The coldest a human can tolerate for lengthy periods

Luxury! Try cryogenic chamber therapy. Guess this doesn’t qualify under “lengthy periods” but it sounds pretty “interesting”.

[Quote=Wikipedia]

The chamber is cooled, typically with liquid nitrogen, usually to a temperature of −120 °C (−184 °F)—although temperatures of −140 °C (−220 °F) or even −160 °C (−256 °F) have been used.[3] The patient is protected from acute frostbite with socks, gloves and mouth and ear protection, but in addition to that, wears nothing but a bathing suit. The patient spends a few minutes in the chamber. During treatment the average skin temperature drops to 12 °C (54 °F), while the coldest skin temperature can be 5 °C (41 °F). The core body temperature remains unchanged during the treatment, however it may drop slightly afterwards. Therapy triggers the release of endorphins which induce analgesia (immediate pain relief).

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Interesting, but it sounds like a really expensive alternative to opioids.

And a really expensive alternative to shovelling my front walk.

There a study done decades ago with Asian (Japanese possibly) children who were raised in an mainly unheated winter household environment, and were not bundled up in heavy clothing. Their rates of colds and flu etc was substantially lower than that of kids in heated houses who were bundled up.