Four weeks into Japan's nuclear crisis, nobody seems able to agree on how far the danger of radiation from the Fukushima Daiichi nuclear power station extends.
Japan has set a distance of just 30 kilometers. The first 20 kilometers of that is a mandatory evacuation zone, the last 10 kilometers a voluntary one.
But the UN's International Atomic Energy Agency (IAEA), plus several other countries say that's too short a distance.
Last week, the IAEA found radiation exceeding its safety standards in a village 40 kilometers from Fukushima. It recommended that Japan extend the hazard zone but didn't say by how much.
Meanwhile, the United States, Britain, and Canada have recommended that its nationals keep a full 80 kilometers away from Fukushima. They calculated that distance based on the possibility that the situation at the stricken nuclear power plant could suddenly worsen and more radiation might be released.
But if nobody seems to agree on how far the radiation threat extends around Fukushima itself, there are just as many opinions over how far the danger stretches beyond northeastern Japan.Japanese Food Products Banned
A worker wearing a protective suit walks near the damaged pit at the crippled Fukushima plant's No. 2 reactor.
Neighboring South Korea and China say they are worried about radioactive fallout moving westward over their territories. China's Health Ministry has found traces of radioactivity in spinach in three Chinese provinces and South Korean media have reported fears about contaminated rainfall.
Farther away, India has banned all food imports from Japan for three months, and Russia has banned imports of Japanese fish.
And around the world, scientists have measured trace-level increases of radioactive iodine in the atmosphere. The trace levels are not harmful to health but have further raised public concerns over how far radioactive particles can travel from Fukushima.
So, how far from Fukushima is a safe distance?
The answer combines both science and politics and that is one reason why it seems to vary so much from country to country.
On the scientific side, there are globally accepted baselines for radiation safety that dictate what levels of exposure are hazardous. But in practice, the application of these international standards can vary with different governments and different situations
According to British-based nuclear physicist Frank Barnaby, Japan's setting a distance of 30 kilometers as the safety zone is a perfect example.
"What [the Japanese] are saying is that within so many kilometers in our opinion the radiation dose given to the population is acceptable to us in the circumstances," he says. "They wouldn't disagree with the international calculations on what the damage is, but the question is 'is that acceptable' [for Japan's interests]?"
A boy wears a face mask at the Shimizu Elementary School in Fukushima.
Barnaby notes that as Japan reels from the damage done by last month's earthquake and tsunami, it appears to be trying to limit the population displacement from the Fukushima disaster so as not to add to its multiple problems.
More than 70,000 people living within the 20-kilometer mandatory evacuation zone have been moved to temporary shelters. But beyond that distance, the population gets denser.
An additional 136,000 people live in the voluntary evacuation area 10 kilometers farther out. Officials have said they should either evacuate or remain in the area but stay indoors.
Most have elected to remain and live normally while trying to cope with the radiation. They drink bottled water because the tap water exceeds safety limits and they wear gas masks outside because the air is also hazardous.
The Japanese government's "cost-benefit" approach to defining a small hazard zone has prompted other governments to take a similar approach, but in the opposite direction.Seawater Contamination
Not knowing how far the radiation might spread in the future, countries are urging their nationals to stay well away from Fukushima and, in some cases, even from Japanese food products altogether.
Japan's seafood has been particularly targeted because, to cool the damaged reactors in the coastal nuclear plant, rescue teams have pumped ocean water into them. As the water cools the fuel rods to prevent them melting further, it also picks up radioactive particles, and some of that contaminated water has leaked back into the sea.
Already, those leaks have caused radiation levels up to a distance of 40 kilometers offshore to soar high above safety levels. The nuclear plant's operator, TEPCO, has sought to reassure the Japanese public that ocean currents will distribute the radiation so widely that offshore areas will quickly fall back to safe levels. But some fishing villages have banned the sale of one species, the mudfish, after detecting above-normal radiation levels in their catches.
According to Barnaby, the rescue teams have no choice but to continue pumping seawater into the reactors, even though doing so violates international laws against dumping radiation into oceans.
"They would argue they have no choice but to keep them cool because the difficulty they have is that if the fuel in those reactor cores melts down, and some fuel has already done that, the radioactive contamination would be absolutely enormous," he says.
The dumping problem could worsen soon. TEPCO announced on April 4 that it needed more room to store the emergency seawater it keeps pumping into the reactors. It said it would make room by draining back into the sea some 10,000 tons of less contaminated water that has leaked into the basement areas of the reactor plant.
As radiation from Fukushima spreads into the sea, radiation also continues to escape into the atmosphere. And that, too, worries other countries, particularly Japan's nearest neighbors.
So far, Japan and other countries in the region have been fortunate that the prevailing winds are blowing the radiation eastward and out over the broad Pacific Ocean.
"If the wind direction changes 180 degrees, then the Japanese and China and other local countries, the Philippines and so on, will be in trouble," Barnaby says. "The information is that the reactors are still spewing out radioactivity and one would expect that in all probability the amount of radioactivity given out will increase a bit.
"And if that is blown back, then there is a problem for the Japanese and the local countries downwind."Risk Of Another Explosion
Japanese officials say that the nuclear plant could continue to release dangerous radiation into the air for several months.
On April 6, engineers announced that they had managed to stop a leak of radioactive water into the Pacific Ocean. But they said they were now urgently refocusing their attention on attempting to prevent another explosion at the power station.
TEPCO said it planned to fill one or more of the four damaged reactors with the inert gas nitrogen in order to prevent bubbles of explosive hydrogen forming inside them. Since the March 11 earthquake and tsunami, uncontrolled explosions of hydrogen have blown the roofs off of two reactor buildings and damaged a water tank under the core of a third.