So if its a little chilly in my room I can still be naked, feeling a bit cold but otherwise ok. My skin may be colder to the touch than normal, but I'm fine, im not going to go hypothermic. So if i can happily sit naked at say 18 degrees, at what external ambient temperature will my body be unable to maintain equilibrium with my surroundings ie my core temperature is not on a downward slope towards death.

To prempt a few questions I would like to assume: 0 wind, therfore 0 wind chill. I am at rest I am naked I am in a sufficently large body of still air that the heating effect of my body on the air around me is insignificant

As a mini follow-up, what is the lowest temperature you could maintain temperature through doing exercise? ie if you were running at 15 km/h, i assume your energy output as heat is increased, so i would assume that you could maintain equilibrium with your surroundings if they were at an even lower temperature