So many different numbers and scales and claims are being bandied about regarding the radiation from the damaged Fukushima power plant, and the dangers it does or does not pose. It is all very confusing. I’ve tried to make sense of some of it below.
I don’t know whether my calculations are valid (there’s many different kinds of radiation; some are more dangerous than others; also people absorb radiation at different rates; and there are no doubt many other complicating factors), but they are what I’ve been able to come up with. If anyone knows better (or I made some calculation or logical error) please let me know.
According to Idaho State University’s Radiation Information Network, average yearly exposure to radiation in the United States is around 3600 microsieverts. This includes radiation from natural sources like radon in the air, or cosmic radiation from the sun, as well as radiation from medical tests. This is equivalent to 360 millirems (e-1)/ 3.6 millisieverts (e-3)/ 0.36 rems ( e-4)/ 0.0036 Sieverts (e-6).
US regulations cap nuclear power plant workers’ yearly exposure at 50 millisieverts (that’s 50,000 microsieverts). (See Table 4 on the link).
Reuters reports that radiation exposure does not pose an elevated cancer risk until it reaches 100 millisieverts (that’s 100,000 microsieverts) a year. Even that level is disputed; the evidence that this amount does, in fact, increase cancer risk is far from conclusive, according to the Health Physics Society. (See the Power Point download on Radiation Effects).
Acute radiation sickness occurs in exposures of over .25 sieverts (that’s 250,000 microsieverts) in the course of a few hours or a day (See Table 2). In the US, rescue workers may be asked (on a volunteer basis) to expose themselves to up to twice this much radiation in "once in a lifetime" life-threatening rescue situations.
So, where do the levels stand from radiation emitted by the Fukushima Daiichi power plant? In 66 measurements from 34 locations in Fukushima prefecture on Thursday Friday, March 17, the median level of radiation measured was 3.7 microsieverts/hour. Even if these levels continued for a full year, this radiation dose does not come close to the threshold for elevated cancer risk (3.7 * 24 *365 = 32,412).
In Tokyo, March 17 readings show levels of radiation at .053 microsieverts/hour. That’s only about 1/70 of the median measurement in Fukushima, so really, Tokyo is fine, too.
However, there are some stations in Fukushima that posted measurements far above the median, and these are quite worrisome. Stations 31, 32, and 33 (all located just outside the 30km perimeter to the northwest of the plant) recorded an average reading of 104.34 microsieverts/hour. The nine measurements from these three stations are by far the highest recorded by any station. At this level, 40 days of exposure increases the risk of cancer (104.34*24*40=100166.4).
Radiation readings come from the Japanese government, and are available here.