My country started distributing potassium iodine tablets to take in case of a nuclear accident or attack, but only to those under 40 years old (with prescription). Why? What happens at 40 so the pills are bo longer useful?
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For background: one of the major isotopes produced by nuclear fission is iodine-131. This might be released into the environment by a bomb explosion or a serious accident at a nuclear power plant, as happened at Chernobyl and Fukushima-#1.
I-131 has a half-life of about 8 days, which means it’s *very* radioactive. The flip-side of that is that it doesn’t last very long — three months (more than 10 half-lives) after the release, it’s essentially gone.
The human thyroid gland needs iodine, and if I-131 is what’s available, it’ll take it, after which it’ll irradiate the thyroid and surrounding tissue. This’ll cause a significant risk of thyroid cancer. But the gland only needs so much iodine, so if people take non-radioactive iodine in pills, they can saturate the gland. In which case, any radio-iodine ingested will, well, go in one end and out the other, without doing noticeable damage.
https://en.wikipedia.org/wiki/Iodine-131
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