The graph shows per capita thyroid doses in the continental United States of Iodine-131 resulting from all exposure routes from all atmospheric nuclear tests conducted at the Nevada Test Site. Notice that the highest levels of Iodine-131 are NOT at the test site. This is because particles carried by the wind get washed out of the air by rain and fall to the ground at various places around the country, controlled mainly by wind patterns. Also note that the half-life of Iodine-131 is only 8.07 days. In 80 days, or about 10 half-lives, only 1 part out of 1,024 parts would remain. Between 1945 and 1963 the US detonated 183 atmospheric tests, which is a test every 35 days. So, for 18 years, there was a steady supply of Iodine-131 being supplied to the atmosphere. People living in Lincoln, NE during those 18 years received a total dose of between 4 to 6 rads, which could be between 6 and 120 rems, depending on the "Q" factor. Regardless, spread out over 18 years the danger from Iodine-131 from US nuclear testing was so low that it was negligible.
A level of exposure that low would NOT necessitate using Potassium Iodide (KI). The Thyroid gland requires Iodine in order to create Thyroxine, which is a prohormone and a reservoir for the active thyroid hormone triiodothyronine. It is involved in controlling the rate of metabolic processes in the body and influencing physical development. Taking KI before and during an exposure to Iodine-131 reduces the amount of I-131 taken in by the Thyroid gland because it become saturated by the normal Iodine in KI. HOWEVER, taking KI does NOT protect against exposure to other components of nuclear radiation, like Cesium-137 (half-life of 30 years) or Strontium-90 (half-life of 28.8 years).
NOTE: For FORTY YEARS the Fukushima power plant has been storing its spent fuel rods in seven water pools, six of which are on top of the reactor buildings. That's about 600,000 (no typo!) fuel rods, each 4 meters long, which have to be kept under water because they are still radioactive. Water acts as a Neutron moderator preventing significant additional fission from taking place. Remove the water and the rods will begin to fission again, generating heat. While not enough to be useful to generate electric power, it is enough to heat the rods up. Most of the Uranium is in the form of oxides, but some is converted by Hydrogen and other metals to Uranium metal, which is pyrophoric .... it reacts spontaneously with Oxygen in the air when it gets hot enough. A Depleted Uranium sabot, when it penetrates the target, gets hot enough to explode, which is why it is such an effective round for a tank gun.
Forty years worth of spent fuel rods setting on top of those reactors in Japan has, as an engineer in the US said, the potential to become Chernobyl on steroids. This is information that most media won't pass along because of the panic it can cause. We weren't "down wind" from Chernobyl, but we are from Japan. If the worse happens exactly how much radioactivity will land on American soil is anybody's guess, but it would be a miracle if some didn't.
The usual dose for KI is 135mg/day for adults. It is non-toxic in these amounts and only twice as toxic as ordinary table salt. It should be taken at least 6 hours before Iodine-131 is expected to arrive and for as long as it is present in the environment. Remember, its half-life is only 8 days. In three months most would be gone unless it is continually refreshed from the source. As I write this (3/16/2011) all the KI sources in the US are out and KI won't be available till the middle of next month. Con artists and gougers are asking up to $500 for what normally costs less than $15. When it becomes available again you should be able to get it from here.
NOTICE: Potassium Iodide is of NO USE in protecting against other radioisotopes, like Cesium-137 or Strontium-90, both of which would probably be present in large or larger amounts with the radioactive cloud that contained Iodine-131, because their half-lives are 30 years, not 8 days. To protect from them you would need to stay indoors or wear safety suits and micron filter masks when going out doors. Drink distilled or reverse osmosis water and wash all food prior to eating if it was exposed to the outdoors.
If, as one expert has said, this incident becomes worse than Chernobyl, then we will have a scary ride ahead of us.
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