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NAVAL AIR FACILITY ATSUGI, Japan — The Navy said very low levels of airborne radiation were detected Tuesday morning at greater Tokyo-area bases in Yokosuka and Atsugi, prompting commanders to direct base residents to remain indoors as a precaution.

At 7 a.m., the aircraft carrier USS George Washington at Yokosuka Naval Base detected elevated radiation levels, according to a U.S. Navy 7th Fleet statement. The Navy said the elevated levels were associated with the disaster at the Fukushima Dai-ichi nuclear power plant, located about 200 miles to the north.

The Associated Press reported that the Navy said Monday that radiation was detected by the USS Ronald Reagan, and that 17 helicopter crewmembers had to be decontaminated after returning from search and rescue duty. The Navy said more crews were exposed to very low levels of radiation Tuesday and had to be decontaminated.

Potassium iodide pills were given to a small number of those crewmembers as a precaution, said Col. Dave Lapan, a Defense Department spokesman.

A level of 0.5 millirems of radiation was detected at Atsugi with similar levels at Yokosuka, said Atsugi public affairs officer Tim McGough. The radiation was detected coming from winds blowing from the northeast, he said.

"We’re not expecting this to last long," McGough said. "Winds should be changing early this afternoon, we’re told."

Officials at Camp Fuji, Camp Zama and Yokota Air Base, all in the Tokyo area, told Stars and Stripes that they had not detected elevated levels of radiation as of about noon Tuesday.

At about 9:25 a.m., Atsugi base commander Capt. Eric Gardner announced the following over the base loudspeaker system:

"According to (Commander Naval Forces Japan) Rear Adm. (Richard) Wren, a third reactor at Fukushima nuclear plant has exploded. Radiation has been detected at extremely low levels at Yokosuka and Atsugi. Radiation is at the levels of a chest X-ray; however, to be to the most extent conservative, I request we all shelter in place, stay indoors and close all ventilation sources and close windows."

Gardner broadcast a similar message at 12:30 p.m., stating that walking outdoors was OK if necessary, while continuing to discourage unnecessary outdoor activities.

Air operations continued at the base during the period of elevated radiation levels.

Atsugi planned a town hall-style meeting Tuesday night at the Cinema 77 movie theater, where Atsugi command officials will discuss the situation with residents.

Yokosuka Naval Base held a similar town-hall meeting Monday night, where Wren told Stars and Stripes that the Fukushima crisis was too far away to force a safety-related evacuation of Yokosuka.

Millirems, the American version of the internationally used microsievert unit, are a measure of biologically effective radiation.

A Hiroshima radiation expert said that the levels of radiation detected so far at Yokosuka and Atsugi are not a health hazard. "The level of 0.5 millirems, which is translated to five microsieverts, is 50 times more than the level that exists in nature," said Masaharu Hoshi, professor of radiation physics at the Research Institute of Radiation Biology and Medicine of the University of Hiroshima.

"The level detected at this time is not a level that immediately causes health hazards," said Hoshi, who studies exposure cases and history in Chernobyl, Semipalatinsk (a Kazakhstan nuclear test site, closed in 1991), Hiroshima and Nagasaki.

"I would say not to worry. Even at Chernobyl, there were only 20 or so people who had direct effect and died of thyroid gland cancer," he said. "The major problem is psychological, the fear of harmful effect that people might harbor."

For perspective, the U.S. Nuclear Regulatory Commission estimates that a passenger on a round-trip flight from Washington D.C. to Los Angeles would be exposed to four millirems of radiation. Hoshi said that male reproductive functions would be threatened at 10,000 millirems.

Joseph G. Young, principal health physicist at Australian Radiation Services, told Stars and Stripes that factors like the rate of millirems, the type of isotope and whether the radiation detected was gaseous determined the seriousness of the exposure.

Navy officials told Stars and Stripes that they were working to answer those questions as of 12:30 p.m. Tuesday. They had not yet responded by 3:30 p.m. Navy officials said Tuesday that remaining indoors cuts exposure by 60 to 70 percent.

"Staying indoors would be good advice to give to people," Young said. "The obvious thing is that you don’t want to breathe any gases or particulate material."

Around noon Tuesday, Tokyo Gov. Shintaro Ishihara said they too had detected elevated radiation.

"I was informed that the radiation levels, which had no change until yesterday, has increased significantly this morning," Ishihara said during a news conference. "However, it is not the level that immediately causes a health problem. I ask all the reporters here to report calmly and accurately."

The radiation readings in Tokyo were 0.809 microsieverts (.0809 millirems) at 10 a.m. Tuesday, said Keiichi Nakaya, chief of Tokyo Metropolitan Government’s Office in Charge of Health and Safety. The reading had dropped to 0.147 microsieverts by 11 a.m., he said.

"Even the highest reading at this time has no health impact," Nakaya said.

Stars and Stripes reporters Chiyomi Sumida and Grant Okubo contributed to this report.

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