Let’s assume that the Fukushima Daiichi reactors collectively manage a plume the size of a square kilometer with radiation levels of 400 mSv/hour. Now, to get comparable numbers we need to get the dose per year:
400 x 24 x 365 = 3.5 million mSv/year.
Now let’s take that as a uniform distribution over 1 km square and spread it out over the whole earth. Divide by 500 million square km (global distribution).
3.5 million ÷500 million = .007 milliseiverts / year
One Japanese reactor site is not going to sustain that level of emission for a week, much less for a year. Also, 400 mSv/hour is probably a peak value not average value. The real numbers are much, much less. So,the absolute crazy-absurd worst case scenario is less than .007 mSv / year globally.
Typical, natural background radiation levels are about 2.4 millisievert (mSv) per year. You are already being irradiated at this moment with 342 times the absolute worst case dose from that reactor. So crunch your iodine tablets if it makes you happy, but people in Japan are suffering from the quake and tsunami damage, not radiation. How about we spend our iodine budget on helping out the Red Cross?
Adapted from Pournelle http://www.jerrypournelle.com/