Not just any respirator, look for a p100 particulate filter, and don't think that just because you've got a respirator you're safe. Melt it outside, you'll know when you're to pouring temp when the zinc smoke is coming off of it. I've poured alot of brass using just a heavy walled steel crucible, I'm sure that the steel will affect the alloy of brass, but for most aspects you'll be more than fine. Don't be scared off, just be carefull and know the dangers, if you're adding brass to the melt, make for sure to heat it up to drive off moisture first, I was adding copper pipe to a bronze melt and learned this lesson first hand, the copper pipe was folded up and had some moisture in it, luckily I was wearing a full face shield and didn't get burned too bad, always have a clear way to run without objects to trip over. You'll be fine, just play it safe, casting metal is a rewarding hobbie in itself.
Zeb