kenjuudo and others:
I too have a chemistry background and I agree. Lead fume inhalation is a non-problem with bullet c asting temperatures. The real value of a face masks is for workers in a lead DUST environment, where health hazards from lead dust particulates is a real problem.
The fumes that you smell when bullet casting are mostly impurities that burn off or scorch at lead melting temperatures. They include oils, dirt, and grease, etc that would normally smell bad even if you were not melting lead.
An exhaust fan or better yet, casting outdoors is a good idea for that reason.