NPCs do not suffer environmental damage, only player characters do. Environmental damage is inflicted once a day at a certain tick. Sorry, I can't remember when exactly it is since it is a time when I'm usually asleep anyway.
It's not exactly a bug, but damage to npcs was something that Zhao had alluded to wanting to add but never did. I think at this stage it would warrant a new discussion to work out the specific functionality of environmental damage. There were some tentative plans to modify it so that damage would be taken upon exposure (when you unboard without the proper gear, or when you remove your gear outside), but it hasn't received any work in ages. Basically, this isn't something that would be easily patched up because it still needs discussion and Sel will need to make a final decision.
The tick is at 17:00 BTW. (Also subject to change in the distant future :p)
"On two occasions I have been asked,—"Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?" I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage