Most tumblers have an open top(mine does), but some are totally enclosed like the Lyman "wet" dish for the Turbo-Twin model. Perhaps they all have one to stop any dust escape? You could rig up a little extractor and vent if you were really worried, I suppose!
Most cast or swaged bullets and slugs I handle are coated with alox or other lubricant or graphite( Winchester shotgun shot) - so direct contact is rare. The same result can be obtained with a patch at the bottom of the air pellet tin moistened with a little Tri-Flow teflon spray.
I suppose swaging is a safer process as regards vapour etc contact/ingestion compared with casting, but lino-type alloy would be a bit difficult to use.
When I was younger I regularily crimped the lead split-shot on my line with my teeth, I would use my Leatherman super-tool pliers now!But...
"Worry makes cowards of us all."
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***Big Bunny***
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