As the condition of the stricken Fukushima Daiichi nuclear power station in Japan continues to deteriorate, nuclear safety experts, government regulators and health physicists are keeping close watch on the situation to determine the danger—both real and hypothetical—that the incident poses to people near the plant.

Japanese authorities have carved the area around Fukushima into two zones, recommending that individuals within 20 kilometers of the plant evacuate and that anyone living 20 to 30 kilometers from the plant take shelter and stay put. Nevertheless, after initially supporting that action the U.S. Nuclear Regulatory Commission (NRC) took the dramatic step March 16 of recommending that Americans within 80 kilometers of Fukushima Daiichi evacuate the area.

U.S. federal guidelines set a much smaller standard perimeter for radiation hazards, leaving some puzzled as to why U.S. authorities would recommend such a large evacuation zone for Japan. (The 80-kilometer radius is larger than that implemented for the 1986 Chernobyl disaster in what is now Ukraine, after which 115,000 people were evacuated within roughly 30 kilometers of the nuclear site.) And although radiation measurements in the vicinity of Fukushima have varied greatly, it appears that exposures outside the 20-kilometer radius have not exceeded levels that would be cause for action in the U.S.

The differences in the responses of the U.S. and Japan to the crisis highlight the lack of detailed information on the rapidly shifting condition of the crippled nuclear plant, the inexact science of predicting what might happen next, and the difficulty of weighing the risks and benefits of taking various types of protective action.

The NRC's recommendation for a widespread evacuation stems not from the present conditions but from projected radiation exposures in a worst-case scenario, according to agency spokesperson David McIntyre. "Basically it was projections by our protective measures team based on what information we had available, what was going on at the plant, weather conditions, and so on," McIntyre says. "They projected into the short-term future and reached the conclusion that in the near term people out to 50 miles [80 kilometers] would exceed the exposure levels at which we would recommend action."

U.S. guidelines recommend protective measures, usually evacuation, if individuals are expected to receive 10 to 50 millisieverts (mSv) of radiation. (A sievert is a measure of ionizing radiation equal to 100 rems; a rem is a dosage unit of x-ray and gamma-ray radiation exposure; one millisievert is 0.1 rem.) The average American absorbs 6.2 mSv a year from natural and man-made radiation sources.

In the U.S. each nuclear power plant is surrounded by an "emergency planning zone," extending 16 kilometers in each direction, which would bear the brunt of radioactive fallout from a nuclear accident. The U.S. regulations are predicated on the assumption that most partial meltdowns would not expose individuals beyond 16 kilometers to dangerous radiation levels, and that even in worst-case accidents "immediate life-threatening doses would generally not occur outside the zone."

Outside the immediate vicinity of the nuclear site the primary danger is not radiation emitted directly from the plant as high-energy photons or other subatomic particles but airborne radioactive material released from a damaged reactor into an atmospheric plume. The material in that plume, as it undergoes radioactive decay, gives off dangerous radiation primarily in the form of gamma rays and can pose additional hazards if inhaled, swallowed or absorbed through the skin to emit radiation from inside the body.

"Internal exposure is very different than contamination you can walk away from," says Jerrold Bushberg, a health and medical physicist at the University of California, Davis. "If there's a plume that passes overhead and some of the material precipitates down, you may be externally contaminated, but it's nothing that a change of clothes and a shower can't take care of."

The "shelter in place" recommendation made by Japanese authorities for individuals between 20 and 30 kilometers from Fukushima Daiichi is intended to minimize ingestion of radioactive material and to prevent skin and clothing from becoming contaminated as the plume deposits radionuclides around the area.

So far, those precautions seem to be sufficient. "Those are appropriate actions to take to keep the dose to the population below one rem," or 10 mSv, says Richard Vetter, a professor emeritus of biophysics at the Mayo Clinic in Rochester, Minn. "If the actions are taken and the doses are in fact lower than one rem, they will experience no health effects. There's a possibility to calculate a statistical increase in cancer in that population, but those are below the levels at which epidemiological studies have shown an effect."

The Japan Broadcasting Corp. (NHK) reported March 17 that measurements registered radiation levels of 0.17 millisievert per hour at one location 30 kilometers northwest of the site. Those levels were anomalously high—at most other locations the levels were roughly 1 percent to 10 percent that much—and would have to persist for 60 hours to deliver 10 mSv of radiation. "Currently the levels over there are not that concerning," Vetter says.

What is more, expanding the size of the evacuation zone can have its own liabilities. "When you evacuate people, you have to make a risk-informed decision," says Stephen Musolino, a health physicist at Brookhaven National Laboratory in Upton, N.Y. "Evacuation is not always the best way to avoid a radiation dose." Sheltering in place is a very effective form of protection, Musolino says, because moving farther from the accident site does not necessarily mean escaping the radioactive plume. "The plume is going to generally go in one direction unless there's a wind shift," he says. "You could evacuate into the plume."

The NRC's McIntyre says that the recommendation for additional evacuations in Japan is not incompatible with the standard 16-kilometer evacuation plan for a U.S. accident. "The 10-mile [16-kilometer] zone was always conceived as something that could be expanded as the situation warranted," he says. McIntyre notes that the NRC's shift—from publicly backing Japan's handling of the accident to publicly recommending a much stronger response—stems from the fact everyone is reliant on the Japanese authorities for accurate, up-to-date information on the situation. "Part of this is the difficulty in obtaining and assessing data. We're dependent on the Japanese for the data, pretty much," he says. "We're trying to get the information and assess it. Yesterday the information we had led our team to conclude that it was time to take action."

Patricia Milligan, a health physicist with the NRC's Office of Nuclear Security and Incident Response, says that the evacuation assessment was "based on the great uncertainty surrounding the situation," including "some of the issues with communications surrounding the event." The Japanese recommendation, she says, "was certainly appropriate. Ours is much more conservative."

Whether or not a turn for the worse at Fukushima Daiichi would endanger residents beyond 20 kilometers, individuals in Tokyo, some 200 kilometers from Fukushima Daiichi, appear safe; those in the U.S. are almost surely beyond danger. "Tokyo is certainly at risk of having some radionuclides blown over in a plume," Vetter says. "I don't think they would be of sufficient concentrations to cause problems." Bushberg of U.C. Davis says it would be very unlikely that Tokyo would see any substantial harm from radioactive plumes. "That's a pretty significant distance," he says.

As for the reported runs on potassium iodide—a drug that blocks the uptake into the thyroid gland of radioactive iodine from fallout plumes—on the U.S. west coast, "I think that's being a little unnecessarily cautious," says Edward Christman, a health physics consultant with Christman Cua Associates in Princeton, N.J., and an assistant clinical professor at Columbia University.

"It's certain that at the moment there is no concern whatsoever for the west coast or Hawaii, for that matter," Bushberg says. "It would be hard to imagine a release so catastrophic that it would endanger people on the west coast."

Vetter notes that sensitive monitoring equipment in the U.S. may in fact detect fallout from Japan, but the levels will be negligible from a human health standpoint. "It's inconceivable that we would have health effects in this country as a result of this accident," he says.