So let me give you some clarity about lead casting..... It's safe if you follow proper safety precautions. If you're casting slugs in a mold, you're going to want a Lee lead pot with the bottom pour spout. Lead is very dense, so all the impurities will rise to the top. You can easily scrape them off with a steel spoon. But position a small fan to blow the fumes away from you. And do it in a well ventilated area. I cast in my garage at home. I open the big roll up garage door, then open the side door in the garage. This creates the conditions for wind flow. And I use a small fan on top of that to ensure all fumes are blown away from me. You can wear a mask for extra precaution, but I don't. I'm confident that there are zero fumes hitting my face.
I recommend using pure lead. I bought mine from rotometals on Amazon. Pure lead is temperamental. Way more so than alloy blends. If you're using a Lee lead pot, you only need to go to 3 or 3.5 on the heat setting. If you go too hot with pure lead it starts to crystallize and crust on the top. But pure lead is very malleable and soft. It will conform beautifully to the lands and grooves of your barrel.
But don't be scared to cast your own slugs. The piece of advice I would give you is buy the proper tools to do it correctly. Unless you're casting big bore slugs that weigh over a hundred grains, you'll find the task of trying to use a ladle to pour the slugs frustrating and inefficient. You definitely want to use the Lee pot with the down spout. I have LOTS of experience with this. So does jwrabbit123. As a matter of fact, he probably owns more molds than anyone on here. He would be a great person to seek advice or chime in on this thread.
Not to discredit Yo. I like Yo. And his concerns are legitimate. You just need to exercise precaution when working with lead. Before I got into air rifles, I was into fishing (and I still am). To this day I cast my own fishing weights. I have many different molds. I'm still here, and I've been doing it for 15 years now. But I've ALWAYS heeded the warnings, and gone out of my way to avoid inhalation of lead fumes.