The stricken Fukushima Daiichi nuclear power plant in Japan has raised alarm over the possible health effects. However, so far, radiation levels outside of the plant remain relatively low and unlikely to cause health problems.

The health effects of radiation depend on the dose a person receives. The acute effects of radiation sickness usually begin when an individual receives a dose of radiation that is one sievert (the standard international measurement of radiation exposure) or above. Most of the workers hospitalized after the nuclear disaster that destroyed a reactor in Chernobyl in 1986 received estimated doses of between one and six sieverts. Because such levels are rarely encountered, radiation levels are most often given in millisieverts (one thousandth of a sievert) or microsieverts (one-millionth of a sievert). For comparison, a chest x-ray delivers about 0.2 millisieverts of radiation, and the average person in the U.S. is exposed to about six millisieverts of radiation per year, about half of which is from natural sources and another half from medical procedures.

Radiation levels at the Fukushima plant have fluctuated widely. The highest emissions levels so far are 400 millisieverts per hour—rates that are high enough to cause symptoms of radiation sickness within two or three hours. But that level quickly dropped, and other readings have been far lower. On Tuesday, measurements at the gate of the power plant ranged from 0.6 millisieverts per hour to 11.9 millisieverts per hour, according to the International Atomic Energy Agency. The levels at the gate were at 1.9 millisieverts per hour on Wednesday, according to the Japan Atomic Industrial Forum. Radiation levels, however, are changing rapidly, and there have been reports of rapidly rising levels due to problems with stored spent fuel rods. [UPDATE, 3/17/2011:  There have been reports that rates of 250 millisieverts/hour have been measured above the plant.]

The readings taken outside the plant don’t necessarily reflect the exposure to people working inside. Levels may be higher closer to the reactors, but workers are wearing protective clothing and using monitors to estimate their personal exposure, which they can limit by retreating to protected control rooms. Japanese authorities recently raised the maximum dose limit on the workers to 250 millisieverts, or five times the annual dose allowed for workers in the U.S.

People exposed to very high levels of radiation in a short amount of time are at risk for acute radiation syndrome, which can be fatal. William McBride, professor of radiation oncology at University of California, Los Angeles, says that at a radiation exposure of about one sievert, a person begins to experience sickness after an initial delay of a day or more. The most common symptoms are nausea and vomiting, diarrhea, and fever, and the illness often resolves within days.

At higher doses, symptoms become more severe and can lead to long-term health consequences or death. Radiation first affects cells that divide rapidly, including blood cells and the cells lining the gastrointestinal tract. At four or five sieverts, the effects can be life-threatening, and may include a need for a bone marrow transplant, or the use of bone marrow growth factor stimulants to avoid death within two to eight weeks. At higher doses, around 10 sieverts, McBride says, the intestines stop functioning properly, and this may cause death within a few weeks. At even higher doses, blood vessels become leaky and the brain is affected, likely causing death within 24 hours.

In terms of potential health dangers from radiation from the Fukushima Daiichi Nuclear Power Station, “the people who are in the most immediate danger from acute and severe radiation doses are those people who are on site at the moment and who are desperately trying to keep the reactors under control,” says Jacqueline Williams, a radiation oncologist at the University of Rochester Medical Center. 

Moving away from the immediate vicinity of the plant, radiation levels drop very rapidly. James Thrall, radiologist-in-chief at Massachusetts General Hospital, says that radiation levels are inversely proportional to the square of the distance from the source: The level at two miles from the source are one-quarter what they are at one mile, and “at 10 miles away, it’s almost an infinitesimal fraction,” he says. Individual exposure also varies widely depending on whether a person is outside or indoors, or shielded with protective clothing. Japanese authorities have evacuated the population living within a 20-kilometer radius of the plant, and have warned those living within 30 kilometers to stay indoors. Some experts say that people living beyond this range have no cause for concern at this time. “This has nothing to do with the general population,” McBride says.

The trickier question is whether lower doses of radiation—well below the threshold of acute illness—could lead to long-term health consequences for those in that area. Thrall says that epidemiological studies on survivors of nuclear attacks on Japan have found that those receiving 50 millisieverts or more had a slightly elevated cancer risk—about 5 percent higher than expected—and that risk seemed to rise with higher exposures. But scientists still vigorously debate whether that risk can be extrapolated down to even lower exposures.

After the nuclear disaster at Chernobyl, the population experienced a surge in thyroid cancers in children. However, scientists found that the culprit was not radiation in the air but radioactive contamination of the ground, which eventually found its way into cow’s milk. Thrall points out that in Japan, this is highly unlikely because the authorities are carefully monitoring the water and food supplies and keeping the public informed, which did not happen at Chernobyl.

Another important factor in determining the potential health consequences is the type of radioactive isotopes released from the plant. Different isotopes have vastly different half-lives; some decay almost instantly, while others persist for weeks or years. Iodine-131, which has been detected at the site, has a half-life of eight days, while caesium-137, also detected, has a half-life of 30 years. Japanese authorities have distributed over 200,000 doses of potassium iodine tablets, which can help prevent the risk of thyroid cancer from radioactive iodine. However, Thrall says, the pills can cause unpleasant side effects and rare serious conditions in people with allergies or thyroid problems, so they should not be taken indiscriminately.

There are major differences between the type of reactor at Chernobyl and the one at Fukushima, according to Peter Caracappa, a professor in the Radiation Measurement and Dosimetry group at the Rensselaer Polytechnic Institute. During an online Q&A hosted by Reuters, he said “the first difference is that at Chernobyl, there was a large quantity of graphite in the core which caught fire and spread contents of the reactor high into the air.” The Japanese plant  uses water rather than graphite. Second, the Chernobyl reactor did not have a containment structure like the ones present at these plants. Such structures, he said, “are designed to contain the contents of the reactor even in the case of an accident. A failure of containment, if it should come to pass, may allow materials from the core to ‘leak’ out, but they would not ‘spew’ out in the same way as Chernobyl.” That could limit how far the radiation spreads.