As an emergency crew attempts to prevent a catastrophic meltdown at Japan's Fukushima Daiichi nuclear power plant, dangerous radiation levels have prompted local evacuations and trepidation across the country, which is already reeling from the massive March 11 earthquake and tsunami.

What level of radiation exposure are the Fukushima workers—and everyday people—facing? Is the situation worse than Three Mile Island—or Chernobyl?

The Fukushima Daiichi crisis is so tense in part because of its unpredictable, seesawing radiation levels.

On Tuesday, for example, an apparent crack in one of the nuclear plant's four compromised reactors likely resulted in "a puff release, perhaps of radioactive steam," said health physicist Kathryn Higley.

"You saw radiation levels around the plant shoot up, although those appeared to have dropped off" soon afterward, added Higley, head of nuclear engineering and radiation health physics at Oregon State University.

At times on Tuesday, exposure levels at the nuclear plant reached up to 400 millisieverts of radiation an hour before plunging to 0.6 millisievert an hour, according to the International Atomic Energy Agency (IAEA).

A millisievert is the internationally accepted measure of radiation dose based on biological effects in humans, according to the IAEA.

Four hundred millisieverts an hour is "a lot—you certainly don't wish to be hanging around there for any period of time," Higley said.

By contrast, the average person receives about one to two millisieverts a year from natural sources in their environments, and one chest x-ray yields about 0.2 millisievert.

Fifty millisieverts is the lowest cumulative annual dose for which there is evidence of radiation-related cancer in adults, according to the World Nuclear Association.

(Related: 20 "unforgettable" pictures of the Japan tsunami and its aftermath.)

What Is Radiation Sickness?

People get radiation sickness—technically known as acute radiation syndrome—when they receive high doses of radiation over the course of minutes or hours.

When radiation is at the 400-millisievert-an-hour level, the nuclear plant workers are at risk of radiation sickness, depending on the length of exposure.

For example, a total dose of a thousand millisieverts in a day—say from two brief exposures to 500 millisieverts—can cause temporary radiation sickness, leading to nausea and decreased blood count but not death.

A few minutes of 5,000 millisieverts would likely kill about half those receiving it within a month, unless aggressive medical intervention was provided.

Once someone is exposed to these levels, "radiation has basically poked holes in cell walls," Higley said. "At some point, the cell simply can't handle any more damage, and at extremely high doses, you lose organ function.

"Still, even with high doses, if you get really good medical care, you can probably survive something like that."

As of now, there are only rumors of people reporting symptoms of radiation sickness, Higley said—"no definitive statements."

As part of the effort to keep things that way, authorities have ordered the public to evacuate to a distance of 12 miles (20 kilometers) from the nuclear power plant. From there to about 19 miles (30 kilometers), people have been told to stay indoors and seal all windows and doors.

(Find out how you can help Japan tsunami victims.)

Radiation-Risk Level "Hard to Tell"



As of Wednesday, some 330 Fukushima emergency workers were trying to keep the nuclear power plant cooled with seawater, to keep fuel rods from overheating and releasing more radioactive material.

To limit exposure, small groups are rushing in for roughly 15-minute shifts before hurrying back out, according to the Associated Press. But even this tag-team approach is no guarantee that the crew will make it out unscathed.

"I don't know any other way to say it, but this is like suicide fighters in a war," University of Tokyo radiology professor Keiichi Nakagawa told the AP.

Oregon State's Higley isn't so sure.

It's "hard to tell at this point," she said. "Reactors are designed with the recognition that there are potential areas where high radiation may exist. They have designed safe areas for folks to work in during emergencies. It really depends on what tasks need to be accomplished."

Right now the most pressing task for authorities is to get the reactors to cool down.

"The heat is driving a lot of the releases, so that is why they are trying to circulate seawater and remove excess heat," Higley said. "Once they have a stable cooling situation, they will look and see how they can better stabilize the plant."

Wider Risks of Radiation Exposure

In the longer term, Japan is concerned about fallout, especially given the country's high population density and small size, said health physicist Vadim Chumak. There simply aren't many places to flee.

A variety of dangerous radioactive materials are present at Fukushima, including plutonium. But the most worrying are cesium-137 and iodine-131. Both vaporize easily and thus can disperse over large distances.

Cesium-137 mimics potassium inside the body, seeking out muscle. Iodine-131 is rapidly absorbed by the thyroid gland and, in children, increases the risk of thyroid cancer.

Iodine pills—which authorities are giving out in the region—can help protect against the effects of iodine-131 when taken before or within an hour of exposure to fallout.

So far Fukushima's radioactive material has not risen especially high in the air. "It's gotten knocked down by wind, rain, and salt spray," Oregon State's Higley said.

"Also, it's sticky. It has a static charge that makes it stick to building surfaces. We're going to find most of the contamination is really close in, adhering to walls and bushes and the like."

Most of the released fallout will decay relatively quickly, said Chumak, head of external exposure dosimetry at the Research Center for Radiation Medicine of Ukraine.

Once the plant is relatively stable, "the best strategy would be just to leave the contaminated site alone for, say, one year," Chumak said. This will let radiation levels drop to manageable levels.

At that stage, the main risks come from ingestion of material such as cesium and strontium, "which will pass through food chains both terrestrial and aquatic," Chumak said.

This is very difficult to stop "without significant alteration of agricultural practices and food supply, though this is a more controllable phase of the accident, and authorities will have more time to cope with this problem."

Three Mile Island, Chernobyl ... Fukushima?

On Tuesday, the IAEA rated the seriousness of the nuclear emergency at Fukushima as a 6 on a scale of 7. By contrast, 1979's Three Mile Island accident in Pennsylvania was a 5, while the 1986 Chernobyl disaster in Ukraine (then part of the Soviet Union) was a 7.

(See pictures of Chernobyl 20 years after the disaster.)

Aside from sheer scale, there are key differences between Fukushima and past tragedies.

For instance, at Chernobyl—the worst nuclear accident in history—the reactor had no containing structure, and the explosion occurred when the system was at full power.

At Fukushima, the reactors have already been shut down, and their containment structures "appear to largely be functioning," Oregon State's Higley said.

Fukushima currently seems more like Three Mile Island, which lost coolant and had a partial meltdown.

Still, experts probably will not be able to quantitatively compare the catastrophes until long after this crisis has ended, "when people have access to the reactor vessels and can look at the cores," Higley said.

"It looks like some damage has occurred. As much as Three Mile Island? Don't know. There are radioactive releases that look to be more than Three Mile Island, but they may be locally contained.

"Only time is going to let us answer these definitively."