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.