No respirator or mask when casting. I keep the garage door open and a fan running to blow the fluxxing fumes away. We are not getting to the vaporization temp of lead. Most of the lead poisoning you read about is from ingestion, ie eating. Just wash your hands thoroughly after casting and you'll be fine.
FWIW, I know of several folks, one is my neighbor, who have documented high lead levels in their blood. None shoots muzzleloaders or cast anything. They have, however, been avid shooters at indoor ranges with modern arms. One of the main ingredients in modern centerfire primers is lead styphnate and that gets into the air, on the floor, and coats most everything at an indoor range.