It’s precisely because elemental iodine is very reactive. The thyroid has follicles, where iodine is pumped into a gel in ion form, and then enzymatically converted into elemental iodine, where it then attacks thyroglobulin to create thyroid hormones. The final hormones are the only molecules allowed out of the follicular lumen back into the blood.

I read it as meaning food waste in the household, as opposed to commercial waste (so still regular per capita, otherwise you’d just write kg/household surely?)