It is agreed that lead when it enters you body is universally bad. However, melting lead should not create lead fumes. Lead melts at 621 degrees Fahrenheit and boils at 3182 degrees Fahrenheit. There is no reason to ever heat lead up to the boiling point to pour sinkers.
Your most likely route of entry will be through ingestion (swallowing) or inhalation of inorganic lead particles or through transdermal (through your skin) absorption of organic alkyl lead.
Your respiratory tract provides the most effective route of absorption because it only depends on the size of lead particles and the metabolic activity of the body. Airborne lead particles that are less than 0.5-1 microns in diameter generally are completely absorbed. Gastrointestinal absorption of lead is less effective and depends on a number of factors, eg, the presence of food in the stomach, the concentration of lead ingested and your general nutritional health
What dose this mean?
It means that you are most likely to put lead into your body when to have some lead dust on your hands and put it in your mouth (eating, smoking, chapstick, nose picking, etc.). Remember lead dust is lead and does not enjoy being airborne.
There are respirators that can protect you if worn correctly. However, wearing a respirator has risks associated with the increased burden it places on your cardio vascular system. The best protection is to not be exposed at all. Ventilation is primary, and should always be employed.
These filters are called HEPA and are effective against metal fumes and particulates down to 0.3 microns.
IMO the most important thing to understand is that you can take every precaution to protect your self, but still bring the lead dust into your home. Dust that is on your clothing or skin can be transmitted to children, spouses and pets that never came close to the pouring.
Just remember that good housekeeping, personal hygiene and common sense in most cases will keep you and your family safe if you choose to pour lead.