millirem as measures of radiation exposure, but they're generally considered to be outdated terms. 1 rem is equal to 10 millisievert.  

Using the newer measure, a one-time CT scan can expose a person to between 5 and 10 millisieverts. An X-ray of the spine might expose a patient to an estimated 1.5 millisieverts. A long, cross-country air flight might expose someone to about .03 millisieverts. A person who smokes a pack of cigarettes a day is exposed to 53 millisieverts each year, according to the National Institutes of Health.

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So far, Japanese officials have reported possible top exposures at the plant of .5 millisieverts per hour, a level that has dropped to perhaps .04 millisieverts per hour, Frey said. While that level is concerning to plant workers, residents who heeded a 12-mile evacuation zone would not be affected, said Dr. James H. Thrall, chief radiologist at Massachusetts General Hospital in Boston.

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“That would only expose nuclear plant workers,” he said. “If you’re even 100 feet away, or 1,000 feet away, the exposure drops dramatically.”

The Fukushima Dai-ichi plant, located off the Pacific Coast, is the most devastated of the four Japanese nuclear complexes damaged by Friday's earthquake and tsunami. A Japanese official said 22 people had suffered radiation contamination and 190 may have been exposed Saturday after an explosion at the plant's Unit 1.  

Japanese officials ordered evacuation of an estimated 185,000 residents within about 12 miles of the plant. Officials have distributed 230,000 units of stable iodine to protect against thyroid cancer in case of radioactive exposure, but it’s still just a precaution, experts noted.

The typical limit for radiation exposure beyond background radiation is about 1 millisievert a year, according to the Nuclear Regulatory Commision. Adults who work in places where nuclear radiation is expected, such as plant workers or uranium hospital workers, are limited to 50 millisieverts per year, the NRC says.

People exposed to 1,000 millisieverts per year are considered at risk for acute radiation sickness, but will likely survive, Thrall said.

“At 10,000 millisieverts, you die rapidly and with a high probability,” he added.

Even if the workers at the nuclear plant in Japan were exposed continuously to .5 millisieverts per hour, it would take about 40 hours before them to reach the yearly limit for exposure. Now that the level has fallen, so has the risk, Thrall said.

Favorable winds
Prevailing westerly winds have helped the situation, blowing any contamination out to sea, experts said. But it’s highly unlikely that contamination will reach the shores of the United States, they add.

Of course, the situation could change rapidly, noted Morin.

"The most serious situation to me would be if there were a core meltdown," he said. "That might cause a significant release of radiation to the environment and that would be one of the biggest things. It doesn't look like that's going to happen right now, but you can't be certain."

In the meantime, the U.S. experts cautioned observers, especially those in the U.S., to keep the situation in perspective.

“There’s very little likelihood of any concern,” said Thrall. “Instead, I would advise people to look both ways before crossing the street.”

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