The fumes are sulfur dioxide, best known as one of the components of smog. Being near a pot of molten sulfur, it rapidly becomes difficult to breathe. If you want to find out what asthma feels like, doing sulfur inlay is probably a decent approximation (and if you have asthma, you should never be around sulfur dioxide fumes, as they can trigger a severe attack). This is not something you want to do in the house, especially if you want to continue living in that particular house. Oh, and after you work with sulfur for a while, it reacts with the bacteria on your hands and forms hydrogen sulfide, giving you the smell of rotten eggs.