My advise. Stay out of the house with lead melting. The only concern about dampness in the garage is when putting lead in a molten pot. It it contains moisture it can blow molten lead up on you. Or if a ladle is coated with marvalux flux it will spew lead all over if not preheated well before putting it in the melt. Marvalux is used to minimize smoking, but my advise is don't use it if your pot will be stored in a damp area or the area gets damp from humidity. Use any kind of rendered animal fat for flux. It will smoke like everything, but is not hydroscopic. To the contrary it repels mositure if some is left there when done casting. For years when I worked in a damp climate, I put a bit of lard on the alloy left in the pot when it got cool enough that the lard would still melt but not smoke. This flowed down around the lead and oil sealed the pot sides from mosisture to prevent rust for unlimited time.
Lead isn't supposed to give off fumes until quite a bit over the melt temp, but cast a few hours in a closed room and you won't feel good. With lots of fresh air carrying the fumes away, no problem.
When shooting lead indoors, be concerned about lead vapor if you are getting the least trace of leading in the barrel. That is caused by lead which is melted by friction, and when it's happening, you are getting serious lead vapors. LBT lube will take you farther from leading than any other lube available, I believe. Of coarse I'm a bit biased, because I spent our life savings and 6 months full time developing it! Check my website if interested LBTMoulds.com