While I agree that a new soldering iron would be the best route to take, using a clean used one won't hurt anything. Lead and tin are metals that by themselves won't produce smoke. It will simply turn molten and possibly roll off. The smoke would be from the flux. Whether rosin or acid I wouldn't want it near my food.
Cleaning would simply be a matter of letting it heat up fully and then wipe off the tip with a wet paper towel or sponge. If you're really worried about it then after the iron cools off, hit the tip area with some sandpaper. The metals are soft enough that they can be easily sanded off.
Btw, I ordered the cold smoke generator from Mac's. I can't wait to try it out on some cheese, garlic, salt, bacon. Hmm.. maybe I should order some extra dust..
Russ