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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It’s triage.
They are still useful to people over 40 if the radiation dose is high enough, but they are more useful to people under 40.
The thyroid is more active and cells are generally more active under 40. Radiation disrupts this and can lead to higher cancer risks or issues if the thyroid is destroyed. Where as the chance of thyroid cancer from radiation generally goes down at around 40 since the thyroid becomes less active with age.
So the drugs are generally more targeted to the most vulnerable populations first.
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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