Are you talking about preps for someone who lives near a nuclear power plant in anticipation of some meltdown? I ask because when people say "fallout" they are often referring to nuclear weapons, and that's a different situation. Radioactive iodine is a primary contaminant from nuclear power plants, but not as much from nuclear weapons.

And don't forget that in both cases, dozens of radioactive isotopes are spewed into the environment from the nuclear reaction. The situation with iodine and thyroid cancer just happens to be a convenient combination to try and block with pills. Taking KI pills is not some anti-radiation pill that protects you from all the radiation. That's only in science fiction.

There are standard guidelines for taking potassium iodide pills, like they hand out to residents near nuclear power plants, and just follow the directions on any commercial KI pills out there or what local authorities tell you if there's some nuclear power plant accident. But I think Fukushima has turned many of those assumptions on their head because it's an ongoing nuclear accident instead of a discrete event with a definite end. I'm not sure how much radioactive iodine is still being emitted from Fukushima (probably not that much compared to early on), but other isotopes like cesium-137 and strontium-90 are still leaking and getting into the air and the ocean.

Oceanographers have noted that the level of radiation in the Pacific was expected to spike and then gradually return to a fairly low level. However, in reality, the level only reduced part of the way and then leveled off at a higher than expected level. That suggests that more radiation is being added to the Pacific on an ongoing basis.

There are plenty of people who supplement with iodine long term for health reasons, but it's a risky thing to do without medical supervision because you can screw up your metabolism and thyroid hormone levels with long term supplementation. Eating more iodine rich foods, like kelp and dulse, though, should not be a problem. People like the Japanese eat a lot of it everyday as part of their normal cuisine.

That's probably not exactly the answer you were looking for, JP.