My friend was a blacksmith, who put some zinc coated pipe flanges in the forge to hopefully melt the zinc off them. This process was probably just a few minutes, just enough to heat the zinc. The fumes formed sulfuric acid in his lungs and basically he drowned in his own fluids. There was nothing the medics could do by the time he arrived at ER. From the smelting until his death was less than 24 hrs.
The same thing can happen if you use a welder or cutting rig on galvanized.
If you are outside and away from the fumes, you are probably ok. The idea is, you have to vaporize the metal to inhale it, not just melt it.
Lead melts at less than 1/4 of it's vaporization point. I've forgotten the vaporization point of zinc, but it's far below lead.
There is alway an element of danger when you vaporize any metal. Many modern steels have components that you don't want to inhale. Just ventilate, and be careful.
IMO, when smelting lead with zinc in it, the "boiling" you see is not really metal boiling but the "nastiness" cooking away.
If you ever have a bad headache and develop a fever after smelting any metal, seek medical attention just in case.....
Also remember that metals are cumulative in the body. It takes a while for the body to rid itself of them, and if you get too much too fast, you can be in trouble.
I've smelted very large amounts of lead for years in battery repair, and so far no problems. I have become acutely aware of the hazards of metals however, and take reasonable precautions....
Ben