system
Given the following post, respond as if you're commenting on Reddit:
human
[Lord of the Rings] Where does the smoke go in an underground place like Moria?

It  doesn't have to be Moria, it is just the most known example. Imagine an actual underground city in a medieval setting (so no electric lights,  also not so much magic). You obviously need a lot of fire, or you just  don't see a damn thing. Wouldn't they get a smoke poisoning sooner or  later? The underground halls might be huge, but they're still closed  rooms, the only openings would be tunnels.

I am asking this question, because I am in the process of creating such a city for my ficitonal world. And then it is an interesting question for fantasy worldbuilding in general, if you think about dungeons in video games and such.
gpt
The mountain above the city is full of air ducts that not only direct smoke out, but allow fresh air inside. Many of them are artfully carved so as to blend seamlessly into the rock above everyone's heads, or are part of the buildings around everyone. Small curves in the vents would also prevent sunlight and rain/snow from falling into the settlement. 

They are so numerous and relatively small that the amount of smoke released by each vent is negligible, and thus the mountain doesn't appear to be wreathed in smoke at all times. 

Keeping these vents clear would be a massive and endless job, but the constant maintenance would ensure that nothing clogs and ends up killing all of the inhabitants.