Experts say the amount of radioactivity emitted by the facility is relatively minor and should dissipate quickly over the Pacific Ocean.

It’s a big ocean between north-eastern Japan and the United States and thousands of miles from the crippled nuclear power plant to much of Asia.

That means there’s little chance - at least for now - that radiation from the shattered reactors could pose a serious threat to the wider world.

Experts say the amount of radioactivity emitted by the facility is relatively minor and should dissipate quickly over the Pacific Ocean.

“Every mile of ocean it crosses, the more it disperses,” said Peter Caracappa, a radiation safety officer and clinical assistant professor of nuclear engineering at Rensselaer Polytechnic Institute in Troy, New York.

However, as crews who were trying to prevent a meltdown abandoned the plant to save their own lives, questions remained about just how much radiation levels would rise and where that toxic material would go.

All along, those at immediate risk were workers inside the plant and the people living closest to it.

If the water level in fuel storage ponds drops to the level of the fuel, a worker standing at the railing looking down on the pool would receive a lethal dose within seconds, according to a study by the Millstone nuclear plant in Connecticut.

If the water level in such storage ponds drops to the level of the fuel, a worker standing at the railing looking down on the pool would receive a lethal dose within seconds, according to a study by the Millstone nuclear plant in Connecticut.

Such intense radiation can prevent workers from approaching the reactor or turn their tasks “into suicide missions,” said David Lochbaum, a nuclear engineer who heads the nuclear safety programme of the Union of Concerned Scientists.

Next in the line of danger would be those who live within a 20—mile (32—kilometer) radius. Areas around the plant have been evacuated for that reason.

“The odds of someone outside the plant getting an acute injury - sick in the next couple of weeks - is close to zero,” said John Moulder, a professor of radiation oncology at the Medical College of Wisconsin in Milwaukee who studies the effects of radiation exposure.

The radioactive particles probably contain materials linked to cancer in high doses, including cesium and iodine. The long—term cancer risk for nearby residents will depend on exposure and cleanup efforts, Prof. Moulder said.

Radioactive cesium and iodine also can combine with the salt in sea water to become sodium iodide and cesium chloride, which are common elements that would readily dilute in the wide expanse of the Pacific, according to Steven Reese, director of the Radiation Centre at Oregon State.

Winds in the area are currently blowing towards the coast because of a winter storm. But that will change to a brisk wind blowing away out to sea at least through Wednesday, he said by telephone.

Still, the forecast offered little comfort to those living in the area - and in nearby countries such as Russia.

The Russian Emergencies Ministry said it was monitoring radiation levels and had recorded no increase.

Many Russians, however, distrust the reassurances, perhaps remembering the Chernobyl disaster 25 years ago and how long it took the Soviet government to reveal the true dangers of the radiation.

“The mass media tells us that the wind is blowing the other way, that radiation poses no threat. But people are a mess,” Valentina Chupina, a nanny in Vladivostok, said in a comment posted on the website of the newspaper Delovoi Peterburg. “They don’t believe that if something happens we’ll be warned.”

The news portal Lenta said that in addition to potassium iodide and instruments used to measure radiation, people in the Far East also were stocking up on red wine and seaweed, which they believed would offer protection from radiation.

Even so, many experts here say that this emergency is nowhere near the level of Chernobyl, the worst nuclear disaster in history.

For one, that reactor’s core contained graphite that caught fire, which blasted radiation high into the air and into wind currents that carried it long distances. The Japanese core is metal and contains no graphite, experts said.

The Chernobyl plant also lacked a heavy concrete shell around the reactor core. And the incident there happened quickly, with little time to warn nearby residents.

So far, the radiation released in Japan has not reached high altitudes, said Kathryn Higley, director of the Oregon State University Department of Nuclear Engineering and Radiation Health Physics.

“In addition, radioactive material is sticky. It has a static charge,” she said, so it will stick to the sides of buildings, and “rain is going to knock it down.”

As a precaution, the World Meteorological Organization has activated specialized weather centres to monitor the situation. Those centres, in Beijing, Tokyo and Obninsk, Russia, will track any contaminants.

Japanese officials said that, early Wednesday, the level of radiation at the plant surged to 1,000 millisieverts before coming down to 800 to 600 millisieverts. Still, that was far more than the average.

Doctors say radiation sickness sets in at 1,000 millisieverts and includes nausea and vomiting.

Damage to blood cells can show up two to four weeks later, said Dr. Fred Mettler, a University of New Mexico radiologist and adviser to the United Nations on radiation safety. He led an international study of health effects after the Chernobyl disaster.

Levels are still likely to be lower away from the plant, said Kelly Classic, a radiation physicist at the Mayo Clinic and a representative for the Health Physics Society, an organization of radiation safety specialists.

The U.S. Nuclear Regulatory Commission says doses of less than 100 millisieverts, or 10 rems, over a year are not a health concern.

By comparison, most people receive about three—tenths of a rem every year from natural background radiation, according the U.S. Environmental Protection Agency. A chest X—ray delivers about .1 millisieverts, or .01 rem of radiation; a CT scan of the abdomen and pelvis is about 14 millisieverts, or 1.4 rems.

If a full meltdown occurs at the Japanese plant, the health risks become much greater - with potential release of uranium and plutonium, said Dan Sprau, an environmental health professor and radiation safety expert at East Carolina University in Greenville, North Carolina.

“If that escapes,” Prof. Sprau said, “you’ve got a whole new ball game there.”

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