I am not a chemist or a physician so this will be strictly the basics...
One of the nasty things about radiation is the effects it has on the thyroid system. If you are exposed to large doses of radiation it saturates your thyroid glands with radioactive iodine (radioiodine) and will most certainly cause cancer.
The idea is to saturate your thyroid glands with Potassium Iodine (which is inert); this prevents your thyroid system from being able to absorb the irradiated iodine. The dosage varies depeding on how the KI is packaged but the FDA recommends 130 mg daily for an adult. The protection the pills offer lasts for a twenty-four hour period.
These pills will not prevent the adverse affects of radiation! They will protect you from this very specific threat to your thyroid system. The pills should be taken for as long as there is radioactive iodine in the atmosphere...ideally you would move into a safe zone and not stay in the area of exposure long enough to have too many of these pills.
Here is a link to a company that manufactures KI under the brand name iOSAT.
I personally have a 2 week supply for myself and the wife on hand. The shelf life of the product is between 5-7 years depending on storage conditions but studies have been done on KI much older than that and it still effectively prevented radiation absorption as effectively as the day is was manufactured. I would replace every 10 years or so just to be on the safe side.
...that is all.
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