Some or all of the ideas in this post have been suggested many times, but not recently. I figured I'd start a new thread here, especially since the recent Siege Update has made these ideas all the more tantalizing to me. This, and poop/waste. But that is for another time.
The suggestions are as follows:
1. To simulate the harmful gaseous byproducts produced by these things in real life, furnaces, kilns, and other workshops that use (or in the future, might use) fuel for heating or burning things should produce smoke (for lack of a better approximation). This should occur while the job at the workshop is being completed. Where the smoke arises should be designated in the raws that define the workshop layout. For example, smoke could be made to originate at the north-most center tile of the smelter workshop where it looks like it should. The tiles surrounding this tile should either be made impassible to smoke, again defined in the raws. Or the tile should behave like the chimney/pipe tile suggested later on (see suggestion 3). This might need some further thought, though.
2. Related to this, smoke physics needs to change a bit for this to work in a way that isn't just cosmetic and that will require the player to make some functional design choices in fortress layout. While doing some !!SCIENCE!! I learned that miasma and smoke behave a bit differently. Smoke, once it propagates upward, doesn't propagate downward. Miasma seems to do so, though. I suggest smoke behaves more like miasma, but propagating upward more frequently than downward. I also suggest smoke diffusing more widely hanging around longer than it does presently for a given unit of smoke.
3. Introduce a chimney/pipe construction that only allows propagation/flow of fluid through it in a given direction and is impassible to anything else (though, maybe not vermin?). This could conceivably be placed where the smoke comes out of the workshop (see suggestion 1) and built in tandem with other pipes to direct where the smoke goes out. This could also be useful for directing other fluids.
4. In order to simulate the requirements of fresh air and upward draft for furnaces without revamping the physics of the game with more complicated and resource intensive modeling of fluids, I suggest at least non-magma smelters, kilns, and glass furnaces require an unobstructed path (perhaps not exceeding a certain length) to the outdoors from where the smoke comes out of the workshop (see suggestion 1) or chimney (suggestion 3). From my understanding, this wouldn't require too much overhead, no? I mean, the workshop could check maybe every 100 ticks or so to see if the path is still clear.
5. Sort of unrelated... Make kilns and glass furnaces able to use wood as a fuel. And kitchens, for that matter, if they ever come to need fuel. These don't require a reducing environment to function, so using charcoal, while it still should be available as an option, is a real waste. To do this, I suggest that, at least, the [FUEL] token in the raws be changed so that one can specify what fuel is permitted in reactions.
This might sound like a big change, but this is largely already possible with some DFHack scripting (at least suggestion 1 is). The only obstacle so far from my attempts is suggestion 2. If the physics of smoke changed, it would be possible to kludge it in with existing tools by the modding community. Anyway, with all these suggestions in place, it might make for some really interesting gameplay, in my opinion, especially with the new difficulties introduced with the new Siege Update. No longer can you just have your smelters and furnaces nestled in the same area as everywhere else without making some sort of compromise beyond access to stockpiles. You need to have some rudimentary ventilation considerations if it is going to be located indoors somewhere. I think this would be neat and is already within reach.