Iodine-131 and Caesium-137 are the radioactive substances we are concerned about being emitted from the nuclear reactors in Japan. Iodine-131 emits beta radiation, blocked by clothing, but when ingested, can cause illness. Potassium iodine is taken to block some of its effects on the thyroid. The half-life of Iodine-131 is also pretty short—8 days or so. Caesium-137 is gamma radiation which can penetrate the body. The half-life of this source is 30 years—a longer-lived problem. Distance and limiting exposure from the source is the best way to protect people from radiation.
At one point, one of the nuclear reactors released 400 mSv (milliSievert) per hour. At the 250-350 mSv (25-35 rem in older terminology) exposure level, we begin to worry about the immediate effects on the body. It appears that the levels of radiation were high enough at one time to make people ill. The age and size of person, length of time of exposure, and inherent biology of the person are the factors determining how sick they would get.
Rapidly dividing cells in our body, like the ones lining our gastrointestinal tracts and producing blood cells in our bone marrow, are the most susceptible to radiation effects—acute cell killing and DNA damage. These cells rely on rapid development and growth of precursor cells to replenish them.
Levels of radiation, around the 250-350 mSv level may cause nausea and vomiting, sometimes limited in nature, but long-term effects described in the next paragraph may result.
At higher levels (around 500 mSv, 3Gy) dysfunction of bone marrow can lead to low white blood count and increased risk of infection, decreased platelets and increased bleeding risk, and decreased red blood cells and anemia. This dysfunction may not be evident for two weeks. Cell-mediated immunity can also be affected, leading to either more unusual or to gastrointestinal infections. At this exposure level, the long-term risk of leukemia and other cancers is high.
Levels of 1000mSv (approximately 10Gy, not seen at this time in Japan) can lead to more immediate effects of overall gastrointestinal failure, diarrhea, and infection. Hospitalization, antimicrobials, and fluids are required for people with this level of exposure. Without medical attention, people with this amount of radiation exposure would die.
Exposure to over 7000 mSv (approximately 20Gy) is mainly fatal; only supportive care is available to make person comfortable.
If there is any good news, it appears that radiation exposure of individuals has been limited; monitoring of radiation levels, screening of potentially exposed people, treating the exposed, and evacuation is also occurring.
There are some reports of three LA Times reporters being ill due to radiation, and that some US military personnel were exposed. I don’t know if these reports are substantiated, but long term monitoring of their immune system is one of the keys to their overall safety.
Here is a chart showing radiation levels and when danger occurs.
Another source that is helpful--http://blogs.nature.com/news/thegreatbeyond/2011/03/fukushima_crisis_radiation_exp.html
Please keep the people of Japan in your thoughts and/or prayers.